Water – Is It Okay to Connect Two Water Heaters in a Series (so that water from one flows into the other)


Our project is to install a 50-gallon hybrid water heater. Our existing 38-gallon electric water heater is too small and we do not know how old it is. It cannot fill our very large bathtub.

The existing unit is located in a small space below a stairwell, behind our clothes dryer. There is not enough space to install a larger unit in that location. It is, however, near the garage where there is plenty of space (after a few modifications). So our idea is to install a hybrid heater there.

Instead of simply removing and replacing the existing unit, we are wondering about the feasibility of continuing to use it for additional capacity. Our thought is to connect the two heaters in a series with the old unit "downstream" from the hybrid heater.

We will add the hybrid heater regardless of whether we continue to use the old unit. We do not know how old the existing heater is, so maybe it would be best to simply remove it. We are just thinking that it probably would not use much electricity keeping already hot water hot, but it would significantly increase the overall capacity. Our bathtub is so large that we are not even sure if a 50-gallon heater will be adequate.

Existing Hot Water Heater

Best Answer

Not gonna work. You’re on electric, and you can’t just spam on more water heaters. You need to have the electric service to power them.

So let’s look at your options (other than, a smaller tub).

Make sure your heater is healthy; e.g. siphon pipe is intact.

A lot of water heaters get their water supply shortened when a siphon pipe rusts out. Have that checked, and also the sacrificial anode while they are in there.

Convert all water valves; raise heater temp.

This will only work if your heater temperature is currently set relatively low to avoid scalding people.

First, every hot-water faucet in the house must have an automatic, thermostatic anti-scald valve on it.

Second, you max out the temperature setting on the water heater’s thermostat. Raising the water temp will give you “more” hot water at the temperature you desire, since it will be blending less hot and more cold water.

What about scalding? There’s another factor on the table now. It turns out that temperatures which prevent scalding (<120F) also cause very nasty bacteria to breed inside water heaters, including legionella. As such, health officials recommend just what I said above - fit all thermostatic valves, then crank your water temp to 140F.

A large-tank “heat pump” water heater

All large electric water heaters these days are required to be heat-pump technology. This makes them a lot more expensive, but they also take a lot less power - so there’ll be no trouble powering it.

The gotcha is that there’s no free lunch; the heat has to come from somewhere. The utility space the heater is in will tend to get cold. That only assists the air conditioner in the summer; but in winter you’ll need more gas heat to help it. If outdoors, in the summer you get lots of cheap heat, but in the winter it may have trouble getting enough heat to function. Consult with a competent installer (not Home Depot).

On-demand hot water

This is really the huge winner, because it’s a small unit that easily will fit almost anywhere. It will let you run unlimited hot water, so filling the tub won’t be a problem. But it’ll require one of two things:

  • Conversion to gas. This is the best plan, because it’s easy to get plenty of heat. My gas unit makes as much heat as a 150-amp electric heater would.

  • Enlarged electric service to the building, since electric on-demand heaters will pull 90A or more.