DHW and Backup heating spec for PH

sas

Hi,

This one is really getting to me.

I'm building (towards) certified passive. I'm not there yet however and with the blower door still to go I'm not taking success for granted.
Also, I'm not convinced that in the depths of winter, 20 degrees is going to be sufficient for us i.e. even if we do hit certification level, comfort may still be an issue.

In summary, I'm planning for a slightly higher than expected heat requirement.

This is the spec that I currently have on paper.

(a) 90 Tubes (installed already so no going back on this).
(b) Supplier of tubes is specifying a 1000 litre stratified tank for both DHW and backup heating. This would have all the necessary coils etc.
(c) 10 kw timber stove, 7 to back boiler and 3 to room. This is what I hope to heat the tank with in winter.
(d) UFH circuit downstairs with heat supplied by 1000 litre tank
(e) Water based heater for hrv supplies on upstairs room (installed).
(f) Very small rads in bathroom and ensuite on North of building.

My concerns.
1. The tubes will give bugger all in winter so I'd be depending entirely on the stove. Not convinced this will be able to keep that much water to a high enough temp i.e. lots of luke warm water
2. The hrv heater needs hot water. I would like to draw this off the tank, I do wonder if the tank will be hot enough to support this.
3. The rads will need hot water, again I'd like to draw this off the tank, I do wonder if the tank will be hot enough to support this.
4. Will the tank be able to also supply the ufh?

Some of this is sparked by my plumber reckoning 1000 litres is insane. Also a recent article in CI magazine where a PH was designed with a dual tank system. A small tank (say 200 litres) which is targetted first by solar\stove for DHW. Once this hits temp the excess dumps to a second 1000 litre buffer tank. The designer of this system comes highly recommended in the Irish PH scene but I can't afford him. The concept being that 200 litres of hot water is alot more usable than 1000 of warm water.

I should state that I'll also have provision to connect an external boiler if needed to the tank e.g. oil. This provision will be a flow and return from the tank room to outside the house via a service pipe in the floor. If needed we can connect to it.

Thoughts?

fclauson

SAS

Here are my thoughts - regardless of being Passive you will need a constant supply of heat in the cold weather - your HeatDemand sheet will show what constant figure your need - this number will vary due to solar gain etc but is needed day and night (because a passive house stays at the same temp)

PHPP works on averages - the dublin weather data assumes a low average of -0.2 - and at that from you HeatDemand page you will get two load figures - one for a cloudy warmer day and one for a bright but colder day.

Now - if the temp drops to -5 and is a dark murky one you will need to pump in more heat to maintain your 20 degress - PHPP does not really allow for this. Also - the PHPP model makes assumptions about internal heat gains - if you do not achieve these (people build bigger houses than the 35Sqm/person model PHPP is built on ) then you will also need more heat.

Also if you solar panales get a few cm of Snow - they will not work a jot (and last winter that may have lasted 2 or 3 weeks !!!!!)

So - you will need heat - your stove will do some of the work-you say it can deliver 3Kw to the room - constantly - but if the water going through the back boiler is cold (e.g. post bath or shower) will still perform still ?

Your tank (assume 1000L) can store about 54Kwh of useful heat - but the physics is not that easy - a tank at 80degrees or a tank at 30degrees - both hold heat - but how much of it is actually useful. Your UFH needs about 28->35 degrees. Your rads etc need more - and your MHRV will need 54degrees.

So in summary - you WILL need a constant dependable reliable source of heat in the cold weather - the solar may not work - the stove will have its output figures measured in a lab at optimum settings - and will need stoking day and night to keep it delivering. If it goes out at 1am then the house will cool and by 7am you house will have a higher demand.

What a I doing
1 - about 6 to 8 sqm of panel (enough to meet Part L)
2 - 500L store (probably)
3 - UFH
4 - bathroom towel rails
5 - oil burner - (although i would prefer gas as boilers can modulate but have been wholly advised against it)
6 - stove in sitting room - your cannot sit around UFH heating drinking your hot toddy

The house has a very low demand - but does have a demand. At -0.2 it reguires 3Kwh - but at -17/no solar gain/all water used up by baths etc it will require 8 to 10 Kwh

Why the above choice - simple - the above will work in any weather - regardless of what temp the house is - it will warm up the house - FACT

Heatpumps or woodpellet all sound attractive - but oil works on the button - on demand in ANY weather - it does not frost up or drop its COP (much) based on outside air temp

Remember my loads is about 10% of a current part L build - so I have done my bit to reduce usage -

BryanF

sas wrote: »

Hi,

This one is really getting to me.

