Can a typical tankless water heater be set to very low temperatures like 70°F/20°C?

Question

Hear me out: I want to put a water heater on the cold water supply to my shower.

I have a thermostatic valve on my master bath shower, which causes a frustrating issue in the winter - my tankless heater can't supply enough hot water to mix with the nearly-freezing cold water to allow me to shower at a reasonable temperature with good water pressure. During the summer, when the cold water supply is more like 80°F/25°C, my showers are fantastic.

My idea to fix this is to warm up the cold water supply to my shower to around 70-80°F in the winter with a small electric tankless heater.

Would an inexpensive tankless water heater allow me to set a temperature this low? I've looked at a few online, but haven't seen any mention a minimum temperature setting.

I've also considered installing a stilling tank for the shower's cold water supply within the climate-controlled envelope of my house, but it seems like that would be just as much of a hassle, not much cheaper than a small tankless heater, and leave me with a limited supply of warm-cold water for showers.

Are there other options that I should be considering?

Answer 1

To answer the title question: No.

From the Rinnai manual:

• RE199i, RE180i, RE199e, RE180e - temperature setting 98 F - 140 F
• RE160i, RE140i, RE160e, RE140e - temperature setting 120 F - 140 F

Searching a bit more, found some Rheem gas 100 F 140 F and a Titan electric 105 F - 120 F.

And now I found Rheem electric 18 kW 3.51 GPM which according to the manual can be set between 80 F and 140 F.

I suspect that electric actually gives finer control here than gas because it can run at a low level and/or be modulated (similar to dimming a light fixture) which is not so easy to do with gas.

80 F is the lowest I found. Is 70 F possible? Yes. But since the primary use case is hot water, that isn't going to be a terribly popular option.

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And now back to my regularly scheduled tankless rant commentary.

Let's see if I can understand this:

• Tankless water heater is unable to produce hot enough water. Meaning, it is undersized for the intended usage.
• Rather than look at a different solution, just go ahead and add another tankless water heater.

That doesn't make much sense to me. If "plan a" doesn't work, why not try "plan b" instead?

In fact, "plan b" is the older, proven, technology - tank water heater. With one exception - extreme space constraints - it actually works very, very well.

Tanked and tankless water heaters do the same thing: heat up water. Tankless has two possible advantages:

• Unlimited hot water, as long as you stay within the limits (flow rate and temperature combination).
• No wasted energy due to loss of heat from already heated water.

The first one has already turned out to not be correct in your particular installation. That may be due to poor specification - i.e., not planning for the flow rate actually desired. Or it may be due to serious limits - particularly if this is an electric tankless heater. Electric tankless water heating uses HUGE amounts of electricity - often equivalent to everything else in the home put together, sometimes 2 - 3 times as much as everything else in the home put together. Really. Adding more electric tankless may not even be an option if your service is already maxed out, and that may be the reason why you have too small a tankless heater for your needs. Or you may have natural gas tankless and a little more capacity (maybe) to add an additional tankless heater.

The second reason is actually highly overstated, particularly by vendors of tankless systems. Touch the outside of a modern (last few decades) tank water heater (electric or gas, doesn't matter) anywhere except for at the hot water pipe, the exhaust duct (for a natural gas heater) or near the burner (for a natural gas heater). It doesn't feel warm, does it? That's because these tanks are extremely well insulated. Is some heat lost over time - of course. Can't escape thermodynamics. But very little. And in the winter it is 100% OK because the heat goes into the rest of your house. It is only a problem (a very minor problem) in the summer.

So the tankless advantages really aren't, at least not in your setup.

A tank water heater takes a few square feet of space, but other than that installs very similar to a tankless water heater. In fact, if it is electric then the installation is much easier because it only needs (typically) a single 30A circuit rather than as much as 3 x 40A circuits. A tank can typically hold 40 to 50 gallons. As you get larger it gets complicated due to government rules designed to push energy efficiency. But the basic tanks are extremely simple and reliable. They can also provide a considerable amount of hot water. Looking things up, I quickly found a 50 gallon electric heater that is rated at 61 gallons in the first hour and 21 gallons per hour recovery with a 90 F rise (e.g., 40 F in, 130 F out).

Answer 2

The part you need is called a mixing valve.

The other answer is correct, but he OPs question is how to warm up the cold water supply.

If you can't find a tankless heater with low enough temperature setting, all you have to do is get a mixing valve. They are normally installed along tankless water heaters in combi oil boilers. With that valve, you can mix the cold and new heater supply to any temperature you want (in between the cold and hot, obviously), then send that mix to your shower to be mixed again using the shower valve.

I would still do as @manassehkatz suggests, but I wanted to add an answer to your actual question.

Answer 3

Are there other options that I should be considering?

Another option you could look at is a waste water heat recovery system. This uses the heat from the water running down the drain to preaheat the cold water on it's way to the shower. They are usually completely passive - just a heat exchanger, no need for electrical power or anything like that.

There are two types - horizontal and vertical. The horizontal ones aren't as good. The vertical ones only really work if you can install them in the room below the shower. They are small enough to go inside a wall.

In a well constructed modern home, showering contributes a good chunk of gas consumption, so they can be a good way to cut bills and cut carbon emissions. As such they go into a lot of new-build homes in europe, and are readily available. As you're in the USA, where carbon emissions are not such a hot topic, I don't know how easy they will be to get over there.

Answer 4

Replace the main water heater. Obviously, it is not powerful enough for the job. I assume this is a gas fired heater, so you need a professional qualified for working with gas lines. You can also consider installing a closed chamber heater that takes air from outside and doesn't create dangerous carbon monoxide while you are at it, they are much safer.

There is also a possibility of thermostatic valve needing cleaning or replacement. The thermostatic valves are designed to fail safe, eg. in case of problems they shoot cold water, not scorching hot one.

If you decide to DIY this yourself regardless, don't put the second heater on the cold water pipe.
Put it on the hot water one. If the electric heater has its own thermostat, put it behind the main heater, so it just warms up the water to desired temp if main heater cant. If its just a dumb heating element, put it before your main heater (I assume your main heater has a thermostat). It will bring the temp to required value and not more.
Don't put it on cold water pipe. Firstly, that pipe probably supplies whole house, and you don't want the cold faucets across house shoot hot water. Also, if both pipes are fully hot, the thermostatic shower head may get confused and you get burned.