Anyone look into heat-pump water heaters? My quest for oil-free house

[Redox]

Nope, can't say I've ever seen anything like that before! Is this a Waterfurnace part? I'm not sure what the purpose is, other than to make it easier to tap into an existing heater. Since you've had problems with it, maybe it's time for a rework? Free heat's wasting as we speak...

Chris

 

[Jerry_NJ]

I"m not sure how much guidance the installation firm got, but Waterfurnace provided what's at the right side of the pictures, including a desuperheater and circulation pump with a defined "in" and "out" and a suggestion that these be connected by pipes to the hot water supply, and in parallel, not series. So, on the left, the installer put a three-way tap into the drain hole of the water heater. This provides for water to be circulated between the HWH bottom (seem good, the hot water will rise) and the dsh while still allowing the third-way on the tap, just a hole in the portion of the tank feeding the desuperheater, with a faucet to allow draining the HWH. As it now sets, I can open the drain faucet and nothing comes out, also nothing circulates in the dsh circuit, best I can tell. My test method over the years has been to measure the pipe temperature, under the insulation sleeve, on the water going to the dsh and on the water being returned, assuming water was moving in the pipes. I believe the expected delta T is only a few degrees, but I would measure none to a heat loss and concluded water was not flowing in that circuit either.

 

[Redox]

Yep, sounds like it is stopped up to me. I assume the ball valves were open while you were checking? I'm not sure I would want to go disturbing it if it is 8-9 years old. I've found the tappings on a DHWH tend to rot out after a while. If I were you, I would consider adding a 50 gal tank in series in front of this thing and pipe the DSH into this. That way, the heat pump can heat the entire water heater up before you even begin to use hot water. Pull the cold water back up the dip tube through the DSH and pump it back into the outlet of the first heater. Just for yuks, put a thermometer on the line in between the two heaters and you can see how much heat you are recovering. I think it's possible to hit 180F on a desuperheater without much effort.

My reasoning for the second heater is to separate the electric heat from the "waste" heat. If you were to overcirculate the cool water back up to the top of the electric heater, it might bring on the electric sooner than necessary. At least if it doesn't work out, you will have a spare heater!

Chris

 

[Jerry_NJ]

Thanks Chris,

Trying to put in a "three hole" tank in front: cold in (tube to bottom of tank), hot out (top dip tube, or flush with top) and drain/flush (bottom of tank). If I put a "T" in the cold water line into the new pre-heater tank, on the cold that has the dip tube to the bottom of the tank, I could run the other leg of that "T" to the input of the DSH. Then I'd return the DSH to the drain hole of the new tank, here I assume the water pressure in the tank is balanced top/bottom so the DSH circulatory pump doesn't have to work against any back-pressure. Then the hot out line would simply connect to the cold input on my regular/normal electric water heater. This would allow me to use the full 1/2" pipe cross section area in the loop through the DSH. Sounds like:
1) a lot of plumbing
2) should work, if the existing system can/did work as it too has to work to circulate water in/out of a tank that has water under pressure stored in it.

Here I note I never verified the original system worked, as there were no easy ways to measure...when I began to wonder and started measuring the surface temperatures of the pipes to/from the DSH I think the system was already plugged up. Other tests I made, while my wife was away, she'd never go for periods without hot water, I turned off the hot water heater when the A/C was being used and didn't use any hot water for the whole day, then measured the hot water coming form the tank...this was all long ago, but I think the first time I did this there was some elevation in the temperature, later there was not.

 

[Redox]

That would work, but I think it would work better if you piped the DSH back into the hot outlet. This will promote stratification which anyone in the "Boiler Room" will tell you is a good thing when it comes to heat storage. This way, the hottest water is always available to the regular heater.

Don't worry about the pressure difference across the DSH as the pressure is in the positive direction while you are drawing water and will only increase the flow rate, if it changes anything at all.

Chris

 

[Jerry_NJ]

Ok, you're suggesting using "T" connections on both the top cold/in and hot/out. I'd then run the cold to the input of the DSH and return from the DSH to the hot. My intuitive on fluid dynamics isn't working (may not exist) well, but it seems that in the static state the DSH circulatory pump would draw water from the "T-ed" tank, not the cold feed/well, and return water to the "T-ed" hot, this water would flow back into the top of the tank as space would be available due to the water flowing to the DSH. Then, as I'm dumping hot water in the top, where it wants to set anyway (stratify level), and drawing cooler water from the bottom of the tank where it sets (stratifies) to feed the DSH there will not be much thermal churn. The cold feed isn't supplying any of this flow. Now when there is a draw on the system, I suppose most of the water flowing into the DSH would be the cold water input to the "T", feeding some of the demand through the DSH and some through the "normal" dip tube into the storage tank.

 

[Redox]

You got it! That has to work better than what you have, IMHO. Let us know how it works. I'd be willing to bet that you could shut off your electric elements for the summer and coast along on waste heat!

Chris

 

[Jerry_NJ]

I'll bring this one back. I got around today to pulling the bottom end of my old/original water heater loop. There is a picture of the loop a few posts back. My main concern was how was I going to drain the water heat so that I could do sweat (solder) joints on the copper pipes. Recalling, I could not get the spigot on the bottom to drain water, all plugged up due to the narrow channels associated with the tap that splits the one drain hole into two channels, one water out to HP, the other water back to the water heater. One of these channels is also used by the drain spigot As seen the the picture a few posts back the water heater HP loop has individual ball valve shut offs. So I was able to shut down both lines. I then drilled a small hole in one of the 1/2" pipes and when I opened the ball valve, very little water came out. So I drilled a small hole in the other 1/2" pipe and when I turned the ball valve open, water shot out of the hole. So, I cut the pipe there, ball valve closed, and connected plastic tubbing to that pipe coming out of the ball valve and fed it over to my French Tile drain in the basement floor. It took some time to drain the tank, but it was finally accomplished. I then pulled the two channel tap, and took some pictures of how it is loaded with calcium (lime) whatever, it is jammed up especially the narrow channel that surrounds the center pipe with provides the other flow direction for water in/out of the tank.

