Solar thermal with in-floor hydronic
Hello,
I’ve read some question threads with opinions running against using in-floor hydronic since the warm toes effect would only happen if the house shell was poorly insulated. For a well insulated house the expensive in floor hydronic could be omitted for a much less expensive system (even electric baseboard). I understand this argument.
I would like to use solar thermal panels to provide at least 50% of the energy required for house space heating and preheating of the DHW supply. From reading on other sites I have understood that in-floor hydronic is the best way to distribute heat collected by the solar panels due to the match in operating temperature. Any warm toes effect is only a bonus. Do you think that is possible?
My plan is for a one story house with a full basement (walkout), approximately 2,000 sq ft foundation, in the Minneapolis, MN area. The in-floor hydronic would be used in the basement slab over 4 inches of extruded polyurethane (meant extruded polystyrene) insulation. The house insulation target is 20 / 30 / 40/ 60 with tight air sealing, good windows, and an HRV system. This solar system would provide heat in the basement where passive solar would not work since the walkout exposure is not southerly. The floor slab and an insulated water tank would provide the thermal storage for night time plus one cloudy day. What do you think of this plan?
I have also considered using sub floor hydronic (staple up tubing with aluminum spreaders) for portions of the main floor. These parts would be under entryways and looping just inside the exterior walls (about 4 to 6 feet). The remainder of the heating would be provided by a standard ducted forced air furnace or mini-split system. I would like the solar panels to provide almost all of the heat and hot water needed for April through October. November through March would require supplementary electric / propane / wood. This is rural property and I have 20 acres of woods and excellent southern exposure at the building site. Any thoughts on this plan?
Thank you for the education in building science. It’s been a fun ride unlearning everything I thought I knew and filling the void with science! (que up Thomas Dolby)
Karl
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Replies
Karl,
You mention that you will be installing "polyurethane" insulation under your slab. Did you mean extruded polystyrene? I hope so. Don't use polyisocyanurate insulation under your slab -- it can absorb moisture.
Your goal of heating your house from April to October with an active solar system is reasonable, unless October is a cloudy month in your location. Here's the problem: you don't need much heat during those months in any case, and if you design a tight, well-insulated envelope with passive solar features (adequate south-facing windows), you will probably be able to satisfy all of your heating requirements that way, without an active system, during the months in question.
Moreover, active solar thermal equipment is very expensive. It's easy to pay $10,000 for a system that supplies just domestic hot water. If you need a system that supplies a significant percentage of your space heating needs, it's easy to spend $20,000 or $30,000. But it's hard to justify the investment because the amount of energy that it yields is relatively low.
Your envelope specs sound good. My recommendation: build a top-notch envelope, with particular attention to air-sealing, and heat your home with a cheaper HVAC system. Instead of spending $20,000 on active solar equipment -- collectors, pipes, pumps, tanks, and electronic controllers -- spend the same money on triple-glazed windows and a Passivhaus consultant.
I was hoping to do almost the same thing, but the problem as Martin notes is the expense of all of the controllers and the professional installers. The first quote we got was over $30K in a house that costs about $1200 a year to heat... not a very fast payback.
However I am still hoping to investigate a manual control system -- sort of an A-B switch -- that could be less expensive. In the early winter and early summer, when the basement temperature is useful for moderating the temperature of the building in general, the solar heat would be fed into a preheat tank for domestic hot water. In the spring and fall when it is not too cold indoors, the solar heat would be fed into the basement (to reduce humidity and store some more heat in the slab).
This does not directly address your question but at least we have some similar issues. In particular, installers do not seem to provide super-insulated buffer tanks for on-demand hot water at a reasonable cost. The economics of solar hot water and hydronic, at least in the U.S., are designed to handle much larger amounts of hot water in an hour than we would really need. I don't know the actual numbers, but if a typical system can heat 90 gallons of hot water in a half hour and we would need only 25, shouldn't the smaller system be a third the cost of the larger one?
Unfortunately, it is not, and that is why the economics of solar/hydronic or geothermal systems might not make sense for smaller homes at this time.
Martin Holladay,
Yes, I meant polystyrene. My typo and thanks for the cost info. I haven't gotten any bids yet. I was hoping to contain the costs by only speccing the 7 months. The winter months in this area are usually cold and overcast. At that cost I'm better off buying a nice wood splitter instead of the solar system.
I agree that using insulation and passive solar is best for the main floor. My thoughts were about heating the basement where I don't have good sun access directly. A good wood or propane fired boiler is less expensive by far than what you've been quoted.
Rich Cowen,
That is exactly along the lines of what I am thinking. I really want the system to add a base level of heat to the basement where temperature range control is not as critical. I just want it warm and dry even though it has poor sun access - nothing fancy needed. The forced air furnace or mini-split would handle finessing the temperature up to the correct range on the main floor (I'm married). Much less need for fancy controls or large panel areas. Preheating the DHW would be a bonus to use the system in the summer when space heat is not needed. A.O. Smith and Rheem-Richmond have solar ready tanks with or without electric boost.
I will look into the pricing more. I really like the idea of using solar for the heat and hot water but the dollars may eliminate it. Thanks for all responses.
Karl,
It sounds like you are on the right path in your thinking.
I'll throw in my two cents' worth - insulating the concrete details of a walkout basement tend to get very tricky. Ideally, you'd like to have all the concrete inside the thermal envelope. It's quite difficult in practice, but can be done with good planning.