I'm building (towards) certified passive. I'm not there yet however and with the blower door still to go I'm not taking success for granted.
Also, I'm not convinced that in the depths of winter, 20 degrees is going to be sufficient for us i.e. even if we do hit certification level, comfort may still be an issue.

In summary, I'm planning for a slightly higher than expected heat requirement.

This is the spec that I currently have on paper.

(a) 90 Tubes (installed already so no going back on this).
(b) Supplier of tubes is specifying a 1000 litre stratified tank for both DHW and backup heating. This would have all the necessary coils etc.
(c) 10 kw timber stove, 7 to back boiler and 3 to room. This is what I hope to heat the tank with in winter.
(d) UFH circuit downstairs with heat supplied by 1000 litre tank
(e) Water based heater for hrv supplies on upstairs room (installed).
(f) Very small rads in bathroom and ensuite on North of building.

My concerns.
1. The tubes will give bugger all in winter so I'd be depending entirely on the stove. Not convinced this will be able to keep that much water to a high enough temp i.e. lots of luke warm water
2. The hrv heater needs hot water. I would like to draw this off the tank, I do wonder if the tank will be hot enough to support this.
3. The rads will need hot water, again I'd like to draw this off the tank, I do wonder if the tank will be hot enough to support this.
4. Will the tank be able to also supply the ufh?

Some of this is sparked by my plumber reckoning 1000 litres is insane. Also a recent article in CI magazine where a PH was designed with a dual tank system. A small tank (say 200 litres) which is targetted first by solar\stove for DHW. Once this hits temp the excess dumps to a second 1000 litre buffer tank. The designer of this system comes highly recommended in the Irish PH scene but I can't afford him. The concept being that 200 litres of hot water is alot more usable than 1000 of warm water.

I should state that I'll also have provision to connect an external boiler if needed to the tank e.g. oil. This provision will be a flow and return from the tank room to outside the house via a service pipe in the floor. If needed we can connect to it.

Thoughts?

can i just ask, your expected heat/hw demand will be far lower than 10kw? so how much on paper is this oversized... the idea of the 2 tanks makes sense (i'd bow to someone with more M&E knowledge) but wasn't there another article in the same mag where 2 x 1000l tanks were installed and only one every used...

sas

fclauson wrote: »

SAS

Here are my thoughts - regardless of being Passive you will need a constant supply of heat in the cold weather - your HeatDemand sheet will show what constant figure your need - this number will vary due to solar gain etc but is needed day and night (because a passive house stays at the same temp)

PHPP works on averages - the dublin weather data assumes a low average of -0.2 - and at that from you HeatDemand page you will get two load figures - one for a cloudy warmer day and one for a bright but colder day.

Now - if the temp drops to -5 and is a dark murky one you will need to pump in more heat to maintain your 20 degress - PHPP does not really allow for this. Also - the PHPP model makes assumptions about internal heat gains - if you do not achieve these (people build bigger houses than the 35Sqm/person model PHPP is built on ) then you will also need more heat.

Also if you solar panales get a few cm of Snow - they will not work a jot (and last winter that may have lasted 2 or 3 weeks !!!!!)

So - you will need heat - your stove will do some of the work-you say it can deliver 3Kw to the room - constantly - but if the water going through the back boiler is cold (e.g. post bath or shower) will still perform still ?

Your tank (assume 1000L) can store about 54Kwh of useful heat - but the physics is not that easy - a tank at 80degrees or a tank at 30degrees - both hold heat - but how much of it is actually useful. Your UFH needs about 28->35 degrees. Your rads etc need more - and your MHRV will need 54degrees.

So in summary - you WILL need a constant dependable reliable source of heat in the cold weather - the solar may not work - the stove will have its output figures measured in a lab at optimum settings - and will need stoking day and night to keep it delivering. If it goes out at 1am then the house will cool and by 7am you house will have a higher demand.

