The Homemade Heat Pump Manifesto

[pachai]

AC_Hacker,
Thanks.
Here's what I am thinking - would love to hear
if this sounds worth considering...

A tiny background - the contractor who is adding 400SF
to the footprint of my house next month...is doing it up the
street today. I saw them using a backhoe today to go
down about 15'.

My thought is to show some of these pictures to my
contractor. He's is a lot like me, in experimenting, etc,
but he has to run a business, and crazy ideas slow him down.
(and heat pump is only the nuttiest...yet :-)

I saw the backhoe dig out 15x30' to 15' deep.
I was wondering - what if he made the hole
30x30', half will be my foundation (carport + basement),
and half will be my trench field.
Then I could do a combination of horizontal and vertical,
and get about 300' of pipe under 7' of dirt.

Alternatively, I could drill horizontally into
the backyard through the retaining wall, but that
would be only about 5'

I just read your suggestion about test holes -
and was thinking along those lines today.
Will hit some kind of supplier tomorrow I guess
to see what I can get.



Thanks
Seth

[pachai]

BTW,
I found a site that answers some of my
more naive questions - for example
why the trench should be dug after the
boreholes.
keram - faq

(note, that reason is one that could be solved:
Trench, plywood platform for boring machine,
then bore. But then again I never did this :-)

or, whether you can bend the pipe in a U shape?
(no, you can't)

Seth

[AC_Hacker]

Seth,

Maybe this will guide your thinking:

It all really starts with knowing the heat load of your house. This will give you an idea of how big your loop field will need to be. It will also tell you how big your heat pump needs to be to move the heat from the earth into your house.

* * *

Heat moves through earth very slowly. I have heard various rates, but it's in the neighborhood of 6 months for heat to move through 16 feet of earth.

So if you have a single vertical borehole going down through the earth, you would be advised to have a goodly space between boreholes. I have seen recommendations of 12 to 16 feet, with 16 feet being cited most frequently. Imagine that you are drawing heat out of a huge cylinder, with the borehole in the middle of that cylinder.

The same principle applies to trenches. So the advice is to space your trenches about 16 feet apart, and the cylinder concept applies here too, except that the cylinder is horizontal and has a radius that is bigger than the distance from the pipe in the trench to the surface of the ground. This would make you think that a trench will not yield as much heat as a borehole. It is exactly so.

So if you're trying to figure out the best layout, think in terms of those big cylinders... you don't want to have them too close together.

Now I don't know what the shape of your property is, but if it were me, I'd have them dig the garage basement and then have them use the machine to dig as many 8 foot deep trenches, spaced 16 feet apart as you could fit on your property... figuring about 12,000 BTU per 80 to feet of trench (with 300 feet of slinky in the 80 foot trench). If you knew the heat load of your house, you'd know how many feet minimum of trench you'd need.

http://www.youtube.com/watch?v=-Q23HJ-jidk

If you really had your program together, you would have your HDPE slinky pipe already spaced properly and correctly laced with nylon tie wraps, ready to go, and when he had finished one trench you and some of your buddies could start putting the prepared slinkies in place and welding and shoveling dirt on top while he was digging the next, etc. Then when he was finished with the digging, he could use the machine to finish burying the trenches.

Of course, this would assume that you had your headers ready, and your HDPE welding skills up to speed.

At one point I had about 12 holes in my backyard, each about 12 to 16 inches in diameter (just the right size for a small child to fall into) and about 17 feet deep. It was then that I realized that I had small children living on three sides of my back yard. I immediately plugged the tops of the holes with chunks of firewood and started planting HDPE loops and filling the holes as quickly as I could...

So you don't want to have dangerous earthworks open too long.

Good luck with your decision. Looks like you have an opportunity there.

Regards,

-AC_Hacker

[pachai]

I was thinking about the exposed holes in the ground.
Firewood is a great idea to block them.
I was thinking - I have a couple of shipping crates.
I could put some cinder blocks in them
and bolt them shut.

[AC_Hacker]

Originally Posted by pachai

I was thinking - I have a couple of shipping crates.
I could put some cinder blocks in them
and bolt them shut.

