No heat pump

Discussion in 'Hybrid Systems' started by ABN, Dec 2, 2011.

  1. ABN

    ABN New Member

    I am wanting to heat my shop floor by running horizontal ground loops of which I have plenty of room. And then just pumping into the slab (pex) and circulating back out. My goal is to just raise the temperature by 20 or 30 degrees. I am not sure if that is realistic or not. I live in western Colorado and the night time air temps are below zero for a couple months. Does anyone know how many feet outside I would need for 1000' inside? thanks
  2. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    As nice as it would be to eliminate the heat pump from geo systems, it will not work. Sorry.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Yep. Won't work.

    Just think about how you could just as readily dump your shop heat in to the ground without a heat pump.

    Now if you lived with a natural hot spring...
  4. Sound Geothermal

    Sound Geothermal New Member

    HZ to the shop floor

    Unless you have hot rocks (geothermal like Glenwood Springs), this won't work unless you want to cool the shop in the winter and heat it in the summer. The earth temp at 6 - 7 feet will reflect the air temp about 35 days previous. Get the system designed by a certified GeoExchange Designer for a system that will work. You will also pick up some nice tax credits.
  5. ABN

    ABN New Member

    I have tried to get a handle on "this won't work", but I am still beliving it will. I am having some confussion with the temp coming in. I was under the impression that the temp coming in around 50 degrees is one of the major reasons for the efficiency of the heat pump. I do not see that the earth temp at 6-7 feet deep will reflect the air temp from 35 days prior. My outside water is five foot down and does not freeze in the winter. So, even if the ground temp were 33 degrees I would think that that the slab would be trying to maintain that temperature which would be well above the now zero degrees outside. I think what is different is that I am not planning on heating my shop to 70 degrees all the time. It may be that I am not heating it for a week or two at a time. So what I would like to accomplish is just to have it at a warmer temperature to start with when I want to go out and work for a day or two or more. I can appreciate that if I were trying to keep the temperature at a workable 60 degrees or so that I could be supplying heat into the ground. I have to believe that in the summer this would help in moderating the temp also. thanks again
  6. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    Your floor slab is already in this ground you are pulling heat from. Granted not likely at the depth you would put the pipe.

    There is no guarantee the temp coming in is 50 degrees. That is just the temperature of the ground at depth. If you had "infinite" pipe, then you could see background temperatures. Or the water was allowed to sit without moving.

    Your heat pump is the engine that drives the temperature one way. In the direction you want: in to the shop during winter; in to the ground during summer. But passive heating and cooling can potentially work. Without a heat pump, you will have just as much flow of btu's out as in typically. You will reach some sort of equilibrium that we can't predict very well.

    I would predict that doing anything in your shop in winter (mechanical equipment) will heat it up to the extent that you will actually be pulling that heat out and putting it in the ground.
  7. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Just trying to be helpful here really.

    Three pros here don't think this will work. You do.

    I guess the only way to know for sure is for you to do it and let us know how well it works.

    If your slab is at 0[SIZE=+1][FONT=Helvetica, Arial, sans-serif]°[/FONT][/SIZE] and you run 50 [SIZE=+1][FONT=Helvetica, Arial, sans-serif]°[/FONT][/SIZE] water through it, after a few days the water temp will probably soon drop to 40 [SIZE=+1][FONT=Helvetica, Arial, sans-serif]°[/FONT][/SIZE] and your slab might go to up to 3[SIZE=+1][FONT=Helvetica, Arial, sans-serif]°.

    Then after a few more the water temp will probably drop to 30[/FONT][/SIZE][SIZE=+1][FONT=Helvetica, Arial, sans-serif]°and your slab might get up to 5[/FONT][/SIZE][SIZE=+1][FONT=Helvetica, Arial, sans-serif]° or maybe 10[/FONT][/SIZE][SIZE=+1][FONT=Helvetica, Arial, sans-serif]°.

    And so on.

    This is all conjecture on my part. Let me know if it works out different for you.
    Last edited: Dec 5, 2011
  8. Blake Clark

    Blake Clark Member

    Know what I'd do for a test? I'd borrow a big 'ol hydronic fan coil and hook it up to a garden hose. (Presumably the water would be at ground temperature) Run 10 gallons a minute or so through the coil and measure the input and output temperature of the water. This would give you some idea about how many BTU's you can put into your shop at these low temperatures and you'd also see if that made any practical impact on your shop temperature.

    At 10 GPM every degree drop in water across the coil is 5000 BTU's, approximately the output of a 1500 watt space heater. Keep in mind, that the rate of heat transfer will go down quickly when (if) your shop approaches the coil temperature. Also incoming water temperature of a closed loop will be significantly colder than your garden hose.

    For your proposed slab system, my guess is that there is not enough heat transfer through PEX (essentially an insulator) at small temperature differentials to make this worthwhile. There are three "exchanges" happening in your proposed system. 1. Heat "flows" out of the ground through the wall of the PEX pipe. 2. Heat is then conducted back through the wall of PEX pipe into the slab. 3. Finally heat is transferred from the slab to the air. Each exchange requires a difference in temperature for heat to flow. Since PEX is plastic, to get any useful amount heat to flow through it there has to be either A: a significant temperature differential or B: a very large surface area. Same goes with the slab. Since air is a very poor heat conductor slabs are typically designed to be about 20 degrees warmer than the air temperature.

    Practically speaking for your slab temperature to approach ground temperature you'd still need a heat pump to move heat effectively. If you have 50 degree ground temp and wanted a 50 degree slab, you would circulate water at 40º underground and through the slab at 60º with the heat pump doing the work to boost the temperature by 20 degrees. Even then, your air temp might still be 30º. These aren't real numbers, but it gives you an idea.
    Last edited: Dec 5, 2011
  9. ABN

    ABN New Member

    Thanks for all the input, I will let you know how good or bad this turns out. Of course I am shut down for the winter so it will be a little while yet.

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