Loop design comparison

Hello,

We are planning a 5 ton water to water geo system and 2 different geothermal professionals have proposed 2 different loop configurations. I am trying to understand the pros and cons of each, and which will be better.

One design uses slinky loops 5000' of 3/4" pipe in 5 trenches. Each trench 3.5' wide, 6' deep, 90' long. 1000' of pipe in each loop.

The other design uses horizontal 'racetrack' loops 2400' of 1.25" pipe in one trench 4' wide, 6' deep, 300' long. Lay four pipes in the trench, backfill 2' and pipes return at 4' deep.

Seems to me there is more volume of fluid in 2400' of 1.25" than there is in 5000' of 3/4". Therefore the heat pump could receive the same gpm while the fluid is moving slower and spending more time in the ground.

But it will take longer to transfer the heat through the larger diameter pipe, and the 1.25" design will be using about 300 sf less ground area.

Any advice or help with this decision is much appreciated.

 

More is not always better.

What is the ground like?

Will you be able to afford the pumping costs?

Mark

 

There are many other things that go into the design such as soil TC, actual btu requirements. The slinkeys strike me as a misunderstanding as they would have to pitch at more than 10 foot/foot. Where are you located?

 

We are in western washington. The ground here has good clay, round rocks and lots of ground water. The 1.25" design is in use nearby and did not require sand to backfill. The 3/4" design says use sand to bed the pipes. Yes the slinkys would have a pretty tight pitch. Pumping costs meaning the additional electricity used to pump more fluid?

 

From what I understand the ground here has good thermal conductivity. Also the heating load for the house is between 3- 3.5 tons. So I was originally looking for a 4 ton system, which is why the 1.25" uses 4 pipes, it was intended for a 4 ton. The 3/4" loops are actually for a 5 ton system that was recommended. So I guess the 1.25" would probably add one more 600' pipe if I do the 5 ton heat pump.

 

With a 3-3.5 ton heat load, you should probably be looking at a 3 ton, vs. a 5 ton. Definitely not more than a 4.

 

I need to get a second opinion on the heat loss. As in hire an engineer rather than take the advice of others. I think 3- 3.5 seems a little low for a 3000 sf home, but the architect designed it to take advantage of passive solar gains.

 

So if it is a 5 ton then, is one loop design 'better' than the other?

 

Sounds like you need to start from square one and get a heat loss/gain. You can use the HVAC calc and do it yourself for $50. You need to be honest with your input information and resist the urge to pad it as there is already a generous fudge factor.

 

I used an online calculator and it gives me 36,595 Btu's heating. I just ordered manual j, hoping I can get it dialed in more accurately.

Mabey I can get away with a 3 ton system.

Or I could do the 5 ton system and use it to heat the guest cottage which currently uses electric wall heaters.

Aren't loops typically defined as 'X' amount of trench and pipe per ton?

With the size and configuration of the pipes and trenches being the variable.

So couldn't a given loop design be applied to a 3 or 5 ton system and adjusted proportionately?

 

Sand is not good. If the ground is a good thermal conductor use it without the rocks for a few feet then add the rock to the backfill.

Mark

 

What was the gain on your load calc? While rules of thumb regarding loops are often employed, one must still do the math. Local guys often know what works per ton in their area but it generally involves the fact that they are not going to throw away 40 feet of pipe if their standard sized loops are a little long nor add 20 feet if they are slightly short.

 

The results said

Total Btu's cooling 25,587
Sensible load 24,094
Latent load 1,493
Total Btu's heating 36,595

 

If those numbders are accurate, then both of the systems you described are grossly oversized. depending on soil conductivity and depth you likely need 600-1,000 feet of pipe per ton.

 

Sounds like they are both in the ballpark for a 5 ton system.

But if I go with the 3 ton system then it will be either-

3 slinky loops with 3000' of 3/4" pipe in 3 trenches 100' long.

or

3 horizontal 'racetrack' loops with 1800' of 1.25" in one trench 300' long

What I am trying to understand is the difference between 1000' of 3/4" pipe in a tightly pitched slinky versus 600' of 1.25" pipe in a horizontal u-bend.

 

Patfogarty,

I could likely come up with 20 loop designs for one client. 20 loop designs that would give you the btu's you need. Sticking to just geometry: depth, pipe spacing, pitch, trench spacing, pipe diameter, etc. all effect your loop design. Some match a given property better (area available, tough digging, loose soils, the chosen fruit tree). But they all give you the btu's (if designed properly). Given the choice, I'll aim towards the less head loss. Lower circulating pumping.

Your just seeing two that contractors have put together.

When I bid, I don't even really know what the end result loop design is going to be. It really doesn't factor into costs that much (unless we're drilling).

 

Thank you, I think I just figured it out. Looking at head loss the 1.25" x 600' is the way to go.

If either design will give the btu's but one is 10x easier to pump it seems like the better choice.

 

Pat:

I am going to mess with your new found knowledge.

Wire to water costs, (Thanks John S, P.E.), are now being considered in geothermal designs. Do not run pumps that use more power than needed to do just enough. I never liked a pair of push pull pumps in a flow center, that made designing easy, but defeated the purpose of energy saving enjoyed with geothermal heat pumps.

I have always been a fan of just enough pumping to do the job.

The loops you describe are way to big for what your home needs. They will need big pumps that eat electrons to make the heat transfer.

In hydronic systems, the force required to overcome the friction of the water moving through the pipes is measured in feet of head. I do not know why, but I get the math.

Consider the manifold.

If one uses a manifold to build a loop field here is what happens.

To size the pump to move the amount of fluid that the geothermal unit needs in GPM, you count the head loss of the transfer piping and just one loop of 3/4" loop. If you need say 300' of 3/4" HDPE per ton and just one of how many 3/4" loops needed.

So you can down-size the pump to the GPM X Head loss for the transfer piping X one loop of the manifold.

Less pump.

Works every time.

Mark

 

Not oversizing loops and using the smallest pump to get the job done makes sense.

But why only count the head loss for one loop?

And why use 3/4" when 1.25" has so much less friction?

I was advised to go with the bigger system because the 5 ton heat pump won't be putting out 5 tons when EWT's get down to say 35* which could happen at the end of a long cold winter. So what I was told is that having more pipe in the ground and a bigger heat pump is cheap insurance against this situation. But I am now seeing the light that this may not be the way to go.

 

Circuits are generally headered in parallel. Thus only one circuit's worth of head loss.