horizontal loop depth question

Reading design guide from the manufacturer of our geothermal heat pump.
It is currently in an open loop configuration, and am planning to convert to horizontal closed loop (in Ohio).

On one page in the guide, they reprint a graph illustrating that deeper in the ground
(12 feet down) the soil temperature swing is more constant throughout the year,
with larger temperature swings closer to the surface.
My assumption then is the deeper the better to bury a horizontal loop.

However, they make a remark, "Pipe depths in the Northern Zone should be 3 to 5 feet.
Excessive depth will reduce the ability of the sun to recharge the heat used in winter."
What might they be referring to? I am confused by this.
Why would 5 foot depth be better than 12 for horizontal loop?

I suspect that maybe they are referring to one of two things:
... the yearly average temperature, which they don't discuss,
maybe it is colder at 12 feet than at 5 feet in the winter?
... they spend a number of pages discussing multiple lines per trench,
whereas I am only interested in one line in a long loop?

 

You're right that deeper is better on the design side. Around 20' and deeper you have pretty constant temps. That being why vertical uses less pipe. Deeper horizontal systems need less pipe than shallow ones.

And minimum depth should be 3-5' (below frostline) - though there is some shallow designs coming from Europe.

Ignore the "excessive depth" statement. Just do. I'd be guessing at trying to justify it. Ok, I'll guess, crappy short looped systems could use the benefit of a summer recharge.

 

I am for the conversion let me know if I can help.

With out consulting Doc, who knows all, I have found that in most of Ohio, 300' of 3/4" per ton manifolded to the equipment, to work well at 1 to 2 feet below frost line. Frost line is where the AHJ says you need to dig to to prevent frost heaving.

One needs to protect machines from freezing, but it is still cheaper that 12'. Going that deep in Ohio brings all sorts of issues. I am not going into that trench.

Mark

 

The statement is silly, that would mean that vertical bores of a few hundred feet don't work at all since the they don't recharge from the sun. That obviously is not true either. In general the deeper the better, but you get to the point of no return for the extra effort. We usually put them at 8 feet depth, and within a month they have recharged in the spring. Heat travels through the ground and also comes from below, and from the sides.

Also in general, a single horizontal pipe does not work, since you must have enough flow for the heat pump, and if you push that 3 gpm/ton through a single pipe, you need and waste a lot of pumping power. Better run your loop design by us here, you might run into a pressure drop issue.

To Mark: 300' of 3/4" pipe per ton might work well in a pond or a vertical system, but not in a horizontal system 1-2 ft below the frost line. Now I understand why you need that much antifreeze!

 

I'm not sure I am able to follow that, but 3 gpm/ton sounds consistent with what I have read.
What do you mean by "waste a lot of pumping power"?

I am preferring a single loop because of fewer pipe fuses to make (and mess up), and I would imagine more likely to succeed at flushing all the air out. Though I am still not done studying yet, so there may still be things that change my mind.

According to my (3 ton) heat pump manual says it requires 5 GPM for water only, ..., or just short of 8 GPM for a 30% solution of NoBurst (which I think is a commercial mix of propylene glycol and what I will be aspiring for). The heat pump as 3/4" copper, and I intend to run 1" piping from the heat pump through the basement wall and have a 2" diameter loop (SDR-11).

The manufacturer has a worksheet I filled out that they run through some loop design software they have, but I must have filled it out incorrectly, because the report they sent back was for a 1" loop; it came out to 1150 feet for 5 foot depth. I resubmitted and requested the same report with a 2" diameter loop and should get a response in another two weeks. I think that at 5 foot depth the 2" diameter loop will probably be around 1050 feet.

I can only estimate the final pressure drop of the system until I get the report back, but if I work with the 1150 foot the calculations come out as 75.9 foot head for 1" diameter loop with 30% NoBurst, or 23.158 for 2" diameter pipe. I am considering the Grundfos UP26-99F (245 watts) circulator pump, which would require three in series for a 1" diameter loop, or just one pump for a 2" diameter pipe at that length loop.

