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

Question

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.

Harper - Reinstate Monica · Accepted 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.

jwh20 · Answer

Yes you can do that but it will be highly inefficient.  A single larger tank is more efficient than two smaller ones with the same total capacity.  This is because they have more surface area between them for heat to be lost from.
So install a 50 gal. tank.  You might consider an on-demand/tankless heater for your tub needs.  That way you will have all the hot water you want when you fill it but won't be paying for holding a large quantity of hot water on the chance you might use it.

ThreePhaseEel · Answer

If you're willing to throw some money at this problem...
As it turns out, there is a heat pump water heater that can come close to fitting in the existing space available for a tank, while putting out the necessary quantities of hot water; however, it costs a few times as much as a typical heat pump water heater does.  If that's not a deal-killer, though, one can solve this without having to juggle two water heaters around, as the 43-gallon Sanden SANCO2 water heater can put up an 80+ gallon First Hour Rating using only a 43 gallon tank that's only about 8" taller and 1" fatter than your existing "low-boy" 38 gallon unit.  This performance exceeds that of basically every hybrid heat pump water heater on the market without needing to burn power in piggy backup electric elements.
The prime downside, other than cost, is that you need some extra plumbing and wiring, as the way the Sanden units work is by piping cold water from the tank into an outdoor unit, similar to that found on a mini-split.  This outdoor unit heats the water and pumps it back into the tank, where it's stored for use; said outdoor unit can be placed in an unconditioned space, as one would do with a unitary/hybrid heat pump water heater, or it can go outdoors outright with the addition of heat tape and/or drain back valves to protect the system from freezing in climates where that's an issue.
Furthermore, the Sanden units use stainless steel tanks, which means they have a warranty that's easily comparable to the best warranties available on traditional hot water heaters, provided chloride concentrations in your tap water aren't excessive.  This also means they don't have anodes to change, and aren't susceptible to problems with conductivity in softened water, unlike glass-lined tanks which rely on their sacrificial anodes for protection.  As to fitting, note that the Sanden tank is side-plumbed, which saves height that's otherwise taken up by having flex piping come out the top of the tank; you'll probably have to put the new tank a bit further forward in the space it occupies than the old tank was placed, but I suspect that's a reasonable compromise for having a hot water heater that can deliver the goods, as a hybrid heat pump water heater, especially one in the 50 gallon range, is going to struggle to deliver the quantity of hot water required in your house without basically being an electric heater, nearly defeating the point of the heat pump on top.

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

3 Answers

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

Convert all water valves; raise heater temp.

A large-tank “heat pump” water heater

On-demand hot water

If you're willing to throw some money at this problem...

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