What a I doing
1 - about 6 to 8 sqm of panel (enough to meet Part L)
2 - 500L store (probably)
3 - UFH
4 - bathroom towel rails
5 - oil burner - (although i would prefer gas as boilers can modulate but have been wholly advised against it)
6 - stove in sitting room - your cannot sit around UFH heating drinking your hot toddy

The house has a very low demand - but does have a demand. At -0.2 it reguires 3Kwh - but at -17/no solar gain/all water used up by baths etc it will require 8 to 10 Kwh

Why the above choice - simple - the above will work in any weather - regardless of what temp the house is - it will warm up the house - FACT

Heatpumps or woodpellet all sound attractive - but oil works on the button - on demand in ANY weather - it does not frost up or drop its COP (much) based on outside air temp

Remember my loads is about 10% of a current part L build - so I have done my bit to reduce usage -

Wow, you never cease to amaze me. What I'm taking from the parts of this that I understand is that you by and large agree with me except you'd go for a smaller tank.

I also agree wit you on the heat pumps, I don't like them. Capital cost is always much higher than an oil burner.

As regards your question on the stove when the cold feed is coming back, that part is covered. The pump set for this specific stove comes with a mechanism to address this. Basically, the hot water pipe leaving the stove has a T on it that connects to the cold return pipe i.e. the outgoing hot water is used to increase the heat of the returning cold water. I do agree though that the efficiencies assume the stove is at full tilt with ideal fuel etc.

Thanks

sas

BryanF wrote: »

can i just ask, your expected heat/hw demand will be far lower than 10kw? so how much on paper is this oversized... the idea of the 2 tanks makes sense (i'd bow to someone with more M&E knowledge) but wasn't there another article in the same mag where 2 x 1000l tanks were installed and only one every used...

Not sure which figure you're asking for here.

Specific Space heating demand: 10 kWh/(m2a)
Heating load: 8 W/m2

I need to reread that issue and check your point.

Do-more

sas if you are worried about your water temperature would you consider fan coils instead of the rads as they can run on much lower water temps?

I'm looking at a fairly similar solution to yours.

I will almost certainly have approx.8m2 of solar but count on them doing very little for me for about 3 and half months of the year.

In terms of the lowest capital costs I am looking at an air to air heat pump (about €1,500 fitted) and the immersion for hot water for those winter months.

I am confident that the air to air can do the job for me in terms of supplying enough heat as we have one in our current house which is no where nearly as well insulated and can have the place at 22deg C if we choose on the heat pump alone most of the time. It did struggle to keep the place at 20 degrees for two days in total last winter. But that was with overnight temps of -17 and never above -10 during the day.

But I am also looking at alternatives for the winter months that won't have me so dependent on electricity prices. The top choice is a 15-20kW wood pellet boiler in the utility room served by a small hopper beside it which I will load with bagged pellets. One of the suppliers I'm considering does a 750 litre accumulator tank. I haven't spoken to them in depth but I'd imagine it shouldn't have a problem keeping the water at above 60 deg C and probably won't be running at anything like full capacity. My original plan was looking at a 500 litre tank but one guy I priced solar panels off did mention the possibility of running two tanks, a 300 solar litre tank and a 200 litre tank connected to it heated by the pellet boiler.

For the space heating in this scenario I would also use a hot water duct heater. I figure the wood pellet boiler will halve my heating + DHW costs compared to the heat pump and immersion but I figure the payback period will be very considerable given the increased capital costs.

So there you go, I'm no help as I am even more confused than you!

sas

Do-more wrote: »

sas if you are worried about your water temperature would you consider fan coils instead of the rads as they can run on much lower water temps?

Actually, I hadn't considered those. Nice idea. Will have to consider.

fclauson

My comment on that is noise

You MHRV can be silenced with ducts and silencers

Fans in a room cannot be

Do-more

fclauson wrote: »

My comment on that is noise

You MHRV can be silenced with ducts and silencers

Fans in a room cannot be

As sas is saying that the only place the rads are to be used is in a bathroom and en-suite I don't think that noise is going to be an issue, certainly not in the way it could be in a bedroom for example. As the heat demand is probably going to me very low anyway fan speeds are likely to be very modest.

