The Homemade Heat Pump Manifesto...
 

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NOTE: This blog string started here. It was suggested to me that it might better belong here:

http://ecorenovator.org/forum/projec....html#post2631

...so I made that change. You might want to want to read both blogs, but the real action is happening at the above link.

Regards,

-AC_Hacker
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Hello all,

I live in an older small house (< 1000 square ft.) built in 1892. I am going through the whole house, meticulously insulating it with expanded poly styrene board. I tore off the lath & plaster, increased the thickness of my walls, and I'm layering in 6 inches of rigid foam into the walls, ceiling, and under the floor.

So I'm thinking ahead to a highly efficient heating system. As best as I can determine, a Ground Source Heat Pump (AKA: GSHP) running warm water though a thin (1.5 inch), radiant concrete floor is my best bet.

I don't have much money, but I am resourceful and very stubborn.

I have found out that the approximate size of a heat pump I'll require is slightly under 12,000 BTU/hr. In HVAC speak they call 12,000 BTU/hr a 'Ton' of Air Conditioning. Your mileage may vary.

I have also found out that here in Western Oregon (Portland), a GSHP will require a borehole about 200 feet deep. It could also be two boreholes about 100 feet deep, etc... Again, your mileage may vary.

I also found out that heat pumps are classed as Air-to-Air, Water-to-Air, and Water-to-Water.

The kind I need is Water-to-Water. The kicker is, that the smallest I have been able to find is four ton (48,000 BTU/hr). So this means that in order to proceed with the project, I'll have to build my own heat pump. In HVAC, bigger is not better... just slightly smaller than big enough is best, economically speaking.

So, is anyone interested in such a project? I have already built a prototype heat pump, it is working and I have tested it and the test results are very encouraging. I have attached photos and performance data at the end of this post.

I am now in the hole drilling phase. I built an earth auger, which would have worked really well in other parts of the country, but in Oregon, where I live the drilling is not so easy. I have increased the power of my auger 20X and I'm getting ready to dive back in.

A properly designed system as I'm describing should be able to operate at an efficiency of over 300%. My prototype has actually shown efficiency over 400%. In HVAC, efficiency can go over 100%, because the electrical energy that is input is not directly converted to heat, but is being used to move heat from one place to another. Strange but true.

So there are four parts to the project:

1. The Ground source loop field.

2. The Heat Pump.

3. The radiant floor system.

I'd really like to get other people interested in this kind of project. Commercial units installed run $15,000 to $45,000 and up. I'm estimating that I can get mine going for under $2,000. Maybe under $1,000. So far I've spent about $400.

So, let me know if there's interest... I have loads and loads of information I'd like to share.

If we are not the people who can re-purpose pieces of junk that are now headed to the scrap yards and turn them into state-of-the-art high-efficiency home heating systems, who's going to do it?

Humbly Yours,

- AC_Hacker

That's a 'cool' project you're in:) I read a book on home GSHP, mostly water-water. It had a few pictures of homemade contraptions with a word on what not to do when doing it yourself. I can't tell you more at the moment b/c a friend borrowed the book.
I'm wondering if installing a window A/C unit backwards would work as an air-air heat pump? It'd heat only one room, so you'd want it only where you spend most of your time, keeping the rest of the house cooler. There is also the question of efficiency - it might gobble too much electricity.

AC_Hacker: Maybe this thread should move to the Saving@Home subforum? There's more on heat pumps there. There is also EcoModder's evil twin, EcoRenovator, with a forum and everything. You can find most of us there:)

Piwoslaw,

The book you mention sounds very interesting. I sure would like to know more about that!

Regarding using an Air Conditioner turned backwards, I have heard of people doing just that. I am currently using a Sanyo Mini-Split heat pump and I have examined it and compared it to an AC of similar capacity. The thing that really stands out is how much larger the area of the evaporator core is on the heat pump, compared to the AC unit. It's over twice the size.

I believe that the amount of heat that can be transfered by a given volume of water compared to a given volume of air is about 3500 to 1.

I think that the energy consumed by a fan to move a unit heat in air is greater than the energy consumed by a pump to move the same amount of heat in water. I don't have a figure for that.

Radiant floor installers figure that there is a 30% efficiency advantage over central air systems. They also have the advantage of no noise, no draft, extraordinary reliability, and greater comfort.

But, all that aside, turning an AC backwards is much simpler, and should be more efficient than using resistance heating.

