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Tandem hot water heaters

Johnthomas's picture

I need extra hot water fast due to relatives moving in soon. I'm considering adding a 50 gallon electric hot water tank in tandem with the existing 50 gallon natural gas tank. The house has four tub/showers as well as two whirlpool tubs although only the showers get used at the same time. Any advice?

(post #98296, reply #1 of 6)

I had that. They were in series with separate cutoffs. I kept a somewhat low temp on the one and regarded it as a pre-heat. Seems like it worked. Consider a timer on the pre-heat so that it only runs at times of peak usage.

(post #98296, reply #2 of 6)

I did some work at a day care school. They had two water heaters. One set at low heat went to little sinks in classrooms. The other went to the kitchen that only teachers could access. It was set at regular temp.

Slightly different. I needed more air volumn than my compressor could pump (and keep the pressure high). I originally thought I would gang the two tanks and have some automatic way to alternate the two twin compressor pumps. One could cool a little while the other pumped then it would work while the other pumped etc.

My Dad (one of those PE's) said, "why not just have them both run at the same time resulting in the run time being cut in half?" So I still have both compressing at the same time with one common air outlet. Could that work for a second heater? Then when company (who will be happy to pitch in for the extra gas/elec right?) leaves just turn one heater off.

Just a thought. Tyr

Edited 5/17/2005 3:32 pm ET by Tyr

Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden.... Roman Poet Phaedrus 15BC–50AD

(post #98296, reply #3 of 6)

Well parallel could work in some cases.

But you need to for them to be balanced in both types of heater, size, and plumbing.

Otherwise you won't get the best use out of the two. One would run out before the other.

And just turning it off won't work it. Needs to isolation valves. And it does not do the water any good to set for months at a time. Stuff can grow.

For series the most "efficient" for the short term would be for water to through the gas heater first, and then the electric. So the electric only operates a small amount to over come standby losses and but come on full when only after the gas tank has run out of hot water. That is probably about after 140 gals of usage. If the electric rates are not too high he could just leave it that way all of the time.

If he want to turn it off he would have to have 3 valves plus the drain valve to isolate it and drain the water out of it.

The other option would be to run the electric before the gas. Then when not in usage he could just turn it off. Cold water would pass through it and keep it clean.

However, when in use it would kick on first as water is being used. But unless the electric rates where very high or this setup was used often it would not cost too much.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #98296, reply #4 of 6)

I have 2 40galloners in series (natural gas).  Initial is set at slightly lower temp.

Do it and don't look back.

I had three kids at home 10 years ago when I did it.  When I think of kids (mine and friends) getting ready for prom, weddings, name it, It worked like a charm.

You get out of life what you put into it......minus taxes.


Edit - I wouldn't use electric to heat water.

Edited 5/17/2005 4:40 pm ET by Marv

You get out of life what you put into it......minus taxes.


(post #98296, reply #5 of 6)

When I started doodling the bypasses out, it occurred that the "idle" tank probably ought to be "in front of" the existing one.  That keeps water circulating through it (less grunge to worry about), and lets the water warm to ambient while "waiting" on the second tank.

What I can't quite decide is how to run the controls.  I suppose you could just have a switch labled "relatives" someplace on the 220 feeder.  I'd set the temps exactly the same on both units, though, if only to prevent any confusion (don't want to set either one to 'scald' by accident).

The "idle" question is one to ponder.  My first thought was to use a tankless or u/c in/near the guest quarters.  My next thought was to connect the tankless to the vent switch; but lighting would be better.  Non-guest use is likely clean-up/prep and will want hot water anyway.

Occupational hazard of my occupation not being around (sorry Bubba)
I may not be able to help you Occupational hazard of my occupation not being around (sorry Bubba)

(post #98296, reply #6 of 6)

Yeah, it can be done and is done with some frequency. You can go either "series" or "parallel". With parallel it's best if the two heaters feed different bathrooms, vs being ganged together, so that you don't have problems with balance. With two identical units you can get pretty good balance, but not with units that are so different.

The advantage of parallel is that you get a slightly higher flow rate, but you're probably limited by service entrance pipe anyway.

Series is more common, probably. The electric can be ahead of or behind the gas unit, depending. The electric first will allow you to turn off the electric (perhaps on a timer) when not needed. Having the electric second, though, gives you more of a "set it and forget it" setup.

With the electric first (and assuming that electric is more expensive than gas where you are), set the electric unit at it's lowest temp, and just use it as a tempering tank.

With the electric second, set both units at about the same temp. The electric unit will rarely run in this case, just serving as a "holding" tank, and will really only kick in when the gas unit can't quite keep up. If you get an electric unit with high-efficiency insulation (the only thing that affects basic efficiency on an electric) then standby losses will be minimal.

If you really want high-volume capacity, the electric unit can be rigged so that both elements can come on at once (generally on two separate breakers), essentially doubling the max recovery rate. (Note that this requires special thermostats.)

In any event, design a set of bypass valves so that either unit can be isolated for service, but keep water flowing through both most of the time.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville