Solar hot water and electric heating

j_the_ostrich

Hi, we are at the planning stage of renovation & extension of a 1500 sq ft / 140 sq m late 1960's bungalow. Large site, easy access, Louth.

Renovation include insulating a suspended floor, changing some interior walls, plumbing, wiring, internal perimeter wall insulation, re-rendering exterior.

Extension is a story and a half of 1350 sq ft / 125 sq m.

At this stage I'm wondering about heating systems.

We are looking for a best option for heating; air to water is pretty much ruled out given the building, gas seems to offer little improvement on the oil system (mains supply not available), oil seems to have poor cost prospects for the long term future.

So solar and electric seems our most accessible option. But having an all electric system is quite new to us and we could do with some advice from experience.

For hot water:

We are planning a 12 panel 5kw solar system with 5kw battery.

For heating

We have decided against a heat pump as the original building has too many costs attached to achieve anything approaching air tightness.

We have been recommended to consider changing from a traditional oil heating to all electric radiators, again assisted by solar and battery.

This would involve additional panels and battery.

Any thoughts, advice or experience with solar water and heating / electric radiators in a +250 sq m house would be much appreciated.

Or if anyone has experience of approaching this in a different way we would greatly appreciate recommendations.

Thanks

John

10-10-20

https://www.boards.ie/discussion/2058295146/solar-hot-water-and-electric-heating

Which profession specifically recommended this approach to go all-direct-electric, may I ask?

I'd worry that they are oblivious to some aspects such as unit pricing or even developments in heating in the recent, eh, 15 years. Did they also calculate the gain from 5kW of panels over the solar year and specifically in Winter when you most-need the power?

Sorry if it seems like I'm being pointed, but I do worry.

j_the_ostrich

Thanks 10-10-20

As you might have guessed the solar installer suggested the electric radiators, so no apology needed, I'm aware of being sold something and the suggestion to go all electric s hardly unbiassed. Your thoughts are welcome indeed.

The truth is I had not considered electric heating and had pretty much decided to stick with the oil system and offset heating costs by insulating. So I felt I needed to ask for some opinions or experiences.

While we have pretty much decided upon the solar generating some electricity and in turn the hot water / immersion tank. The system in mind is 12 solar panels 5kw, generating electricity for the house, used by appliances and immersion tank, and excess stored by battery. Topping up the battery if needed off the grid at night on lower unit rates, for use during the day. Cost €10,500. A projected saving of about €250 bi monthly (average) on electricity bill, Payback time for this about 7 years at todays unit prices.

The question is whether an addition to the system; a solar / electric radiator heating system instead of the oil and traditional radiator system would be a good or bad move.

We are at the planning stage so all options remain open.

Thanks again

John

graememk

You dont need an air tight house for a heat pump. As long as the system is correctly sized, re Heatpump and radiators. But as your changing the plumbing etc you almost have a blank sheet. And you will have a well insulated house to boot.

Heatpumps are just a heat source like oil, gas or electric.

Pure electric only is expensive. 1kwh of electric = 1kwh of heat. 1 Litre of oil has 10kwh of energy but due to boiler losses and type of boiler it could be 7-9kwh of heat.

If you took a poorly performing heat pump, with a COP of 2, thats 1kwh of electric and 2 kwh of heat.

As you have a well insulated house, and the scope to put in big radiators, you could easily be looking at a COP of 4-5 1kwh of electric = 4-5kwh of heat.

As for solar, 12 panels is a bit skimpy...

TLDR Pure electric heating is expensive. Heaters may be cheap running costs are high.

10-10-20

IMHO, going completely for electrical heating in this way is retro-grade as even if - let's say the unit price rises and you end up pulling far more from the grid than you wished for - you don't then have a fall-back. Direct heating such as this (if it's resistive elements) have an efficiency of 1.0 (one unit of energy for 1kW of heating) while air-to-water (underfloor) can be as good as between 3.0 to 4.2 (3.0 to 4.2 units of heat for each unit of energy). So I'm not joining the dots. I think somebody got their wires crossed.

Just on the solar topic - is the roof-area maximised with 5kW, is that the limit, or is that just a lower budgetary bound? I'd be looking to get more than 7kW to be honest if I was you, and probably 1:1 on the battery. Nobody here will tell you otherwise, the mantra here is "the best solar system is the largest installation of the cheapest..." and hence the adage that you should always seek to maximise the panel count while you're up there installing them. No doubts about it.

slave1

Put up more panels, you need to max your Winter generation if going electric heating in any form, the steeper the slope the better also

j_the_ostrich

Thanks you all

Thanks Graememk, after some research I have been put off proceeding with a heat pump and UFH and would only proceed with a heat pump with caution after specialist assessment. I've been reading testimony's of very high electricity costs for some. The house may not be suitable, and installation costs including retrofitting high. Like anyone I'm trying to calculate the installation costs against savings in fuel. The tipping point to being wary of this have been the testimonies of very high electricity bills with an air to water system.

