# UFH and Solar



## thepool700 (30 Jan 2011)

Has anyone ever used their solar heat into their underfloor heating. 

last few days have been getting about 40c out of solar'

would it be best if put 30c into the ufh and have floors warmed up so they wouldnt be wasting as much oil in getting temps up in the evening when ufh is switched on

has anyone done this?
is it possible?


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## villa 1 (31 Jan 2011)

A complete waste of time and expense in my opinion, that is, unless you cover your roof with panels(which you can't) and install a buffer tank and then compliment the system with a back up heating source for all those gloomy cloudy days.


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## Shane007 (31 Jan 2011)

Yes, this is very possible and I have done so on many occasions. As Villa1 states, it must be done in conjunction with a buffer tank. The size of the system on the roof will depend on the quality of the system & "real" output of that collector. For winter solar gain, I would only install evacuated tubes. Flat plate will have too much heat loss in winter period as outside ambient temperature plays a big part with flat plate.
Systems I have installed with success have been 60 tubes feeding 500 litre single coil buffer tank, backed up with high efficiency condensing oil boiler. A 500 litre buffer tank will give you approx 17.45kW of energy with a ∆T temperature difference of 30C (70C Tank Temp - 40C UFH) and more if higher tank temperature is achieved. Therefore, if you are using underfloor heating, and the heat loss of your house is say 50w/m² and your house is 200m², then this will give you approx 1.75 hours of continuous output. If you leave underfloor on during the daytime, the energy is slowly taken from the buffer tank and then boosted by the oil boiler back up.
Even using this buffer tank without solar will save your oil bill. A 35kW oil boiler will heat this tank in approx 30 minutes and will give you nearly 2 hours of energy for 30 minutes of output from boiler.
A bigger tank will give a longer output time, however, will require a bigger input to heat in same time. You will also require a much larger collector area and then space and economics play a large role. Make sure house is well insulated to minimize the heat loss, do not rely on solar to heat but to contribute and have an efficient boiler to back up the system.
I would not recommend solar to heat radiators as the demand from rads just makes it too inefficient. 
I have a client where we installed the above and the client prefers to leave a trickle heat all day from solar and then on sundown, the oil kicks in. They informed me this has been using 1.5 fills of oil per annum.
Also if you can, increase the pitch of the collector for more winter solar gain. You can also pipe a direct circuit to your existing cylinder to heat domestic hot water in summer months from the buffer tank so you are not wasting the excess heat produced in summer months. Ensure you fit a heat dump or a collector tube with a temperature limiting device to avoid stagnation during summer months. (Or build a swimming pool for the heat dump!!!)
If you already have some tubes, then you could use a 3 port diverter valve on the solar circuit and in winter months divert to the buffer tank. A small contribution to heat gain is better than none. Most solar control panels, even the basic ones, have this facility to program to heat 2 tanks. Steca's SC200 has a built in timeclock and can run multiple tanks and pumps.


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## Shane007 (31 Jan 2011)

BTW, in December, my tubes were covered in snow and were still sending down 18C.


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## quentingargan (1 Feb 2011)

I beg to differ. The problem with solar space heating is that it works best when you need it least. To my mind, it just doesn't stack. 

The concept of solar space heating originated in countries where there is a higher level of daylight during winter months. The following chart shows daylight energy for the months of the year in Dublin and in Austria. You can see that they are similar in the summer months, but in witer, they have twice as much daylight as we do...









The point is that in December, we get 0.47kw hrs per sq m of horizontal ground. Slightly higher than this on a sloping roof. Even if the system efficiency was 100% (which it isn't, particularly in winter) the max you can get on average per day in December is about 3kw Hrs per day, worth about 21c if replacing oil or gas. 

The better insulated your house is, the less it will work. If you have a badly insulated house that needs heat from August to April, maybe the system will stack to some small extent. 

The only times I have installed this is where there is a buffer cylinder there already for some other purpose, such as a log gassifying stove. I'm also not a great fan of UFH personally because the response time is so poor. If the sun comes out and you get some solar gain during the daytime, you don't get the reaction. 

Solar panels make sense for domestic hot water, because they supply free hot water during the summer months when your central heating is knocked off. During those months, your hot water would otherwise cost you more because it either comes from an immersion, or from using a boiler very inefficiently to heat a small cylinder in your hotpress. 

During winter, the solar will pre-heat your cylinder so that the oil system has less work to do. On a good day, it will bring the water to full temperature. Paying extra to get an occasional bit of extra heat from it is just not worthwhile, and there is a better bang for your buck from insulation and other works.


