under floor heating with buffer

rich071

hi guys , i have new build with ufh up and down stairs ,liquid screed on both floors , i have a 500 litre buffer with coil , solar panels and main heat source is going to be a firebird enviromax condeser a rated burner , trouble is my plumber is saying i only need a c26 model burner its a 90k btu boiler and i questioned the size of the it thinking it should be bigger but he said im just wasting money fitting even a 120/150 as the ufh runs a low temp and the 90k boiler is all i need to run my system as cost effectivly as possible , he said the buffer will first be heated and the boiler will run till this is a set temp , then after that my boiler will only need to top up the buffer when it calles for heat , this much i understand i think ha , but its the boiler issuse i need piece of mind with as i dont want to be whipping the boiler out in a few months cos it under powered , any surgestion or advise please ,
thanks
richard .

tony underfloor

your plumber is 100% correct. Your buffer is a thermal store that is charged by whatever energy source is available ie Solar /Oil boiler. Ideally the buffer stores warm water @ 70 -80 degrees and the supply water temperature required for underfloor heating systems is circa 35 degrees,

Assuming a return water temp to the buffer of 30 degrees the store has a energy potential of 100 kw. That means it can service a 20kw load for 4-5 hours. A small boiler will easily keep up with demand and will run more efficiently

Well done to your plumber

rich071

hi tony ,
thanks for your input , i feel more at ease now with the 90k boiler , i thought my man was just taken the cheapest option for himself and bagging more profit for himself , turns out he is looking after me then so , aw credit to him he has a great system in the house from what i understand of it , i will have it running this wk and i will post comments as to how it runs , im glad to hear its all ok with the 90k boiler ,
thanks .

heinbloed

Richy071 wrote:

hi guys , i have new build with ufh up and down stairs ,liquid screed on both floors , i have a 500 litre buffer with coil , solar panels and main heat source is going to be a firebird enviromax condeser a rated burner

A correctly sized UFH needs no buffer tank. The floor screed IS the buffer.
Only badly designed heating systems need a buffer tank.

Tony underfloor says:

your plumber is 100% correct. Your buffer is a thermal store that is charged by whatever energy source is available ie Solar /Oil boiler. Ideally the buffer stores warm water @ 70 -80 degrees and the supply water temperature required for underfloor heating systems is circa 35 degrees,

Assuming a return water temp to the buffer of 30 degrees the store has a energy potential of 100 kw. That means it can service a 20kw load for 4-5 hours. A small boiler will easily keep up with demand and will run more efficiently

Well done to your plumber

That's wrong. The OP's plumber is incorrect.
The OP wants to use a condensing boiler. With a flow temperature of 35 degrees for the UFH the tank will never reach 80-90 degrees Celsius.
Where would that come from?!

A modulating condensing boiler correctly sized needs no buffer tank. Esp. with a huge buffer like UFH in screed.
The extra amount of water to be heated up needs extra energy. 500liter content of the buffer tank might be twice as much as the water volume of the UFH......
Either the boiler is three times oversized by accident or the plumber has a try on the incompetent consumer. Note that most plumbers are bad with numbers, only the check counts for many.

@ the OP:

Get an independant person like for example an energy advisor to check these numbers. They seem to be more than absurd. Grotesk is a mild expression.

Pete67

I'd imagine the buffer is maintained at 70 - 80C and supplies heat to the underfloor at ~30C via a mixing valve. That way the buffer tank can store solar heat when available for later use. Oil boiler could be used if the buffer drops below say 50C and that way the boiler would alway operate in condensing mode. Makes sense to me anyway.

The required output of the boiler is dependent on two things, heat loss from the building to be heated, and projected demand for domestic hot water, and the OP has not provided details of either.

heinbloed

Well, first the 70-80 degrees Celsius warm water has to be produced. Since there is no demand for this temperature this is wasted energy.
The UFH demands 35 degrees Celsius. So there is absolutly no reason to put the boiler at an output of 70-80 degrees Celsius.

With a correctly sized heating system there is no need for a buffer tank.
The smallest output demand determines the boiler size.
Assume the smallest demand is 2 kw then the boiler would start at 2 kW plus possible distribution losses.

A buffer tank at 70-80 degrees Celsius (during the heating season) makes the solar thermal collector obsolete. There wouldn't be any use for the ST system. Turning on the ST circulation pump at such a storage/buffer temperature would drain the stored thermal energy into the environment. Paid for out of the pocket of the occupier.
There are expensive ST toy installations which would deliver more than 100 degrees Celsius, but if we feed the collector with 80-90 degrees Celsius the efficiency of these ill designed toys would suffer even more badly. As well as the material itself.

The OP didn't give much information, as said already.
So I assume with what we know so far that the ST system should run seperatly from the UFH.