Certain brands of fan coils use impellers rather than fans, these have the advantage that they are generally quieter and should not pick up dirt on the impeller surface whereas a fan which accumulates dirt over time is likely to become unbalanced and cause bearing wear which leads to added noise and eventual failure.

On the issue of your concerns about being able to heat the house to 22 deg C or whatever during very cold weather, what about adding an electrical UFH cable near the top of the slab when you are installing the pipes? I know it is not energy efficient but you would only need a very light cable widely spaced to be sure you can boost the indoor temperature up to the required level.

It shouldn't cost much to install and would in all probability never be used but if you are looking for a low cost solution to give you peace of mind it might be worth considering?

The other option is to consider an electrical duct heater rather than the hot water one. Again not as energy efficient but if is only going to serve as a back up source of heat rather than the primary source then energy consumption is likely to be very low in any case.

MicktheMan

sas wrote: »

Hi,

This one is really getting to me.

I'm building (towards) certified passive. I'm not there yet however and with the blower door still to go I'm not taking success for granted.
Also, I'm not convinced that in the depths of winter, 20 degrees is going to be sufficient for us i.e. even if we do hit certification level, comfort may still be an issue.

Sas,
My tuppance worth.
Sounds like you are not fully convinced by the passive house concept's ability to deliver comfort when needed most.
As I'm sure you know, Tomas O'Leary (Mosart) has built and lived in a passive house in Wicklow through the last number of winters and perhaps he can give you more info on comfort levels etc.

On your heat demand worries, what I would do is:
Trust phpp (but make sure accurate climate data is used;))
Move in and allow house to dry out. As you know, the first year will need more kWh to heat while structure is drying. If needs be, buy 1 or 2 2kW oil filled electrical rads to supplement heating. These rads are inexpensive to buy, mobile and in a passive house can be located anywhere (not necessarily in the room being occupied).
Later, if it turns out that the house needs significant extra heating input regularily then set up the various systems (now) so that an oil boiler or something can be easily added if needed in the future.

Imo, if an extra 2-4 kW are needed for a week or two a year, then the cost of installing & maintaining a ch boiler/oil tank etc is not justified.

sas

MicktheMan wrote: »

Sas,
My tuppance worth.
Sounds like you are not fully convinced by the passive house concept's ability to deliver comfort when needed most.
As I'm sure you know, Tomas O'Leary (Mosart) has built and lived in a passive house in Wicklow through the last number of winters and perhaps he can give you more info on comfort levels etc.

I do trust that if the house is built close to the spec it is designed to in the PHPP, that it will work 99% of the time. I am a selfbuilder though and not an experienced PH builder. There are no guarantees that I'll be close enough. I'll feel a little better in few weeks if the blower door goes well.

On Tomas O'Leary, he's managed to create a market and positioned himself at the head of it. If his house isn't working he'd be the last one to admit it. They did have to install rads in a couple of rooms I read in CI. I have also been left underwhelmed by him on the couple of occasions I've seen him present. This is my personal opinion however and I could be way off the mark.

MicktheMan wrote: »

On your heat demand worries, what I would do is:
Trust phpp (but make sure accurate climate data is used;))
Move in and allow house to dry out. As you know, the first year will need more kWh to heat while structure is drying. If needs be, buy 1 or 2 2kW oil filled electrical rads to supplement heating. These rads are inexpensive to buy, mobile and in a passive house can be located anywhere (not necessarily in the room being occupied).
Later, if it turns out that the house needs significant extra heating input regularily then set up the various systems (now) so that an oil boiler or something can be easily added if needed in the future.

Imo, if an extra 2-4 kW are needed for a week or two a year, then the cost of installing & maintaining a ch boiler/oil tank etc is not justified.

I'm likely to push ahead as I originally described above. My tank & solar supplier is going to meet us on site next week to walk through our spec more thoroughly.

I am definitely adding provision for some time of external boiler (e.g. oil) during construction. This will consist of a flow and return to the tank from outside the perimeter of the house. Then as you suggest, we can add it later if necessary.

