Using a second water heater as an indirect fired tank?
I have two water heaters. A 50 gallon HTP Phoenix Light Duty propane water heater, and a 50 gallon electric resistance water heater. The two heaters are plumbed in series, electric water heater first, then propane. To make a long story short, some things didn’t work out as originally planned, and now I’m paying an extra $15-20/month (and wasting source energy) because I’m heating up a bunch of water with electric resistance right next to a high efficiency propane water heater.
I’d like to stop doing that. I have a couple of extra constraints:
*I need to be able to fill my huge garden tub with somewhere around 70-80 gallons of hot water. So I do need at least some of the added capacity of the electric tank.
*I need fast recovery times (okay, just want, but it’s part of why I forked out for the fancy HTP in the first place and I’m not giving it up without a fight). So just swapping the sequence to propane->electric isn’t acceptable unless I have a good way to bypass the electric tank while it reheats.
The obvious, easy improvement is to install a mixing valve and then crank up the temperature of the propane heater and turn down the electric heater, shifting a larger percentage of the total heating onto propane. But that only reduces the problem, it doesn’t fix it.
But then I got to thinking… couldn’t I just circulate water between the two tanks, using the electric tank as an indirect water heater? That could meet all of my criteria, and as long as I insulate the lines between the two tanks well, it would be very efficient. Plumbing it would be trivial; the only tricky part would be the control logic, so that it doesn’t steal capacity from the tank while it’s filling the tub (or at least, not too much).
Has anyone here tried a similar scheme before? Any advice for products to use, or how to rig it up?
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Replies
I think you should test using the Propane tank only and see how it goes. IT's 50 gallon capacity, plus recovery rate while filling, plus the liklihood that you are mixing some cold water in to the tub may get you what you need. If not, doing propane only with a mixing valve so you can increase the temperature of the tank (and therefore the capacity) while still having safe 120 degree water coming out might get you to where you want without the complexity and costs of two working tanks.
Also running the water through the electric tank turned off will probably temper your incoming temp somewhat.
Tim,
I agree with Albany's advice.
That was actually the first thing I tried. With that, after about 6 minutes of filling it starts running cold. I can then stop the water and wait another 6 minutes for the tank to recover, and then finish filling the tub. It does work, but it's an unwanted extra delay for a tub that already takes a long time to fill.
Question is , what do you have the temp set for in the electric heater ? This is very important since your Phoenix is a modulating - CONDENSING unit whose efficiency is dependent upon incoming water temp being cold . In other words , your zeal to make something more efficient has bitten you .
The warmer the water entering the unit the less efficient it operates . The greater the amount of condensate product , the more efficient your combustion will be .
Try taking them out of series . Plumb the electric tank as if it were the sole source , set the temp to 90-100* , upon leaving the electric plumb the hot (leaving) line to the cold side of the mixing valve mentioned below . This will get you far beyond where you want to be .
I advise to boost the Phoenix Storage temp to 140* (min) , use a fail safe mixer set to 120* . This will allow the electric tank more time to preheat and increase your SYSTEM capacity and capability . Forget about control logic and added expense that will only frustrate you , this is physics and it is not difficult unless you think you can fool Mother Nature .
Taco 5000 series mixer or Caleffi 521 series are what I would recommend . Both can be purchased with a temp guage and I also recommend that .
Hmm. By itself, that doesn't really fix the problem - half of my water heating would still be performed by electric resistance. But I could easily put in two mixing valves, one as you describe and a second one for the rest of the house that takes actual cold water; then I'd only be using the electric resistance to help fill the tub.
It's a disappointingly mundane and straight forward solution, though. I'm still open for more elaborate and convoluted schemes, if anyone else wants to apply some creativity :)
The second mixing valve that takes "ACTUAL COLD WATER" as you say is certainly an option . Here is the problem , the greater the ratio of cold the more hot will be required to meet your desired temp . This does nothing to leverage or increase your storage capacity .
Maybe you could use the second mixer for a dedicated hot line to the tub and employ a flow switch which would close the switch to the electric heater only when flow is detected in the dedicated line to the tub . Hmmm , is that complicated enough ?
I had not a clue that this whole set up was just for a tub and am even more flabbergasted that you believe something more elaborate or convoluted will offer a better result . I guess something that benefits the WHOLE Home and not just the tub is undesirable .
Sorry to have wasted your time .
Other than the tub, my hot water capacity requirements are unremarkable and can easily be met by either tank, and the impressive recovery capacity of the HTP could feed my low flow showers indefinitely. If I didn't have the tub, I wouldn't have two heaters (and this is not a particularly uncommon situation for people with large garden tubs).
I wasn't saying that something more convoluted would give a better result - it would just be more fun to implement :)
edit: I should also note, I already basically have a dedicated hot water line for the tub - the tub filler is on a wall shared with the utility room, so it's the very first branch off my hot water trunk; plumbing it to use a separate mixer wouldn't be difficult.