Hi

I'm building a new home and excavation will begin in about a week and half.  I had no intention of doing geothermal because this is a very tiny downtown lot...but late last night I thought maybe some benefit could be obtained.

I have room to do roughly an additional 600 sqft of excavation to a depth of 9ft. It would be backfilled with an additional 3ft putting the bottom of the pit 12ft below the surface. Does a 12' deep 600 sqft pit provide enough space or volume for horizontal tubing to even meet the needs of 1 ton? 2 tons?

If my horizontal runs are 6" apart(and I have no idea what the minimum can be) that would allow for 1200ft of pipe at a depth of 12ft. I could backfill 4ft on top of that and lay another 1200 ft of pipe and then repeat that again after another 4ft of backfill for a total of 3600ft of pipe at an average depth of 8'. Sorry I'm a total newbie as this wasn't part of the plan but is that enough horizontal pipe for even the smallest of applications? Is that too much pipe for the volume of earth(600 sqft x 12ft)?

The new house has a footprint of only 750 sqft but including the basement its 3 storeys plus 300 sqft of conditioned space in the attic. Not all of the basement is conditioned(roughly 1/3 is an semi underground garage...Im on a slope) So its about 2300 sqft of space but vertical(750 sqft footprint). Its a very well insulated new house with unobstructed south exposure for passive solar gain.

Im in a cold climate in the winter(Edmonton, Alberta, Can).

So am I dreaming? Could I get enough out of that pit for just the basement?...or is it a ridiculous thought for a small downtown project. (Vertical drilling would be too expensive).

Thanks

If I dont have enough volume in my pit for a minimal application...maybe I could still implement an "earth tube" type setup to preheat my fresh air intake for the HRV. Maybe this is the wrong forum :)

You can get some heat out of any amount of earth. But with a small lot, a medium sized house and the high "first ton" cost of geothermal, it probably doesn't make as much sense as more insulation. Or maybe even PV solar (for electricity, not heat).

Don't bother with an earth tube - a HRV is already pretty efficient.

Thanks for the reply jonr

Where I live in Alberta natural gas is very cheap and is significantly less than electricity so GSHP or ASHP don't make a lot of sense. But I was doing some last minute thinking...if I can get a free 6000 cubic ft pit and lay down the tubing myself it might be worthwhile to rough it in now while I have the chance. I can worry about compressors and pumps and gas prices years from now. Im just wondering if I should take advantage of the opportunity which will only happen once(next week) to rough things in for cheap.

For the earth tube I would have gone with the glycol/air exchange type since i wouldn't have to worry about air quality or condensation. Since I wouldn't need a compressor and the excavation is free and the pipe is cheap...I figured why not? I dont have much to lose in that scenario. My only concern is that I read the HRV is optimized for cold air so preheating the air would make it less efficient thus eliminating any gains.

When I first saw your post, I thought, he must have Natural Gas and I see now that you do.

We have a make-up air pipe that runs under ground and also connects to a solar hot air collector. This design 'could' connect to a HRV...Our stable ground temps are 41f or 5 c. In our design, we are using exhaust only ventilation.

In our previous house where we had an HRV, I did not like being the 'Window Policeman', shutting the windows all the time. Maybe if I had the HRV tied to the security system, it would not have run if the window loop was open... And we like the windows open, so it works out.

Your warmest incoming air will likely give you your best efficiencies.

Thanks for the reply nooboo

That is an interesting setup. I assume you are running some sort of fan strong enough to overcome the friction of your ground pipe and meet the needs of your furnace? If so is that fan triggered when your blower comes on? Is there no danger of not getting enough air and having a CO problem? I know zero about HVAC and I'm coming from a 100 year old house, full of holes, no insulation and standard low efficiency furnace.

With only exhaust ventilation are you putting your house under negative pressure or do you have your windows open a lot?

"Your warmest incoming air will likely give you your best efficiencies"

According to this research report that is not the case with an HRV http://www.cmhc-schl.gc.ca/odpub/pdf/67558.pdf?lang=en

"Interaction with the heat recovery ventilator (HRV) The literature shows that strictly from an energy point of view, there is very strong evidence that combining an EAHX and a HRV provides little benefit in heating mode. Both devices are trying to raise the temperature of the incoming air; but the temperature rise caused by the EAHX makes the HRV work less efficiently, so the combined energy gain is much less than the sum of the gains of each system working independently. This is also confirmed by several experimental studies which have shown that when an HRV is present, the advantage of combining it with an EAHX is minimal from an energy point of view. "

Maybe my best bet is to use a ground loop strictly for cooling as my cooling requirements are much lower

Posted By DaveJJ on 25 Apr 2014 04:55 PM


Thanks for the reply nooboo

That is an interesting setup. I assume you are running some sort of fan strong enough to overcome the friction of your ground pipe and meet the needs of your furnace? If so is that fan triggered when your blower comes on? Is there no danger of not getting enough air and having a CO problem?

