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Solar boosted hydronic space heating

(19 posts) (11 voices)

Started 4 years ago by ross
Latest reply from TonyT

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ross
Member

Hello

Issue 95 of ReNew had a great article by Michael Harris on solar boosting of hydronic space heating systems using evacuated tube panels. It was early days when the article was written and the need for a follow-up of the performance after a winter was noted.

Has anybody got any "hands-on" experience with this method of boosting that they would like to share? Does it work, is it economical, any tips, etc?

One thing I've picked up so far is that solar boosting is perhaps better suited to floor slab heating than wall mounted radiator systems because the temperature of the circulated water can be lower for the slab heating. However the house I want to use hydronics for has cavity brick walls and timber flooring, so the initial idea is to lay the heating coils onto the walls and render over them (or batten out, fill and and put cement sheet over) ... a sort of "wall slab". Any comments on this idea?

Thanks
Ross

Posted Monday 28 Apr 2008 @ 7:35:21 am from IP #
ghostgum
Member

Some non-expert thoughts from someone who has hydronic heating, but who only had it installed 1.5 months ago.

Can you add more northern windows to let in more heat in winter? Particular to let sunlight onto a brick wall?

You can put hydronic under a wooden floor, with insulation below that, but it doesn't work as well as radiator panels because the wooden floor insulates things a little. My calculation is R=0.16 for the floor boards, but you there is an additional R value between the hot water and the bottom of the floor boards.

The days when you want the most heating are often the cloudy days. So to heat on those days you need to store heat from sunny days. I wonder if a water tank is an effective way to do that. Unless you can store heat for several days, the solar boosting may be very expensive for very little return.

One of the houses covered in Santuary magazine had a glass room, with provision for moving the warm air into the rest of the house during daytime, then closing it off at night.

Posted Friday 9 May 2008 @ 3:53:54 am from IP #
jdduff
Member

Good to see someone else grappling with active solar heating. We are building (using an architect and builder) a stabilised limestone/earth house in southwest WA. We are finding it very difficult to get practical information about things such as solar hydronic heating, solid fuel boosting of solar hot water, best flooring materials (we would like polished concrete but wonder if it might be disfigured by cracks in the future). We would like to hear from others building an energy efficient house, and would like to hear about active forums with good local information.
John Duff

Posted Thursday 22 May 2008 @ 4:54:36 am from IP #
8688john
Member

Hi, I have been grappling with solar hydronic heating as well. We are building a new loft style home, slab on ground, with external walling of Timbercrete [R2.5 super insulator type]. The basic house is rated by Firstrate5 at a bit over 7 stars.

My first set of number crunching was to look at the feasibility of solar hydronic heating that could ride through a cloudy day; that means to gather enough heat energy during one winter day to last for 2 full days. The implication was that I would require a hot water storage tank of several thousand litres holding water at up to about 80 celcius, plus up to around 40 square metres of solar panel - depending on assumptions made. This system would also require a heat exchanger to provide water to the slab at a suitable temperature, therefore 2 circulation pumps and all the control systems etc. With this type of system concept, the system size and cost rises sharply the more you try to approach the holy grail.

Now I am thinking of something simpler. This system would operate directly to the slab from the panels, so it would operate whenever the panels are raising the water temperature, using a simple controller. An override would be required to turn off in warmer weather! This system relies on the slab for storage. It will work least when it is colder so supplementary heating cannot be avoided (not for my spouse anyway!) One possibility would be to use a heat pump to boost the slab water.

The advantages of this system approach seem to be that:

- simple
- water collection and delivery is at the lowest possible temperature, which maximises panel efficiency.

The possible disadvantages include:
- time lag during day before heating is apparent
- not so easy to integrate supplementary boosting?
- problems with cycling of slab temperature??

I do not have access to useful data on panel efficiency. What I am interested in is the relationship between heat gathered as a function of inlet water temperature and ambient temperature (at a given solar insolation) for flat panel and evacuated tube collectors.

My last resort is to buy a panel of each type and run my own side by side trial - an extravagant way to find the data however. Does anyone have data to save me this indulgence?

There is a very good book on hydronic heating systems "Modern Hydronic Heating, 2nd Edition". Refer to http://www.hydronicpros.com/publications/index.php?id=24

I borrowed this book from Monash University via a friend. It is now returned, and I would love to get it back again! This book includes data that enables a calculation (a good estimate anyway) of the water temperature in your slab versus the heat delivery. By matching this information to the solar heat collection it should be possible to estimate the water temperature at which the system would tend to stabilize.