Your point of air to water v's and electrical system is well taken though. And thank you for the performance ratio's, this is an area I'm learning little by little.

And thanks you again 10-10-12 and slave1

There is a generous south facing roof space so a 5kw / 12 panel solar system is just a lower bound, I'll heed the advice and appreciate the mantra and will go for a larger system with 1:1 battery.

10-10-20

https://www.boards.ie/discussion/comment/120554717#Comment_120554717

Just a clarification on the battery part - I would not necessarily match the battery to the panels 1:1 right the way up, but if you size the inverter right you can add battery capacity in the future when the price-point is right.

j_the_ostrich

Thanks you very much, much of this project has been ruling out the missteps and getting system foundations right for the future.

graememk

The bill electric bills need to be taken with a pinch of salt.. there's so many variables in play there.

But if you went full electric heating the bills would be even scarier.

The rules for getting the most out of a condensing oil boiler and heatpumps are the same.

Go with a condensing oil boiler but the lower you keep your running temperatures the more efficient it will be.

Doing the right thing and figuring things out when have time and opportunity to change things

NattyO

Have you looked at the Hone system in Mayo? Can't recommend or not, but saw it on a housing development in Offaly. Uses PV for electricity and solar thermal for heating.

j_the_ostrich

Thanks Graememk, a very useful rule of thumb, Do you think I should try to oversize radiators, keep them running at a lower temperature with a greater surface area?

And thanks very much NattyO, the tip on Hone look very interesting.

I'll post again with what we come back with.

10-10-20

I have to link HeatGeek here. A very passionate perspective.

https://www.youtube.com/watch?v=9fcsgrrHSQ8

Sorry it's very techy, but sit through it.

denismc

I'm just wondering if you have considered get a professional energy survey done? If done right it will it will take a lot of the guess work out of what you are trying to do.

Your distrust of heat pumps seems to be based on anecdotal evidence, a heat pump may or may not work for you.

An energy survey carried out by a professional engineer ( not your builder) should determine if it is suitable for your project.

As others have mentioned you have no way of telling if the current electricity tarrifs will be there in the future, so the cheap night rate electricity that your basing your assumptions on may not be there.

Also the output from you PV panels in Winter will be a fraction of what it is in summer.

j_the_ostrich

Thanks again 10-10-20 a great source of information, much appreciated

I've calculated the cubic m of each room, number of outdoor walls, window size, room above or below, but have not found a table recommending kw per cubic m. Is there a good rule of thumb?

j_the_ostrich

Thanks Desnisme

Completely agree, I'll get an assessment done by a professional.

So far it seems when I go looking for someone to assess what system would best fit with the house I find they describe they can only advise on the system they sell / install, and can't advise on other systems. In other words I haven't found a surveyor to compare between systems.

It's still early days and this is a process of getting to terms with best options.

graememk

https://www.boards.ie/discussion/comment/120555381#Comment_120555381

If you really want to get into the nitty gritty of it

Here's the DEAP manual, although that is good for reference values due to the age of the house, and standards of the time (its also what the BER is based on)

It's harder to calculate what the final "insulation" of what your walls will be, unless you can find U values for the external insulation that you are getting.

https://www.seai.ie/publications/DEAP_Manual.pdf

Using that you can fill out the heat loss spreadsheet

https://www.seai.ie/tools/SEAI-Room-Heat-Loss-and-Radiator-Sizing-Guidance.xlsx

then you can find the heat loss of the room, you only usually have to do the external walls as it assumes joining walls to other rooms are at the same temperature so no heat loss.

That will give you the heat loss in watts

You can then look up the spec sheets of radiators. It will list outputs at different Temperatures, as far as I can remember the temperatures listed is the difference in temperature between the room and the temperature of the radiator, eg 50

So if the room is at 20 c and the radiator is at 70, it can put out 1000

As the value get smaller the less power it can output to the room, hence the need for bigger radiators on lower temperature systems.

It's very hard to do a rule of thumb.

DC999

There’s a dedicated heat pumps thread here so likely worth scanning that to see numbers for winter heating. Or add a post to it asking about kWH usage in winter for heating. Afaik @ECO_Mental runs a heatpump in a new-build detached house with likely a high BER. He might share his winter usage for heating and hot water. But would give some barometer.

As others have said, if you go electric without a heatpump it’s a 1:1 ratio. Any electric rad is 1kWh of electricity = 1kWh of heat. You lose the 2 or 3 times multiplier (called COP on heatpumps due to the refrigerant boost).

