Radiant Flooring / GeoThermal / Solar
Statements
– We are building a house and are putting radiant flooring on both floors between the joists.
– We will install solar panels after we are finished building.
– Our house will be a tight house with the Atlas LCi-SS system installed in place of the OSB, PolyISO and wrap.
The question is one of efficiency. Since we are using solar should we also use geothermal? I understand that geothermal is more efficient, etc but if we are going to go solar anyway then should we worry about getting every piece perfect? ( ie get 80% of the gain for 50% of the cost instead of 95% of the gain for 100% of the cost).
We live in Virginia and will be on a mountain where it will be COLD and windy. So by using an electric boiler for the floors the <32 degree days will not force the traditional system to go into Emergency heating. For where we live this is the prime driver of high electric bills. so Radiant flooring, solar, boiler, traditional system or Radiant flooring, solar, geothermal I’m unwilling to get rid of the solar energy since where we are moving I understand that the power is often out. thanks
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Replies
To get the HVAC right requires calculating the loads then looking at the solution options, not defining the solutions first.
Without knowing the heating load, you won't know what the temperature requirement is for the floors, and thus the water temperature requirements. Above the subfloor radiant floors don't require water as hot as between the joist systems, and that makes a HUGE difference in efficiency and sizing of geothermal system, or even when/where back up heat would be needed.
A hydronic air source heat pump would be a cheaper alternative to ground source, and still 2-4x as efficient as an electric boiler.
Running a heat pump off batteries won't get you very far during a power outage without a dauntingly expensive battery array. A small EPA rated woodstove might be worth considering.
You're likely to need some amount of cooling even in the mountains in VA. Ductless mini-split heat pumps can do both, usually at a fraction of the upfront cost of geothermal, but it won't heat your floors.
So, run a Manual-J on the house design, maybe even modify the design to reduce the loads, then figure out what it really needs to heat/cool the place.
Michael,
Q. "I'm unwilling to get rid of the solar energy since where we are moving I understand that the power is often out."
A. Almost all grid-connected photovoltaic (PV) systems provide no electricity during power outages. As Dana noted, it's possible to design a system that will power your house for a few hours -- or, if your bank account is large enough, even a few days -- but such a system requires a very large, very expensive battery bank, and the batteries require regular maintenance. (The cells have to be topped up with distilled water.)
Q. "Should we worry about getting every piece perfect? (i.e., get 80% of the gain for 50% of the cost instead of 95% of the gain for 100% of the cost)?"
A. I have no idea what a "perfect" system would mean for you, but in my opinion, cost is always a consideration. Before balancing cost on the one hand with what you can afford on the other hand, you need to think about your goals. To paraphrase Amory Lovins's famous statement, your goal is not to own a geothermal system or to install PEX tubing in your floor assembly; your goal is to have hot showers and cold beer -- or, in your case, to make sure that your home is comfortably warm during the winter.
If you agree with my statement of your goal, then I don't think that a ground-source heat pump system or in-floor hydronic tubing should necessarily be near the top of your list.
One of the major disadvantages of a ground-source heat pump (besides its very high cost) is that such a system must be individually designed for each house -- you need a one-off design -- and it must be assembled on-site from a great many components by contractors of varying skill. It's fairly common for these systems to provide disappointing performance. As I've written before, the types of problems that cause these performance failures can usually be solved, as long as you have patience, buckets of money, and access to consultants and engineers. Here at GBA, we've heard about plenty of problems related to undersized ground loops, oversized pumps, or surprising soil characteristics; any one of these factors can lower the expected efficiency of the system.
In your climate, ductless minsiplits (or ducted minisplits) perform very well. They operate even when temperatures drop well below zero. They cost a fraction of the cost of a ground-source heat pump system connected to PEX tubing installed in your floor assembly.
I imagine that it's a good idea to install PV, but check with your local utility to determine whether the utility offers a good net-metering contract before committing to such a system. And make sure that your designer knows that you need a very large south-facing roof without any dormers, valleys, chimneys, vent pipes, or skylights.
Here are links to three articles that you may wish to read:
All About Radiant Floors
Are Affordable Ground-Source Heat Pumps On the Horizon?
Getting Power From Solar Equipment When the Grid is Down
I have great concern about the long term prospects for the geothermal (ground source heat pump) industry. In our area there were a number of contractors that jumped into geothermal 5-7 years ago, even buying little drilling rigs of their own, and they are out of business now. And that was with a 30% tax rebate. With that rebate expiring it's questionable how the economics can work for anyone.
Good comment by Dana that your location in Virginia might be better served by a hydronic air source heat pump. Here John Siegenthaler compares the two technologies, air source and ground source: https://blog.heatspring.com/john-siegenthaler-reviews-heat-pumps/
Again as Dana says, it might be iffy that you can make hot enough water with a hydronic air source heat pump for an underfloor radiant system. Even with slab/thin slab radiant heat that could be questionable; that issue severely limited our choices of floor covering in our geo heated home.