I'm possibly deluded but one of the upsides of my taking forever to build this is that the major parts of my structure are bone dry already. Ok, I've to plaster and pour a screed downstairs and that will bring alot more moisture back into things. I am hopeful however that given our March deadline for move in that by the time we get to winter, the structure will be mostly dry.

sas

Do-more wrote: »

On the issue of your concerns about being able to heat the house to 22 deg C or whatever during very cold weather, what about adding an electrical UFH cable near the top of the slab when you are installing the pipes? I know it is not energy efficient but you would only need a very light cable widely spaced to be sure you can boost the indoor temperature up to the required level.

A friend of mine that recently moved into his PH has this down as the biggest waste of money from his build. He doesn't believe they are good enough to lift the temp in his house at all. I've already poured my screed upstairs and down stairs I'm happy the UFH will do the job assuming we can isolate it's effect on my big tank from the DHW.

Do-more wrote: »

The other option is to consider an electrical duct heater rather than the hot water one. Again not as energy efficient but if is only going to serve as a back up source of heat rather than the primary source then energy consumption is likely to be very low in any case.

I'd have gone with this but my system supplier doesn't have one and the water one has already arrived

Do-more

sas wrote: »

A friend of mine that recently moved into his PH has this down as the biggest waste of money from his build. He doesn't believe they are good enough to lift the temp in his house at all.

Good to know not to go too light on the cable so. I'm considering to add an electric underfloor cable in my slab, not so much for ongoing heating requirements but to aid the drying of the slab before I glue down wood flooring on most of the ground floor. It will only cost a few hundred euro and could be a good insurance against any possible moisture damage to €10,000 worth of flooring.

I used to work in a big renovated stone mill building where the only form of heating was electric underfloor heating in the slab. I ran on night rate and we never had a problem with comfort levels in the building.

sas

Do-more wrote: »

Good to know not to go too light on the cable so. I'm considering to add an electric underfloor cable in my slab, not so much for ongoing heating requirements but to aid the drying of the slab before I glue down wood flooring on most of the ground floor. It will only cost a few hundred euro and could be a good insurance against any possible moisture damage to €10,000 worth of flooring.

I used to work in a big renovated stone mill building where the only form of heating was electric underfloor heating in the slab. I ran on night rate and we never had a problem with comfort levels in the building.

Ah, you see he paid alot more than a few hundred euros. This country...

I'm told that concrete needs to be given a day per mm.
Hoping to get 80mm screed down in the next few weeks. No flooring until feb.
Plastering started yesterday so the build will have had good opportunity to dry.

Will still likely have to invest in a moisture meter.

It never ends does it?

Do-more

sas wrote: »

Will still likely have to invest in a moisture meter

Don't waste money on the sort that have two metal prongs which are designed for checking the moisture content in wood they are of no good for checking moisture levels in concrete and may give you a false positive.

The accurate way to check moisture levels in concrete floors is with a hygrometer specifically designed for the purpose. One or more holes need to be bored in the centre of the slab and a special plastic plug with a cover inserted which the head of the hygrometer fits into. These need to be fitted a number of days before the first moisture reading is taken, as obviously the heat of the drilling will have effect on the moisture level around the hole.

It's not a cheap piece of equipment but talk to your screed supplier about testing, if they are any good they should have the required equipment and do the test for you. (Possibly not F.O.C. though)

sas wrote: »

It never ends does it?

Some day, hopefully!

superchip

hey sas,i have just got my system up and running last friday.i have gone for 60 tubes for dhw with any excess heat to dump into 500l buffer,11kw stove,oil back up,ufh down stairs,will finish upstairs down the line..post recession.i just thought id throw out the idea of a 45mm pour on screed ,thats what ive gone for and im delighted with how fast i can alter the temperature.i have insulated well so i can leave ufh nearly off during the day and once i get home and light the stove the ufh heats up in no time,i thinks it makes more sense than trying to keep temp up all day when house is empty.i know its early days for me but i will keep u posted on how efficient it works out if u like.i also have not had any issue with hot water yet but it really hasnt been tested yet. an oil boiler will cost you less than a grand,it has to be worth putting it in at this stage even if u think you wont have much need for it.