I don't plan to reuse this two channel tap, so I install the simple drain spigot in the bottom drain hole. The tank can now be drained/cleaned via that spigot. As discussed earlier in this trhead, I plan to tap into the cold line in, a 3/4" copper pipe with shutoff, and into the hot line, another 3/4" copper pipe. I will install "T" in those pipes with a 1/2" outlet. I'll then reuse the two ball shutoff valves, one in each line and plumb those two lines (from the Ts) to the loop going to/from my HP. I should get that done sometime this week, and as we are still in heavy air conditioning season I will be able to test to see how much "free" heat I can capture for my hot water system. My wife is away for a few days, so I can experiment without any worry about a cry for hot water.

Picture, full picture of three way tap, looking at the business end, end that sticks into the water tank, the area around the threads is where one of the water channels is located, it is fully coated over with deposits. Last picture is a closer look.

 

[Redox]

EEEEWW. That's gross! Are you on a well?

Looks like you found the problem. I have never seen a fitting like this. Have you looked at the circulator for this contraption? I hope it's not burned up. Maybe a drain valve in the HP loop will make future work easier?

Good luck and let us know how it works. I want temp readings! ;-)

Chris

 

[Jerry_NJ]

Hi Chris, with your change in avatar it took a while to recognize you. That's why I'm going to tap in with 1/2" in and out. That should not get plugged. As for the heat exchanger, it has bee turned off for some time, years. It could have burned up before I turned it off, but in fact some small amount of water was getting through and the HP is "processor" controlled with some active maintenance programs running. There is an indicator showing the status of the super-heat exchanger, but that may be nothing more than it is "on/off".

EDIT: the big problems I see now is cutting into the 3/4" in/out to the water heater. These pipes run against the overhead joists, no up-and-down free movement. Trying to insert a "T" with no space to move the pipe will be problematic, I may have to break into another joint to get the freedom required.

 

[Jerry_NJ]

Here are two pictures of the revised plumbing to connect the heat pump DWH via 1/2" pipes in/out of the existing water heater. I reused the two ball valves, they are needed for "bleeding" the loop. One picture shows the details of the splice into the cold and hot water pipes of the water heater. The other picture shows how the two new lines travel to get down to the original loop plumbing going in/out of the heat pump.

The weather is not hot, so no test yet to see how well it works. I have trouble getting my mind around the fluid dynamics involved, but it seems obvious to me that when the heat pump low capacity pump starts to pump water in the loop, it will draw water out of the cold water feed side, but that will not be water coming from the well/pump/tank as the other side of the pump goes to the hot water side of the tank where it puts "pressure" on the water in the tank. This seems to be no different than the "factory design" that used the drain hole to talk water out and around the HP loop. The improvement I' have is dedicated 1/2" plumbing in and out.

I note in passing I had some problems getting leak free solder (sweat) joints. Must be old age, seems I didn't used to have any trouble. Most joints were new pipe and fittings, a couple were reconnects to existing pipe and they don't leak. I did clean, flux and get hot, sill some leaks that had to be redone, three times in one case.

 

[Jerry_NJ]

Some operational data.

It hasn't been the usual hot August in NJ, so the A/C hasn't been on a lot. I did run some A/C however, while my wife was out of town, with the electric water heater off. The HP did provide some warm water, but never got real hot, but then the A/C was running only briefly and periodically.

My wife is home and the water heater is back on, but I did have the "need" for some A/C today so I took some temperatues of the In and Out pipes (1/2" copper - see pictures in previous post) and found with the A/C on high speed (a two speed unit) the delta temperature twas about 5 degrees (example: 105 our and 100 in) and when in low speed the delta was about 3 degrees (example: 103 out and 100 in). I have no idea of how much water is flowing in the loop, but assume the dedicated 1/2" copper line will allow several gallons per minute. Thus a Delta T of only a few degrees represent a sizable BTU transfer.

In any case, the design discussed on this forum was implemented and it appears to be working. Hope the saves me enough KWH to justify the expense (an additional $30 approximately) and time, at least 3 hours given I had some trouble getting two sweat fittings to stop leaking.

 

[Jerry_NJ]

Design Error!!

After running the connection I have shown above with pictures with the HP in A/C mode it became obvious that for some reason the water delivered to tap upstairs wasn't hot, in fact it was only warm sometimes. Thinking on this for a while I came to the obvious, now that I see it, design error I built into the HP loop. As you can see from the pictures the hot water tank out (hot) and in (cold) are in parallel with the heat pump loop. So, if the A/C is running when a demand is made on the hot water some of the water delivered to the tap is derived directly from the cold water inlet to the hot water outlet, via the HP, which is not putting enough heat into the water to operate as a instant/demand hot water heater. I will take the line off of the hot water outlet and instead return it via the drain tap at the bottom of the hot water tank. This can be done easily by simply cutting the 1/2" copper return line from the HP at any convenient point, attach a hose outlet and put a piece of hot water rubber hose that can be used with a washing machine, for example, between this new hose tap and the hose tap on the drain valve. If this works well I can later plumb with a more permanent line. I say this knowing that I have used such rubber hoses on my hot water feed to our cloths washer and they have held up for many years, say 10 or so.