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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?

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    What kind of tankless is this that can't keep up?
    – KMJ
    Commented Sep 15, 2023 at 3:10
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    I don't think it's an issue with the existing tankless heater itself - we can have two showers running off of it at the same time without problem, and it supplies a ton of hot water to our tub. I think the issue is more the inlet capacity on the hot side of our shower valve. Commented Sep 15, 2023 at 3:51
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    do you know the model of the thermostatic valve so I can look up if it is manually adjustable to provide more hot water
    – Traveler
    Commented Sep 15, 2023 at 5:06
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    It's probably going to be cheaper/better to fix your shower valve then. A shower valve is, what, $300 if it needs to be replaced? Meanwhile just doing the plumbing for an additional stage of tankless is likely to cost you that.
    – KMJ
    Commented Sep 15, 2023 at 5:31
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    IIRC any system design that is meant to store (or even keep in pipes) semi-heated water should be done with the risk of legionella in mind... Commented Sep 16, 2023 at 22:45

4 Answers 4

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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.


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).

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    The existing tankless heater only serves our upstairs (master bath and kid's bathroom), and is natural gas. I'll take a look at its capacity tomorrow, but I suspect the issue is more with the inlet capacity at the shower valve than the capacity of the heater itself. I can still get hot water from other faucets served by that heater when my shower is sad. If that's actually the case, I guess it may be possible to just replace the valve on the shower with a higher-capacity alternative, but the shower system is pretty integrated and that seems like it would be difficult. Commented Sep 15, 2023 at 3:59
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    I've got a second tankless for the downstairs Commented Sep 15, 2023 at 4:01
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    @DrewShafer tankless is based on capacity for a certain temperature rise and GPM. Are you sure it has the capacity when your water is cold to keep up with heating?
    – KMJ
    Commented Sep 15, 2023 at 5:32
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    And tankless systems have an altitude de-rate, so won’t perform as well in Denver as Boston. Have you bumped the output temperature up as high as it can go?
    – Jon Custer
    Commented Sep 15, 2023 at 14:21
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    Well, you've talked me out of this particular harebrained idea. Thanks for the advice! Commented Sep 15, 2023 at 22:25
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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.

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    Another name for the same part is a "tempering valve" The tank-heated hot water has to get to a certain minimum temperature to kill off contaminants etc. This whole question feels like the classic XY problem.
    – Criggie
    Commented Sep 17, 2023 at 23:52
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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.

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    Carbon emissions are a very hot topic in the US. In fact, that's the thing - they are such a hot political topic that (for better or worse) little actually gets done outside of California. Commented Sep 15, 2023 at 14:08
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    These can be so effective that in some commercial settings (dorms, locker rooms, etc.) it is entirely worth pumping the drain water up to run down through the heat exchanger where there isn't space below for the totally passive version.
    – Ecnerwal
    Commented Sep 15, 2023 at 15:24
  • In the southeast U.S., at the moment, the cost to buy the vertical exchanger and retrofit it into a single stack is about $3k, then another $1.5k for each additional stack. (Source, personal research for recent renovation.)
    – bishop
    Commented Sep 15, 2023 at 15:29
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    @bishop they much cheaper here, and gas/electricity is more expensive. But they still take 3-5 years to pay back on a new build, more for a retrofit. They mostly get fitted to meet government mandated efficiency targets. For the OP, I assume they would be much cheaper than a new boiler (which OP suggested). I know the USA has cheap water heaters that wouldn't be close to legal over here, so maybe there isn't as much price difference as I first thought. Probably not the cheapest option still, but sometimes a combination of reasonable price and an environmental benefit wins.
    – Jack B
    Commented Sep 15, 2023 at 17:58
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    @PCLuddite Maybe hot topic was the wrong phrase. I concede there's lot of talking going on, just not much doing. I live in the UK, annual per capita carbon emissions are 5T, down 50% from the mid 90s. The USA is 3x that and down only 25% from the mid 90s. There clearly isn't the kind of drive that would make WWHRS readily available at reasonable prices, which is what I alluded to in my answer. This is a US-centric stack, so top answer here is to install a direct-electric water heater - suggest that here and anyone who understands plumbing would look at you like you'd just kicked a puppy...
    – Jack B
    Commented Sep 17, 2023 at 12:41
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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.

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