Homemade Heat Pump Water Heater (R290)

[launboy]

What are you planning to use for a defrost system? Also, do you have a circulator pump or some system to distribute the heat from the HP tank to the other tank? Otherwise the resistance still has to maintain that tanks heat, although I guess the heat loss through the tank isn't much.

Adam

[AC_Hacker]

Congratulations on the good job.

It was very kind of the dehumidifier folks to make such an attractive shroud for your heat pump!

Really, that's about as straight-forward a solution to improving the efficiency of a water heater as I can imagine. Well done, really well done.

As I recall, you saved the water heater you used from scrap hell. So you might not know its age. At any rate, someone mentioned way before on the 'Manifesto' thread, that having a heat pump instead of 220 volt heating elements, would be much less stressful on the inside of the water heater core, because the temperature cycling is less extreme, and also because you don't have high amperage elements cycling inside the tank, that can accelerate corrosion. I have no way of proving those assertions, but I pass them on along because they sound so intuitively correct.

Keep us posted on the measured efficiency.

Also, it should be a pretty easy matter to keep track of energy costs over a few typical months and compare that with the cost to run the propane heater, from similar months.

Best,

-AC

[Hv23t]

Originally Posted by launboy

What are you planning to use for a defrost system? Also, do you have a circulator pump or some system to distribute the heat from the HP tank to the other tank? Otherwise the resistance still has to maintain that tanks heat, although I guess the heat loss through the tank isn't much.

Adam

Yes your right the resistance elements will have to maintain the temp in the primary tank. I cant imagine there being much loss there. I'm going to the hardware store tomorrow to pick up a couple tank blankets to insulate them even further. My defrost system is just going to be a recycling timer controlling the compressor. It will take some trial and error but my first setting will be 1 hr on and 5 minutes off. I've done it manually a few times and seems to work well enough. The refridgerant in the lines going to the tank are in the 100-130deg range. When the compressor turns off and that heat distributes itself thru out the system the ice melts pretty fast. I also just ordered a digital temperature switch that I'm going to set to bypass the heat pump altogether when the air temp gets below 45 deg f. And then it will use the 4000 watt 240 volt element, that is running on 120 volts so is only 1000 watts. And then of course the primary tank has its 4500 watts worth of heating elements to keep the warm water flowing.

Hv23t

[Hv23t]

Originally Posted by AC_Hacker

Congratulations on the good job.

It was very kind of the dehumidifier folks to make such an attractive shroud for your heat pump!

Really, that's about as straight-forward a solution to improving the efficiency of a water heater as I can imagine. Well done, really well done.

As I recall, you saved the water heater you used from scrap hell. So you might not know its age. At any rate, someone mentioned way before on the 'Manifesto' thread, that having a heat pump instead of 220 volt heating elements, would be much less stressful on the inside of the water heater core, because the temperature cycling is less extreme, and also because you don't have high amperage elements cycling inside the tank, that can accelerate corrosion. I have no way of proving those assertions, but I pass them on along because they sound so intuitively correct.

Keep us posted on the measured efficiency.

Also, it should be a pretty easy matter to keep track of energy costs over a few typical months and compare that with the cost to run the propane heater, from similar months.

Best,

-AC

Yeah the water heater was set to go to the scrap heap. The only reason it was being discarded was do to a remodel of the room it was in. It is 15 years old but we don't seem to have problems with rusted out tanks around here. My previous homes heater was 26 years old with no probs. the propane heater I just took out of my parents house was 29 years old. When I hooked the hose up to the drain port to empty it the water came out purfectlt clean, no signs of rust. And that was the first time it had ever been done.

Hv23t

[launboy]

Originally Posted by Hv23t

The refridgerant in the lines going to the tank are in the 100-130deg range. When the compressor turns off and that heat distributes itself thru out the system the ice melts pretty fast.

Hv23t

This is basically what I found with my Hottub HP last winter, even in below freezing weather, with the condensor in 104*F water the ice melted on it's own pretty quickly, but the heat didn't last long enough to dry the coil so the melt water just refroze on it. If you keep you only run your system above 45 you shouldn't have that problem though. Honestly you could probably run it lower than that. It would be interesting to see where the balance point would be on a small HP like this. Most commercial units it's around 35*F.