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## Shane007 (4 Feb 2011)

Hi Quentingargan, I find your figures a little off the mark. The figures will vary depending on the location in Ireland. If you go to http://re.jrc.ec.europa.eu/pvgis/apps/radmonth.php?lang=en&map=europe click on Dublin, choose data to Irradiation at opt. angle (37°) you will find a very different result. This opt. angle will be in the region of most roof pitches in Ireland. You are doing your calculations based on "Horizontal Ground" whereas most installations will be located on a roof pitch of approx 35°, giving far better results than horizontal! Horizontal plane will lose much of the solar gain, especially in winter time and is more suited to summer time, when the sun is at 60°. You must remember that at winter solstice, the sun is at 14°.

*Dublin:*
Jan: 1.202kWh/m2/day, Feb: 1.818, Mar: 2.885, April: 4.123, May: 4.818, June: 4.614, July: 4.720, Aug: 3.935, Sept: 3.445, Oct: 2.318, Nov: 1.419 and Dec: 0.889. 

This will give an annual average solar irradiation for Dublin as 3.021kWh/m2/day. 

Go further south in Ireland and the figures increase, for example Cork = 3.129kWh/m2/day. and Wexford = 3.161kWh/m2/day

Salzburg in Austria set to the same parameters will give an annual solar irradiation of 3.540kWh/m2/day, therefore not such an enormous difference as your graph claims and certainly not the twice as much daylight.

I have just returned from one of my clients in Wicklow whom I installed my previous post specification and they informed me in the past 12 months, they used only one fill of oil. The house is an old thatch cottage with 3 foot thick stone walls with underfloor heating, 26kW Grant Vortex oil boiler, 500 litre buffer tank and 60 evacuated tubes (ground mounted on frame set to opt. angle as thatched roof).

I am not trying to knock what you say and I have much respect for your many very well informed posts, but in my opinion, as long as the installation is installed fit for purpose, not "oversold" and with the correct installation and management parameters, solar contribution systems can play a contributable role in an Irish home.


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## quentingargan (5 Feb 2011)

Hi Shane007. 

Interesting post. Thanks. I still beg to differ though...

Yes - horizontal is a bit misleading in winter, but then I also allowed 100% efficiency. However, if your roof pitch is 30 degrees, or even 45, you will still lose efficiency in the winter. 

It isn't just a matter or comparing total annual solar gain between the locations - the overall difference isn't that much, but the winter difference is quite a lot. 

The best way to judge this is to use computer simulations and see what the monthly contribution will be for a given angle. We have run such simulations in T-Sol for systems at 45 degree pitch to see what the winter gain was, and it really didn't represent a good return on investment that was worth spending extra money on. 

That said, I have put solar space heating into houses that already had a 1500L buffer tank there to serve a log gassifying stove. 

Solar, to my mind, is great in the summer when it is displacing hot water that would otherwise cost you up to 15c per Kw Hr using an immersion. The payback falls apart in winter with the double-whammy of a smaller solar contribution which is displacing oil or gas costing only half that price. 

I owned a thatched cottage for some time, and to be honest, I found it quite light in its heating requirements. So it is hard to gauge. I've also heard of other people doing this, but their build quality was so far above the norm, that again, it is hard to assess why the heating bill was so low. 

But most houses with a 60 tube system find that it strugles to bring the bottom of the cylinder above 40 degrees much of the time in winter, and while I know solar efficiency falls off as temperature rises, that isn't quite so true of tubes. 

But I agree you haven't over-sold. What worries me is when I see really sophisticated systems being sold with sophisticated control and buffer tank systems.


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## Shane007 (5 Feb 2011)

Yes, we seem to both agree and disagree. T-Sol, I find can be somewhat misleading from reality also. I have done T-Sol calculations with results being far too oversized. 

I find it is much better to use a smaller buffer tank and using 1,500L buffer would need a serious amount of input. The 500L will also use far less energy with the back up heating system. The downside is you have less energy store, but I feel the balance is swayed towards the smaller and cheaper system and it still makes a useful and worthwhile contribution.

I would only advise solar contribution to be used on UFH and certainly not on radiators. That particular client leaves UFH heating on from midday to evening trickling 25C - 30C through the circuit and then uses oil to boost at night for a couple of hours. The couple of hours at night from the buffer tank only uses approx 40 minutes of oil burner input.


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## Shane007 (5 Feb 2011)

quentingargan said:


> Yes - horizontal is a bit misleading in winter, but then I also allowed 100% efficiency.


 
I forgot to mention that the calculations I quoted are based on an efficiency of 76.7%. Kingspan's HP250 evacuated tubes has a solar keymark certification of n0 (n zero) efficiency 76.1%.


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## quentingargan (5 Feb 2011)

Yes, but that efficiency only applies at bang on noon. Flasks have a lower n0 efficiency, but this has to be multiplied by the IAM and it all averages out about the same. 

