HPWH improvement

[Mikesolar]

OK dudes and dudettes,
Since people have been having success with the heat pump water heaters from old window shakers, I have designed a mod that will make it more usable.

The issue results from the fact that trying to heat up a 60-80 gal tank with a 1-2kw heater takes a long time. The element (condenser) in the bottom has to heat up and mix the whole body of water before the top gets warm enough to use. We have this problem in solar all the time with indirect tanks.

The solution is to set up a thermosyphon system with a sidearm HX, as shown in the drawing. The condenser will heat up the small volume of water which will move directly to the top of the tank and is replaced by cold water at the bottom. This will create usable hot water in the shortest time, and eliminate the need to make a special fitting for a tank (although a fitting will be needed for the HX) and the issue of getting the coil into the tank is eliminated.

Just thought I would share this.

[NiHaoMike]

Take it a step further by adding a pump and then you can put the unit where the added cooling would be more useful. That's the design I adopted.

Another solution is to add a second coil near the top of the tank and connect it as a desuperheater. The commercially available units likely use a design like that.

[jeff5may]

But what if the hot and cold water pipes are both at the top of your water heater tank? Most newer units are built this way.

[Fordguy64]

Just use the drain.. A lot of the outdoor wood furnace guys use this idea for heating water

[NiHaoMike]

The added thermal resistance would likely reduce efficiency unless you added a pump to keep it very low.

How often is it that additional cooling right at the tank would be a benefit? I most often see hot water tanks in the attic or garage in my area, so it would be trying to uselessly cool unconditioned space.

[Mikesolar]

No need to use any pump on this system. The the water in the HX won't start to rise until it reaches the temp higher than the water above it and it will need to be quite warm to get to the top. This is where you want the heat to be but a convection current will only happen if the HX is at the bottom. A pump will just force what may be cooler water up to dilute the already warm water at the top of the tank so it is less useful. Having the HX at the bottom is natural protection and gets the best efficiency from the system and I have done it 100s of times with solar.

I always disconnect the top intake and use the drain. One electric tank manufacturer did some tests and showed a 10% improvement in efficiency by doing this. Also, the dip tubes do wear out.

[Zooomer]

Originally Posted by Mikesolar

I always disconnect the top intake and use the drain. One electric tank manufacturer did some tests and showed a 10% improvement in efficiency by doing this. Also, the dip tubes do wear out.

Can you elaborate on that statement? How do you get 10% more efficiency out of a resistive heater?

[AC_Hacker]

Originally Posted by Mikesolar

OK dudes and dudettes,
Since people have been having success with the heat pump water heaters from old window shakers, I have designed a mod that will make it more usable.

[* I do seem to recall someone who posted an idea similar to yours, but as I recall, he wanted to spiral-wrap his tube-in-tube HX around the outside of the water heater. I don't recall him ever reporting back that he'd had success (or not). *]

Mikesolar,

I think that this is an excellent idea. I can see every reason why it would work well.

You are correct in that it solves the condenser-inside-the-tank issue. However the condenser inside the tank does have the advantage that there is almost zero lost energy, as all the heat goes into the water. Therefore your external circuit would need to be excellently insulated to be competitive.

The next step would be experimental proof...


-AC