Heat-pump modification idea

[Oval_Overload]

Heat pumps are a great climate control device, as I'm sure everyone is aware. They do not create heat, but move it from one place to another.

Ground coil systems work very well but are very expensive. Systems that have coils in the open air are much cheaper, but don't work well in extreme Minnesota cold. Would it be possible to compromise, placing the coils in a root cellar or subterranean garage?

~Jim

[Piwoslaw]

Originally Posted by Oval_Overload

Would it be possible to compromise, placing the coils in a root cellar or subterranean garage?

That would freeze the air in that garage pretty quick. Look at it this way: with ground source, there is a very large thermal mass in a relatively small volume, while air has very little thermal mass, so very large volumes of it are needed. Hence, the fan the blows air over the heat exchanger of an air source hp.

In the setup you proposed, there is a small volume of air enclosed in a cellar, so the heat pump would fairly quickly rob that air of most of its heat. Yes, the air would gradually warm up thanks to the heat seeping through the walls, but that takes a while, so it will be the bottleneck. If the walls of the cellar had the same surface area as the total area of the tubing that goes into the ground, then you might be close.

I believe that the most cost effective ground source (other than laying your tubing on the bottom of a lake) is a buried spiral just below the freezing level. This give less stable temperatures, but the ground work is cheaper than drilling.

[Snax]

One contractor I talked to simply refuses to install a drilled ground source system. There's too much potential for what can be difficult and expensive repairs - if it doesn't cost an arm and a leg to drill the hole(s) to begin with. (In other words, backhoes and tubing for horizontal laying are relatively cheap.) They steer people to conventional pumps if a horizontal system won't work. Otherwise, the payback period may exceed the life of the system.

[Daox]

We have a guy over on EcoRenovator that is doing a complete DIY heat pump system. Drilled his own holes and everything. Its very impressive. His goal is to stay under $2k for the entire system, and he might even stay under $1k.

[Piwoslaw]

Drilling holes gets more expensive with depth. Maybe the best bet is 5-10 easy to drill holes instead of 1-2 very deep holes?

[Snax]

Interesting concept.

I wonder if there are contractors putting in hybrid horizontal-vertical systems that way. Instead of one 100' hole, it would be pretty simple to drill 10-10' holes. They'd probably really need to be closer to 15-20' into the ground with a short tract of excavation 5-6' deep into which to lay the horizontal part, but it would certainly help with a shortage of horizontal ground space.

Of course it would all depend on when and where they hit bedrock.

Drilling only 10' or so, it's conceivable that a person could do that with a portable auger and extensions using a smaller bit.

[Daox]

That is exactly what AC Hacker has done on EcoRenovator. I think he is using 17' holes.

[Snax]

Yikes! He has gone to allot of trouble to make that system work!

I keep looking at our existing air to air system and thinking that it is already a great starting point that doesn't really even need to be hacked directly, but rather an efficient means to exchange ground source heat with the exterior unit is all that it would take to drastically improve efficiency overall. The exterior exchanger could even remain air to air by positioning a second ground source exchanger around it, using the surrounding air as both a medium for extraction and storage as well as transfer.

I doubt the efficiency would be as good as a strict GSHP, but it would probably split the difference between the two and allow a retrofit of existing equipment that would not compromise integrity or reliability in any way, allowing the ASHP to continue to function normally even if the ground source loop fails.

In other words, no messing with solder, refrigerant, or things that could potentially explode!

[rbrowning]

If you built a simple solar water heater that heated a fairly large tank of water stored in your basement, couldn't this be used to augment your ground heat source?

If memory serves me well (seldom) there was an experiment similar to this in Anchorage back in the late 70's. They were able to heat a house for the winter w/o a furnace. But I believe that the thermopile was rather massive.

The idea if drilling shallower holes is great except where the ground freezes down 3-4 feet in the winter time. You need to be well below that in order to find a use able amount of heat.

[Piwoslaw]

If you have A LOT of well-insulated thermal mass, then you could use the solar heater to store the summer's heat until winter. Using the ground as a "solar thermal battery" in this way has been done (citation needed).