[AC_Hacker]

Originally Posted by Hv23t

...I also just ordered a digital temperature switch that I'm going to set to bypass the heat pump altogether when the air temp gets below 45 deg f. And then it will use the 4000 watt 240 volt element, that is running on 120 volts so is only 1000 watts...

Hv23t,

I think that having a temperature controlled cut-out is a good idea.

However, there is a curve that results when you plot COP vs. air temp, and COP drops as air temp drops. When the COP approaches 1, it is definitely advantageous to switch to resistance heat.

Here is a plot I found on the web just to show you the shape of the curve. The temperatures here are in C not F, and the place on the graph where COP = 1 will most likely be very different from yours, but the idea is very similar to yours.

I guess my point is that 45 degrees seems to me to be an arbitrarily high point to set your cut-out, and that even though the COP is declining, it's still a lot better than resistance heat.

But exactly what your curve looks like will depend on more factors than I can possibly guess. You can get a rough idea by monitoring the system as ambient temp changes.

By the way, there is a very timely conversation going on right now on the 'Manifesto' thread that concerns R-290 efficiency. You should check it out, as it pertains exactly what you are doing.

Best,

-AC

[Hv23t]

Thanks AC I'll check out the manifesto thread. I picked 45 deg as a starting point for no particular reason other than on my geospring Heater at 40 deg it switches to resistive heating automatically. So I was just using a starting point somewhere around its. I'd imagine they switch over then for recovery time reasons. But I'll play around with it to see what's best. Since its just a preheat tank as long as its over 1 cop I'm still ahead.

Hv23t

[Mikesolar]

My fridgie friends from the UK all seem to be concerned about sulfur content in BBQ LPG. They seem to think it will harm the copper components long term. I've no idea but has anyone looked into it?

[Hv23t]

Originally Posted by Mikesolar

My fridgie friends from the UK all seem to be concerned about sulfur content in BBQ LPG. They seem to think it will harm the copper components long term. I've no idea but has anyone looked into it?

I'm not sure of the long term effects either. But when I first charged the dehumidifier and ran it for a couple days, I released the propane so I could turn it into the heat pump. The filter dryer seemed to filter out the sulfur as the propane had no more smell. So I'm guessing it wont be an issue,
Hv23t

[AC_Hacker]

Originally Posted by Mikesolar

My fridgie friends from the UK all seem to be concerned about sulfur content in BBQ LPG. They seem to think it will harm the copper components long term. I've no idea but has anyone looked into it?

Yeah, this can shorten the life of a system in general, the compressor in particular. I don't have any data as to how much. A properly set-up heat pump that is installed out of the weather can last decades. To get this kind of life, you'd want to go the distance with the best refrigerant you can obtain or upgrade, and scrupulous attention to water removal, etc.

But for hacking and learning, BBQ-gas works. But for your Opus, you'll want to use best practice.

The problem is sulfur + water which can make sulfuric acid, which is the real problem.

A good vacuum pump is used to create a vacuum deep enough that water will boil off, and hold the system vacuum long enough so that the water can be removed. This means a vacuum in the sub 200 micron (or equivalent) range. Good equipment can do this. A micron gauge will tell you if your equipment is good. I thought my equipment was good until I got a micron gauge, then I found out that my vacuum pump was not creating a sufficiently low vacuum. I now have an inexpensive, but new vacuum pump that does the job.

Also, some of our most industrious fiends have used HVAC filter/dryers to remove sulfur content. The noxious rotten egg smell (Hydrogen Sulfide) that is present in BBQ-gas is an add-in (Capton) for safety, because propane is odorless. So filtered BBQ-gas will be nearly odorless.

You should be able to locate 'refrigerant grade' R-290 that is properly dry and sulfur-free, but it's not cheap.

If you search the forum, you should be able to find postings of how others have done it.

Best,

-AC