My setup is unique...I have in-floor slab heat with a GSHP, so my design is quite a bit different than yours.


We have passive make-up air in our design with Panasonic Whisper Vents. Here is a decent write-up of a design (not with whisper vents):

http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/exhaust-only-ventilation-systems



Our house has been balanced and the system works.

Yes, there is a decent fan to bring the air in through the 30 feet or so of pipe. I have it set on a timer so that the fan is only coming on when the solar wall is getting sun. I might get it on an electric eye, or a solar powered fan or a diff temp controller.

I know zero about HVAC and I'm coming from a 100 year old house, full of holes, no insulation and standard low efficiency furnace.

With only exhaust ventilation are you putting your house under negative pressure or do you have your windows open a lot?

"Your warmest incoming air will likely give you your best efficiencies"

According to this research report that is not the case with an HRV http://www.cmhc-schl.gc.ca/odpub/pdf/67558.pdf?lang=en

"Interaction with the heat recovery ventilator (HRV) The literature shows that strictly from an energy point of view, there is very strong evidence that combining an EAHX and a HRV provides little benefit in heating mode. Both devices are trying to raise the temperature of the incoming air; but the temperature rise caused by the EAHX makes the HRV work less efficiently, so the combined energy gain is much less than the sum of the gains of each system working independently. This is also confirmed by several experimental studies which have shown that when an HRV is present, the advantage of combining it with an EAHX is minimal from an energy point of view. "

Maybe my best bet is to use a ground loop strictly for cooling as my cooling requirements are much lower

You have passed me in the acronym race as I do not know what EAHX without 'googling'...is it entering air heat exchanger?


You know there are some people who think boiling water will freeze quicker than 5c water:}


There are more ways than one to handle your project; keep us posted!


Brian

Posted By DaveJJ on 25 Apr 2014 04:55 PM

Thanks for the reply nooboo

That is an interesting setup. I assume you are running some sort of fan strong enough to overcome the friction of your ground pipe and meet the needs of your furnace? If so is that fan triggered when your blower comes on? Is there no danger of not getting enough air and having a CO problem?

I have in-floor slab heat with a GSHP, no furnace.


We have passive make-up air in our design with Panasonic Whisper Vents. Here is a decent write-up of a design (only not with the Panasonic whisper vents):

http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/exhaust-only-ventilation-systems



Our house has been blower door tested and the system works here; it's a good tight house, despite the passive vents.

Yes, there is a decent fan to bring the air in through the 30 feet or so of pipe. I have it set on a timer so that the fan is only coming on when the solar wall is getting sun. I might get it on an electric eye, or a solar powered fan or a diff temp controller. No big danger of a buildup of CO, I am mostly electric, but the wood burning stove is the one concern.

I know zero about HVAC and I'm coming from a 100 year old house, full of holes, no insulation and standard low efficiency furnace.

With only exhaust ventilation are you putting your house under negative pressure or do you have your windows open a lot?

"Your warmest incoming air will likely give you your best efficiencies"

According to this research report that is not the case with an HRV http://www.cmhc-schl.gc.ca/odpub/pdf/67558.pdf?lang=en

"Interaction with the heat recovery ventilator (HRV) The literature shows that strictly from an energy point of view, there is very strong evidence that combining an EAHX and a HRV provides little benefit in heating mode. Both devices are trying to raise the temperature of the incoming air; but the temperature rise caused by the EAHX makes the HRV work less efficiently, so the combined energy gain is much less than the sum of the gains of each system working independently. This is also confirmed by several experimental studies which have shown that when an HRV is present, the advantage of combining it with an EAHX is minimal from an energy point of view. "

Maybe my best bet is to use a ground loop strictly for cooling as my cooling requirements are much lower

That may be the Economic Law of Diminishing Returns in action...
You have passed me in the acronym race as I do not know what EAHX without 'googling'...is it entering air heat exchanger?

There are more ways than one to handle your project; keep us posted!


Brian

"That may be the Economic Law of Diminishing Returns in action... "

The greater the Delta T (difference in temperature) the more efficient any heat exchanger is at transferring heat. So yes, by elevating incoming air temperature you reduce the efficiency of an HRV. However, if the temperature of the incoming air is higher because of that, who cares. If you for instance, with a solar set up, were able to get the entering air to 70F, do you really care that you tanked the efficiency of the HRV?

I'm not a fan of earth tubes. They are offering little to a client of mine in an experimental installation. I think I would skip the GHX under the house as well. I'm with Jonr invest in PV solar. This is the advice I give to folks in my AO that have small lots and NG.