I think I have the perfect location for an open loop system. I have a 1 acre shallow pond, would like to irrigate from the pond and the overflow from the pond would make a nice creek through our property.

I am putting in the drain line myself, I have not picked a  geothermal contractor yet. But they tell me I will need a 5 ton heat pump for the house and later maybe I will add a 5 ton for the attached garage. I figure I will use 4" schedule 80 for the drain, and run the downspouts out the same trench with a separate 4" line.

I am at 45 degrees north west of Minneapolis. It gets cold here.

Should I combine the downspout and geo drain into one 6" drain for the 170' run to the pond?

How many GPM will it discharge?

How deep will I need to bury the lines? Can gravity drain lines freeze-up?

Then what do I do when I get to the pond?

Any advice?

Closest major city and manual J load will allow me to answer your questions.
Joe

Why use Schedule 80?  If you bury the 4" or 6" pipe deep enough so it will not freeze, then I would think that Schedule 40 will not get crushed by heavy equipment.  Now if you already own the Schedule 80, then it makes sense to use it.  Although the amount of water from the two heat pumps can be calculated, we do not have any way to know how much rain water will be added to this pipe since we do not know the watershed.

A 6" diameter pipe will carry a lot of water and be less likely to stop up.  You probably need to somewhat filter the water from the roof before it goes into the pipe.  I think you will save money on materials and save time by using one 6" pipe instead of two 4" pipes.  The force of the water from the heat pump should help keep the pipe clean.  My only concern would be to not let the pipe freeze up where it empties into the shallow pond.

Figure about 3 GPM per ton - so 15 GPM for 5 tons. Keep all the lines below the frost depth (perhaps in the 5' range). Does the pond maintain some water in it in the winter or does it become a solid block of ice?

3 GPM/ton is what we figure for closed loop. 1.5-2 for open.
Geo pros will agree.
I agree with Alton that < sched 80 will suffice. Look at sched 30 or other inexpensive drain pipe.
J

I live near Minneapolis, and recently removed an 8 ton open loop system of 10 years. I will share some things I learned:
Make sure you test your water quality and quantity. My installer didn't and I had tons of problems with iron bacteria, pump replacements, discharge pipe freeze up, etc.

I would suggest a flexible(pex) discharge pipe that you insulate . The flexible pex will allow for soil movement due to freeze/thaw--PVC won't flex.

My installer used the geothermal taco valves which were fine except for the slow drip that slips past them. This became a reason for freeze up when the geo was shut off for 6 hours at a time during load control by the electric cooperative.

Buyer beware of open loops in freezing climates!!

> 1.5-2 for open. Geo pros will agree.

Using joe.ami figures can leave you with freezing exit water. Not to mention reduced efficiency.

Just why do you think an open loop system, running 1 1/2 to 2 GPM, is less efficient? Exactly what will make the "exit water" temperature below freezing?

Bergy

Posted By jonr on 20 Aug 2010 08:52 AM
> 1.5-2 for open. Geo pros will agree.

Using joe.ami figures will leave you with exit water below freezing - ie, a non functional system. Not to mention reduced efficiency.

Hmmm...So what in your experience as a geo design expert, installer owner has led you to conclude this jonr?

Certainly not the following excerpt from ClimateMaster's all products tech guide....".....adjust the valve until the desired flow of 1.5 to 2 gpm per ton is achieved. .....NOTE: when EWT is below 50*F, 2 gpm/ton is required."
This is from the engineers who designed the equipment.
I have units in operation at the flow rates I describe. You of course do not have geo installed anywhere including your own home.

Was this always true? No. A while into last century flow rates were higher due to poorer heat exchanger technology.
That's the trouble with posing as an expert based on research only and no field experience; you have to check the expiration date on the info you use to support your attacks on pros.

For others;
3 gpm/ton is the closed loop requirement. As well water may approach 20* warmer (higher EWT) less flow is required.
System will not operate with leaving water temps (LWT) below prescribed temps and has temp controls that cyle compressor off to prevent freezing "exit water".
Yes higher rates of flow can increase COP (efficiency). That is always true in open loop. 3 gpm gives you higher efficiency than 2 while 10 gpm/ton raises it even more. The folks who designed these systems have determined a minimum gpm (to best reduce water requirements and preserve well pump life) they do not object to higher flow, but suggest it can be wasteful if overdone.
j

I am sorry, I misspoke, I bought 400' of a drain-line that says "3500# crush" from Menard's. It is gray, built in coupler and the receipt says ASTM 2729. Someone else told me is schedule 80, and what do I know about pipe. Is that okay for this?