The efficiency of the ST system, it's capabilities and the actual demand must be calculated. It makes no sense to run the ST storage tank with a heatexchanger coil connected to the UFH.
The fresh water would arrive with 10 degrees Celsius, demand for DHW is about 40 degrees Celsius.
The correctly sized boiler for the UFH will deliver 35 degrees Celsius.
Minus the energetic transition from boiler flow temperature - heatexchanger - DHW we will get only 30 degrees Celsius at the DHW side, in the buffer tank. Minus the loss through insulation and distribution we will get a maximum temperature at the tap of around 28 degrees.
Running a boiler with 26 kW output (if I understand the OP correctly) to create a temperature rise at the DHW side of 18 degrees Celsius with a flow rate ( average household demand ?)) of who knows is extremely wasteful.

Better get the necessary temperature raise by a combination of ST - electric resistance - UFH fed storage tank.
Or more simple, cheaper: ST plus electricity.

From what I see with the UFH/boiler-buffer 'calculation' in the OP's post the ST energy calculation is propably not done as well.

So better get an energy advisor in. Someone who doesn't want to sell something, who checks the financial investment AND energetic efficiency situation.

rich071

Pete67 wrote: »

I'd imagine the buffer is maintained at 70 - 80C and supplies heat to the underfloor at ~30C via a mixing valve. That way the buffer tank can store solar heat when available for later use. Oil boiler could be used if the buffer drops below say 50C and that way the boiler would alway operate in condensing mode. Makes sense to me anyway.

The required output of the boiler is dependent on two things, heat loss from the building to be heated, and projected demand for domestic hot water, and the OP has not provided details of either.

hi pete , the house has 110mm gavitiy pump with beaded insul , and all external wall have 50mm thermo boards 32+12mm , joist have 200mm rockwool , all timber studs have 100mm rock , ceiling joints and attic space has 300 of brown loft earth wool , and as for hot water , theres just the two of us at minute , maybe a shower in the mornigs , but deafo 2 showers in the evenings so hot water comsumption wouldnt be a whole lot ,
i dont really understand all the working of the plumbing yet , but it was my choice for the buffer tank as i had intended on geothermal , had my holes drilled and lined ready to go , and i heard nothing but bad reports on the cost of the running of it , so i diverted back to oil at last minute , with the option of going geo in years to come , hence the buffer , but my plumber has the buffer plumbed that i can by past it with two levers , he did tell me in first place that i didnt need it , but i had already paid 1200euro for it so i said stick it in and if it doesnt work out i just by pass it then , should of said all this in start , hope it makes sense
thanks again

rich071

heinbloed wrote: »

Well, first the 70-80 degrees Celsius warm water has to be produced. Since there is no demand for this temperature this is wasted energy.
The UFH demands 35 degrees Celsius. So there is absolutly no reason to put the boiler at an output of 70-80 degrees Celsius.

With a correctly sized heating system there is no need for a buffer tank.
The smallest output demand determines the boiler size.
Assume the smallest demand is 2 kw then the boiler would start at 2 kW plus possible distribution losses.

A buffer tank at 70-80 degrees Celsius (during the heating season) makes the solar thermal collector obsolete. There wouldn't be any use for the ST system. Turning on the ST circulation pump at such a storage/buffer temperature would drain the stored thermal energy into the environment. Paid for out of the pocket of the occupier.
There are expensive ST toy installations which would deliver more than 100 degrees Celsius, but if we feed the collector with 80-90 degrees Celsius the efficiency of these ill designed toys would suffer even more badly. As well as the material itself.

The OP didn't give much information, as said already.
So I assume with what we know so far that the ST system should run seperatly from the UFH.

The efficiency of the ST system, it's capabilities and the actual demand must be calculated. It makes no sense to run the ST storage tank with a heatexchanger coil connected to the UFH.
The fresh water would arrive with 10 degrees Celsius, demand for DHW is about 40 degrees Celsius.
The correctly sized boiler for the UFH will deliver 35 degrees Celsius.
Minus the energetic transition from boiler flow temperature - heatexchanger - DHW we will get only 30 degrees Celsius at the DHW side, in the buffer tank. Minus the loss through insulation and distribution we will get a maximum temperature at the tap of around 28 degrees.
Running a boiler with 26 kW output (if I understand the OP correctly) to create a temperature rise at the DHW side of 18 degrees Celsius with a flow rate ( average household demand ?)) of who knows is extremely wasteful.

Better get the necessary temperature raise by a combination of ST - electric resistance - UFH fed storage tank.
Or more simple, cheaper: ST plus electricity.

From what I see with the UFH/boiler-buffer 'calculation' in the OP's post the ST energy calculation is propably not done as well.

So better get an energy advisor in. Someone who doesn't want to sell something, who checks the financial investment AND energetic efficiency situation.

hi mate i not sure what the ST is ? as i said im not up to speed on the plumbing lingo so to speak , but i will on all the advice here get a indo energy adviser in this week or so and go tru what the best option is for me , but does the fact that i diverted back from geothermal to oil and had the buffer justify haven it , it can be bypassed as my plumber did say i didnt need it
thanks for all advise .

heinbloed

Hi rich071!

hi mate i not sure what the ST is ?