Making a loop field can be a big project and it can be of long duration.

I think it's a good idea to minimize risks to yourself and to others.

-AC_Hacker

[pick1e]

Hey all, thanks for sharing all this information. I found this thread searching for info on fixing up air conditioners I've only been able to read up through the first dozen pages so far though.

I too am headed down the DIY GSHP path. I think my project should be easy to accomplish. I have a 1 car detached garage that I would like to heat a little bit. I'm not living in it, it's just a workshop, so I'd just like to keep it from freezing in winter, maybe 45 degrees or so would be good, and raise it up to 60 or so when I'm working out there. I hope to use a window air conditioner unit and a water ground loop.

I have some questions though. What's the deal with straight down and up loops in PEX tubing? What kind of flowrate are you using? I would rather dig less holes and use some copper tubing, but I guess I'm curious if that's worth it, maybe the thermal transfer rate in the soil is the limiting factor? I mean suppose you buried the same size loop of copper instead of PEX in the same hole, would you still be limited to the same amount of heat gain?

I'm not a HVAC guy but I do have an engineering background so I know the thermo basics. I borrowed a heat transfer book from a friend (mine is long gone) and am trying to figure how much tubing I need... But I don't know the temps of the in and out water until I rig up a test I guess. Any insight here? What is a reasonable temperature of a cold evaporator? I could use that as a theoretical for max heat gain I suppose. I didn't know if I could use the ground temp as the theoretical exit temperature though. Do you guys with working systems see the exiting water reach ground temp or still slightly below?

I guess a big and most immediate question I have for you HVAC gurus (the answer to which could through a big monkey wrench into my plans) is can I extend the tubes leading to the evaporator coil? My plan was to literally add a few feet so that I could just hang the coil in a tub of water kept warm by the ground loop.

Thanks for any insight

[AC_Hacker]

pick1e,

Glad we finally connected. I'm posting this reply in the "Manifesto" thread, but there's no telling where it will end up. I hope you can find it, it took me a couple of hours to write it.

Originally Posted by pick1e

What's the deal with straight down and up loops in PEX tubing?

In the beginning, copper and also iron pipe was used for the purpose of radiant floor heating, but after years of service, corrosion killed the pipes and was an expensive nightmare to fix. PEX was developed in Europe to remedy this problem, with the PEX being buried in concrete. PEX, being plastic was impervious to corrosion and has become the industry standard. It is true that PEX is a poorer conductor of heat than either copper or iron.

Imagine the 'thermal chain' (don't google this term, I just made it up) as being water-to-PIPE-to-concrete. In that chain the PIPE is thin and the rest of the chain is very long, so a reduction in thermal flow due to going from metal pipe to PEX pipe causes a reduction in thermal flow, but the reduction is very small, compared to the entire thermal chain. PEX is very easy to work with, is also cheaper than pipe and lasts longer than most buildings.

PEX begins life as HDPE (High Density Poly Ethylene), but then goes through subsequent stages of processing that make it stronger but no longer able to be joined by heat fusion (AKA: welding). HDPE can be welded, which is a very good feature because the weld is as strong as the parent material, and no metal (no corrosion) is used in the joint. Thermally, PEX and HDPE are very similar.

This is why HDPE is so popular for geothermal loop fields.

The 'straight down and straight up' part is that, as Vlad has observed, drilling holes is easy. There are successful installations done where people use a catapillar tractor and plow a hugely deep furrow and 'plant' HDPE and cover it up, all in one pass. But straight down straight up is more common. The buried slinky method is very good, depending on your location and requirements.

Originally Posted by pick1e

I would rather dig less holes and use some copper tubing, but I guess I'm curious if that's worth it, maybe the thermal transfer rate in the soil is the limiting factor?

As you probably know by now, the thermal advantage to using copper and DX (AKA: direct expansion) is about 15%. So you would need less loop field. The thermal transfer rate through soil is very slow, on the order of 2.6 feet per month.