Any tips regarding types of heating pipes (materials) and issues faced with slab construction and contractors??

Regards, John Heywood

Posted Wednesday 4 Jun 2008 @ 12:11:21 am from IP #
SueDunbar
Member

We went to the Melbourne Home Ideas Show in March this year.
Lots to see. Here is one site that was particularly interesting for this topic.
http://www.maassproducts.com.au/

Posted Sunday 22 Jun 2008 @ 5:11:55 am from IP #
Craig
Member

I've designed our house to work off hydronic solar using water heated to 40 degrees C. Haven't got the solar component working yet but have been heating the renovated part of the house using 35 to 40 degree water - I simply doubled the size of the radiators that would normally be used (2 of the biggest triple radiators available). So far have proved that this temperature water is quite satisfactory. In the process of installing solar.

Posted Wednesday 9 Mar 2011 @ 11:01:45 am from IP #
TonyT
Member

Winter is when house heating is needed.

Summer is when a solar water heater captures most heat.

This mis-match can be solved by using a grid-connected solar ELECTRIC system.

The electricity captured in summer can be fed into the grid and converted into dollars.

The dollars can be used to buy electricity when it is needed for heating in winter.

With careful use of the 3 times Feed In Tarrif, for the same number of dollars that you earned in summer you can buy in winter up to 3 times the number of kWhr that you fed in during summer.

Next: a heat pump water heater can be used to convert one kWhr of electricity into 3 kWhr of heat.

Last: if the heat pump runs at night on the off-peak half-price electricity tarrif, each kWhr of energy that was captured as solar electricity can be used to deliver 18 kWhr of heat.

For comparison, one kWhr of energy captured by a solar water heater is used as ONE kWhr of heat. Also, the captured heat is lost through the insulation of the storage tank if it is not used within about 48hours.

It may seem unbelievable, but it is true:
solar electric plus a heat pump can deliver up to eighteen times the heat that you capture with a solar water heater !

Even if the total cost of the solar electric system plus the heat pump hot water heater is 3 times the cost of a solar water heater, you can obtain
(18 times the kWhr of heat / 3 times the capital cost) = 6 times the heat energy
for each dollar of capital invested in a solar water heater.

Without using a boster element, the heat pump can produce hot water on cloudy days and at night.

Using a solar water heater, for cloudy days you will use electricity in the electric booster element. You will also need a larger insulated storage tank.

Posted Wednesday 9 Mar 2011 @ 9:10:42 pm from IP #
sun2steam
Member

TonyT, generally I agree.
But there is some creative accounting involved. Just compare the collector area of a PV system to the collector area of a hot water system.

If you have sufficient roof space, no problem. However most Australian roofs don't because it appears that most people here love these energy wise stupid wrinkled roofs with 12 or more different small triangles as roofs.

Posted Wednesday 9 Mar 2011 @ 10:52:55 pm from IP #
Eco
Member

Craig, I am interested in putting in a system myself. I agree with you that the best way to do solar hydronic is to use lower temperatures as the efficiency of the collection panels decrease as the temperature difference between air and water increases. So at these low temp differences flat panel efficiencies may be adequate, even in winter.

The problem with lower temperature is that it contains less useful heat. With 40 degree water you may be able to extract heat down to a water temperature of say 25 to 30 degrees. (Which gives rise to another problem - your internal panels may work well at 40 degrees, how well would they work at 30 or 25 degrees. And having to boost the water at 35 degrees defeats the purpose.) With sixty five degree water extracting down to similar temps you have over double the useful heat contained in the water. The consequence lower water temperatures is the water storage tank needs to be a a lot larger.

I estimate it is necessary to store enough heat for 2 - 3 days to carry over between sunny winter days. Again a larger tank is necessary and also collection panels. Of course can always have back up water heating for those days without sun.

The other issue with low water temperature is legionnella bacteria - 40 degrees is the perfect conditions for these blighters. It may or may not be an issue with an enclosed water tank, more of an issue with an open tank. I am looking for an additive to counter this - suggestions any one?

Probably the best approach is build the house to be as energy efficient and solar passive as economic (say 8 Star) and then rely on RC Aircon system to provide heat when it gets cold and overcast. And yes you can have a PV system (or Green Power) to offset that energy use. However to design and build a completely solar hydronic heating system is still very enticing.