Agree also on energy modeling, it's essential. As I look back at the evolution of our house the energy modeling was something like this:
1. Owner targets desired building envelope topology.
2. Owner utilizes inexpensive/user friendly Manual J computer program pass 1.
3. Architect and owner refine building envelope.
4. Owner utilizes inexpensive/user friendly Manual J computer program pass 2.
5. "Final" revisions to building design.
6. Hire consultant to energy model home and design HVAC system.
7. Evaluate actual building materials suppliers: SIPs, windows, etc.
8. Owner reads Manual J cover to cover, builds Manual J spreadsheet for greater detail when selecting actual building materials, determines that downsized HVAC system (geo compressor) is within reach.
9. Hire GC/HVAC contractor.
10. GC and owner further refine building materials/techniques.
11. HVAC contractor utilizes commercial energy modeling software.
12. HVAC contractor and owner energy models converge.
13. HVAC system as designed by consultant constructed, but at slightly scaled down size based on final energy modeling.
OK, OK, at the end of the day I might have been able to skip a few of these steps and end up with nearly the same outcome. (Fortunately my wife doesn't read this forum.) The point is that this is an iterative process.
Dana gave good advice [edit: so did Martin and Andrew, didn't see them before I first posted]. Note that radiant floors with an electric boiler cost just as much to run as emergency heat with a conventional ducted heat pump system, assuming both are designed and installed well. If you want to avoid high peaks in electric consumption when it's cold outside, a better option is to select a heat pump that works down to very low temperatures. Mini-splits that work down to sub-zero F temperatures are common; the Chiltrix air-to-water heat pump is another option that gives you the flexibility of using floor heat if you want, along with super-efficient mini fan coil units to provide room-by-room heating and cooling without the need to run ducts.
You'll need to think carefully about what you want to work during a power outage, and for how long. Your PV system will need a battery to work during a power outage, and it would take a large, expensive battery and a bigger PV array to get through a week-long power outage that happened to occur during a cloudy and cold weather period. But if you don't power the heating system during an outage, you could get away with a much smaller battery and no up-sizing of the PV array. You could do that simply by planning on allowing the house to get cooler and wearing sweaters as needed, which might not even be noticeable for a 6-hour power outage, but would be increasingly more like a camping trip if it dragged on for days. Or you could use a wood stove as Dana suggests. Or you could use the Chiltrix air to water heat pump with a huge water tank (e.g., ~500 gallons) to provide a backup reservoir of heat you can draw from by running low power water pumps from the battery without running the heat pump.
Note that the better your insulation is on the building, the longer it will stay warm with no heating system running. For a given target of how long you want it to stay reasonably warm, it might be more cost effective to put in super-insulated walls and super-windows rather than a giant battery or a giant water tank. Saving scrap wood from the construction project and having a stash up of that ready to burn in the wood stove is also pretty cost effective.
This discussion really typifies the pragmatic, detailed advice GBA provides.
Michael ,
All good advice so far . Pay no mind to Martin's epic POS , "ALL about Radiant floors " , simple fact is that Martin doesn't know all that much about radiant floors with the exception of poorly designed and installed ones that were done , I suspect , with only budget in mind . They can be very well designed and done by the right people , You'll just have to locate them . You can contact me for assistance with that . 732-581-3833 . Best bet would be , look for designers and installers who belong to the Radiant Professionals Alliance . The most qualified 300+ guys that you'll find . Dana is absolutely right about systems installed directly over the subfloor .
I seriously recommend that you think about a quality generator for those outages that may end up being extended for any number of reasons . No one really knows how long an outage will ever be .
Solar thermal with storage is a very tried and true and proven method also . It can even be done passively , much better at a fraction of the installed cost than GSHP. Look up Thorsten Chlupp . There was also a very spirited debate right here quite some time ago , maybe Martin could throw up a link to that one , Solar thermal is really , really dead .
A wood stove is always something that can heat a home using little fuel .
As always , make sure the building is well insulated and sealed as a first step .
Andrews advice is pretty good also . The one thing you may want to consider though are the steps . Getting a team of highly qualified contractors involved in the design process right at inception saves alot of time , bickering and nonsense . When everyone has input nobody gets forgotten about . This is called Integrated project design and delivery , works very well in every instance . Design the systems for the occupants , not the house . Build the home so it is conducive to health and efficiency .
Find those contractors who are able to model and design their own systems for a more seemless approach and to have one person to hold accountable if it does not work . Systems designed by one and installed by another that go wrong will only cost you money and gray hair .