 

You touch the point that is the issue here. If you use 1" pipe you need 3 circulators in series, wasting a lot of pumping power. If you use 2" pipe, at 8 gpm you Reynolds number drops down to less than 1500, which will unlikely work well to allow enough heat exchange, you simply have slow linear flow. There is a reason why parallel loops were developed.
Your heat pump manufacturer does not seem to be very evolved. What brand are you using?

 

I want to say I have 5 runs that are 5-6' deep with about 600-650' of slinky in each for my 5 ton unit. I am in northern Ohio and the loop field seems to do well. I am pulling EWT right now of about 35*-36* as we approach the end of the really cold season, it is holding up well. With it being a new loop last winter and the horribly cold winter we had, my loop temps still stayed above 32* EWT. So, that said, with the coldest winter is a LOOONG time in Ohio and a new loop field, the loop did fine being that shallow. I am just an idiot user, not a professional, so take my comments with a grain of salt!

 

Have you priced 2" srd pipe vs 3/4 or 1"?

Do you know how hard it will be to work with 2" during install?

For my system I didn't want fusion joints at all. (To cheap to buy the tool)
My 3 ton unit has 3 loops of 1" srd 800' long. I brought the tails into the house and used a manifold inside.
This also made filling and flushing easy.
The 26-99 pump purged out one loop at a time with ease. All I had to do was install valves on each line so the full pumping power can flush one loop at a time. (No need for a flush cart)

Only down side is 6 holes through the basement wall.

 

Sorry, I am not familiar with that acronym, is that the temperature entering or exiting the heat pump?

I will have to read up more on the Reynolds number, the manufacturer lists sources for the material in their manuals and design guide, I will have to look for where they referenced the Reynolds number. The design guide mentions the Reynolds number in reference to propylene glycol near a cold threshold loses efficiency at heat transfer, but for the most part they just offer a table for the minimum turbulence required for heat transfer based on type, size of pipe, and fluid solution used. They list SDR11 2" diameter to require a minimum of 7.0 GPM @ 25 F for a solution of 30% NoBurst (and 2.4 GPM @ 40 F for water only).

Their design guide does not recommend any design in particular, but it is evident that they prefer parallel horizontal or vertical.

I won't mention it, so that their reputation cannot be affected by my ignorance and lack of experience. The heat pump was already in operation when we bought the house, I'm just attempting to make what is there work better for my needs. Currently we only use it in the summer for cooling, when renovations are completed I will insulate the duct work and use it some for heating. I have considered maybe only using water in the closed loop and continue to only use the heat pump for cooling.

Yes.

No. I have no experience, but I have looked up the weight for linear foot. I imagine it will be difficult but doable.

Hhhhmmmm.... I had not thought of that. Thats interesting, I will definitely reevaluate. What spacing do you have between the 6 holes?

 

Entering water temp after cycling through the loop.... Leaving water temp out of the system is around 31-32. I think last year I had entering temps of very low 30s and the leaving water was in that 26-27* range but last winter was BRUTAL and it was the first winter cycle the loop had been through. In year 2 with the addition of the hot water buffering tank, the settling of the soil, and the tweaking of the system, my electric bill has dropped 25% from corresponding months in year 1.

 

Milkweed,

 

Once upon a time in a Galexcy far far away, no one knew Doc would be able to Know what was needed in each geological area to make a good geo system possible.

In those days we made judgment calls and if we got it right we smiled. If we did not then our customers died of cold.

Frost line is the point where it never freezes.

 

150' of borehole (300' of pipe) per ton is a good number for vertical boreholes, but not for a horizontal system 1-2' below the frost line, not matter how good your ground is. Either you tonnage is way to high, and you units are way oversized for your load and hardly turn on, or you indeed better use a whole lot of antifreeze.

 

Heatpump manufacturers are usually good in manufacturing heat pumps, not designing geo systems.

It does not help if you are withholding information, than you are wasting everyone's time.
What size is your system?

 

let me save you a bunch of time. Don't waste any more thinking about 2" pipe. It is a terrible idea.