Posted Wednesday 9 Mar 2011 @ 11:54:29 pm from IP #
TonyT
Member

sun2steam

I agree that one square metre of solar hot water panel absorbs more energy (above ? 70%) than does one square metre of solar electric panel (below ? 20%).

My point is that using solar electric plus a heat pump can give up to 6 times the heat energy return for each $ of capital invested.

Do you dislike corrugated iron or is it the layout of the roofs - or both ?

Posted Thursday 10 Mar 2011 @ 12:57:15 am from IP #
sun2steam
Member

I find the design of the roofs just wrong. what we need to maximize harvesting of solar energy are large, uninterrupted, sloped roofs facing north, and not these roofs with dozens of triangular areas pointing into all directions.

Posted Thursday 10 Mar 2011 @ 1:02:09 am from IP #
TonyT
Member

Should function follow form or should form follow function ?

As an engineer I have been certain that form must follow function.

My artistic wife has been equally adamant that function must follow form !

As we age our viewes are moving slightly closer together

Posted Thursday 10 Mar 2011 @ 1:10:33 am from IP #
munter
Member

TonyT - I can see how the numbers proposed above could work but I believe they take the most optimistic view on the viability of a combined PV/heat pump collection system. When I did the comparison (we are planning a solar hydronic scheme) I came up with a different conclusion to yours. The arrangement seems worth considering though as using PV solves the issue of excessive collector size in summer and allows you to gain some benefit from the collector panels all year round. An alternative, less optimistic set of numbers would assume:

No gain from feed in tariff (the 60c deal is no longer available for new systems in NSW and represents an artificial aberration on the real economics of the comparison). Worse yet, if I now added panels to our existing set I would lose the 60c tariff for the original panels! Taking this off removes a 3 x multiple.
Either no offpeak electricity usage or the inclusion of a large storage tank as assuming off peak electricity restricts heating to certain hours which may or may not be appropriate for the building structure. Taking off a reduced electricity cost removes a 2 x multiple ie, based on your previous figures the PV/heatpump approach comes out at about the same cost.

That said, if it comes out at about even then it definitely appears worth looking into further because the relative costs of PV vs thermal components are always changing.

I guess there are a few extra positives for the heat pump:
The use of a heat pump provides for the option of using hotter water and therefore regular sized radiant panels rather than oversized panels or in-slab hydronics.
Use of a heat pump should also allow for the easier integration of domestic hot water into the heating circuit as the water temperature can be sufficiently high to be used inside the house
If you don't restrict the heat pump to off peak you can heat the house any time you like which may be difficult with solar thermal and storage type arrangements
A heat pump is mechanically more complicated than solar thermal but it is also fully engineered/tested unlike a home-built solar thermal system that might have various teething troubles. It would seem more difficult to repair a heat pump compared to a solar thermal system that is comprised of relatively simple parts.

S2S - wait till you see the amount of north facing roof our renovated house will have! The long axis of our building will run east-west and we have a continuous north sloping roof along the entire length. There is going to be plenty of room for collecting sunshine.

Posted Thursday 10 Mar 2011 @ 1:37:57 am from IP #
TonyT
Member

munter said:
"I can see how the numbers proposed above could work but I believe they take the most optimistic view on the viability of a combined PV/heat pump collection system"

Agreed:
the $ return on $ investment in a (solar electric plus heat pump) system can be UP TO six times the return on investment into a solar hot water system.

If the 60cent Feed In Tarrif is not available and if low tarrif electricity is not available then the figures are much less favourable.

However, with a solar hot water system, the necessity to use booster electric heating on cloudy days can still make (the solar electric plus heat pump) the cheaper form of heating.

At home we can access the 60cent/kWhr FIT and also J-tarrif electricity for ? 10cent/kWr so our choice was clear.

To maximise the return from the capital $ we have now invested to generate PV electricity and to heat our domestic hot water, our heat pump will run for up to 24 hours each day in winter and the additional cheap heat will be used through hydronic wall-panels to heat two small rooms at the lowest possible running cost.

The hydronic heating is increasing our capital outlay for parts-only by about $4000.

Parts include:
two Grundfos UPS20-60N circulating pumps including unions $450ea, Sondex SL34-BR25-10 heat exchanger including unions $400, four Newstar radiators including valves $300ea, two Newstar heated towel rails including valves $250ea, 50m of Hepworth 18mm PEX pipe at $3.50/m plus $1/m to use polyurethane "noodles" for insulation, press fittings for PEX pipe at ? $7 each, 36m of 12.7mm copper pipe at $7/m. Other PEX pipes were cheaper but they needed special tools costing hundreds of dollars.