I am told if I have no ponding/ pee-traps in the line, and it exits the pipe above the water line, it will not freeze. What do ya'll think?

I see that joe.ami, the self proclaimed geo expert already agrees that his suggested 1.5 gpm rate is too low for this case. His information seems to go bad within hours. But that's the problem with non-engineers that don't actually understand the physics.

You might squeeze by with 2 gpm in some areas of Minnesota - but not others - do the math. Or design for about 3 gpm, meet all specs and reduce it later if you find the performance acceptable.

Okay, but my biggest concern is for the drain line not to freeze and also I don't want ice flows everwhere.

ICF,
I need heat loss calc as well as closest city to answer your total gallons question.
If you are referring to a 3-4" pipe such as that used in a septic drain field, that is what I would suggest.
It is inexpensive and unlikely to be obstructed by water flow as long as it is even slightly pitched (regardless of where it discharges). Barring critters camping out in the pipe of course.
I replaced a 25 year old heat pump that discharged in the middle of a customer's back lawn (in 2" sched 40). Grade pitched away and unit never froze up. Fact is it's really hard to freeze moving water. Even harder if it is self venting (say 4" pipe).
Not getting the "ice flows" concern.

Jonr, All I bring to the table is multiple industry certifications, 6 different heat pump manufacturers' endorsements as well as MI mechanical, boiler and inspector licenses, 1000's of training hours, mandatory continuing ed requirements, more than 20 years in the field and hundreds of design/installations....oh and by the way 100's of satisfied customers.
Your background was............? Your source for this imaginary 3gpm open loop requirement was......?
Quit posing as a geo expert. Try your hand at "askalawyer.com". You'll be less likely to mislead folks there.
j

The thermodynamics associated with the GPM / ton open loop bickering is fairly easy to work through and explain:

1 GPM happily works out to right at 500 pounds per hour, a handily round number. A nominal ton of HVAC is 12000 btuh, another handily round number

If we ran just one GPM per ton we'd get a delta T of 12000 / 500 = 24 degrees. In other words, LWT would be 24 less than EWT. Obviously then EWT would need to be 32 + 24 + a few degrees for a safety factor. At minimum an EWT of 60 would be required to support 1 GPM per ton

Jumping up to 1.5 GPM per ton - the math works out to a water delta T of 16. That's probably allowable, with a safety factor, for EWTs down to 55 or so.

Jumping up again to 2.0 GPM per ton - the math works out to a delta T of 12. That's OK for EWT of 50 or a tad less, maybe 48 or thereabouts.

I hope the foregoing helps with understanding this issue.

Posted By engineer on 21 Aug 2010 03:55 AM

If we ran just one GPM per ton we'd get a delta T of 12000 / 500 = 24 degrees.

That's only for quad-ubend + GeoClip + ultragrout systems with infinite COP.
More pedestrian installations get only 2/3 to 4/5 of their BTUs from the loop,
so a realistic delta-T would be more like 15°F to 20°F at one GPM per ton.

At 1.5 GPM/ton (as recommended by the dummies at CM, WF, etc.), delta-T
of a RealWorldSystem™ would be no more than 13°F.

duplicate post - deleted by Looby

> 2.0 GPM per ton - the math works out to a delta T of 12. That's OK for EWT of 50

Now factor in that someone north of Minneapolis will have less than 47F EWT and can be as low as 40F. Doesn't always work even with the BTUs from the electricity and no margin.

The Delta - Ts I calculated were essentially worst case scenarios - note my use of "nominal" ton. As EWT drops, so too do COP and actual tonnage, both of which factors will reduce Delta-T to below nominal ton values.

In reference to the flow requirements of an open loop system, the installation manual of the specific heat pump under consideration might be useful.

At least one manufacturer I’m aware of (Climatemaster) does not like ≤ 40˚ LWT’s for its open systems.

This is due to the potential of the refrigerant temperature being as low as 32°F [0°C]
with 40°F [4.4°C] LWT, which may lead to a nuisance cutout
due to the activation of the Low Temperature Protection.
JW3 should never be clipped for standard range
equipment or systems without antifreeze.

Source: climatemaster.org/downloads/Submittals/LC356.pdf

Regards,

Masoud

Can't argue with that