Geothermal AC

[2011laramie]

Hey guys and gals, got a bit of a different question. Were building a new house next spring and i got a hair brained idea for a geothermal AC system.

Were gonna end up having 1100ft of trench for waterlines from well to house, house to shop, and house to garden. I had a 10ft hole dug last week for a different project and the ground temp was 10C / 50F. I was thinking of running 3 500ft 3/4" geothermal pipe loops in these trenches and running back to a fancoil in the furnace duct. Then circulating the ground temp fluid into the fan coil as an AC system.

I have to dig the trenches anyways for water, so it would only cost me the pipe and a circ pump and a fancoil. The house will be a sip wall design with R32 walls. So i dont think it will need to much AC anyways.

Would this work as an alternative to getting an AC unit?

 

[cthulu]

Subscribed, interested to see what you learn.

 

[yeldogt]

You should check to see if your system design would work as a conventional geo AC in your area.

What you are trying to do is not new -- I remember reading about people trying to do this 40 years ago with super insulated dwellings. It's all about capacity - the heat exchanger being unconventional/ large w/ high water flow. You can't just use a standard coil and expect to get the same heat transfer as when gaining the mechanical help of a refrigeration cycle.

I don't remember all the points -- the lack of installations should be a warning.

If your loops can provide the needed heat sink for a conventional system at a reasonable price -- I would do it. What's the anticipated heating cooling load ?

 

[Fixin'Stuff]

Read this: https://www.garagejournal.com/forum/showthread.php?t=366622

 

[toplessHO]

you wont get near the performance from that as you would with a refrigerant system.
figure about 4x the heat sink(slinky pipe) and evaporator size. You wont get near the humidity removal either even with those increased sizes. Why not just add a water source heat pump and have some heat in the winter too. Keep in mind that ground water temp is the median temp of your area,which means it gets pretty cold in winter.

 

[LS6 Tommy]

All the Geo systems (and that's not too many. It's more or less a technology that has been given up on.) I've ever seen had their ground source about 25' deep straight down. IDK if that will change things as compared to running it horizontally in a more shallow hole.

Tommy

 

[Jackfre]

The issue is heat transfer. You would have to keep the piping moist for any performance. You then get into the characteristics of the soil and if it will allow the temp to travel. There have been a lot of geothermal projects that started out great, but ended up cold capping themselves.
About the best source on geothermal is the Geo-Heat Center at Oregon Institue of Technology.

 

[kd3pc]

you need an engineer - the depth is shallow for our part of the world, as is the coil lengths. And that system needs to work with the inside stuff to transfer heat correctly.

This is not the place to DIY, as it may work, OK - or it may not work at all...

engineered systems, installed correctly are 2 or 3 times as much as conventional HVAC and with the new high efficiency units...the return is just not there in many cases

bests

 

[2011laramie]

Hey thanks for the replies, ive been doing some reading on this and found a few things.
on the loop length for a ton of cooling capacity it looks like some people say 500-600 ft per ton. So i ballparked my idea at around 3 tons or 30000btu of cooling. Figured that would be lots just to remove some heat out of the house.

The ground conditions of moist soil to wick the heat away, i live and a gravel pit/ sand bar. The dirt is dry everywhere. We dug the 10ft deep hole last week and no moisture at all. That will not help the system

It was just a simple idea of circulating ground temp fluid at 10C/50F through a fan coil or even just circulate through the infloor heat pipe and cool the floor down, as i gotta dig the trenches anyways. Its not supposed to have any heating value. I like a nice gas furnace too much to consider going with a heat pump.

 

[LutzTD]

the problem is mostly that a 10C coil does not produce 10C air. The temp drop is a function of the difference in air temp and time of exposure. It might work if you can get a couple sets of coils so you make a few passes, since you have less temp difference you need to expose the air longer. The other issue is that you wont get any moisture out of the air with higher temp coils. So you would have to run a separate dehumidifier. The best solution is a water based heat pump rather than an air based heat pump. The unit will use your 10C water and with a water to air heat exchanger it does the job of the outside coil. for a small amount more add a desuper heater and get free hot water.

 

[theoldwizard1]

Read this: https://www.garagejournal.com/forum/showthread.php?t=366622

As stated in the mentioned post, using horizontal loops is NOT the best solution. If they are not properly sized, they will freeze the ground solid to the bottom of the trench. Then the return water temperature is too low for the heat pump.

Worse, fixing the above condition is difficult/impossible

Either oversize your horizontal loops by 50%-100% OR install vertical wells. It may be more expensive, but if the return water is too low (<45F) you can easily drill another well.

 

[James-W]

Personally speaking, after much reading on the subject I tend to think that for the cost of doing it versus the savings you get, it isn't worth it. Then too, many times the system doesn't work very well and you get little or no use from it.

 

[2011laramie]

As stated in the mentioned post, using horizontal loops is NOT the best solution. If they are not properly sized, they will freeze the ground solid to the bottom of the trench. Then the return water temperature is too low for the heat pump.

Worse, fixing the above condition is difficult/impossible

Either oversize your horizontal loops by 50%-100% OR install vertical wells. It may be more expensive, but if the return water is too low (<45F) you can easily drill another well.

I dont see how it would freeze the ground in the summer especially when not using a heat pump, just circulating fluid into a coil in the ducting.

I think ill ditch this idea totally and just run a basic AC system. Good furnace with a nice AC unit is more than enough for alberta. And nothing beats the simplicity of a hot water tank.