Geothermal integrated into our radiant floor?
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  1. #1
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    Question 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 02:32 PM.

  2. #2
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    Jun 2008
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    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?

  3. #3
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    Nov 2007
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    ?

    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.

  4. #4
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    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?

  5. #5
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    Nov 2007
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    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!

  6. #6
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    Thanks for your input bc, I greatly appreciate it. Now I'm going to have to look at plan B.

  7. #7
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    Quote Originally Posted by bc3141 View Post
    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.
    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?

  8. #8
    Join Date
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    NE PA
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    I have installed a couple of geo based radiant systems. The downside is that the water will only get to 120-125 deg, so the radiant system must be designed for that temp.

    Using a water-water geo heat pump, you feed a hot water storage tank, which can also have an electric or other form of backup. In the summer you use valves to switch to running the chilled water into a chilled water air handler to do the AC for the house. Radiant loop aside, the rest of the install is not too much more than other geo installs.

    paul

  9. #9
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    Quote Originally Posted by tecman View Post
    I have installed a couple of geo based radiant systems. The downside is that the water will only get to 120-125 deg, so the radiant system must be designed for that temp.
    Thanks Paul, I did setup the system to run with-in those temperature parameters, tubing spaced 4-6 inches apart. Would there be any advantage to running another loop under the existing loop in order to gain more heat at a lower temperature setting from the geo? Would it basically be like doubling the amount of tubing to transfer the heat while using less electricity to transfer the heat from the outside loops to the heat exchanger inside. Kind of like adding another baseboard heating element in series to a room.

    Quote Originally Posted by tecman View Post
    Using a water-water geo heat pump, you feed a hot water storage tank, which can also have an electric or other form of backup. In the summer you use valves to switch to running the chilled water into a chilled water air handler to do the AC for the house. Radiant loop aside, the rest of the install is not too much more than other geo installs.

    paul
    I like the idea of having a storage tank. Would I be able to have it installed in line with my oil water heater and use that as my backup system? It's brand new (year old) so removing it wouldn't be a thing I would like to do. Would water still flow through it if the oil burner was turned off? Could I be able to just use the oil water heater as a storage unit?

  10. #10
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    I am not an expert on radiant systems, however the design temp should be 120 deg. I don't think adding another loop is required if you follow the rules for a given temp.

    As for the tank, we have installed 80 gallon electric tanks. You need some volume to store hot water. We use the electric elements as backup/emergency heat with some mods to the t'stats. The system needs to be a separate system from drinking water. Normally antifreeze solution is required to prevent freezing at certain times. The control for heating is an aquastat in the HW tank to control the geo unit for heat, and circulators from the tank to control the loops. Additional zone valves are used to switch loops for heating and cooling.

    paul

  11. #11
    I had trouble finding much information on designing the ground side of geothermal.
    I designed and built my own geothermal unit, to heat my shop's radiant floor, out of refrigeration parts I had laying around.

    Works pretty well.

  12. #12
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    Jun 2008
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    Out of spare parts? Have any pictures of your creation?

    I need to finish remodeling in my house and I'm thinking of getting a geo next year. I think I would be better off running the ducks now before I close up walls and add the extra insulation to the attic. Any one have any ideas in what size duck work I would need to have installed for a 1800sqft split level ranch? I don't really need ac in the lower half since it stays so cool so I'm figuring that a return line might be in order to pretty much just filter the air.

  13. #13
    Quote Originally Posted by nevea2be View Post
    Out of spare parts? Have any pictures of your creation?
    I have pictures somewhere. It just looks like any other refrigeration circuit. The only thing I had to purchase new was the copper tubing. The rest were things I picked up here and there.

    It took a lot of time, engineering, physical labor, and a just little money.
    I am really an electrical guy, so refrigeration came easily to me. It was a fun project.

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