Ignore the talk of smart rads, they are simply electric rads that you can turn on with an app. Pure marketing and no efficiency gain. Can get a €10 smart plug and do the same on a €40 oil filled electric rad.

I ran some electric rads this year to try it as a test. Used 500kWh a month for a small part of downstairs to a temp lower than most would like. Was a 5x5m sitting room and 3x3 WFH room (that was only for about 10 hours a day. Then it was unheated). Our hall was unheated which most people wouldn’t want.

You still need a main heating system – be that oil, gas or heatpump. Individual electric rads are wildly expensive. And I don’t say that lightly. I say that as someone who wants to turn off our gas boiler.

If you can get underfloor heating as part of the refurb, that acts as a ‘thermal store’. Basically it slowly heats the layer of concrete above it and then pushes out heat during the day. Don’t have one but supposed to be very comfortable in the living room space. Heatpumps can run that, or gas, oil…

 

DC999

https://www.boards.ie/discussion/comment/120555295#Comment_120555295

That cheat sheet for rads is class. I sussed out this year that our 80 yr old, small, leaky house could run a heat pump. Don't know how much it would cost, but we heated downstairs from 2 small electric rads. So our existing rads would work with a heatpump.

Dayor Knight

https://www.boards.ie/discussion/comment/120555617#Comment_120555617

Yes on the underfloor. We got it under our extension floor when extending about ten years. It works really well on low temperature water circulation (gas boiler). Have it on for a few hours early in the morning and again in afternoon. Stores up the heat nicely and releases gradually, like one big storage heater under the extension living area. I did a rough estimate on the gas usage back in the Autumn and found that having this on for 5 hours was using about the same as the system for the rest of the house was using in one hour. That may not be exactly accurate but it was in that ballpark anyway. Because it's low temp (30 to 40) it seems like it might be suitable for a heat pump, though you'd still need an alternative for the rest of the house I think.

Maybe some food for thought there..... hope it helps.

j_the_ostrich

I cannot thank everyone enough.

I've had a quick scan through the DEAP and thank you Graememk I think I can with some time get an idea of heat loss in different areas and it certainly will help me understand the various insulation for the retrofit in particular. Some time with a spreadsheet should at least prepare for a more professional assessment.

I'll follow the thread on heat pump's DC999 thank you Perhaps we had ruled these as not suitable prematurely and this has been a great discussion that help us rule out the electrical radiators.

Thank you Daynor Knight, we have wondered about putting in UFH in the extension, and leaving the original building heated with traditional radiators, your experience has encouraged us.

The discussion, expertise and time everyone has taken is so much appreciated. With costs these days we need all the help we can get to get it right first time and have as few slips ups as possible.

I'll post again when we get some way further into the process.

dbas

https://www.boards.ie/discussion/comment/120556208#Comment_120556208

Download the SR 50 heat loss sheet.

Version of it here.

https://berassessors.com/wp-content/uploads/2020/05/MIKE-TEAHAN-VER.-SR50-Heat-Loss-Calculations.xlsx

Fill that in. Sounds like you've the hard work done. Fill in your measurements that you already have for each room. Use the resulting heat loss to sound out heat pump suppliers. Some great split systems out there.

You will need that sheet filled out by a heating systems designer at the end of the job or your BER will be hammered. That being said, that's how to size domestic heating systems in Ireland

ECO_Mental

@j_the_ostrich

See below graph of my heat pump electricity consumption (kWh) for the last 14 months. This is a 7 day moving average just to smooth out the ups and downs. I live in a 230m2 A1 new build house with underfloor heating upstairs and downstairs. I also have 6.1kWp solar PV system. My heat pump total consumption for the year 2022 was around 5000 kWh. It does all the heating and hot water.

I don't have any schedules set to come on on night or anything if it needs heating or hot water it will come on. The room stats are set to 20-21 in the living rooms and 18-19 in the bedrooms and naturally enough between solar gain and people in the house generating, heat most of the heating comes on at night anyway. My usage would be al lot less if I could stop my two teenage kids not have half hours showers at least once a day....

Looking at the graph during an average winter day Id say I use between 20-25kWh/day, that big spike was that really cold week or two we had in December last so that was an extreme exception, the HP was working a good bit but I think we got down to -6c or -7c for over a week and never got above freezing. During the non heating months March - late October it will use about 6 kWh/day...

I am very happy with the A2W heatpump Samsung/Joule (12kW output) I have a 300 litre tank and 300 litre Aquabox and my kids have yet to run it cold even though they try their best. I do try and make people have showers when the sun is shining....

My advice is try and get your house as airtight as possible, I am just at 2ach/hr but you will also have to install MHRV for that level. I would also consider not bothering getting the battery, with the FIT payments and the cost of them its just not worth it. Put up as many PV panels as you can. If your house is insulated and airtight then heating it becomes easier.