We are doing a master bathroom renovation and are considering adding a tankless hot water heater to supply just that bathroom. Our current 50 gallon hot water tank runs out after about 2.5 consecutive showers. The problem is we are on oil so we're limited to electric tankless water heaters which have limited GPM.

We will be moving our laundry into the master bathroom (there is already a supply line and 240V connection), we'll have a freestanding tub and a shower. I may be overthinking this and the laundry may be able to run on cold water alone (verifying with the manufacturer), but electric hot water heaters are pretty limited and it's unlikely we'd even be able to run the shower and tub at the same time from what I understand.

I'm wondering if there's a way for the on tankless heater to leverage hot water from the tank when it's unable to meet the GPM demands.

  • Are there any models that can take hot water effectively as a pass-through to augment hot water demands?
  • Alternatively could we feed the tankless heater into the hot water supply line?
  • Is there anything like a thermostatic mixing valve (but the opposite) that can use water from a secondary supply line if the main supply line (in this case the tankless heater) falls below a certain temperature?
  • One possible approach (not what you're asking about doing) is to raise the water temperature, and then install a tempering valve to get it back down to a non-scalding temperature - since that mixes "too hot" and "cold" water to make "hot-enough" water, you get more "hot-enough" water from a fixed volume heater. Another "trick" is a shower-drain heat exchanger to warm the cold water line to the shower from the water going down the drain.
    – Ecnerwal
    Dec 1, 2020 at 14:35
  • Yeah I think we will want those tempering valves either way, but we have a 1 year old and eventually plan to have one more so this means we will need to install these everywhere which may end up being more expensive and less efficient.
    – Jordan
    Dec 1, 2020 at 14:40
  • As I have been mentioning a lot lately watch out for internet only electricians. There are quite a few here some are quite long winded and just today one of them gave dangerous advice that violated code because he had done it in the past. So be careful!
    – Ed Beal
    Dec 1, 2020 at 21:29
  • 1
    @Jordan, keep in mind that you don't have to put mixing valves at the point of use. Another common arrangement is to put one large one right after the water heater, simply to extend the runtime. So e.g. your tank stores water at 160F, and immediately when it exits the tank, that water is mixed with cold down to 120F, and then distributed throughout your house. You'll get more 120F water this way than if you set the tank at 120 directly, with no electricity required.
    – Nate S.
    Dec 1, 2020 at 23:05

3 Answers 3


Do the simple thing ..

The simplest thing is to add more electric-fuel tank for heating and storing water. You'd do a bit of math to figure out how many gallons you want to be able to consume in a window of time and how much recovery time you'll allow for the heaters to come back up to temperature before the next demand cycle hits. With that one can estimate whether it's even possible to get there from where you're at, and if so, how many tank water heaters you should have in service.

.. or else engineer a solution

You can save a little space and energy by using an electric instant heater to supplement the hot water stored in the tank. You're going off the well-beaten path, though, so a bit of design (aka engineering) will be necessary. Like any custom design, you may find that the design parameters weren't quite right and the solution requires a bit of in-field adjustment.

Following is just one approach of many. This approach focuses on minimizing the load the master bath shower places on the tank heater. It assumes a plumbing topology in which the tank heater continues to directly feed all hot water consumers except the master bath shower. The master bath shower would be fed by the outlet of a tankless electric heater; the tankless heater is fed by a mixing valve; the mixing valve is fed from the tank heater and cold water.

  1. Figure out what is the hottest temperature you'll require at the master bath shower and how many GPM will be needed.
  2. Look at the electric instant water heater's data sheet; find its temperature rise graph. Determine how many degrees temperature rise the heater can provide at the GPM selected in step 1.
  3. Subtract the temperature rise of step 2 from the target water temperature of step 1. This is the required input temperature at the tankless heater.
  4. Adjust the mixing valve so that its output temperature is equal to or greater than the input temperature found in step 3.

This arrangement balances two goals:

  1. provide more water (GPM) at the target temperature than what the tankless heater could do on its own
  2. use the tankless heater to truly supplement or extend the runtime of what the tank heater could sustain on its own

I'm assuming your tanked heater is set to 140F because that is CDC guidance on preventing legionella and other bacteria from growing in the heater. Also, raising storage temp is the first thing you do when trying to extend runtimes.

But that increases risk of scalding which calls for thermostatic faucets. Turning the heat down to protect babies from scalding will shorten shower runtime, but it will also raise legionella, and babies don't like that either.

How it's going to work

OK, so your tanked heater gives you 4 phases of hot water:

  • phase 1, AMBIENT (say 70F). The supply pipe had hot water in it some hours ago, but it has cooled down to ambient temp of whatever you heat the building. This "plug" of cool water must be pushed out of the pipe before hot water will arrive.
  • phase 2, HOT (say 140). Now the water heater is working as-designed. This is the part of the cycle you actually use.
  • phase 3, TEPID (say 90). the water heater is near exhaustion, and the remaining hot water is mixing with new cold water being introduced from the street/well.
  • phase 4, AMBIENT (say 70). It's cold 50F water from the street, with the water heater putting its available 23A into heating it, which gives about the same rise as the tankless, giving 70F. Too little.

So let's put a tankless heater downline of this. Let's say at your flow rates, the tankless is big enough to give us a 20 degree rise, and has a shutoff at 110F because with tankless, there's no worry of legionella and other bacteria. So what do we get?

  • In phase 1, the tankless warms the "ambient" water to "tepid". :b
  • in phase 2, the tankless shuts off entirely. The incoming water is above its set point.
  • in phase 3, the tankless warms the tepid water to a useful 120F, but this isn't for very long. And it's useless anyway, since the mix is getting colder and colder, and you're constantly having to adjust the manual knobs. This is no better than it was before.
  • In phase 4, the tankless is back to "tepid".

