sled4fun

I am not a math major (but I did stay at a Holliday inn once) Anyway I have been contemplating installing a ground loop cooling system in my home. Mainly because I have a bit of property to use for it and a nephew with a backhoe to dig the trenches. For those of you that do not what this is. It is a trench about 3' wide and 6-7 ft deep and then you coil about 1500' of 1" special tubing that will lay on the bottom of the trench (like a flattened out slinky) Then you install a Water to air exchanger (kind of like a car radiator but looks like an a/c evaporator core on steroids) to the end of the piping and inline with you air return duct for you HVAC. The theory here is that the ground @ 6-7 ft will be a constant 55F and when you circulate the glycol water mixture through it and into the house it can keep your house cool without running the A/C.

Now this is where Geothermal guys will say I need a heatpump! I do not want to use a heat pump.

I just want it to work with just the fan running on my existing 4 ton HVAC and I will have a small water pump installed to circulate the coolant through the ground loop. I was also thinking of a solar water pump and solar operated fan that would run most the day.

The question I have for you Math gurus is. If I have a 2500 SF single story home with all vaulted cielings and built out of SIP (structurally insulated panels - like an ice chest) The house is very efficient. The outside temps are 110F at times and I am trying to cool the place down to maybe 75F if possible.

How big of a Water to Air exchanger should I need? I was thinking 18" diameter x 18" thick.
How much ground loop?

Purplezr2

I would help but am not sure what equations are need to figure this out. I'm going to have a math minor in a year, so I know my way around number, just not house cooling, if you have the equations need, I can run the numbers.

BC847

It's been a long while since I've been to the Holiday Inn but I think you're gonna need a LOT of pipe for the ground heat-sink. The air to water heat exchanger's gonna have to be pretty big as well. And then there's the expense of having to move the fluid through all that plumbing.

The reason is there's not much of a temperature difference between that heat source (the home) and the heat-sink (the earth). With using a heat-pump, you're substantially increasing the temperature difference (delta/T) and can therefore recognize a better heat transfer per given area of heat exchanger.

And using the heat-pump will allow you the benefit of heat for the home if needed. One such alternative might be to use the system to heat domestic hot water when not heating/cooling the home (ground heat > hot water).

Fronty Owner

Not a Math major, but one of the local MEs
BC hit it, you dont have much of a delta T (temperature difference) to pull the heat out of the house.
Granted, if you R-rating is high enough, and with other sources of natural cooling (ie windows open at night), then you can stay fairly cool.
If your serious about this and have the resources, add solar panels to the south side roof to drive a heat pump with the heat pump cooled by the ground loop. This will drive the delta T up on the heat pump making it more effecient. This will give you greater cooling capacity too.
If you dont need the heating cycle of the heat pump (Sacramento is a bit north in CA, but not bad), go with a standard AC with the condenser coil encased in the ground loop water resovior.
About the only place your going to get away with your plan is in an arid location where you can evaporate the water after its cooled.

scuzman00

I am an A/C contractor here in Houston. I don't have any experience with geothermal or ground loop systems in that I work mainly in hospital environments. Get on the web and look up "Heat pipe technologies". They may have the answers to the questions you seek. I know that they manufacture a system of refrigerant pipes that you sink into shallow wells that alleviate the condenser coils of a standard A/C system that uses refrigerant. It is supposedly way more efficient than the standard air/refrigerant condenser. Another route is to utilize ponds in lieu of the wells for your condenser loop.

Good Luck
Gary

Redleg

Just a little info for referance. My boiler stove holds probably about 300 gal. It's above ground, but out of the sun. I circulate the water in the summer and it'll keep the house about 70 on hot, muggy days. Probably closer to 80 when it's pushing triple digits. Cut's down a lot on AC use. I have some mods planned out to improve insulation and circulation that may bring ot down more. For comparison, the friend that helped me with the construction had a 1000 gal boiler in a pole barn. Heated and cooled a 2 story farm house. Never got above 70 on the hottest days. I don't run glycol due to potential problems if you spring a leak. Constant circulation with few dead/stagnant areas (eddies) will limit most if any rust and calcium. My Pex has a thin coloring of rust on the inside after over 2 years use, no build-up and no calcium, useing shallow well water. Glycol might be better for you though since it has cooling properties. If I had it in my system when heating, they say up to 20% loss in BTU's.

high bid

kind of sounds like the "swamp coolers" theory people use in dry climates.

Fronty Owner

I think that is what he is trying for without the evaporation, humidity, or swamp smell...

sled4fun

Fronty Owner has it right. Not a swamp cooler design at all.

The bottom line is........will a ground loop heat sink of 55F cool a house of this size to a comfortable level without using an A/C or heat pump unit. I figure that I have about 33,000 cubic ft of air in the home to move through this water to air exchanger and try to cool it down.

Fronty Owner

Do you know the R-value of your walls and ceiling?

