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RB855 12-29-13 12:49 AM

Another DIY HP Waterheater (pics and info)
First new thread from a lurker, this is mostly a documentary to spread what I have learned. I have been slowly poking at a heat pump waterheater project for years now. Iv lost count of how many alterations have happened along the way. The quick and dirty info of its current state: 12k r-410A heatpump window unit. Swapped to r-134a for better working pressures. TXV evap control, series/parallel piped BPHE r-410a rated, small 4-8gpm taco pump, chinese heatpump waterheater controller, and some fancy pipe work.

My design goal is a outdoor remote piped unit (funny to think cold air is a byproduct!), at least a 2.0 COP, maximum water temp 130*F (115-120* desired), and totally automatic.
The current iteration is achieving about 2.5COP at 115*F water temp at 80*F ambient, moving approx 5800btu. I literally just charged it with 2lb of gas and let it run. There may be some improvement to be found in a proper subcooling adjusted charge as it felt like only the desuperheater exchanger was doing all the work.

(Please be patient with links, my server is underpowered for what I demand of it)
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I have updated my site and images have moved. Please use this link

I understand the cautions of using BPHE in a potable water circuit, but they saved a lot of space, and just one could handle the full load. They are "wired" in series on the gas side as a desuperheat/condenser, so even if one fouls, it will still operate fine. The BPHE are piped in parallel on the water circuit, counter flow of the gas circuit. I have also included a fine mesh strainer on the inlet of the unit. I had trouble deciding to stay with hot gas defrost as it does show some loss of hot gas temp when running, and I'm not sure how low an ambient this unit will still be efficient. The controller has inputs for safety disables, so I have included a high and low pressure cutout, as well as a water pressure cutout (no sense in it running if the waters off! Also prevents dry run, and shut down on plumbing failure). The filter dryer acts as a receiver for the txv in this arrangement, convenient! I decided to use the original evaporator in the recent variant for a few reasons. This gave me a 3 speed lower wattage motor, a much quieter blower, separates the condensate from the components, allows me to seal up the component side and insulate the box, instead of every component by itself. The unit is a 12k unit, and I'm only moving about 6K of heat, so the evap should be capable of lower ambient temps. It has significantly reduced unit noise, and also offers the ability to duct the cooled air. Im seeing about a 25-30*F delta coming out of the cold side on full speed.

Here is the growing gallery of photos of *invalid url*
Please excuse the lack of any naming scheme. I know what is in my head lol.

I have updated my site and images have moved. Please use this link

The previous phase of this project, you will see in the gallery I was attempting to use the old condenser as the evap. Trial and error showed some need to change the coils circuitry, which resulted in some very messy brazing, mistakes, unbalanced circuits, and other things that made things not very nice. The fan was also loud, single speed, and some 150watts by itself. It also *invalid url* drew the air around all the components[/URL], wasting significant heat. Also resulted in a lot of *invalid url* un-tamed piping jumbled into one corner [/URL]around the compressor. And further, all the components were sitting in a pan of condensate. While this arrangement worked ok, after a couple years of fooling with it, I decided to go the route of the original evap. I work on PTAC and windowshakers these days, so getting another of the same model was a matter of going out back to the scrap pile lol. The first version is a mess of a learning curve and years of fiddling. The newest version was put together in 2 nights, and tested in an afternoon. Yay experience!

I will update this thread as things advance, and feel free to ask anything you may have questions about. The spreadsheet pictured is just something I slapped together, ment for limited data calcs. But if there is interest, Ill buff it up and post a link. You feed it time, temp, amps, volts, and water capacity, it does the rest. Iv spent enough time fooling with this thing to learn a thing or two. Just don't expect an expert reply lol.

RB855 12-29-13 01:06 AM

*Saved post*

I forgot to mention, the goal of this unit is to store heated water in a un-powered electric 50gal waterheater. I may leave the elements in, and use a outdoor ambient thermostat to transfer from HP to Elect in <1.0 COP operating temperatures.
Also to note: Switching to r-134a on a r-410a compressor proved fairly significant capacity loss from the compressor. I can only assume the 410CP uses a smaller displacement pump than a 134CP, likely due to the very different pressure and mass flow rates of the 2 working gases. There is probably a slight performance hit as the motor is slightly over-sized now, but that also offers a bit extra overhead to work with.

jeff5may 12-29-13 11:54 PM

Looks like an awesome monster machine to me! It looks as if you've done lots of mods to the original design. I have some questions:

So how does your hot gas defrost perform? There have been a couple members interested in the process, not usually used in consumer machines.

Since you live in Florida, how come you don't use this for air conditioning as well?

