Thread: Water heaters in series

I was in a home today with two 50 gallon hot water heaters in series (atmosphereic venting type). The hot water discharge of one hot water heater fed the cold water inlet of the other, the discharge of the 2nd heater went to the house. One hwh was off (at the gas valve), the other was turned down (vacant house). Has anyone seen a setup like this, and what does the plumbing code say about it? I have seen dual hot water heaters where the cold water feed T's and feeds both heaters with separate hot water discharges (as in one for the MBR and one for the rest of the house) but never two in series.

Thoughts?

Thanks, all.

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Originally Posted by Michael McElligott

I was in a home today with two 50 gallon hot water heaters in series (atmosphereic venting type). The hot water discharge of one hot water heater fed the cold water inlet of the other, the discharge of the 2nd heater went to the house. One hwh was off (at the gas valve), the other was turned down (vacant house). Has anyone seen a setup like this, and what does the plumbing code say about it? I have seen dual hot water heaters where the cold water feed T's and feeds both heaters with separate hot water discharges (as in one for the MBR and one for the rest of the house) but never two in series.

Thoughts?

Thanks, all.

If different size tanks, in-series is correct. Same size heaters, first heater will wear-out quicker than the other one.

Originally Posted by Bill Parrish

If different size tanks, in-series is correct. Same size heaters, first heater will wear-out quicker than the other one.

Yes and no. The first tank can be set low to preheat the water for the second tank, or it can be turned off. A tank full of water left sitting in the house will warm to room temperature, so less energy for tank #2 to heat the water. This is a good idea if you are on a deep well system. Google 'tempering water heater'.
Tank #1 can be turned on when a lot of consumption is expected, but just sitting there, it will last a long time, IMO.

See attached:

Parallel or series is a choice one needs to make.

True, the first heater may fail sooner than the second in a series setup, but there are advantages for the series setup in that it can provide more hot water for a longer period without running out because one is set to preheat the second.

If someone is actually concerned about one tank failing before the other, all one needs to do is plumb in a couple of valves and every six months (when the time changes to/from Daylight Savings Time) turn the valves - you have now swapped the first and second tanks, in 6 months turn the valves again, you have now swapped them back - that will take out the 'the first tank will fail early' disadvantage ... if one even considers it a disadvantage worth worrying about.