Cheers,

-AC_Hacker

I'd probably have to translate it for you, since it's in Polish. The author is a local heat pump installer (and, I believe, a professor). Hopefully I'll have the book back in a few months (my friend is looking for ideas to ecorenovate his summer house), then I'll scan some pics and translate a few interesting bits.
I wanted to exchange radiators in our house for floor heating to raise the efficiency of a heat pump system, but it turned out that our lot isn't good for a heat pump (small and lots of tall trees), so the floor system wouldn't pay for itself. But the installer (actually the author of the mentioned book) told me that a radiant floor system should have at least 8 meters of pipes per square meter of floor. Maybe in your area the numbers may be different (the units definately are...), so find out about that. Hope that helps some.

--Adam

> ...but it turned out that our lot isn't good for a heat pump
> (small and lots of tall trees), so the floor system wouldn't
> pay for itself.

If you can borrow or build your own well drill, a vertical borehole loop field can be reasonably inexpensive. Also, lots of heat can be had from a small lot. Is your problem with tall trees the issue of drilling through the roots?

> ...a radiant floor system should have at least 8 meters of pipes
> per square meter of floor.

Interesting... Most of the hydronic systems in the US consider 6 inch spacing to be the minimum. Your specification works out to about 5 inch spacing.

If everything else is equal, your spec of 5 inch spacing should prove to be more efficient.

By the way,there was a free design tool that I used called "Hydronic Explorer 2" by a company called Slant Fin. (Slant/Fin | High Efficiency Boilers, Baseboard and Radiant Heating) . You may still be able to find it. Easy to learn, very informative.

Cheers,

AC_Hacker

The closer the spacing, the more pipes you get in, and the lower the temperature, which does raise efficiency. Also, with more water in the system you might not need a buffer tank.

As for our heat pump: Our lot is small with lots of shade year round, so we'd have to drill. On top of drilling costs, local laws require permits and fees for drilling, while the soil under us is dry (wet is better for GSHP). The guy I talked to said that maybe it'd be worth it if we were building a new house, but not when retrofitting an old one. For a fraction of the cost we could add insulation and exchange our oversized natural gas furnace for a smaller high efficiency (107%) model, and then the bills would not be much higher than with a heat pump and radiant floors.
We will get a heat pump, but an air-water model for heating water with waste heat from the ventilation. That, plus a new ventilation system and solar panels for hot water, will be in a new thread when we finally get that project going.

Piwoslaw,

> On top of drilling costs, local laws require permits and fees for drilling,
> while the soil under us is dry (wet is better for GSHP).

The local laws and permits issue is why I built an ELECTRIC earth auger. It's very quite.

Around here, a finished borehole (drilling + pipe +grouting) that will yield 12,000 BTU per hour, costs $7000 (3.5 KW per 24,660 PLN) and would be 200 feet deep (61 meters). It could also be two 100 foot holes, or four 50 foot holes, or eight 25 foot holes, etc.

$7000 is a lot of money for me, that is why I built an electric earth auger (see photo below). I think I can actually do 25 foot holes. The auger shown on the photo uses a quarter-horse drill motor (187 watts) and a 25:1 gear reduction. I didn't get very far drilling dry, then I discovered using drilling mud which required using a pump to send drilling fluid down the hollow drillling pipe. This of course required making a drilling fliid swivel arrrangement, and a drilling fluid recirculation system. I discovered the drilling mud secret late in the summer, and it greatly improved my success, but then cold weather overtook me and I had to stop.

My gearmotor auger, at a quarter horse (187 watts) is really absurdly small. Most homemade augers are in the range of five horsepower (3730 watts) or bigger. Here in Portland, Oregon the ground is very difficult to drill. Lots of big rocks deposited by glacial deposits during the last ice age. However, even though it's slow, hard work, I have gotten down to 17 feet. If worse comes to worse, I can succeed with a loop filed that consists of 18 holes that are 17 feet deep each. In the mean time, I'm scouting around for a larger gearmotor for my drilling setup.

Earth structure really has a lot to do with it. Even my tiny 187 watt gearmotor auger would do a great job if the earth conditions were favorable. About 30 miles from here, there is sand that goes down about 85 feet. If that were my case, I'd have been done last summer.

Oh, and I should mention that I made a T-handle hand-powered test drill, which used domestic water pressure to flush out the drill cuttings. I was actually able to get down nearly 12 feet. With favorable conditions, it would be possible to do the loop field by hand!

The photos:
Gearmotor-Auger-2(small).jpg - This is a close-up of the gearmotor auger showing the first swivel which didn't work very well. It had O-ring seals and so much friction that I lost almost 1/3 of my drilling power (which wasn't much to begin with). Also seen is a garden hose feeding the auger. With the soil conditions here, sand and not much clay, cave-ins were a problem after maybe 5 or 6 foot depth.

