A plumber recently told me that its perfectly common to keep the old 40 gallon tank water heater in series with a new tankless water heater.

The motivation is to decrease the time it takes to get hot water to a faucet. And also to then have endless hot water once it gets there.

As I understand it, the tankless pumps hot water into the cold water inlet of the tank heater, and that in turn pumps out to the house via its hot outlet. The hot water either waits in the tank (which adds heat to keep temp constant) or passes through if a faucet is.

Heating happens in the tankless unit, and is maintained in the tank unit until use.

Does this make sense to do?

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    When you state “task” heater I think of a small 5 gallon or less at the point of use in this case yes it is common to keep them for instant hot water especially when at the far end of a house. If full sized located close to the tankless it would defeat the purpose of a tankless.
    – Ed Beal
    Commented Dec 27, 2020 at 20:47
  • If you are going with a tankless, is there some problem you anticipate that this set up will address?
    – Alaska Man
    Commented Dec 27, 2020 at 21:02
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    I edited all "thankess" to "tankless". We do routine edits like this all the time... it's a tankless job, but I don't mind :) Commented Dec 27, 2020 at 21:35
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    I am very tankful for your help.
    – Misha AM
    Commented Dec 27, 2020 at 21:39
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    Isn't the whole purpose of a tankless heater is to do away with a holding tank? A holding tank that needs to be re-heated?
    – ojait
    Commented Dec 27, 2020 at 23:28

3 Answers 3


The question is

Does this make sense to do?

My answer to that is, Based on how you worded your scenario, This does not make sense to me. I can not wrap my head around how it would be in parallel. What you have described is two units in series. (one after the other)

A tankless water heater is an on demand unit. It only fires up and heats water when a hot water faucet is opened.

If they were in series:

If the "tank", installed after the tankless, is not heating water then all the water in it will be cold. (if it is heating water then what is the point of having the tankless)

When you open the hot water faucet the tankless will fire up and start sending hot water to the faucet but it will have to push all of the cold water in the tank out through the faucet BEFORE any HOT water gets to the faucet.

  • Thank you. I made edits based on your answer.
    – Misha AM
    Commented Dec 27, 2020 at 20:49
  • Thanks but it does not clear anything up for me. Why have a tankless feeding a tank?
    – Alaska Man
    Commented Dec 27, 2020 at 20:51
  • "Heating happens in the thankless one, and maintained in the tank one until use." This defeats the purpose of having a tankless.
    – Alaska Man
    Commented Dec 27, 2020 at 20:52
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    I suppose in theory you could do it the other way around. Tank heater first set at a low temp, sending that warmed water to the tankless so the tankless does not have to work that hard. But that is still a needless use of energy, which is, usually, why you go with a tankless in the first place.
    – Alaska Man
    Commented Dec 27, 2020 at 21:00
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    Thank you. I was reading more about this. Maybe what I need is a recirculating pump and faucet valve? Pump that can be activated on a schedule?
    – Misha AM
    Commented Dec 27, 2020 at 21:15

Your core concern is the amount of time it takes between opening the hot faucet and getting water that is actually hot. This plan will do nothing for that.

The delay is caused by the volume of water (inventory) that is in the pipe in between the water heater and the faucet. That pipe is exposed to ambient? Outside? air, and has cooled off. That water must be physically pushed out of the pipe by hot water. It's like a big plunger.

You can figure out the inventory of water in the pipe by taking its interior cross section x its length. For instance 50' of 1/2" pipe* contains 0.57 gallons of (now cold) water. Going out a 1.5 GPM low flow showerhead, it will take 23 seconds for that to vamonos.

The cure is to make the pipe routing more direct, smallify it, or move the heater closer to the faucet. Each step down in pipe size reduces pipe volume by about half (and thus wait time) - however, too small and that restricts flow too much.

Restricting flow can actually help tanklesses, because a major customer-satisfaction issue with tankless is when the customer draws water faster than the tankless is able to heat it satisfactorily, so they get tepid water - and the customer does not understand the technology, so they refuse to do the one thing that'll fix it -- to reduce flow rate. So they reach impasse and blame it on the tankless.

Note that a tankless->tank arrangement will only make this problem worse, because the tanked heater will assure the first 40 gallons is OK, then will put a 40-gallon "delay" between action and consequence. So as hard as it is already for customers to connect "reducing flow" to "increasing temperature", this "40 gallon delay" will be far more confusing. I would expect bizarre customer-satisfaction issues with this.

Putting the tankless after the tank would mean you can ignore flow rate issues until the tank empties, as the tankless will not operate at all. Once the tank goes tepid, the tankless will be doing "all the heavy lifting", and then flow rate will affect heat. Again, very complicated, you shouldn't have to take a training class to understand your water heater.

* 1/2" squared x pi/4 = 0.2 square inches or 0.00137 square feet.... x 50' = 0.06 square feet.... x 8.33 gal/sq.ft. = 0.569 gal.


There are tankless heaters designed to operate in conjunction with a storage tank, but the old tank heater would not work for this use. And a basic standard tankless water heater does not have the controls to work in conjunction with a storage tank.

There are published designs for using an electric tank water heater as a storage tank in conjunction with a tankless heater, but they require several extra valves and controls.

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