Richard,
In general, our GBA policy states that we should debate technical issues rather than hurl insults. If you want to introduce a technical argument, that's fine.
Describing an article as an "epic POS" and noting that "Martin doesn't know all that much about radiant floors" is not a technical argument.
Richard,
OK, I can accept the criticism -- which is hardly technical -- that you think that my article should have had a different title.
Your other point -- "Fact is that radiant floors will work in most buildings regardless of construction type when designed and installed properly" -- is one that I certainly agree with.
Although I think that radiant floor heating systems are oversold, I noted in article that "radiant floors have an excellent reputation. Many customers report that this type of heating system is comfortable and quiet."
You'll have to pardon me Martin .
My points were not meant as a technical argument as much as a Technical opinion , based on facts known by many in the hvac realm . An article entitled ," All about radiant floors " , which contains mostly opinion and very little fact should be open game whenever it is mentioned or posted . It should also probably contain all information about radiant floors , after all it is titled "All about radiant floors" .
Fact is that radiant floors will work in most buildings regardless of construction type when designed and installed properly . There are in fact occasions when radiant alone cannot heat a space , room , area , without help from a supplemental source . These occasions should be able to be identified by the designer , whom , might I add should be the installer . Situations like this are also almost always due to a choice someone made while designing the building . If that is not the case and a similar situation occurs over and over, possibly due to location of said building or some other factor , then it is always my recommendation that Radiant is not suited for this building .
At the end of the day , if one wants the healthiest , most efficient , that has the best TCO he may seriously want to consider a hydronic based system of some sort and a DOAS . They can be done very well , are comparable in cost if one wants to perform all the math , and remove refrigerants from the built environment , they can even be powered by Solar Pv .
Please don't take my comments personally . If someone else wrote a paper with that title it would be them I challenged . I am only here and comment the few times that I do to help your readers Martin , that is the goal , Correct ?
I hope the radiant and geothermal naysayers aren't correct because we just built a house with geothermal and radiant heat throughout. We live out in the country and I did not like the choices of propane or oil and having an all electric house appealed to me. We have a wood burning fireplace as backup in case the electricity goes out and a small generator for the fridge and well pump.
When designing the house I heard some horror stories from coworkers on their propane cost for the winter and that is what got me thinking about geo. I don't know, maybe it was a mistake...stay tuned...but now I can wave at the the fuel truck as it drives past our house down the road.
Mel,
There are, indeed, many advantages to building an all-electric house. In general, though, ductless minisplits + PV is cheaper (and keeps you just as warm in the winter and cool in the summer) than a ground-source heat pump system.
Mel: Don't count me as a "nay sayer", count me as a "Do the math! sayer". You can spend a lot of money on a less than satisfactory result if you don't take the time to really design it. At the risk of repeating myself:
Committing to radiant between the joists without first knowing the heat load or what's supplying that heat would be a mistake. You HAVE to know what that load is before embarking down that path to get there most cost-effectively.
Committing to hydronic geothermal without knowing the total heating/cooling loads and water temperature needed for the radiant floor would also be a mistake (a potentially very expensive mistake.)
It's possible to heat & cool a high-R house with electricity very cheaply, but it's also possible to overspend (by large multiples!), on the mechanical systems if you haven't really run the numbers first.
I hope that the "...wood burning fireplace..." is actually an EPA-rated wood burner, which is at once a much higher efficiency and much lower polluting type of heating appliance.
If your system is already installed you'll just have to run it through it's paces and see how it does. If it keeps up with the loads it probably won't be expensive to run, even if it wasn't cost-optimized at the initial design phase. (There are exceptions to prove the rule, however.)
Mel- I don't know anyone around here who would recommend a propane fueled heating system. The primary criticisms of geothermal are the high cost and complexity. I opted for mini-splits as a much more reasonably priced and simpler system for my all-electric house.
That being said, I hope your system proves to be the right choice.
No worries everyone! My wife likes the comfort of the radiant and how quiet it is. So far so good on the geo too. I'll report back next year and let you know how I feel after we live with it for a year and I get an idea of our monthly cost. Keep our fingers crossed!
Dana, it is a high efficiency EPA rated wood burner...sorry, I should have made that more clear.
since power reliability is in question I think the best bet is to make the shell as efficient as possible. Then plan windows for winter solar heat gain. With there strategies you have minimized the heat load. Then have a small generator that can help carry the house through the power outage. A minisplit could provide the most efficient heating.
I dont have experience with radiant heat but it would seem if the heat loads were very low then the home would be more comfortable without the need for the expensive radiant system that only provides heat. I would assume radiant heat covers up for poorly insulated homes that leak a lot of air. The poorly insulated walls would result in walls that drain radiant heat from the occupants while the radiant floors counter that. Super insulated walls and ceilings would result in higher mean radiant temperatures and better comfort.