Also, you have to consider the geometry of heat flow. In the case of a bore hole, you have a pipe in the center. When you first begin your extraction, you get more heat from the earth immediately surrounding the pipe, then it is heat-depleted and needs to get heat from earth further away, etc. And as time passes, there is progressively less and less heat extracted, and the curve gets flatter.

This is a graph I did of a heat transfer test in my back yard. If you turned the chart upside down, you'd get a fair idea of what thermal transfer from a borehole looks like:

So if you put in a sufficiently large loop-field, you will be able to get the heat transfer rate you desire.

The is is not a real simple issue, but the CLGS Installation Guide (#21020) which is expensive, is the best source of information I have seen. The book goes over this issue with explanations, charts and formulas.

Well drillers who have worked on geothermal bore holes have some idea of what's required and of course, GSHP designers know. Call some in your area, they'll tell you what you want to know (especially the well drillers).

Originally Posted by pick1e

I mean suppose you buried the same size loop of copper instead of PEX in the same hole, would you still be limited to the same amount of heat gain?

In the short term, you'd see more heat from a copper field, in the long term it would be the same.

[pick1e]

Originally Posted by AC_Hacker

Here's a diagram of my Loop Field...

*wouldn't let me repost the image or link so the post I'm referring to is #120 on page 13.

Maybe I'm reinventing the wheel here but wouldn't a system be better off with the loops arranged in parallel rather than in series? When I see that diagram my first gut reaction is that the first loop will be doing the most work while the remaining loops will be doing far less work, logarithmically, as the delta T decreases along the line. By the time your water gets to the final loop, that loop is probably not doing much of anything, so there may be heat in the ground waiting to be picked up but that's just sitting there because there is no delta T to transfer it.

Thinking about it another way, maybe it comes out in the wash when the system reaches steady state? If the first loop has transferred all the heat it can, and the surrounding ground approaches Twater, then the deltaT of the second loop is higher and that one starts doing the heavy lifting...

Just curious if this has been considered. 8 years ago I could have probably calculated the efficiency, but I no longer have my differential equations book and certainly don't wish to acquire one

[pick1e]

Originally Posted by AC_Hacker

In the beginning,

I like this intro

Originally Posted by AC_Hacker

Imagine the 'thermal chain' (don't google this term, I just made it up) as being water-to-PIPE-to-concrete. In that chain the PIPE is thin and the rest of the chain is very long, so a reduction in thermal flow due to going from metal pipe to PEX pipe causes a reduction in thermal flow, but the reduction is very small, compared to the entire thermal chain. PEX is very easy to work with, is also cheaper than pipe and lasts longer than most buildings.

I understand where you're coming from here but according to Fourier's law, Q=kA(dT/dx) is just as dependent on dT as it is on k. In other words the thermal transfer rate of the soil shouldn't be static, rather it will vary with dT. And if the outer wall T is closer to the inner wall T the heat transfer from the ground will be higher.

In my experience, builders prefer fast and cheap over correctness so I tend to think their argument for PEX is much more dependent on the "very easy to work with, is also cheaper than pipe" than the thermal properties argument. I think it is quite possible that the metal degradation argument is overblown. I don't know of anybody who complains that their city water service pipe has deteriorated and is leaky after 50 years?

Originally Posted by AC_Hacker

The 'straight down and straight up' part is that, as Vlad has observed, drilling holes is easy. There are successful installations done where people use a catapillar tractor and plow a hugely deep furrow and 'plant' HDPE and cover it up, all in one pass.

This I understand, I guess my point was wouldn't it be more efficient to have spirals or heat conducting cross members or something of the sort. I'm not expecting to see you make spirals out of HDPE, although that would be cool , I guess I'm talking about more of a manufactured thing here... But my point is has any formation other than two vertical pipes been tried? Are you getting the same transfer out of two vertical members as opposed to four or some other configuration?

Originally Posted by AC_Hacker

As you probably know by now, the thermal advantage to using copper and DX (AKA: direct expansion) is about 15%. So you would need less loop field. The thermal transfer rate through soil is very slow, on the order of 2.6 feet per month. Also, you have to consider the geometry of heat flow. In the case of a bore hole, you have a pipe in the center. When you first begin your extraction, you get more heat from the earth immediately surrounding the pipe, then it is heat depleted and needs to get heat from earth further away, etc. And as time passes, there is progressively less and less heat extracted.