 

I agree. You have to understand that you are designing a heat exchanger to to ground. You are missing the importance of surface area in the design of your HX.

100 ft of 2" PE DR11 has a surface area of just over 62 sq ft per 100 ft of length.
100 ft of 3/4 PE DR11 x 3 loops has a surface area of over 82 sq ft per 100 ft of length. That is over 32% more SArea relative to length and over 100% more SArea relative to contained volume.

 

Don't get me wrong, I'm not holding on to my original thought of a series horizontal loop.
Though I feel I am near completing my study as though I were to install a series horizontal,
so I am going to continue with that for now before I reevaluate with a parallel horizontal.

I agree with that.
The design guide that the manufacturer of the heat pump in house seems to be an amalgamation of university
publications and considerations of pros and cons of the various choices to be made.
They include tables for the most common choices (pipe type, pipe size, fluid choice, layout according to site and soil, et cetera).
Once the choices are made they have a form to fill out to send them to run
through loop design software to match loop lengths.
I guess I am saying that the manufacturer of my heat pump seems to be sufficiently thorough enough for me.
Thats just my impression.

It is a three ton heat pump, with an optional heat recovery for water heating, which is installed.
It only has one speed, is that called single stage?
I suppose I don't see why would knowing the manufacturer be helpful if my
questions are general to any heat pump.
Even if it is the least desirable model of a least desirable manufacturer, I'm going to live with it.

Other than docjenser's point of the Reynold's number, then why?
... setup? heat transfer efficiency? cost?
Though so far in my study, it seems that to get 2 feet per second flush through a 2 inch diameter
pipe that would come to nearly 21 gallons per minute. That seems scary even with a flush cart.

Please correct me, I thought the comparison would/should be:
(1) 300' series long 2" diameter loop for 186 square foot
(3) 100' parallel 3/4" diameter loop for 82 square foot

 

1) I don't think you ever wrestled with 2" pipe.
2) How do you handle 1200ft of 2" on a roll, my guys usually cry if I have more than 100ft on a roll
3) if you don't get it as one piece (again 1200 ft) you have to fuse multiple pieces together, now you need a fusion tool
4) one you have a fusion tool you might just fuse a 1.25" header with 3/4" tees, with a few couplings

100' 2" SDR 11 pipe has 68 sqf of surface area
100' 0.75" pipe has 29 sqf of surface area

3 x 600 ft (1800 ft) of pipe would be a good number for a 3 ton horizontal loop field (522 total sqf)

To get the same surface area with 2" pipe, you would need 768 ft of pipe.

But that is not the whole story, since now you are connected to lesser ground. The limiting factor is not the pipe surface, but the lesser thermal mass of the ground. So I am not saying that 1150' of 2" is not an adequate length for the heat exchange, just good luck to trying to install it. Plus it is so much more expensive than 3/4", and you run into a turbulent flow issue.

If you have 3 ton heat pump, 8 gpm is a good number to design for with a glycol mix of 25%.

Do yourself a favor, get some one who can fuse the pipes together for you, and use 0.75" pipe for the horizontal loop.

 

I found an online source for PE4710 2" diameter 500' palleted. I imagined I would need to cut it in half at least, possibly smaller lengths.
This is/was one of my pondering questions in my checklist.
I had thought about buying a small section to practice socket fusing and pressure testing before committing to buying the full amount of pipe, and to also get an idea of the ability to move it to where I would need it... anyway

I've worked with PE3608 3/4" and 1/2" for water distribution before, no fusing only barbed and threaded connections. I imagine that the weight and flexibility of PE4710/3408 3/4" is probably similar.

What sort of trenching are you referring to? One pipe per trench, so (3) 600 foot = 1800 feet of trench; or two pipes stacked, so (3) 300 foot = 900 feet of trench?

 

3 trenches, 300' each, pipe running out and coming back in the same trench, supply and return pipe about 3' apart in the same trench.

Again, once you fuse the 2' pipe, your can fuse the manifold and move on with your life.