Karl
With radiant in general it's better to heat a small enough area that the floor can actually feel warm before the house gets over heated (assuming a good envelope) so use a reqular solar thermal DRAIN-BACK system with a 180 gallon reservoir or so and just add a few extra panels. The basic 180 gallon system cost is only increased by the additional panels at approximately $1,000 each. then use a flat-plate or tube-in-tank heat exchanger to draw heat off the solar tank and into the floor, but don't use a whole lot of floor area, maybe a quarter of the house or so, the mud room, master bath, monopoly playing area in front of the fireplace, maybe under the dining room table and the center floor area in that basement. Run these areas on a 110 volt t-stat designed for electric strip heat controlling an efficient Wilo three-speed pump and you have a reasonably affordable, if not exactly cheap, way to gain some edge season advantage.
Kevin Dickson,
A very good point. I have been back and forth with my GC about insulation details. He is a very good person but this will be his first superinsulated project and much of this is new to him.
I have seen some of the other houses being built in this area (not his projects) and I could literally see into the house through the gaps in the insulation (could almost stick my hand through some gaps - this was the rigid insulation outside of the sheathing). I decided at that point that I would have to handle all of the insulating and air sealing myself. I will post my design details for critique when I get them done.
Michael Chandler,
Thanks for mentioning the drain back system - that is the one that I was thinking of using (can never keep the two straight). My initial plan is to have outer loops (near the exterior walls) and inner loops of PEX tubing. I can then adjust the heat distribution with manual valves and a multi-speed pump as you said. This may end up as a small initial system with expansions added later.
Michael --
This is an interesting idea... part of it is that I want to swap out some dry-rotted floor joists before doing radiant and not having to heat the entire floor would be a big positive so that we don't have to tackle the daunting task of replacing all of the floor joists all at the same time, where some of these joists are used to run existing plumbing.
Are there any more detailed specs for the system you describe?
We don't have a super tight envelope, and we do have thick subfloors that the heat must pass through, so I am thinking that a "ring of heat" around the center post in the basement ceiling would accomplish a lot.
If you put the extra money into optimizing the building envelope, then you shouldn't have a need for mechanical heat in April and October. If the envelope is planned out well enough, you probably won't have a need for mechanical heat for most of March and November either!
John Semmelhack,
Good point and I do plan on having a high quality envelope using passive solar for the main floor. I have difficulty believing that this will help the basement temperature much in April or October. I had planned on using solar for heating DHW (if the costs work out) and have read that in-floor hydronic is a good match for solar (just add a few more panels to the system). We do have some very sunny days in the winter and that heat collection would be a bonus. Poorly heated / ventilated basements in this area become dank, smelly, uncomfortable places and I'm trying to avoid that without burning more fuel than necessary.
If I were to prioritize the solar heat use it would be
1. Preheat or heat DHW.
2. Space heat for basement.
3 Space heat for main floor. This is not required as I'm fine with using other systems for this.
I wouldn't bother with the staple up under the wood floors, just a warming ring in the center of the basement and an extra couple of panels on the drain back system. I have a schematic of a simplified radiant floor system w/ demand water heater backup in my "things I learned at Joe's house" post and a bit more at my own company website
Hope this is helpful rather than confusing. .
Michael Chandler,
Thank you - that info is very helpful. I am always interested in "lessons learned from the trenches" type info. One question - why the warming ring in the center of the basement instead of looping around the perimeter? I assumed that the perimeter of the slab would be colder (more heat loss) than the center area.
Karl, if you are a DIYer you might explore http://www.builditsolar.com/ and join http://tech.groups.yahoo.com/group/simplysolar/ . There is a tremendous amount of information on building either active or passive solar systems much less expensively than commercial systems. And the folks who run the website and Yahoo group are very friendly and helpful.
Just closing out this topic for me.
After everything is done my decision is to eliminate the in-floor hydronic from the design. Here is where I ended up in my head after reading lots of material here and on various other sites looking for real world experiences.
1. Higher efficiency - This seems to be more true in poorly insulated houses, or is just a lot of marketing talk. Either way it's not applicable to our house.
2. Zoning capability - Zoning can be done by many other heating methods.
3. Fashion - In-floor hydronic is fashionable in this area. Yes it's a stupid reason but I'm human so thhhhhhppppppt. To be bluntly honest this may have been the strongest reason.
4. Recommendations - Lots of people recommend it but mostly for the warm feet effect which will not happen in a superinsulated house.
5. Solar - This was the last sticking point for me and really why I posted the question. I want to incorporate as much solar heat as possible and this is a good way for a conventionally insulated house. Once I really looked at the foundation heat loads from HOT2000 it becomes apparent that in-floor hydronic is overkill. I am convinced that there are better ways for this house.
Other negatives include the price and cast in concrete installation. I have an inherent distrust of things that cannot be accessed for service, repair, or replacement. I really wonder if all of the now new in-floor loops will have problems in 25 years. I have seen so many cracked slabs (much more so than our 30 year old rambler or 90 year old farm house - very ironic) in brand new housing that I have a hard time trusting it. Also I wonder about damage during remodels. With all of those loops what are the odds that a nail or saw will find them? Murphy's law has chewed large chunks out of my rear end before.
Finally in this area A/C and humidity control is a really good thing and can be combined into the heating / ventilating system for a much smaller total amount of money. The money savings can be put into other things for this house.
I may have forgotten some other points but those are the important ones to me. (edit to add with mini-splits, forced air, or radiators the floor covering does not affect the heat transfer as it apparently does with in-floor.)
Thanks for the discussion,
Karl
Jack Woolfe,
I have definitely been reading through BIS.com and am amazed at what people are doing and learning lots. Great site. I hope to contribute back with whatever small things I do.