Hello, I was told it would cost too much to install a full geothermal system in our vacation home in WI to see a payback in energy savings so I'm doing what I call a "poor man’s" geothermal system. During construction I installed 300' of 1" geothermal tubing along the basement foundation ICF wall 10' below our courtyard driveway before we backfilled. This section will be used to heat the radiant floor system installed in the garage floor. During excavating of our septic system we buried another 300' coil under our septic tank which is buried 15' deep and covered it with 2' of compacted screening dirt before setting the tank. This loop will be used to heat the garage radiant floor system in 4" of concrete poured over Spancrete planks that form the ceiling of the basement. I'm hoping that these two systems along with passive solar heating from southern exposure will keep the 1000' loft above the garage from freezing when we close it up during the winter months. I will also install a high efficiency forced air heat pump to help heat & cool the loft when we are there for a faster comfort levels. The system I installed is $20K more. I'm now searching for a reliable low voltage re-circulating pump to keep the water moving 24/7. I don't need a very large pump since I don't want the water to flow too fast so it has time to transfer the heat from the ground and since I want to run it continuously plus it needs to be low voltage so that it’s energy consumption is minimal. Any suggestions or links you can share would be appreciated. Thanks!

Flow center products.com
Eric

Low voltage does not equate to low energy consumption.

Do a head loss calculation at required flow and specify only as large a pump as is needed

I'd have to see a load calculation to be confident that the described system would 'carry' the loft safely through a cold snap - some emergency backup able to safely run unattended would be excellent insurance against pump failure or lack of capacity

It sounds like you are hoping that just circulating water from the ground loop through the floor loop will keep the place warm. With out a ground source heat pump, you have little chance of success.

Posted By flyboy on 02 Oct 2010 09:10 PM
I don't need a very large pump since I don't want the water to flow
too fast so it has time to transfer the heat from the ground ...

Low flow rate will reduce heat transfer, not enhance it. Fuggeddabout
temperature, it's BTUs that count -- and any heat exchanger will move
more BTUs at high flow rates than at low.

Also, independent of the above, you want the flow rate high enough
to achieve turbulent flow. At low flow rates (laminar flow), the fluid
in the loop forms a static "boundary layer" against the pipe walls,
and this retards heat transfer.

As engineer cautioned, low voltage does not equate to low power --
ignore the volts, pay attention to the watts.

Sorry to say, I must agree with geodean on your chance of success.
600 feet of pipe won't deliver much heat, especially with such low
delta-Ts across both the source and load heat exchangers.

You should just design your loft so that it can be easily freeze proofed - and consider a remote control system to turn on the heat/AC before you get there.

OK... all sounds like good advise. The back up electric heat pump will be my backup during those cold snaps but probably will winterize the place just in case there's a power outage or equipement failure. I will install a internet temperature monitoring & video system to check in on the place while it is in winter hibernation. Will keep you posted on how much elect it takes to keep it >50deg. Terry

The heat you get out of this system will go up as the room gets colder. Looby is right - more flow means more heat transfer. But there are power outages...Perhaps the loop needs some anti freeze in it.

600 ft of pipe with a 2f temperature difference between the fluid in the pipe and the ground gives over 8,000 btu - like an electric heater running full time. Qc = L(tg – tw)/R with an R of 0.14. You need to get at least 2 - 3 gallons per min to get turbulence if you use even a bit of anti freeze. You have 2000 sq ft of floor, if it gets to 1 deg F over the room temp, then you will get 2000*3*1deg = 6000 BTUs/hr out of the floor. So to me it looks like you would get perhaps 2000 - 8000 BTU/hr out of the system. (so like a 600w to 1800W heater going all the time). You only get this heat if the room is cold, like 40f.

You do not want to install a heat pump - freezing the ground at 10' deep might not be nice for your foundation.

Try using a heat loss calculator and then see how much less heat you need as you turn the thermostat down from 80f to 40f (although the heat loss calculators i see on the internet don't usually like low thermostat settings!).

It may be that there is no way to do a heat loss calc for 40f using a normal calculator. As an example, heat is usually lost through a concrete slab - but since your desired temp is below the average ground temp you might gain heat from the slab, especially for the first few months of late fall/winter. Also the heat gain from a southern exposure goes UP as the temperature in the loft goes down. Then there is the thermal mass - which sounds nice and high for you - which helps with the cold nights. How much heat will your septic add to the whole system?

Large, thick window blinds on all (or all but south?) windows. (ie remove most of the windows from the heat loss calc). No large areas of rug - or roll them up for the winter.

Posted By TomAndersen on 03 Oct 2010 02:41 PM


You do not want to install a heat pump - freezing the ground at 10' deep might not be nice for your foundation.

Installing a heat pump does not mean that you will freeze the ground.  It might happen or might not happen, just depends on how much heat you pull out of the loop.

My loop never drops below 44° all winter.

I don't think freezing of the foundation will be a problem since I did put a couple of feet of fill above it and the 300' of tubing. I also don't plan to hook up this founation loop to a heat pump but if it doesn't seem to save anything with just the recirculating pump then I will probably hook up the loop that goes to the septic to a small geothermal heat pump. The rough energy calculation for this loft is 2 Ton but his program couldn't factor in this pour boy geothermal & the ICF walls. Any suggestions which loop I should use for the basement & which for the garage? I was planning to use the septic loop for the garage since I'd like the most efficent one to help heat the loft above it but can alwasys switch it around if better and to seperate the basement from the foundation loop if wiser. Another factor I forgot to mention which might make it smarter to swiitch the plan is the open stairway to both the basement & loft share the floor of the basement so I was hoping that enough heat would rise from the basement floor to help heat the loft level. The garage is more insulated (R-13 in ceiling & wall to stairs plus 5/8" sheetrock) and probably won't help much from the sound of the BTU Tom came up with. The plumber who installed the radiant floor heating in the basement told me that using antifreeze fluid reduces the heat transfer a bit making it less efficient so thought running water in the lines would be better. But the thought of it freezing has me spooked so may need to use it and accept the lower savings.

I will accept reduced efficiency and peice of mind in regard to freeezing every day of the week.

Suggest methanol antifreeze for minimum loss of efficiency,
and maximum affordability.

Caution: pure methanol is rather flammable; mix outdoors
to about 20% by volume for 15°F freeze protection.

I would not use that 300ft loop under the septic tank for Geo.