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 http://www.left-mind.com/home-projects/pics/old-hot-water-heater.jpg

  • 1
    I would recommend removing the word "Series" from the question and title since a lot of people will think you're trying to wire two heaters in series, which is a terrible idea and not what you're asking about (you want to wire them in parallel) – cat40 Jun 24 at 1:17
  • 1
    @ThreePhaseEel, We have a 200 amp panel. The existing hot water heater circuit has two 30 amp breakers with aluminum wires. A local electrician recommended the hybrid type when we asked about tankless. We are willing to pay more to get a water heater that provides the amount of hot water we want at a lower operating cost. We are not sure if we should get a 50 gallon or 80 gallon unit. . – BigBlonde Jun 24 at 3:36
  • 1
    @blacksmith37, We really want to use that bathtub. – BigBlonde Jun 24 at 3:37
  • 1
    @ThreePhaseEel, The space where the existing tank resides is under stairs, so the over-tank space is slanted. The tank is 33" high and the water pipes enter/exit the wall at 46" directly above the tank. The electrical box is a few inches above the pipes. The space is 35" wide. There isn't room for a larger tank. The small space is the reason we initially considered a tankless heater, because one of those would easily fit. Those use a lot of power, though, and our goal is to make the home energy efficient. – BigBlonde Jun 24 at 13:12
  • 1
    I appreciate all the helpful comments. Thinking about this discussion, we're leaning toward removing the old tank and extending the water and electrical to the new tank's location in the garage. I looked into making a junction for the electrical and understand that it can be done in compliance with code. The water pipes can also be extended. The floor of the garage is 16" lower than the floor of the house, so I'm wondering if that might be a concern. – BigBlonde Jun 24 at 13:24

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.

| improve this answer | |
  • 1
    Excellent information. We have decided against on-demand hot water. We could raise the temperature, though. It's not very hot now. In fact, we don't even mix cold water at all--ever. I will look into anti-scald valves. I'm sure we don't have any. I was only vaguely aware of them, but understand their purpose now. – BigBlonde Jun 23 at 19:48
  • The heat pump hot water heater will go into an insulated but unheated garage. I think it will be an excellent location most of the year. It gets very hot in there on sunny days, so a little cooling would be helpful. I suspect we'll need the heating elements during the winter. I read that 45 degrees is their minimum temperature. As far as the initial higher cost, we're okay with that. – BigBlonde Jun 23 at 19:57
  • @BigBlonde -- what physical size is your existing tank, by the way? (heightxdiameter) – ThreePhaseEel Jun 23 at 23:50
  • See my answer, above. Thanks. – BigBlonde Jun 24 at 13:41

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.

| improve this answer | |
  • That could be a problem. Code has outlawed electric water heaters that large, except for heat-pump models. – Harper - Reinstate Monica Jun 23 at 18:53
  • 1
    What do we mean by "large"? We can still buy 80-gallon, non-hybrid, electric water heaters where I live. That's an academic question, though. We are planning to install a hybrid unit. – BigBlonde Jun 23 at 19:43

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.

| improve this answer | |
  • It looks like an excellent water heater, but you're right, it's costly. If it made the job significantly easier, I would be interested, but it doesn't. With either this or an all-in-one hybrid heater, I would still have to run pipes and cables to a second location. If I were to go this route, I'm thinking that the heat pump could go in the same location as I intend to put the new tank, so maybe this would be a better use of the available space. Removing the old tank would leave a large amount of unused space behind the clothes dryer. All that said, an 80-gallon unit costs less than half. – BigBlonde Jun 25 at 12:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.