We need to know how much heat we are going to need to remove from the house?
Ideally, your probably looking for about 3 tons (36000BTU/hour or about 15 HP) of cooling, more would be better.

Raspy

The one I worked on, but did not fully design, used 3200 lin. ft. of the ground tubing. It was four 800 ft loops spread as you describe and connected in parallel at about 10 ft down. It used two Grundfos 2664 or 2696 pumps in series to circ that loop. The house is a Rastra-Block house and very efficient.

I don't think you'll get a large enough differential for a long enough time to satisfy your cooling needs without a heat pump (as was used in the one I'm describing).

Here's something really important!! Don't fall for the old "55* year round" story. Dig a hole and measure it for yourself. In the one I worked on, in Nevada, the temp was 62* clear down to 200' that I measured. Temps can really vary.

Another factor is the ground material surface contact. We used special dirt that was finer than the native material to give better heat transfer. In the Sacramento area you could have anything from rocks to silt.

I can't give you exact measurments on the heat exchanger. But if you want a room temp of, say 75* with a ground temp of 55* (and slowly rising as you deliver energy to it), you are only looking at a 20* differential at the max. And the house won't come down to 75* without blowing cooler air of about 60* or so. Meanwhile, you have other incomming heat from people, appliances, lighting, opened doors, windows, etc.

I have a hard time seeing how it will work very well without a heat pump. Be sure you know your ground temp before you get too far into it.

I know you don't want a heat pump, but consider that it would also heat in the winter and make hot water too! If you do go with one be sure to size it much larger than the theoretical calcs would call for! I usually consider about 7 BTU/sq ft. for heat loss on modern homes and 10 BTU/sq ft as the minimum size to actually heat. My designs for radiant heat use 25 BTU/ft as their minimum. And remember, I'm talking about heating. You want cooling and must overcome the extra heat sources mentioned earlier. Take your square footage X 10 BTU. In the heat exchanger chart, look up that number of BTUs in the temp range of water and air you are considering. Now you have the heat exchanger needed based on your assumptions of temperatures. Are your assumptions correct in the real world? If so you're there, or at the absolute minimum sizing.


John

sled4fun

About R40 in the walls and R50 in the cieling. The house is very tight and air infiltration is very very low. If I cool the house down to 75F in the A.M. turn A/C off and it sits through a day of 110F max temp. When I get home the temp has only risen 3-4 degrees. We hardly ever use the heater in the winter as it does not get that cold in the Sacramento area for a long period of time.

sled4fun

Raspy. I hear what you are saying about overbuilding due to the low temp. differential there is. I look at it with an automotive background mind. If the radiator on the car (ground loop) is way oversized the controlling factor will be the thermostat for engine temp (house temp) If the house (engine) has a lot of surface area to allow adequate absorbsion of the heat into the system. And if I have a very large (overkill) ground loop. I feel that by adjusting the volume/amount of air circulated through the house and/or the flow of water through the groundloop I can get it to a point of best efficiency.

By the way my ground is that good old red clay and fist to watermellon sized rocks.

I have also heard that the system can be sort of supercharged by installing a fresh water line 12"-24" above the actual ground loop and turned on for a few minutes to aid in dissipating the heat from the ground loop to earth.

Raspy

Sled,

Maximum heat transfer will occur at maximum flow rate. On the ground loop side don't try to get a large differential between the hot and cold, try to minimize the differential. With reasonable horsepower pumps this is the best efficiency. The heat transfer rate to the ground from the piping is greater with a larger difference in temperature to the ground and a small difference between supply and return. If the returning water is cool then near the point where it exits the ground is has a small differential and therefore a low transfer rate. Like in your car analogy, the engine will run the coolest with the thermostat removed because it has the maximum flow to the radiator, which means the radiator is uniformly hot and at it's most effective.


John

Fronty Owner

The total heat removed by air condition chilled-water installation can be expressed as
h = 500 q dt
where
h = total heat removed (Btu/h)
q = water flow rate (gal/min)
dt = temperature difference (oF)
That will tell you how many Btu/hour of heat you can remove with your setup.

The sensible heat in a heating or cooling process of air can be expressed as
hs = 1.08 q dt
where
hs = sensible heat (btu/hr)
q = air volume flow (cfm, cubic feet per minute)
dt = temperature difference (oF)

The latent heat due to moisture in the air can be expressed as:
hl = 0.68 q dwgr
or
hl = 4,840 q dwlb
where
hl= latent heat (btu/hr)
q = air volume flow (cfm, cubic feet per minute)
dwgr = humidity ratio difference (gram water/lb dry air)
dwlb = humidity ratio difference (lb water/lb dry air)

ht = hs + hl
= 1.08 q dt + 0.68 q dwgr

This value will give you your total heat load of your house.

the heat load of your house needs to be less than the heat removal capacity.

Now, Im gonna go look up how big of a radiator you need in your house and your ground loop.
Your making me remember how to do things I haven't done in a long time.