I currently have a tankless, natural gas hot water heater on the side of my house. I want to preheat the incoming water using a solar water heater. Is there any concern with having the input supply water being "too hot"? The input supply coming off of the solar heater would be variable in the range of 90 - 130 deg F.

I looked in the Rinnai specifications for my gas hot water heater for such a value, but there doesn't seem to be one. I assume this means it's OK to do.

  • Doubt if there would be anything on the inlet side. Would be more concern how the outlet works.
    – crip659
    Jun 15, 2022 at 19:41
  • Where will your solar tank be? I don't think "solar tankless" is workable, I see several huge problems with that. Jun 15, 2022 at 23:03
  • @Harper-ReinstateUkraine solar preheat can be a thermal store with a heat exchanger - so there is a tank, but it's part of a closed loop. How this fits with definitions of "tankless", I don't know, because I'm used to different terminology
    – Chris H
    Jun 16, 2022 at 7:40
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    There is almost certainly a maximum temperature limit for the incoming water, but it's probably not lower than the maximum possible output temperature for the unit since changing materials between the cold and hot sides would be highly likely to develop leakage problems far more quickly with only a small cost savings.
    – Perkins
    Jun 17, 2022 at 19:36

4 Answers 4


A Rinnai tankless gas water heater does indeed have a temperature sensor and regulates the outgoing hot water temp. based on incoming water temp. So the burner in the Rinnai has the ability (and the brains) to ramp up to accommodate different supply water temperatures and will maintain constant (set-able) out flowing hot water temp. Unless the incoming water supply is hotter than the outgoing hot water setting. In this case a mixing valve is required otherwise there is a chance for scalding at showers, sinks, etc.

We use Rinnai tankless heaters in this same scenario (especially for off grid homes). We pre-heat the water with a solar water heater setup (with the solar heater's heating element power turned off). The outgoing hot water from the solar heater feeds the supply to the Rinnai tankless. This setup can serve both domestic water supply and supply hot water for radiant floor heating.


The tankless doesn't have a valve to block flow. So the pre-hot water will roll right through it. It'll add heat if it's under-temperature.

This means the water could be scalding!

Tankless heaters are not heat pumps, it's not going to go into reverse cycle and chill the overly hot input water.

This is where complacency gets you. One might think "Since tankless never heat water above 120F, I don't need 'Delta style' thermostatic faucets that control spigot temperature". Surprise. Your solar hot water can and quite probably is scalding temperature. I think 130F is optimistic (i.e. some days it will be higher).

So you will need to fit thermostatic mixing valves (i.e. your basic Delta joystick style faucet) at all your water outlets.

  • @manassehkatz-Moving2Codidact -- most tankless heaters modulate to a constant output temperature (they just slam into the "add 0 heat" stop if the incoming feed's hotter than the setpoint) Jun 16, 2022 at 0:17
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    @manassehkatz-Moving2Codidact -- my understanding is that it'll heat "just a little" if you have it set to 110 and are feeding it 100 deg water (for instance), otherwise you'd have serious problems with people in Phoenix getting scalded in the summer Jun 16, 2022 at 4:25
  • please note that thermostatic valves do exist, to avoid scalding. I have a setup in my home where such a valve has three inputs: solar, hot from boiler, cold from supply; the valve mixes the three supplies to keep a constant output temperature, which I can set. Jun 16, 2022 at 14:55
  • Are there mixers that would be suitable to put in the line before the heater, covering all faucets at once?
    – erickson
    Jun 16, 2022 at 17:30
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    @erickson it would have to be after the heater, but yeah... they're available. But then you end up "putting a GFCI on a GFCI" as it were, since many/most modern faucets are thermostatic mixing valves. E.G. any modern shower faucet. Jun 16, 2022 at 18:23

Since no one else has addressed the question at hand, I will share my thoughts about the equipment aspect:

Is there any concern with having the input supply water being "too hot"? The input supply coming off of the solar heater would be variable in the range of 90 - 130 deg F.

In general, water supply plumbing and equipment is temperature agnostic. It would be silly if a manufacturer used subpar components on the inlet side compared to the outlet side. Think about the instant-hots that people install under their sink; they heat the cold line water until hot water from the tank can make its way to the fixture.

If your plumbing is PEX then just make sure to keep the water temp under 180°F.

Realistically, you should contact the manufacturer and confirm this concern with their specific piece of equipment.

In one PDF I was able to find the flow-rate curve based on inlet water temp but it only addresses 70°F or lower since that would be the major factor in figuring out if you will be able to get enough GPM to suit your needs.

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    "It would be silly if a manufacturer used subpar components on the inlet side compared to the outlet side." While I completely agree, that doesn't stop all companies from doing it. Just most.
    – Mast
    Jun 17, 2022 at 18:35
  • @Mast I never claimed that this company nor any company is immune from silliness =)
    – MonkeyZeus
    Jun 17, 2022 at 18:38

Depending on your solar heat efficiency and temperature setting on the tankless heater, it might not turn on at all.

There can be a scalding problem with solar heaters. If there is no water flow for a while the temperature will rise quite a bit. So the first few minutes the temperature could be scalding, but it drops fast.

Just make sure you have enough flow and pressure coming from the solar heater.

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