I don't know I still take issue with the 2.6 feet per month factor. It seems quite counterintuitive, it should be variable with the temperature difference just like any other material.

Originally Posted by AC_Hacker

This is a graph I did of a heat transfer test in my back yard. If you turned the chart upside down, you'd get a fair idea of what thermal transfer from a borehole looks like:

So if you put in a sufficiently large loop-field, you will be able to get the heat transfer rate you desire.

I'm still not sure about these. You're only measuring the temperature of the water, or the temp of the inside of the pipe. I would love to see these values plotted along with the temperatures of the OUTSIDE of the pipe. Maybe that can be my valuable future contribution to the information here

I just think it's being overlooked that some of these figures come from a steady state operation at specific T values, whereas the whole idea of a refrigeration cycle is to maximize the heat transfer by maximizing the dT.

[AC_Hacker]

pick1e,

Your questions are good ones... but there comes a point where you gotta put your shoulder to the wheel. No matter if it's picking the method that seems the most viable to you, or doing your own experiments to actually learn first-hand what's going on...

Originally Posted by pick1e

I understand where you're coming from here but according to Fourier's law, Q=kA(dT/dx) is just as dependent on dT as it is on k. In other words the thermal transfer rate of the soil shouldn't be static, rather it will vary with dT. And if the outer wall T is closer to the inner wall T the heat transfer from the ground will be higher.

Yes, this does seem reasonable. I'd suggest doing a test with real materials, measuring it and finding out what the performance difference actually is.

Originally Posted by pick1e

I think it is quite possible that the metal degradation argument is overblown. I don't know of anybody who complains that their city water service pipe has deteriorated and is leaky after 50 years?

The design life of galvanized iron water pipe is 40 years. Mine lasted longer. It took maybe 50 years for the iron pipe that lead to the house to start leaking and need to be replaced. It was probably 60 years old when I replaced all of the iron pipe in my house with PEX. Yes, the iron pipe was leaking. So when you say, "I don't know of anybody..." you do know one person, me.

You might also consider that in the case of radiant floors, running heated fluid in metal pipes increases the chemical reaction rate, so that a 40 to 50 year life can be drastically shortened.

Additionally, you might want to consider what is actually involved in replacing pipes in a 3 or 4 inch thick concrete slab, especially if it is to be done every 20 to 30 years.

Originally Posted by pick1e

...I guess my point was wouldn't it be more efficient to have spirals or heat conducting cross members or something of the sort. I'm not expecting to see you make spirals out of HDPE, although that would be cool , I guess I'm talking about more of a manufactured thing here... But my point is has any formation other than two vertical pipes been tried? Are you getting the same transfer out of two vertical members as opposed to four or some other configuration?

Check out the slinky method. Also the previously mentioned GSHP manual gets into these issues.

Originally Posted by pick1e

I don't know I still take issue with the 2.6 feet per month factor. It seems quite counterintuitive, it should be variable with the temperature difference just like any other material.

I guess it all depends on the delta, doesn't it. If the ground temp were varying by a large amount, like maybe 500 degrees, I'd have to defer to your argument. But how much is it really varying? The whole idea behind geothermal is that you get down to a level of stable temperatures. By the way, I'm basing my numbers on studies done on seasonal passive heat storage, using dry earth as a storage medium.

Originally Posted by pick1e

...You're only measuring the temperature of the water, or the temp of the inside of the pipe. I would love to see these values plotted along with the temperatures of the OUTSIDE of the pipe. Maybe that can be my valuable future contribution to the information here

You may have found your purpose in life.

Originally Posted by pick1e

I just think it's being overlooked that some of these figures come from a steady state operation at specific T values, whereas the whole idea of a refrigeration cycle is to maximize the heat transfer by maximizing the dT.

I think it gets back to 'how great is the delta T' over the annual period.

Good questions, good luck with your research.

-AC_Hacker