RB855 12-30-13 01:00 AM

The defrost is time/temp based. If the controller sees the evaporator below 25*F for 90minutes, it will enter defrost. At this point, the controller turns off the evap fan, and energizes the reversing valve. It will stay in this mode until evap temperature reads 45*F, then return to normal operation. I think the max defrost time is 5 minutes (which is a very long time!). This is nearly identical to defrost cycles on your typical household heatpump, with the exception the controller exposes all the time and temperature variables.

I considered using it for a/c, but tell me if my logic is sound: In spring/summer/fall we often have outdoor temps in the 70-95 range. It will almost always be warmer outside than inside on these seasons, which are in my favor for higher evap temps. The humidity load will also do wonders. The 2-3 months we call "winter" will take a hit on efficiency, but not sure if it would be cost effective to "cascade" the unit via the whole house heatpump. That, and space inside is limited, placing the unit in a living space (or ducted). What Could work is duct the air inside during the summer, supplying conditioned fresh air to the house (hmmmm )!

Edit: I re-read your question, and it looks like your asking how well it works, not How it works. Honestly, it hasn't been cold enough to get the evap below 45*F lol. I will say that forcing defrost, the coil goes from 45 to 130*F in about 45seconds, burns off the condensate in a hurry. Once the unit has stabilized, the compressor holds a Lot of heat, and is immediately available to burn off the evap. The heat load of the water in the BPHE also offer a huge supply of instant heat to the system. Do note the capillary bypass and check valve around the TXV for this to work. Don't forget this vital part! (wana know why I have a highlimit? LOL)

NiHaoMike 12-30-13 06:32 PM

Why R134a instead of R290 or R433b/ES22a? What you essentially did was a double "Davuluri Treatment". Your max temp is going to be limited by the discharge temperature limit of the compressor, for which R290 (or R433b, which is mostly R290) would do better than R134a as it doesn't superheat as much on compression. (Counterintuitively, that's actually more of a problem at low ambient temperature since the compression ratio goes up.) The reduced suction pressure avoids compressor overload at high ambient, but just going from R410a to R290 will do it for any reasonable ambient temperature.

You can get a good boost in efficiency by swapping the PSC fan and pump for ECM. As a bonus, a variable speed pump lets you take advantage of stratification by drawing cold water from the bottom and putting in hot water at the top of the tank.

BTW, for best efficiency, operate at 105F or so most of the time and use remote control (with a timer) to boost it up to 140F for washing dishes.

stevehull 12-30-13 06:56 PM

My dishwasher has a built in heater that boosts water temp up if it is not 120 F was used to "old school" where you ran the tap closest to the dishwasher for a few minutes to "get the water hot" before running the dishwasher.

Today, I actually lowered the water temp to 110 F, but I notice that it does take a while to get water hot in the shower (wasting water).

Such are the trade offs . . . .


NiHaoMike 12-30-13 07:10 PM

The amount of cold water sitting in the lines is the same regardless of the temperature in the tank.
Also, if you plumbed the unit into the hot and cold lines under the bathroom sink, it can also operate as a hot water recirculation setup. For those in cooling dominated climates, the summer efficiency gain from having the unit also operate as an air conditioner outweighs the loss during the winter. (And even then, if dehumidification is needed during the winter, it can still be a net gain.)

stevehull 12-30-13 07:56 PM


Originally Posted by NiHaoMike (Post 34389)
The amount of cold water sitting in the lines is the same regardless of the temperature in the tank.
Also, if you plumbed the unit into the hot and cold lines under the bathroom sink, it can also operate as a hot water recirculation setup. For those in cooling dominated climates, the summer efficiency gain from having the unit also operate as an air conditioner outweighs the loss during the winter. (And even then, if dehumidification is needed during the winter, it can still be a net gain.)


I am confused. Is "the unit" mentioned twice above, a dishwasher? And if so, how can it act s a dehumidifier in my hot/humid summer?

Lastly, how can it be used as a hot water recirculation setup?

Am confused . . . .


NiHaoMike 12-30-13 07:59 PM

The unit is the heat pump. As for how it can work as a hot water recirculation setup, if you have it connected to the hot and cold lines under the sink, you can run the pump to move the cold water in the hot line to the cold (needs a reversible pump, thus not practical with off the shelf parts) or just run it to put hot water in the lines.

RB855 12-30-13 08:24 PM

The reason for r-134 is because it's readily available, I already have 30lb as well. I will never run above 130*F, 115*F makes for good shower times and temps. Also the higher super heating I think is in my favor, since heat is my goal. Not too concerned about overloading the comp as its already running at half it's intended capacity. Dishes and cloths are done in cold water, and I don't have a dishwasher. The controller does have a timer function, which I may use to prevent nighttime running when the ambient is lower. I'm not seeking absolute returns, so that saves me a Lot of money on ecm motors. The evap blower wouldn't be interchangeable anyways. Don't go by where the lines cross for cop vs. Temp, it was basically intended to show relation and returns at x temp. As for possible winter dehumid... my winter relative humid is usually below 20% and my sinus crys. Lol

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