Will a tankless water heater be damaged by connecting to house hot water?

Question

I bought an EcoSmart POU 6T tankless water heater. Can I run this as a hot water booster by connecting it to the house hot water line without damaging the heater?

There is a similar answer here but I'm asking something different.

I know that this heater is too small by half for the application. I'm trying to make the best of a less-than-ideal situation. I originally purchased a larger unit - the EcoSmart ECO 11 - but had to change to the smaller unit after learning that my house service could not support the additional 60A load. So I'm using the smaller unit despite knowing that it won't do everything I want.

As others have asked, I'd like to hook the heater up to the house hot water line instead of the cold water line. What I'm hoping for is that when I turn on the faucet I'll get 85 degree water for 3 minutes instead of 60 degree water for 3 minutes; and once the hot water from the tank gets to the faucet I'll have 120 degree water and the heater will just let the hot water flow through.

EcoSmart says the unit is designed to be attached to the cold water line, and hooking it up to the hot water will damage the unit and void the warranty, which sounds like corporate CYA to me, but I'm cynical so I'm looking for a second opinion.

Will hooking up the heater to the hot water line damage the unit?

EDIT A well-regarded community member has called me out for nastiness when I called their useless non-answer a useless non-answer. I'm not sure how else I can respond.

I was very clear in my question that I know what I'm trying to do will be sub-optimal. I am not looking for answers telling me that what I am trying to do will be sub-optimal, because I already know that. If you want to tell me that what I'm trying to do will not work well, then please skip answering. If you can answer that I shouldn't do what I'm trying to do because it is risky, dangerous, or illegal, that's a useful answer.

Answer 1

Unfortunately some require that frame challenges be labeled. This is a frame challenge.

Yeah, the problem is that 85F tepid water sucks a lot more than you think in a shower. You won't enjoy it. But try it, set your main water heater down to 85F, empty it of hot water, let it recover to 85F and see how you enjoy it. You will perceive it as "it NEVER warms up".

Speaking of that, running it at 120F is a mistake, as that can breed legionella and other bacteria. Yes, this is at odds with avoiding scalding, so thermostatic valves need to be needed, easiest done at point of use.

What would really work is a satellite tanked heater near your shower. It doesn't need to be large, its supply will be replenished quite soon from the main tank and the slug of cold water will be diffused into it. I would go for a heater about 3x the number of gallons it flows before it gets warm.

Actually, since British electric showers are 8.5, 9.5 and 10.5kW in size, I would keep the 11kW, and find the space in the Load Calculation by using Load Shed on other loads (y'know, like a 4500W tank water heater, dryer, or other thermal storage load.) Harper mutters to himself something about Y-wires and NC magnetic reed switches needing 10 ampere-turns... nobody cares Harper... nobody cares...

Answer 2

As usual there is a temperature probe on the inlet, and a pressure sensor on the outlet. If it is smart it will adjust heating power depending on inlet temperature, but there will be a delay until the temperature probe registers the change in temperature so it could still heat the hot water for a moment and burn the user. Only way to know is to try it out!

I don't see anything that would be damaged by hot water, it's all metal...

...except the triac, which is cooled by the cold water pipe. At 30A it's going to dissipate about 30Watts which is quite a lot ; there is no way to calculate the temperature rise without more specs about the waterblock. So it may overheat and burn, or maybe not. Normally the heating should stop once the hot water arrives, so maybe not.

It's only 6kW, so if you want to heat the water by 25°F (60 to 85°F) or 15°C (15.5 to 30.5°C) you will have to restrict flow to 6000/(4180*15) = 0.1 l/s or 1.6 gallon/minute which is a bit wimpy.