So as you can see, the tankless contributes nothing except adding a few seconds in phase 3, which is a fairly short phase as the tanked heater runs out.

Not what you were expecting

You are imagining that you can extend the tanked heater's time by mixing its 140F water with 45F street water, giving 85F water which you deliver to the tankless. Then, the tankless raises it the last 25F, giving you 110F water at the tap. Sure, that plan will give you hot water for a lot longer time. But you'd have to install special equipment to do exactly that.

Now you are reluctant to simply install mixing valves at your various points of use, so I have to wonder what you are expecting to accomplish on such a low budget.

Keep in mind the way tanked heaters work

A tanked heater is designed to have the absolute minimum possible mixing in the tank. As the tank is used, hot water moves upward in the tank "like a piston" with cold filling the bottom of the tank. That's how it works through the long phase 2.

After most of the hot water is used up, the "piston" of hot water is too thin to remain cohesive, and it starts to inter-mix with the cold. That's when you enter phase 3, the flow turns tepid, and gets colder as the layers mix more.

You shut off usage since tepid water does not satisfy. And then over the next 30-60 minutes depending on sizing, the heating elements go to work to raise the tank of water to target temperature so you can start the process again.

That means putting a tankless on the input of a tanked water heater is also of little value. The tanked heater is doing its best to keep hot from cold separated; changing the "cold" to "cool" does nothing in phase 2 and very little in phase 3.

The right way to use tankless

To make tankless work for Americans, you either need a really, really big tankless coming off 400A service... or you need to combine 2 things:

  • A low-flow shower head or other restriction so people cannot out-draw the supply, and
  • A tankless heater that is large enough for the planned flow, yet supportable with available electric service.

That is why 30A "electric showers" work in the UK; they restrict flow to where the 30A heater can keep up, and they do it all in a single package.

If you prefer your old legacy high-flow showers and faucets, the consequence of your choice is that tankless will not work for you unless you can greatly upsize your service. Your better plan might be a second tanked heater. But the thermostatic valve situation still needs to be sorted out.

Tankless doesn't store hot water so it doesn't have a legionella problem.

  • I love the way you make stuff yup and use a lot of words to make it sound true. Point of fact I have added quite a few point of use units to extend water heater tank life and to reduce problems with a single tankless. being a professional electrician I have real life experience not made up as your examples.
    – Ed Beal
    Dec 1, 2020 at 21:26
  • Is there some ongoing rivalry here? 😅 The answer did sound quite reasonable and detailed to me. I’m not looking to go cheap, but I’m not looking to unnecessarily spend money. I’d rather augment my water supply (even if it comes at a greater cost than a second water tank) if it’s more efficient and not prohibitively more expensive. Ideally I’m minimizing energy waste. Yes you could make a comment about long hot showers.. 🙃 I’m still working on being more like Wim Hof 😃
    – Jordan
    Dec 1, 2020 at 21:33
  • Just another add ground water temp of 70 shows you have very limited experience as in the us the average is much closer to 50 the extreme south yes it gets into the 70’s but the north is in the high 30 to low 40’s or an average of closer to 50 my particular area is low 50 but ranges from 50-54 but I know this from installing geothermal systems.
    – Ed Beal
    Dec 1, 2020 at 21:37
  • @EdBeal That's what I said, street 50F, with the tanked heater putting best effort in giving it about a 20 deg rise. yielding 70F. Dec 1, 2020 at 22:12
  • But an in-line point of use is not at ambient I read the 70 wrong but after your comment below my answer you definitely have not done this exact thing and I have several times.
    – Ed Beal
    Dec 1, 2020 at 22:26

I have had to add point of use electric on demand water heaters in quite a few homes. The advantage to these small flow units is they heat to the set point so if the water is cold coming in they go full blast if they are set above the incoming water they bring the temp up and extend the length of time the tanked water heater lasts, with this said unless you are using very low flow shower heads they are not usually enough for a hot shower by themselves , but I use them in my barn for example with cold water it warms the water enough to bathe the horses , these units take a 30 amp 240v service but can help and they don’t break the bank.

  • Yep this is why I’m looking to see if it’s possible to simply use them to augment the hot water supply to that bathroom. I should note from the basement we have a completely separate supply line for that bathroom that is easily accessible.
    – Jordan
    Dec 1, 2020 at 15:00
  • 1
    Make sure the one you get can be turned higher than your current set point. There are a couple of brands that only heat to 120 others 160f
    – Ed Beal
    Dec 1, 2020 at 15:09
  • I don't get the usefulness of the point of use heater. While the tanked heater is delivering good temps, the PoU will shut off entirely. Only when the tank is near exhaustion will the PoU kick on, and after the tepid water runs out, the PoU will be doing ALL the heating. Either it can keep up, or it can't. Dec 1, 2020 at 20:22
  • @harper It’s simple they run out by adding a point of use as you did not understand what I typed make sure the point can go above your current set point they will effectively bring the temp up a few degrees even with a full flow conserving some of the tank volume making more showers possible.
    – Ed Beal
    Dec 1, 2020 at 21:20
  • @EdBeal OK, so you're setting the set-point way high, like 170, and toasting it up from 140 to 160 and then they're cooling it back down at the mixing valve. I can see where that would indeed extend the tank range... but 160F is seriously dangerous so that mixing valve better be tip top. If you're starting from a lower tanked heater setpoint like 110 or 120, that would be safer but why not just turn that up to 140? Same effect, no tankless needed. Dec 1, 2020 at 22:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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