Dual Water Heaters - parallel or series

Parallel or Series both have advantages and disadvantages. Series will cause the water heater with the city supply water to constantly work and work hard while the second water heater in series will rarely come on since its being feed hot water all the time. It’s advantage is a constant supply of hot water however, be prepared to replace the first in line water heater often.
If you decide to parallel the water heaters each will receive equal amounts of city/well supply water to both tanks and both share equal loads “if perfectly piped/installed correctly”. There should be plenty of water for everyone depending on the size of the tanks “let say” two 50 gallons, however, the water supply is dependent upon personal demand, which may run out. An oil and/or gas water heater would recover quickly compared to an electric water heater. I don’t recommend either route - On-demand tank-less / instantaneous should be considered. In most instances - 184/205 gallons per minute should suffice. Location is key. If possible, center the WH between equal distances or between plumbing fixtures.

In my experience, there does not appear to be any significant effect on the lifespan of the lead water heater in a series installation. Corrosion of the tank is the main factor in water heater aging and both tanks remain filled with hot water. The gas control valve, being the only moving part hardly contributes to wear. Calendar time, not run time appears to be the primary aging factor. If you have statistical data or a manufacturer statement that says otherwise, I would be interested to see them.

Also, you can’t just stick a tankless water heater in place of any tank type water heater. The typical tankless water heater has five times the peak fuel demand of a tank type water heater (i.e., 200,000 BTU vs 40,000 BTU). One must address the adequacy of fuel supply as part of any conversion plan.

1 Like

It would seem in parallel if one shot craps you still got one,

While in series if one went down, started leaking, etc you may have to shut off both ??

Yes, but you need 4 valves. I rarely, as in almost never, see more than 1. With 4 valves, you can isolate and even change one while never disrupting the other.

Chuck …

Like you said, I saw isolation valves on a series connection about 11 yrs ago

Wouldn’t 4 isolation valves potentially create a hazardous condition? Yes, there are advantages for install / replacement, but if Joe Homeowner fiddled with them and cut off the valves - best case scenario is the TPRV would blow. Worse case is the TPRV would fail and you’d have a rocket that could launch through the floor, ceiling, and roof.

I thought “best practice” is just one cutoff - on the inlet side of the water heater (or the 1st heater if in series)

Ask the fool who posted right before you… … …

1 Like