Do-more

What sort of temperature drop are you experiencing in the house with the heating off during the day Superchip?

superchip

i turn it down to about 15 or 16 and then back to 19 or 20 once im home.it takes around 15 min

Do-more

superchip wrote: »

i turn it down to about 15 or 16 and then back to 19 or 20 once im home.it takes around 15 min

But is your actual indoor temperature dropping to 15 or 16 degrees during the day?

You say you have "insulated well" could you describe what insulation you have and other important factors such as windows etc.?

Do-more

@sas Have you arrived at any decision on the size of your thermal store yet?

I have only just started to email suppliers about solar panels having narrowed down a list of the best performing panels available at the best value.

The first guy I got a reply from today is suggesting a 90 tube set up of heat pipes which is working out at only slightly more expensive than the 60 U tubes I had enquired about. He is saying that u tubes are good in small installations but you can run into problems with pressure drops in large set ups so is saying that the heat pipes would be better. Can anyone make any comment on that?

He is also saying that with a 90 tube setup I should have at least 1000 litres and preferably 1500 litres of storage. Hence I am wondering what you have concluded.

If I go for such a set up I am then wondering what is the most efficient way to heat the water in the winter, two possibilities are a wood pellet stove giving 8kW to heating water or a 15 or 20kW wood pellet boiler. Which we be more efficient the stove running for longer periods or the boiler giving a bigger blast of heat at less frequent intervals?

fclauson

check out for sizing

cir-strategy.com/uploads/Nuosmaa.pdf

I have now relationship with this company - I just thought some of the numbers and details where interesting

sas

Do-more wrote: »

@sas Have you arrived at any decision on the size of your thermal store yet?

The first guy I got a reply from today is suggesting a 90 tube set up of heat pipes which is working out at only slightly more expensive than the 60 U tubes I had enquired about. He is saying that u tubes are good in small installations but you can run into problems with pressure drops in large set ups so is saying that the heat pipes would be better. Can anyone make any comment on that?

He is also saying that with a 90 tube setup I should have at least 1000 litres and preferably 1500 litres of storage. Hence I am wondering what you have concluded.

If I go for such a set up I am then wondering what is the most efficient way to heat the water in the winter, two possibilities are a wood pellet stove giving 8kW to heating water or a 15 or 20kW wood pellet boiler. Which we be more efficient the stove running for longer periods or the boiler giving a bigger blast of heat at less frequent intervals?

Funny, decisions are things that I make when I literally have run out of time!

At the moment I'm waiting to meet up with the supplier.

He is still adamant that the 1000 litres with 90 tubes will perform very well.

As it stands I'm still planning on pushing ahead with what I originally posted.

We are leaving 1 inch insulated pipe (flow and return) from near the tank to outside the perimeter of the house. If all else fails we'll connect a boiler of some sorts.

Bottom line is that we favour a stove over a boiler for the visible flame in the living room.

This is the furthest from the open fire I was prepared to compromise.

beyondpassive

It seems that quite a few people out there building 250-300msq SSHD 13-18kWh/m2.a detached houses are going for a similar spec. 60-80 tubes, 1000L thermal store/heat bank tank with stratification and heat exchange coils or plates to underfloor and DHW. Boiler fed with 15-20kW condensing oil boiler or condensing modulating LPG boiler. Optional supplementary solid fuel stove with pressurised input to thermal store. How many are going for such an arrangement.

The store accumulates solar thermal heat and incidental use of the stove, it also allows the boiler to fire up as long even burns without needed to cycle up and down to match demand. The underfloor can have a 12 loop manifold downstairs and 6 upstairs with boost circuits for bathrooms or towel rails. For underfloor and hot water demand demand, where the temp of say 35 degrees cant be got from the middle of the tank or 55 degrees from the top of the tank for hot water, the boiler is called on to to provide the heat. Once a week the legionnaires function bring the top of the tank up to 50 degrees. Here's where the 200l tank comes in. Rather than heat the entire store to 55 and 60, you could just heat a smaller 200l tank for hot water. Has anyone found any benefit to having this 200l Tank?

Another issue we are having is that for Part L compliance, 70-80 tubes on a good facing south will lead to the overheating and stagnation of the solar fluid and high pressure in the system, this isn't desirable unless the system can store or disperse the excess heat. Many suppliers are advocating installing no more than 60 tubes and writing an exemption into the architects opinion on compliance with building regs where the renewables quotient of part L is concerned, they haven't met the prescriptive requirements of the technical guidance but can be said to comply with the intent of the regulation. Anyone any views on this

Do-more

Have you made any estimates on how often and for how long you will need to fire the stove sas?

