Geothermal heat on a mountain?

May 23, 2008 10:14 AM

Geothermal heating: what happens if you live on a mountain?

My chimney got crushed by sliding ice last winter (see previous Ask here), and now that I've got contractors coming back with repair bills of $3000+, I'm thinking that maybe it's time to ditch oil altogether and look at other alternatives.

One thing I'm VERY interested in is geothermal heating: the costs are daunting, I know, but the long-term benefits are awesome.

But I live on a mountain, and I'm not having much luck finding out how geothermal heating actually works in terms of raw geo-physics. Will the fact that I live at a higher place relative to the earth's core than somewhere that isn't on a mountain mean I will have to drill deeper than if I lived somewhere lower?

As a side note: if anyone has any interesting heating information pertinent to living in Quebec, I'm all ears.

posted by Shepherd to home & garden (14 comments total)

You probably mean geothermal heat pumps, as opposed to geothermal heat, which is more like hot springs and the like. Geothermal heat simply cannot be done on most budgets, unless the area you're in has geothermal activity close to the surface.

But as for geothermal heat pumps, living on a mountain is irrelevant, geophysics are irrelevant. You really just need a large mass to draw heat from. The typical heat pump is an air source pump, which is less efficient because the air temperature is not always conducive to your heating/cooling needs and air isn't particularly dense in terms of heat energy. With geothermal or ground source heat pumps, the earth is almost always at a more advantageous temperature (warmer in winter, cooler in summer).
posted by electroboy at 10:30 AM on May 23

Here's a pretty good link explaining geothermal heat pumps.
posted by electroboy at 10:34 AM on May 23

Electroboy is right on. Wikipedia has a decent article about geothermal heat pumps too.

Another useful search term would be "ground source heat pump".
posted by beandip at 10:40 AM on May 23

Unless that mountain you live on is an active volcano, geotherrmal heating in the sense Icelandic folks use the term is probably not an option for you. However, heat pump systems which use wells to extract energy from the earth might make sense, depending on the hydrology of the rockpile on which you live. For the heat pump solutions, the practicality of the situation is almost entirely defined by your local water table, because it is water in the ground that really is the efficient store and transport mechanism for stored Earth energy; if you'd have to drill more than 100 feet to hit water, the plumbing and pumping costs for the heat pump solution preclude practical economy, unless your electric costs are unusually high. And even in cases where the ready presence of ground water near the surface makes a heat pump type system technically feasible, its characterstics of operation (generally slow heating and near frequent running), and the fact that you generally need some backup heating system to keep from freezing when the power goes out, may make the whole thing moot.

But here in coastal Florida, where you can dig a good well with a shovel in an afternoon, and the climate is favorable, heat pumps are all the rage.
posted by paulsc at 10:44 AM on May 23

Ground water isn't a requirement for a geothermal heat pump. There are water source heat pumps, but it's not necessary.
posted by electroboy at 10:54 AM on May 23

"... There are water source heat pumps, but it's not necessary."
posted by electroboy at 1:54 PM on May 23

Technically true, perhaps, but the OP lives in Quebec, where the frostline is from 1.2 to 3 meters below grade. Putting in a horizontal loop geothermal heat pump system gets prohibitively expensive if you have to trench down as much as 10 feet. So, if you go vertical with your collection loops, you're drilling wells. If they are dry wells, I think it is pretty unlikely you'll get economic amounts of heat flow from the ground, with conventional equipment.
posted by paulsc at 11:13 AM on May 23

Popping back in to say yes, I meant heat pump systems rather than raw geothermal heat.
posted by Shepherd at 11:21 AM on May 23

if you'd have to drill more than 100 feet to hit water...

This cannot be universally true.

I don't pretend to be an expert on this, but they are drilling for geothermal heat pumps on the lot next to my house at this moment, and I was talking to the developer about it a couple of days ago. There are probably several setups and the details probably vary, but here they are going way past ground water. The aquifer is at 80-90 feet -- they blow right past that (casing the bore with 6in steel pipe until they are into rock) and end up going way down into the bedrock -- total depth 640 feet. The depth is related to the "tonnage" -- the heat capacity that you are trying to get out/put in (winter/summer). They have drilled a bunch of these holes (there will be seven town homes), I think one per unit.

Obviously, as developers they don't have to wait to amortize over the long haul, but it must make financial sense or they wouldn't drill so deep.
posted by Quinbus Flestrin at 11:23 AM on May 23

Let's just say there are a lot of variables that will contribute to the efficiency of his theoretical system and leave it at that.
posted by electroboy at 11:23 AM on May 23

You could also look at hear for more info on heat pumps. However, taking all your heat from this will be tough & expensive.

Have you considered wood sounds low tech, but they do amazing things with wood these days, (would pay off in like 4 years).

The effectivness of any of these changes will be significantly magnified by ensuring that your home is as well insulated as possible. This is absolutly the most achievable thing you can do, even if your mountain home is well insulated already. If your house is as well insluated as it possibly can be (or you can afford to make it) it should take very little to heat it. In fact, many Passive houses are heated by the sun.
posted by munchbunch at 12:30 PM on May 23

I've heard of a few houses in QC that use this technology, and it seems to work.

You can get a grant from HydroQuebec!

This Quebec guide to eco-construction (en français) explains the different types of systems, and has a link near the bottom to another article on choosing the right system for Quebec.
posted by MissSquare at 12:39 PM on May 23

The majority of heat pump installations anymore are closed loop, meaning that the fluid (water with something added to lower the freezing point) in the ground loop is recycled continuously rather than pumped from a ground source and discharged. For a residential unit, you would typically dig a series of horizontal trenches about 10 feet deep and lay the necessary length of piping - anywhere from 250 to 1000 feet per ton (ton being a unit of power equal to 12000 BTU/hour). As someone pointed out above, this might be difficult in your area. Ask contractors for quotes for the required length of trench before making any assumptions based on this thread. My boss had his trenches dug and then installed the piping and backfilled on his own. I've never heard of vertical wells for a residential installation, but I work on large public projects in the midwest, so my experience likely is not applicable. Besides open and closed ground source heat pumps, air source heat pumps are an option in more temperate climates, but almost certainly not yours.
posted by Derive the Hamiltonian of... at 3:01 PM on May 23

Derive is correct. And, one of the Dirty Jobs on the TV show was drilling closed loop vertical "wells" for geothermal heat pumps in some housing development. I guess it's easier when you don't have land to lay trenches and/or you have well drillers around. The drill the hole, push a tube down it and then fill it with grout or mortar or something like that. Once it hardens, you are using the Earth as a heat sink. Extract heat in the winter, shed heat in the summer.

I believe this IS the future of HVAC...
posted by gjc at 6:34 PM on May 23

Well, I'm meeting with a company rep tonight, and I'll report back if anything of interest comes up.
posted by Shepherd at 6:12 AM on May 26