When it all works, I hope to stop chopping firewood

Posted Thursday 10 Mar 2011 @ 9:16:32 am from IP #
Ueee
Member

I am too looking at hydronic heating, using radiators. We are renovating and our house is very poorly orientated for solar gain in winter (not a consideration when bought). We have a large crawl space under house so access in not a worry. I was thinking of using a stainless tank 2-3000lt insulated with polyurethane foam- open system- using coiled copper pipe as a heat exchange for hot water+instant gas boost/tempering valve. The beauty of an open tank is you can ad as much water as you like to balance out the no of panels you have and keep the water at an appropriate temperature for the radiators.

TonyT where did you get your pex from? all the systems i've looked at need tools to crimp etc and are not worth the cost for one job. What do you think of the newstar rads? I was looking at these too.

Also does anyone have any suggestions where to buy Evac tubes and manifolds from-Run on sun is the only supplier i can find who sell just the tubes without tanks etc.

Posted Wednesday 30 Mar 2011 @ 9:40:27 am from IP #
munter
Member

Have a look at fx Larkin. They have lots of diy solar stuff.

Posted Wednesday 30 Mar 2011 @ 9:40:55 pm from IP #
TonyT
Member

munter

Thanks for sharing your logic.

Have you decided between installing evacuated tubes or a heat pump ?

Even without the 3x Feed In Tarrif I think your points make the heat pump more attractive.

Recently we had the compressor in a 30 year old chest freezer replaced.
It had worked constantly for 30 years.
I think that it cost about $300 to replace.
Hopefully the compressor in the Siddons will give similar service.
Unlike some heat pumps, I am told that the Siddons is well-engineered and that the compressor is not under-sized.

Ueee

To avoid buying expensive (hundreds of dollars) special tools that I am unlikely to use again, I am using un-insulated PEX pipe called Hep20 and made by Hepworth.

It is called 18mm but the bore measures only 14mm.

The cost is $89 for 25 metres.

Most of the Hep fittings are about $8 each.

It is more expensive than other PEX pipes but it is EXTREMELY easy to use. And 25m rolls mean that I will not have a lot left over.

The PEX pipe is simply pushed into the fitting without even loosening the nuts.

The 12.7mm copper pipe is also just pushed into the fitting.

I paid $40 for a special tool to cut the PEX at 90 degrees. It is useful but not absolutely necessary.

For insulation I was going to use swimming pool "noodles" but eventually I am using "Armaflex" at about$2/m because it was not much more expensive.

Posted Sunday 10 Apr 2011 @ 9:57:36 pm from IP #
Ueee
Member

Tony T,

Thanks for the Hep details, My local bunnings sells the straight lengths of pipe but no fittings!! I noticed FX Larkin sell Pex al Pex plus fittings which appear to be nut and olive type, but will have to check that no tooling is required.

Don't be so sure on the longevity on a compressor-we have several florist shops, 2 coolrooms and 5 fridges. Sometimes they all work fine but every now and then we have had endless problems with compressors and fans, Keep in mind your freezer was made "Back when things were built to last" and i wouldn't expect that life out of a new item. Also check the warranty on these things, many manufacturers are now offering 10 years or more, but when you read the fine print they will cover parts only, and normally the callout fee and labor is worth more than the part(s).

I Noticed in your list of hydronic parts you didn't include a manifold- are you just running the radiators in parallel each with a flow and check valve?

Has anyone considered trying to use a thermo siphon instead of a pump on the HWS (heat pump etc) side of their heat exchanger?

The sudden cold snap here in Canberra has certainly made me think about hurrying this along! So much for not turning the heater on till ANZAC day.

Posted Tuesday 12 Apr 2011 @ 10:18:09 am from IP #
TonyT
Member

Ueee
Each radiator is fitted with a thermostatically controlled valve on the inlet plus a manually set valve on the outlet.
The circuit will include 4 radiators (each Type 22 and 600mm wide by 900mm high) plus 2 small (500mm wide x 735mm high) towel rails all in parallel and without a manifold.
Everything will operate 24hrs/day and 7days/week during winter.
It will be off during summer.
After filling the circuit with rainwater it will be completely isolated.
It will have an 8 litre expansion tank and 2 automatic air release valves but no auto-fill valve and no check valve.
The Grundfos circulating pump will run at about 20kPa and 0.4L/s.

Posted Tuesday 12 Apr 2011 @ 11:38:47 am from IP #

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