ST=Solar Thermal (system)

i diverted back from geothermal to oil and had the buffer justify haven it , it can be bypassed as my plumber did say i didnt need it

Your plumber is right then, this option/his recommendation wasn't clear from your first post. If the tank is already placed you could still use it for a solar thermal storage. And by-pass the central heating system.
Is there an option to include an electric heating element at the top of the tank, to top-up the temperature for showering etc?
If not there are electric showers now available which work with pre-heated water, specially designed to be combined with solar thermal systems. It might be an ideal combination, but ask your energy advisor, or the plumber.

Good luck!

L driver

Hello,
Sorry for jumping on board but why would a buffer tank be suitable for geothermal and not oil? I am looking at a geo system with the gshp going directly to the ufh and to a buffer/thermal store for dhw? Confused again:(

heinbloed

@ Ldriver:
Only an over-dimensioned thermal source would be used via a buffer tank.
Ask your engineer/plumber why he wants to sell you a buffer tank.
A correctly sized heating system needs no buffer.

A thermal storage tank is a different issue.

rich071

L driver wrote: »

Hello,
Sorry for jumping on board but why would a buffer tank be suitable for geothermal and not oil? I am looking at a geo system with the gshp going directly to the ufh and to a buffer/thermal store for dhw? Confused again:(

hi L driver
im no expert , but i was intalling a buffer tank when i was going to use geo and from what i was to believe the buffer was to store hot water at the nite rate of esb , there for not being required to turn on the heatpump during the day at the higher rate of esb ? am i rite in thinkn tho i can still use the buffer , but its just not required , but thats why my plumber has the by pass in place , so it can be just diverted .
thanks for every1s help all the best
richard .

goodcraic

All,:(
Built a house two years ago

---

See report on my Oil Boiler combined

I have a Oil Boiler combined with Solar panels feeding a 500 L buffer tank in my garage this is linked via a heat link pipe to my house. I have UFH upstairs and down and I have a HWT in my hotpress.....

At the time of build, I was going to use wood pellet but a) costs and b) word of mouth put me off.

However, so a year on and how are things !!!! Well the Oil for UFH is not the way forward as the system is too expensive to run full stop ! And yes I have prob oversized my system! But the costs to run is ridiculous !

The main issue is that you have to leave you oil burner on automatic to keep heating / topping up the buffer tank.... All demand is drawing of this buffer so when you don't require the heat is on....

My solar system is fine and does manage to assist the buffer but the temp you need for the UFH is max 35 - 40 deg and not 70 deg for Hot water.

I am now at the stage of by passing my Buffer and just using the Oil to heat my HWT.

I am also considering a Air source heat pump - Sized correctly and combining this with my solar on the same buffer for just the 35 - 40 deg ?????? This would service the UFH only ?

Regards,
Liam

L driver

Hello,
Why can't the oil boiler modulate down as Heinbloed has said to suit the UFH? This is all a bit IRISH.
Sorry doing an edit, still can't see how the oil boiler can be that more efficient direct to the UFH that it would make a huge difference to your bills... check the controls on your buffer set up perhaps.
If oil is expensive for your UFH so will everything else... what is your heat demand?

rich071

goodcraic wrote: »

All,:(
Built a house two years ago

---

See report on my Oil Boiler combined

I have a Oil Boiler combined with Solar panels feeding a 500 L buffer tank in my garage this is linked via a heat link pipe to my house. I have UFH upstairs and down and I have a HWT in my hotpress.....

At the time of build, I was going to use wood pellet but a) costs and b) word of mouth put me off.

However, so a year on and how are things !!!! Well the Oil for UFH is not the way forward as the system is too expensive to run full stop ! And yes I have prob oversized my system! But the costs to run is ridiculous !

The main issue is that you have to leave you oil burner on automatic to keep heating / topping up the buffer tank.... All demand is drawing of this buffer so when you don't require the heat is on....

My solar system is fine and does manage to assist the buffer but the temp you need for the UFH is max 35 - 40 deg and not 70 deg for Hot water.

I am now at the stage of by passing my Buffer and just using the Oil to heat my HWT.

I am also considering a Air source heat pump - Sized correctly and combining this with my solar on the same buffer for just the 35 - 40 deg ?????? This would service the UFH only ?

Regards,
Liam

hi there
you have me worrying now that the oil will rob me , i have two seperate plumbers both friends of mine look at my system and they both tink its a good set up and shouldnt use any more than 2-3 tanks of oil a year ,
what size boiler you using , and how much oil you going trough a year , but i will ride it out for a year see how i get on with it , if its not cheap to run i will look at diverting back to geo or something else ,
thanks .

DoneDL

There is nothing wrong with the theory of buffer tanks, the problems are sizing and heatloss. The theory that if you have a large store of hot water with little loss being created by solar heat that is then used to run an underfloor system seems pretty good but I would say that you need to do your sums well.