I assume that you would need to keep the stove fully stoked with the fire raging to get the full 10kW output?

Has your stove supplier given you any idea of what the likely hot water production/room heating will be with the stove ticking over i.e. if you light it and leave it unattended for a few hours?

I'm just trying to get a handle on what direction I should go in myself. I have estimated that in December & January when I have peak DHW and space heating demand from a source other than solar the combined demand is about 1200kWh each month so an average of about 40kWh per day.

Approx. 75% of the demand is for space heating but averaging that out over the day it comes to approx. 1.2kWh/hr.

I'm looking at a wood pellet stove with a 80/20 split so at full output providing 2kW of heat to the room it looks like it would be slightly over heating the house in those months but not by much.

But of course the worst possible overheating is likely to occur in the shoulder months when you still need to run the stove but the heat demand is lower. In October for example my space heat demand only averages at about 0.27kWh/hr or in November at 0.75kWh/hr. so it seems like in these months when you have to fire a stove as their isn't enough solar available then there is a real likelihood of overheating the house when the stove is running.

Not having lived in a low energy house before I have no feeling for how sensitive they are to overheating, but looking at it like this it seems to make a case for a small wood pellet boiler just heating the water rather than a stove?

Do-more

@beyondpassive Do you think it is justified to install UFH if your space heating demand is at max. 2kWh/hr. as I would expect a house of the spec. you describe above would have?

Seems like a sledgehammer to crack a nut?

fodda

Do-more wrote: »

@sas

If I go for such a set up I am then wondering what is the most efficient way to heat the water in the winter, two possibilities are a wood pellet stove giving 8kW to heating water or a 15 or 20kW wood pellet boiler. Which we be more efficient the stove running for longer periods or the boiler giving a bigger blast of heat at less frequent intervals?

If you burn 50kw worth of fuel and both appliances are 60% efficient and are running at maximum efficiency then you will only get 50kw - 40% = 30kw of recoverable heat even if one is running for 12 months or the other runs for 12 minutes.

To help size your thermal mass/store/buffer tank ... http://www.grow-your-own.ie/woodboilers.html#using_a_thermal_store_or_buffer_tank_with_your_boiler

Do-more

fodda wrote: »

If you burn 50kw worth of fuel and both appliances are 60% efficient and are running at maximum efficiency then you will only get 50kw - 40% = 30kw of recoverable heat even if one is running for 12 months or the other runs for 12 minutes.

That's the question isn't it?

sas

Do-more wrote: »

Have you made any estimates on how often and for how long you will need to fire the stove sas?

I assume that you would need to keep the stove fully stoked with the fire raging to get the full 10kW output?

Has your stove supplier given you any idea of what the likely hot water production/room heating will be with the stove ticking over i.e. if you light it and leave it unattended for a few hours?

I'm just trying to get a handle on what direction I should go in myself. I have estimated that in December & January when I have peak DHW and space heating demand from a source other than solar the combined demand is about 1200kWh each month so an average of about 40kWh per day.

No idea. I'm going with the "suck it and see" approach. The 3 kw won't overheat the room and if it starts to, we can open double doors into the open kitchen living room etc. I did the math some time ago and if I was right the stove would take 8 hours approx (at full 10kw output) to heat the entire 1000 litres to 60 degrees.

My basic theory is that in winter we'll get little from the solar. The stove will be lit specifically to heat DHW and also to bring the living room to "watching a movie" comfort levels.

I do assume the stove has to be at full tilt for 10kw output.

I haven't asked the supplier for any info. I've had little or no success with this in the past so it's fallen off my list of "things to do". I can only take so many fudged answers.

The bottom line from my experiences is that everyone thinks I'm nuts, the systems I've speced couldn't possibly meet our demands etc. Yet here (on this forum) it could be "over specced"

The plumbing industry in Ireland in evolutionary terms has recently "discovered fire".

Do-more wrote: »

I'm just trying to get a handle on what direction I should go in myself. I have estimated that in December & January when I have peak DHW and space heating demand from a source other than solar the combined demand is about 1200kWh each month so an average of about 40kWh per day.