Swivel(small).jpg - Here's the second generation swivel which worked much better. Once I realized that I was going to need to use drilling mud, I started taking my swivel more seriously.Hhere I'm using 3/4 pipe skimmed down to fit 1" ID sealed ball bearings. A happy fit was that the OD of the bearings just worked inside the plastic T-fitting. The whole swivel cost was about $23. Commercial ones are $300 or so. The friction is almost non-existant. Works great.

transfer-test(small).JPG - Here I'm making a test hole to see what the rate of thermal transfer is from the earth to the water.

Best Regards,

-AC_Hacker

It's true that it takes a lot more energy to move a unit of heat in air than in water. For its mass and density, air is pretty viscous so it takes a lot of power to shove it around, and for that matter the conduits we have for moving it, unlike plumbing pipe, are generally pretty rough on the inside. If you could have it, about the best material for moving large volumes of air would be large-diameter PVC pipe - which is really expensive.

My smallest HVAC system here at work uses a 1/3hp motor for running the blower. Obviously none of this has been optimised for the best possible performance but it isn't the worst case scenario either. So that blower moves around 800cfm, which after a little math with a REALLY GENEROUS fudge factor equates to about 300 pounds of air per hour.

My largest water heater has a permanent circulator pump, in this case about 1/2hp so it isn't a direct comparison, but that's moving, assuming the lowest numbers on the scale, over 400 pounds of water per MINUTE. Now, you can't heat water to as high a temperature as air, but water's heat capacity knocks air into a cocked hat.

I've thought about projects like yours from time to time. The one I kept coming back to was, what if I got a small window-mount heat pump unit, detached its exterior coil and built a water box around that? Then just circulate your heat exchanger water from the ground loops into the water box. It's quick and dirty, but I don't see any big reason why it shouldn't work. You get to eliminate half of the fan noise from the AC unit by pulling off that outdoor fan, and redirect that fan's controller to operate the ground loop pump. Since the water is entering and leaving via insulated pipes, you can place the heat pump pretty much wherever you want it.

As you say, your mileage may vary.

elhigh,

> ...what if I got a small window-mount heat pump unit,
> detached its exterior coil and built a water box around that?
> Then just circulate your heat exchanger water from the
> ground loops into the water box.

I think it would really improve the efficiency of the unit, you should do this!

I actually considered making a water box around the air heat exchangers also for my project, but I intend to use this to heat my house, so I took the route I took.

A couple of things you might consider, the amount of tubing and fins, etc that the air exchanger uses is way bigger than you need, it will still work, though. I've been looking for some rule-of-thumb that I could use to make a switch from air to water, so I'd know where to start. I seem to recall that I saw on some boat refitting blog, someone saying there was a 50:1 difference between tubing in air and tubing in water. A buddy of mine is fitting a small refrig into his boat and using a condenser that is actually in contact with the ocean water. He said the condenser tube was only a few inches long for his refrig. So maybe the 50:1 ratio is close.

Another thing is that if you plan to use this unit for a long time you could get stuff growing between the fins, in which case you might need to carefully remove the fins all together, to prevent fouling.

Another suggestion is that your unit, however big or small it is will be putting out a surprising amount of heat, so you'll need to have sufficient pipe in the ground to absorb all the heat. The rule of thumb where I live is that a 200 foot hole is required to transfer 12000 btu per hour. Look on your AC to see what the BTU rating is. If, for instance it is 8000 btu, the equation would be:

Required Hole Depth= (8000/12000) * 200

So your hole would need to be at least 133 feet deep.

You could also use 10 holes, each 13 feet deep. They should be separated by at least 8 feet each.

The earth has a lot of heat to give or, in your case, to absorb. But it does it slowly on a per foot basis. So you need more feet of pipe in the ground to get the heat flow rate you desire.

But it definitely sounds like a worthy experiment that we can all learn from.

I'm currently working on a detailed post, spelling exactly how to convert an Air Conditioner or De-Humidifier to a working heat pump.

So stay tuned...

And best luck on Eco-Modding your AC,

-AC_Hacker





'

What if you did that with your fridge? That's been floating around in my head for a while. If you already have some kind of ground source piping in the house, then sticking the refrigerator's coils into that would not only raise its efficiency (as would external insulation), but would also add some extra heat to the source of your heat pump. In fact, you can go one step further and notice that the temperature inside the fridge is close to either the temperature of your ground source, or of the return, so why not put the whole fridge in a water jacket? No compressors, no coils, no noise.

I've also been wondering about how much heat can be stored underground? Say you dump your A/C heat into the ground during the summer. How much of it will still be there when you need it in the winter, and how much will dissipate? That probably depends on your climate. Here, heating season is 5-7 months, while A/C might be needed no more than 2-6 weeks. Say you have solar panels that help with heating or hot water, but which are too big for summer. Would it be worth while to pump the sun's heat into the ground all summer?