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Originally Posted by kerry
Seems to me that there must be a temperature below which the system cannot function to heat the house given the constant heat of the ground.
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I'm assuming you are talking about ambient air temperature. If so it is all in sizing the ground source heat pump. I have a 5 ton pump. There are 5 loops associated with it. For a 6 ton pump there would be 6 loops and so on. I went and looked at the stickers on my unit at the temp ins and outs of the coolant entering and exiting the ground. These were taken on 7/28/11. In cooling mode it came out of the ground at 70F and entered the ground at 79F. In heating mode it exited the ground at 68F and entered the ground at 59F. I'm sure these temps would be different if they were measured today in the winter. I place my hand on the tubes (pulled back the insulation) and both felt cool. I have an infrared thermometer in the garage. Perhaps I will check sometime.
There may be a limit but I would think the limitation would be the size of the loop field available to extract heat out of the ground. I know in my location they were able to bore horizontally which costs less (smaller machine used). In areas with less of a lot size footprint they drill vertically. I know there has to be X amount of space between each loop. But it is not much. They drilled all loops in one direction spanning maybe 100yds separating the end of the outermost loops (they use GPS like technology and mark them with flags--see below). They could have easily had many more loops horizontally, not to consider vertical loops if my system had needed it.
Of course the further North (or S in the S hemisphere) one gets the cooler the average Earth temperature is (near the surface) so I'm sure that factors in as well. I believe my salesperson told me that average temp in FL is 65F and here in MN it is 52F.
Quote:
Originally Posted by kerry
I know my sister in law had a head pump in Tennesee with a set of electrical coils which would/could be kicked in when temperatures went below X.
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Sounds like this was an air source heat pump. These work similarly but are not near the efficiencies of a ground source heat pump since the air source heat pump takes the energy out of the air, and knowing one tries to cool in the summer when the air temps are hot and heat in the winter when air temps are cool they do not work as well as the ground since the ground is cooler than the air in the summer and warmer than the air in the winter.
Quote:
Originally Posted by kerry
Do you have any idea of what that temperature is on your system?
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Which temperature? The coolant in the loops? If so, see above. The air temperature change across the heat exchanger in cooling mode on the same date above was 70F air in and 51F air out. In heating mode on the same date it was 70F air in and 96F air out. Again I'm sure those values are different in the winter.
Another very interesting part was how the drilled the loops into the house. As I mentioned earlier they use GPS like technology to locate the head of the drill and guide it that way. Basically they dug a 20' x 10' hole about 20' deep in my yard about 50' from my house. They bored down a bit but mostly horizontally 250' out and back then pulled the hoses through the holes.
Now for the cool part. When all 5 bores were done they turned the drill around and bored towards the house. Using GPS like technology in which they use a surface "detector", one guy walks along (and IN the house) to signal to the drill operator which direction to go via walkie-talkies. Then when they are at the correct location the drill is turned upwards into the house. It is amazing how accurate they are.
Then in the hole in my yard they connects the two large tubes coming out of my house to the 5 loops in my yard. Then they filled the hole back up.