Approx. 75% of the demand is for space heating but averaging that out over the day it comes to approx. 1.2kWh/hr.

I'm looking at a wood pellet stove with a 80/20 split so at full output providing 2kW of heat to the room it looks like it would be slightly over heating the house in those months but not by much.

But of course the worst possible overheating is likely to occur in the shoulder months when you still need to run the stove but the heat demand is lower. In October for example my space heat demand only averages at about 0.27kWh/hr or in November at 0.75kWh/hr. so it seems like in these months when you have to fire a stove as their isn't enough solar available then there is a real likelihood of overheating the house when the stove is running.

Not having lived in a low energy house before I have no feeling for how sensitive they are to overheating, but looking at it like this it seems to make a case for a small wood pellet boiler just heating the water rather than a stove?

Don't know enough about this to comment. I'm playing with the idea of an oil boiler to provide the same cover you describe the pellet boiler. My issue with pellet boiler is the capital outlay and the unreliable stocks of pellets here.

A friend of mine moved into his passive house (10% bigger than mine) about 3 months ago. He's been lighting the stove I'm purchasing and is very happy with it and it is heating his 500 litre tank and keeping them supplied. Granted the solar is helping too. The stove has never come near over heating the room it's in.

Do-more

fodda wrote: »

If it burns fast a furious then it will most possibly efficient.

Not according to the graph that beyondpassive posted http://www.boards.ie/vbulletin/showpost.php?p=75380226&postcount=10 (not a log gasifier I know but skip to my next point.)

fodda wrote: »

But wood pellet? Open to market forces even more, have you considered log batch boilers?

We are back to sledge hammers to crack nuts again! My total space heating and DHW demand that can't be provided by solar is approx. 5.7kWh/a. according to the rule of thumb sizing guide which you posted earlier I should be installing a 2000 litre thermal store and a 40kW log gasification boiler!

Yer gas alright!

beyondpassive

Do-more wrote: »

@beyondpassive Do you think it is justified to install UFH if your space heating demand is at max. 2kWh/hr. as I would expect a house of the spec. you describe above would have?

Seems like a sledgehammer to crack a nut?

Agree Its overkill and not needed upstairs, I used to advocate underfloor to tiled areas only, but the extra costs to underfloor the whole slab are minimal. Towel rails upstairs are an effective solution. The underfloor throughput is there for 2 reasons, the low fat cake factor and the mother in law factor.

Low fat cake. Its half the fat so you can have twice as much. You've done the hard yaka with the fabric first approach, your space heat and hot water bills are down under €800, so why not have toasty radiant floors and if you can have 22 degrees as a set point, why not. Mother-in -law, when herself comes calling and asks did you forget the fireplace, you can point to the balmy 24 degrees on the thermostat in January and say its always summer in here. Comfort is perception and for some that's 17 for others that's 24 degrees.

Of course the third and real reason for all the underfloor is to dump excess solar in summer and especially to prime the structure with free solar heat from the solar thermal store in early Autumn. The underfloor can be set up to distribute solar heat in the shoulder months which is keeping the middle of the tank at around 35 degrees, thus shortening the weeks when you have the boiler doing all the work.

Air source heating is an elegant solution, but I don't believe its effective in high thermal mass builds. My biggest concern though is that I don't think our Passive modelling is accurate, firstly we simply haven't enough measured data to correlate the modelling with the measured. Average annual running costs seem to be accurate but some months data like December 2010 and January 2011 were nearly 250% higher usage than predicted. Snow on the collectors and condensate leaks in HRV didn't help. My point is you need to design your system for these extreme months and that is at least a 7kW load for heat and hot water. A German Passivhaus expert told me last week: 'design your building with PHPP by all means and with Meteonorm weather data but don't rely on this for your heating system design as it will be grossly undersized.'

So, its a moderately sized sledgehammer.

sas

beyondpassive wrote: »

A German Passivhaus expert told me last week: 'design your building with PHPP by all means and with Meteonorm weather data but don't rely on this for your heating system design as it will be grossly undersized.'

Well that's just fabulous!

This is doing little to counter my "Don't build a passive house" stance.