How can I mix tank and tankless hot water to feed my bathroom?

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.

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.

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