Overall system efficiency, including pipework etc., is usually much lower, typically quoted on T-Sol at being in the mid-30s. Q


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## Shane007 (6 Feb 2011)

quentingargan said:


> Yes, but that efficiency only applies at bang on noon. Flasks have a lower n0 efficiency, but this has to be multiplied by the IAM and it all averages out about the same.
> 
> Overall system efficiency, including pipework etc., is usually much lower, typically quoted on T-Sol at being in the mid-30s. Q


 
No it does not only apply to midday. It is the percentage of efficiency at any given time whilst the energy is being produced. Nothing to do with time of day. It is based on an energy input of 1,000 watts per m² from the sun. Losses are taken at different points of the tube/panel/collector and shown at various intervals as n0, a1 and a2.

Further more, T-Sol calculations giving a overall efficiency of mid-30's is typical of an over-sized system. With an efficiency of mid-30's, I bet your solar fraction is very high, probably in excess of 60%. 
It is like putting 100 tubes into a 100 litre cylinder (exagerated to make the point, of course), then my efficiency will nose dive, but my solar fraction will be huge. 
This would be very good at producing energy, but as very over-sized, it is very wasteful of energy so therefore a low efficiency. It is better to design a system that has a more even balance between solar efficiency and solar fraction. Ideally, 50:50.


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## BMD (27 Feb 2011)

Hi Shane,

I was very interested to read your comments. I am putting in system similiar to that which you describe (solar and UFH). My house is 2900 sq ft. 

Based on this size what volume of a buffer tank and how many tubes would I need. 

What would the approx cost of the tubes be?


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## Optimistic (27 Feb 2011)

*Re. Solar tubes*

Hi, Be sure to do your home work on the efficiency and output of the tubes you decide on installing. Request it, it will be written in the speck of each ones blurb. Believe you me there is amazing differences in the output between various brands as  I discovered when researching for myself. I have taken flat plate off my roof and put on tubes, that I am very happy with the output so far. 
Take care and all the best.



BMD said:


> Hi Shane,
> 
> I was very interested to read your comments. I am putting in system similiar to that which you describe (solar and UFH). My house is 2900 sq ft.
> 
> ...


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## Shane007 (27 Feb 2011)

BMD said:


> Hi Shane,
> 
> I was very interested to read your comments. I am putting in system similiar to that which you describe (solar and UFH). My house is 2900 sq ft.
> 
> ...


 
Many factors will have to be considered before designing this system and whether or not it will be worth installing in the first instance. Great care must be taken in order for it to be fit for purpose and I would only recommend going down this route if many considerations are catered for. It is not just a matter of sticking a buffer tank in with 60 tubes and off you go. Many other parameters must be installed, otherwise the gain will be quickly outwayed by the losses:

1. Are you installing UFH both downstairs and upstairs? If you are combining UFH with radiators, I would not have the radiator circuit drawing its heat from the buffer tank as this will cool the buffer too quickly. 

2. You will need a cylinder for DHW and that can bypass the buffer tank in winter months, taking its heat directly from the boiler. In summer, it should take its heat from the buffer. This will take a good design and control system. It will be complex, in that all certain situations must be catered for, for example: 

Buffer tank is cold, UFH heating requests heat, buffer stat will not allow circulating pump to function to stop cold water pumped through UFH circuits, it calls in back-up heat source and switches on pump when temperature of buffer reaches pre-determined minimum temperature.

3. What is your back-up heat source, i.e. oil boiler, gas boiler, ground source heat pump, air to water heat pump, etc.?

4. The size of the buffer tank will depend on how much of an energy store you will require. The larger the energy store, the larger the input of energy you will require. Larger is not generally better as this will have an extremely poor efficiency during summer months as the buffer will only be used for DHW. I find that 500L is an accpetable, viable and economical option. A 500L buffer tank with a ∆T of 30°C will have a thermal store of approx 18kW.

5. What is the overall heat loss of the property in watts/m²? The lower this value the longer the buffer tank energy will last. 

6. The amount of tubes you will require to heat a 500L buffer tank will depend on the specification of that tube, the azimuth in relation to south and the pitch of the roof that they are to be installed upon. Do not use flat tube for winter solar gain. Flat plate absorbers are not in a vacuum therefore surrounding ambient temperatures will kill the solar gain. Only select tubes for winter gain. 

7. The cost of tubes and installation, etc. will very much depend on the quality of the materials used and of course on the competence of the installer. Other factors will be type of materials used, such as copper pipework versus stainless steel, glass lined stratified buffer tank versus standard, basic control system versus high end, etc. etc.

8. Is your UFH system installed already or in the design process? Some of the controls I speak of can be encorporated into some UFH systems, such as Heatmiser. A secondary control system will of course have to be installed at the buffer tank side, encorporating three port diverter valves, a number of monitoring temperature probes at different levels of the buffer and four circulating pumps, some of which will be standard and others being self modulating.

