Geothermal integrated into our radiant floor?
Last year we put in a new 3 zone radiant floor heating system, most of the work we did ourselves with the exception of the Bock 40BSCE water heater and oil tank. Of course when we started this oil was at $2.45 a gallon. The estimated heating requirement's need for our house was 73,000BTU's/Hour. We cool with two 10,000 BTU A/C units on each end of the house, one end it's a little to much while the other its just not enough by a small margin.
Our house is a 1800sq ft. split level ranch in Massachusetts. In the summer the cooling is only done on the first floor since the bottom floor only gets up to 71-72 when it's 98 degrees outside while the slab beneath our floor doesn't go past 63 degrees. In the winter the heating system has no problem with keeping the house warm at 73 degrees, the slab beneath the concrete floor doesn't drop below 50 degrees. Note; we didn't use the bottom half of the house so the temperature was kept lower during the second half of the winter since we were rebuilding the downstairs and not using it except for doing laundry.
This year will be our second full year in our new home. First year we used the electric baseboard heaters that were installed when they built the house in 1986. Second year and first full winter we used our new oil heating system. This year we will be using it again but will be greatly improving the insulation in the attic from a R-38 to a R-60 and sealing up around the windows and light fixtures in the house. Bought a thermal infrared temperature gauge and used it through out the house to find areas that needed to be addressed this summer. Last year we spray foamed the whole bottom half of our house during our reconstruction.
In the bottom half of the house we laid down a foam board insulation then added our radiant tubing and re-bar then added 3 inches of concrete on top of our concrete floor. Note; the temperature gauge is located beneath the 3 inches of concrete.
I see that in some soil locations that grout is added to the tubing to increase the heat transfer from the ground to the liquid in the tubing. I was wondering if our setup would be better since we are using 4 inches of concrete instead of grout to help assist in the temperature transformation.
We were wondering if it was at all possible to add geothermal to our house by using the Radiant tubing that we have installed in our concrete floor? We have a four manifold system with four loops of 300' each and was wondering if we could use three of them (900') to heat up the new geothermal system while connecting the other loop to one of the other zones (I know the 10% rule) and using it to help assist in warming up the concrete in turn helping to warm up the water temperature in the other loops so the system wouldn't have to work as long to heat up to the correct temperature.
Last edited by nevea2be; 06-15-2008 at 03:32 PM.
Could I use this setup to maybe preheat the water before it goes into the tank to be heated up to 120 degrees? Would it increase our savings by using less oil?
I don't fully understand what you're proposing - do you want to try to pull heat from the slab to act as a source for the geo? If so, this is unlikely to work - you generally need a much, much larger heat sink/source than the footprint of a house foundation provides. Also, if you're insulated under the slab, the 3" of concrete on top will not store much energy - you'd probably drop the temp well below freezing in a very short time. Or are you talking about using the lower, uninsulated slab? I've seen geo loops put in under foundations before, but the conditions and loads have to be just right - in most cases it is a recipe for disaster. Even if the floor could pick up enough heat form the ground, 900ft of PEX isn't even in the ballpark of what a 73,000btu/hr heat pump would require for a loop.
Sorry if I've got your idea wrong - it wasn't completely clear to me from your post.
I keep reading where some companies use 100' of tubing per ton all the way up to 600' per ton. There doesn't seem to be a set amount written any where. I'm figuring that I would use a 3 or 4 ton system for my house.
I would be using the tubing above the insulation. With the heat off downstairs the temperature never dropped below 50 degrees beneath the insulation. Wouldn't using the "recycled" or "run off" of heat from that one loop help improve the heat within the slab?
We are at 73,000 Btu/hr now and that number should decrease with the improved sealing of fixtures and windows and the extra added insulation in the attic. If 73,000 is the number I need to heat, then wouldn't a heat pump be some where between 50-80% of that figure?
Rules of thumb can be very dangerous especially for Geo systems - loop lengths vary hugely with load, climate, soil, type of system, etc. etc. A loop field should really be designed by a pro for the specific application. That said... In Mass with a horizontal closed loop the rule of thumb would be 600' per ton at the low end, possibly a lot longer depending on the factors above.
However, that's not really the issue here. If you are proposing to use the insulated slab as a heat source, it just won't work, especially with insulation underneath. Your house loses a certain amount of heat depending on the inside temperature, outside temperature, and insulation/infiltration. To maintain temperature in the house, an equal amount of heat must enter - currently that comes form burning oil and a small amount from solar gain, people, etc. In a geo system, about 1/3-1/4 of the heat comes form the electricity running the compressor and the rest from the loop field OUTSIDE the house. Since your slab is insulated underneath, it is essentially part of the house - what you are proposing would be similar to placing an air-air heat pump condender in your basement or trying to cool a room by putting a window air conditioner in the middle of it.
You could move some heat from the basement to the rest of your house, but the basement would rapidly cool down and then there would be no more heat available to move. It stays warm now without heat, but as soon as you start pulling heat from it it will cool down very quickly.
Let's say your slab is 900sqft. by 3" thick. That is about 225 cubic feet of concrete or about 32,000lb. It will store about 6400btu per degree of temperature - therefore if the slab starts at 70F it will take only 3.5 hours to drop it down to 30F at your design load!
Thanks for your input bc, I greatly appreciate it. Now I'm going to have to look at plan B.
Question; what size tubing would be used in a closed loop here in New England, I've read them being anywhere from 3/4 of an inch to 2 inches. Using your 600' per ton as an example would it be more feasible to go with a 1000' run per ton instead? Would you be able to gain more heat from it or would it just be wasting to much more electricity do to the larger size pump needed to handle the additional flow?
Originally Posted by bc3141