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Old 10-12-16, 01:20 PM   #1
IdleMind
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Default Hpwh 2 (heat pump water heater #2)

This is my second small heat pump water heater build. I am starting with a 6 gal 110v electric water heater that I found on craigs list for $45. I have 2 dehumidifiers in my basement. One is about 13 years old and uses r22, the other is about 5 years old and uses R410a. The older one has lousy performance compared to the newer one as measured by water removed per watt-hour. Perhaps the newer one has a better control system. Anyway I am using the old one for this project.

The first step was to grind off the scale from the steel bottom and then tin it with an acid core solder. Here's a photo after tinning.


I am soldering a coil of 3/8 copper tube to the bottom of the heater. I started with a 20 foot coil and got 15.8 feet of working coil soldered in place. My first build had only a 2.5 gal tank which wasn't big enough to put the coil on the bottom. On that one I used about 18 feet of 1/4 copper tube soldered to the side of the tank. I think this will be much better. Here it is after taking outside and washing off the flux residue.


Next I covered the coil over with silicone caulk as a first layer of insulation.


Then I reattached the top & bottom covers and injected with great stuff, I kind of over did it, the covers buckled out a bit as it expanded and cured.


I have the dehumidifier modified so more photos to come soon.

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Old 10-13-16, 05:44 PM   #2
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This photo shows where I am at now.


The 2 dehumidifier HX's are plumbed together in series to make a larger evaporator as was suggested in other threads.

I removed the original squirrel cage fan which drew 80/85 watts and replaced it with a 200mm computer case fan with 18v power brick that draws 14 watts and seems to push as much or more air.

Added permanent gauges, service valves, and two flare connections.

Charged it with BBQ gas and have run a few tests to determine that the fan I am using is really better than the squirrel cage fan.

One of the flare connections may be leaking so I need to attend to that next.

Then I will be doing some bench tests to collect data and calculate efficiencies. I hope that I can determine efficiency at various water temps and refrigerant charges.
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Old 10-13-16, 06:35 PM   #3
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Idlemind

That bottom HX method is a beautiful idea. You don't mind if I use it.

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Old 10-14-16, 07:19 AM   #4
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Where's the decal???? You know a monster case fan won't work as well unless it has a good looking decal on the hub. Now the fan won't know how to act...

All joking aside, my guess is that you are going to have to experiment with metering devices on this unit. Since the two stock heat exchangers are combined, the original cap tube won't pass enough mass for both of them. System efficiency will suffer, and finding an optimum charge will be an exercise in futility. You will either need to shorten the cap tube to better feed the new evaporator, or swap it out with another metering device.

Since you have a successful build under your belt, I suggest a TXV for this unit. I'm guessing the unit has a 5000 btu or smaller compressor in it, and that you will be operating this unit it conditioned space (no freezing conditions). I found an ideal TXV for this purpose here for $31:

Emerson 1/2 Ton R-22 TXV 3/8 X 1/2 flare



This little baby will hot rod your unit! It is a commercial duty valve, has adjustable superheat, and has a sensing bulb charged for medium temperature refrigeration. Where your other unit could possibly gain efficiency (or not) by right-sizing the cap tube, you would eliminate that factor in this unit with the tx valve. If you felt the need, you could adjust the superheat on the valve to get a better feel for how the unit performed.
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Old 10-14-16, 08:21 AM   #5
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The fan decal is on the other side, hope it still helps. Those fans also work better with some cool blue LED's.

I really appreciate the suggestion on the TXV valve. I have run this a few times and think I am a long way from getting it right. I expect this will be on my bench a long time before putting it to work. I intend to do some testing as is, then I will give a TXV a try.

Here are some tips on the condenser coil.

The tank is made from hot roll steel which has scale which not take solder. Before tinning I used an angle grinder to remove the scale.

For tinning I used Oatley #95 flux which contains tin powder which helps a little. I used an acid core lead/tin solder to tin and to solder the coil down. A propane touch is hot enough to do this job, but it takes awhile to get going, but once it's heated up it goes faster. It can be difficult to get the solder to bond to the steel. If the steel is not taking the solder use a piece of steel wool dipped in flux to rub it in. I showed a photo after tinning because that's a real important step.

When soldering the coil down, I used a piece of wood to press the tube tight to the steel as I worked my way around. Then I took it outside and tried to clean off the acid flux with soapy water and scrub brush and lots of hose water. Given enough time the acid can cause corrosion.