9. Where will the buffer tank be located and how far will this be from the UFH manifolds?

Apologies if I cannot be too specific, but the design of this system is complex and really cannot be done over a forum.

Finally, if you PM me, I can email you a layout of what I am talking about. Hope this helps.


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## pache (26 Jan 2012)

Shane007 said:


> Many factors will have to be considered before designing this system and whether or not it will be worth installing in the first instance. Great care must be taken in order for it to be fit for purpose and I would only recommend going down this route if many considerations are catered for. It is not just a matter of sticking a buffer tank in with 60 tubes and off you go. Many other parameters must be installed, otherwise the gain will be quickly outwayed by the losses:
> 
> 1. Are you installing UFH both downstairs and upstairs? If you are combining UFH with radiators, I would not have the radiator circuit drawing its heat from the buffer tank as this will cool the buffer too quickly.
> 
> ...


 
Hi shane,if i pm you can you email me this layout also?

Pachie


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## Shane007 (26 Jan 2012)

pache said:


> Hi shane,if i pm you can you email me this layout also?
> 
> Pachie


 
Of course, if I can help. Just include the specifics as I outlined above.


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## pache (27 Jan 2012)

Hi shane,
Im not looking for a layout for any specific job just the layout for the job mentioned above as you said its pretty complicated stuff!! and i would hold a great interest in the electrical controls of such a layout,
Cheers,pache.


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## Shane007 (27 Jan 2012)

OK, no worries. I think I have already posted this layout. Just be careful on how you control the system, in that what determines each part of the system to operate at what time of year. For example, in winter months for hw demand only, it is better to divert the flow & return from the boiler to bypass the buffer tank. It would be wasteful to heat 500L buffer tank to heat a 300L dhw cylinder. In winter, the buffer should only be used to heat space heating.
In summer, the solar should be diverted to heat the dhw and once the 300L cylinder has reached temperature, the circuit is diverted to the buffer as an additional store. Each day this will be topped up and in periods of prolonged low solar gain, the buffer can then be pumped to heat the dhw cylinder.

It is all to do with controls and how they are installed and operated.

http://www.askaboutmoney.com/showthread.php?t=164265

This layout may not suit if both the buffer & the boiler are installed remotely in a garage and the dhw cylinder is in the house. Another layout will have to be intalled.


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## thepool700 (30 Jan 2012)

Back again... thanks shane007 for all feedback was great help. Would you have a  layout for a buffer & the boiler installed remotely in a garage and the dhw cylinder is in the house.


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## Shane007 (30 Jan 2012)

I will have to draw it up for you. Where will the dhw cylinder be (upstairs or down)? You will need some extra electrical controls to be able to switch between winter hot water and summer hot water.


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## Charlie 07 (31 Jan 2012)

how much would the above cost approx? (not including the installation of solar panels?)


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## thepool700 (1 Feb 2012)

thanks shane hw cyclinder will be downstairs.


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## mckbre (13 Nov 2015)

Hi Shane007,

What you are describing is exactly what I'm after!, have being trying to find info for months! I really would appreciate it of you could help with a piping schematic for 500 buffer and 300 DHW. Will the 500 be open vented for the stove?

I'm electrical so can get my head around the wiring, like you say, you just need  to pre-empt all possibilities and include for them.  

Would appreciate your help.

thanks


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## Shane007 (20 Nov 2015)

Hi mckbre, if you want to send me a pm with your email address, I can help you with a drawing.

There are some decent controls that can greatly improve the performance and the operational criteria of thermal stores. Resol do one that I have used in the past that has middle demands, i.e. goes to NC above a given temperature and goes NO over a higher set temperature, so you can call in demand at say 40C but off demand at 70C. 

For open vented stove, depending on the stove "system" output, I would consider a heat exchanger system such as the Firebird and seal the rads side, but there are many options.


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## Elixabet (28 Apr 2022)

Hi,

I have a similar challenge I have just bought a house in the Algarve with lots of spare land adjacent to my garden. I would like to heat UFH, DHW, Pool and Hot Tub, I would like to look at the possibility of using thermal solar, I have no space restrictions for the panels and would assume I could use a air source heat pump as back up and supplementary heat. With 300 days a year of sunshine it seems very tempting.


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## Leo (29 Apr 2022)

Elixabet said:


> Hi,
> 
> I have a similar challenge I have just bought a house in the Algarve with lots of spare land adjacent to my garden. I would like to heat UFH, DHW, Pool and Hot Tub, I would like to look at the possibility of using thermal solar, I have no space restrictions for the panels and would assume I could use a air source heat pump as back up and supplementary heat. With 300 days a year of sunshine it seems very tempting.


You may be better posting this on a Portuguese forum where others would be familiar with local legislation.


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