The tank bottom is domed up in the center so that is inlet to the condenser so as the gas condenses it flow down hill to the outlet.

If I were to do it over I would not coat it with silicone, I think the great stuff is enough.

I used 3/8 tube instead of 1/4 to have more interior surface area for the gas to condense on. It is starting to seem to me that it may have been better with 1/4 tube, I will have questions about this later.

Putting the condenser inside the tank has got to be much more efficient, but this method allows me to keep the resistance heater which is a nice backup.
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Old 10-15-16, 09:31 AM   #6
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I will be doing some tests on this system but have concerns about accuracy. AT times I get temp readings from my cheap thermocouple readers that has to be way off so I don't trust them at all. I just ordered some cheap thermometers to use instead, at least I expect these to be repeatable. It will be a few days before they arrive and then I will solder copper thermometer holsters to the lines coming out of the evaporator and compressor.

When I start this up it can take up to ten minutes for the temp out of evaporator to drop to 50F and the condenser is also slow to heat up. Once it gets past this start up period it seems to run OK. During this start up the pressures are both quite low. It is very inefficient during this time. Will a TXV help with this issue.

I have seen disscussion on this forum about possible differences between BBQ gas and R290. I think the P-T chart for my BBQ gas is about 8 psi offset from R290. This might be my measurement error. So I am going to try to develop my own P-T chart. Has anyone tried this. I will be working on this in the next few days and will come back with results.
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Old 10-15-16, 10:41 AM   #7
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When I built the HP Hot water tank for my parents I had concentrated on the temp output, suction and current draw of the compressor. I knew that the high side would vary greatly with the temp. I would very slowly increase the charge and monitor the current draw. With a cheap receptical watt meter I would get an idea to the efficiency. I would check on it every few days and teak the charge looking for the sweat spot. The unit had a cap tube and I had used BBQ propane. The cost to operate the machine was about $20.00/month about the same as the rental fee for there previous propane fired tank.

Unlike the compressors previous life of a de humidifier the temps are fairly steady at room temp. The HP hot water heater with the introduction of cold water could start at 40 Deg.F and limit out at 140 Deg.F. Thats quite a swing and the pressures will reflect this. Thats why I like to concentrate on the current (Watts) as the compressor is moving a near fixed volume of refrigerant.

As you would monitor the temp increase and the power consumed (watt Hours) you can calculate the Coefficient Of Efficiency with that volume of water for that particular refrigerant charge. For the temperatures and energy input there will be a sweat-spot. This is called R & D and this is how the big-guys do it in their labs. I have found that propane consumes much less energy to move an amount of heat compared to R22.

Have fun

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Old 10-17-16, 01:57 PM   #8
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randen--I will be doing a bunch of tests to learn about energy use vs heat into water, but first I want to show some data I generated to get my own PT chart.

I made a test chamber from 3/4 copper tube and caps with pressure gauge per this photo.


I added some of my BBQ gas to the test chamber and submerged it in hot water. I took temp and pressure readings and added amounts of cold water to lower the temp and repeated the measurements. See photo.



I also looked up the pressure of R290 from a published chart. All data entered into spreadsheet table and charted.


Neither thermometer or pressure gauge are calibrated so you should be skeptical of these results. But it it interesting that my measurements aligned so closely with published data at high temps and diverged as the temp dropped and always in fairly smooth curve. If you zoom in you will see it's not a perfectly smooth curve.

It may seem like a redundant exercise, but I have been getting some SH of -3F that seem wrong so this tends to explain that. I also want to try some propane butane blends and being able to create a PT chart for an unknown blend interests me.
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Old 10-17-16, 02:40 PM   #9
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jeff5may--you recommended a TXV valve and I am about ready to order one. I also found one on Amazon, it seems smaller than the one you recommended. Would you mind taking a look and let me know which of these two makes best sense to you. This is a really small system, it draws around 400 watts.

https://www.amazon.com/SPORLAN-BFVE-...ords=txv+valve

https://surpluscityliquidators.com/p...-r22r407c.html

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Old 10-17-16, 03:50 PM   #10
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Different valves, same supply house. The Amazon listing comes from surplus city, so it could be found on their home site, saving you a couple of bucks worth of handling fees...

Other than that, the one you posted is externally equalised, which would need a line plumbed between the valve and the suction side of the circuit to satisfy the valve. The one I found is internally equalised. They both have flare connections and a "c" charge in the sensing bulb.

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