I have a 50 gallon hot water tank that is heated by natural gas. I've been thinking of changing over to an on-demand hot water system. Here's the problem. I live in Montana. It's winter. And I just measured the temperature of the incoming water from the main at 39°. Is there an easy way to simply preheat water using ambient air temperature in the basement before it goes into the hot water on demand unit?

I've read other posts that say you need to run it into a non-insulated large container that the water would be used to feed the on-demand tankless system. Are there any on demand systems that would easily bring that incoming 39° water to 110° or would the flow be reduced until the hot water is hot enough to be discharged. Or would the water just flow through the unit and come out warm instead of hot. I'm not sure of the internal mechanism of the on-demand systems.

Thanks in advance for your input

  • Normally when the outside feed is colder in Winter, you just turn the flow down a bit to compensate. On the simpler units, that's all the temperature control on the front does, adjusts flow.
    – Tetsujin
    Jan 6, 2023 at 12:15
  • Is the basement heated? If so, is it heated to 110°F? If not, how do you expect to get the incoming water heated to that temp? You can preheat the water by making a very large air-water heat exchanger. It could be as complex as flowing it through a number of car radiators (working in reverse), or as simple as running a lot of pipe through the heated basement. Either would give the water time to absorb heat from the basement air. However, you'd also be cooling the basement, which means you're heating water with your HVAC system instead of a water heater...
    – FreeMan
    Jan 6, 2023 at 12:31
  • @Tetsujin Don't modern units adjust the flow rate automatically because they prioritize temperature over flow rate?
    – MonkeyZeus
    Jan 6, 2023 at 15:54
  • @MonkeyZeus yes they adjust, see my answer
    – P2000
    Jan 6, 2023 at 16:01
  • 2
    Think long and hard about why you want to switch to an on-demand water heater. It costs more, has complex valves and electronics, has to be flushed regularly, might need a dedicated gas line, no electricity means no hot water. My 50-gallon serves my family of 4 perfectly. You should check if a lifestyle change is a more appropriate solution to running out of hot water.
    – MonkeyZeus
    Jan 6, 2023 at 16:07

5 Answers 5



Not unless you rig up some sort of continuous flow radiator system, anyway. If you're talking about an air-source heat pump, that would be odd--you don't normally run one heater off another (your home's furnace). If anything, you'd use outdoor air.

The problem is that water is many times more dense than air--a factor of hundreds. It would take a massive amount of airflow--or more time than you have available in your usage cycle--to increase the temperature of water even slightly. You've have to run hundreds of feet of pipe through your basement (or install finned piping) to give enough surface area, time, and volume to have an appreciable difference in temperature at the heater.

Use your existing heater as a low-temp preheater. The energy loss will be minimal.

Think about what you're actually proposing here. Your plan is really replacing your water heater with your home's furnace. The heat comes from somewhere. Why would you do that? Energy lost through the wall of the water heater contributes to the home's heat anyway. What's the point?

  • Actually, there ARE systems which replace the water heater with the home's furnace, though they don't replace the tank; it's called "indirect hot water". If you have a high-efficiency furnace (either gas or oil), this can be considerably more efficient than having a separate burner under the tank. Usually the tank operates as its own heating zone, often given priority over others because losing hot water is considerably less pleasant than temporarily having the house drop a degree.
    – keshlam
    Jan 6, 2023 at 22:34

Just do the math.

  • Incoming water temp: 39°
  • Basement temp: 70°
  • Storage tank capacity: 50 gallons
  • Storage tank temp: 70°
  • Desired on-demand temp: 110°

So if you take a 10 minute shower in the morning at 2.5 GPM (10 x 2.5 = 25 gallons) then your storage tank will meet halfway between your incoming and tank temp, 54.5°. Each minute of showering requires an additional and compounding 1.5° of heating from your on-demand system; 1 minute requires 1.5°, 2 minutes requires 3°, 3 minutes requires 4.5°, etc...

If you took a 20 minute shower then you would have depleted all your ambient temp water. Additional water use such as that for dishes and laundry will be heating water from 39° to 110°.

Can your 50-gallon storage tank effectively raise the water temp from 39° to 70° overnight using ambient air, I don't know how to perform that calculation.

The on-demand heater's temperature curve simply shows you what GPM the unit can produce based on the incoming temp so if you change the incoming temp then you'll get better GPM, simple as that.


With a gas heater, you won't need that.

The things are ridiculously powerful - 200,000 BTU/hr. It's difficult to imagine you needing that much hot water.

Let's crunch some numbers.

There's a lovely unit called the "BTU". Most heaters say they're rated in BTU, but actually the heaters mean "BTUs per hour".

A BTU will raise 1 pound of water 1 degree F. So let's think:

For math's sake, let's think about a flow rate of 1 gallon per minute. (A shower is 1.25 to 2.5 GPM). That is the same as 60 gallons per hour. Now, a gallon of water weighs 8.3 pounds so 1 GPM is 500 pounds of water per hour.

Now you want a rise in temperature from 39F to 110F. That is 71F rise. So... 500 pounds of water per hour x 71 F rise = 35,000 BTU per hour.

35,000 BTU is also around 10,000 watts if we're using electricity, or 42 amps.

Remember, this is all for 1 GPM.

If you don't know how many gallons per minute your shower or faucet uses, well, YOU decide that by how wide you open the faucets! But if you want "typical", set the faucets to your usual flow and get a 1-gallon bucket and a stopwatch.

Anyway, that probably won't be necessary for you. Since 1 GPM takes 35,000 BTU/hr, a 200,000 BTU/hr gas tankless is going to deliver nearly 6 GPM.

If you were dealing with an electric heater, then it's pretty simple, you need 10,000 watts per GPM that you want. So if you feel a need for 4 GPM you'll need a 40,000 watt tankless heater. "That's a WHOLE LOT" yes it is. That's why with electric, we think real hard about reducing the need for flow by changing to low-flow showerheads, or placing the electric tankless right at the point of use (e.g. if 10,000 watts for an electric shower works for the British, it ought to work for a bathroom with a 1 GPM low-flow head). A side effect of "multiple tankless at the point of use" is very fast hot water without need for wasteful recirculation systems.


Your continuous water heater will either have a simple minimum temperature requirement (and I'd think as long as the water is liquid it would work), or it has a graph specifying flow rate or output temperature as a result of input water temperature.

Here is an example from Navien, which shows a flow rate limitation depending on the required temperature rise:

enter image description here

Ref: https://www.navieninc.ca/downloads/npe-a-s-manuals-installation-manual-en

For instance, at an output temperature of 120F (no scalding), and an input of 40F, the rise is 80F, and the flow is limited 5 GPM. That's good for about two or three fixtures.

For efficiency reasons you could recover waste water heat to raise the input water temperature, as mentioned in other posts, such as How to pre-heat hot water tank inlet?


There is an additional problem with doing this, beyond cost and effectiveness:

You will be cooling your house as you heat the water!

This is actually a problem with a typical installation of an electric heat pump water heater - the heat that goes into the water is coming from the air.

In most areas (Montana is a bit of an exception) this is not a problem in the summer, because then you are taking heat out of the air that you would otherwise be paying to cool anyway - i.e., a win-win situation. But in the winter (in Montana, that is most of the time :-) ) you are taking heat out of the air, which you then need to add back in. So really no gain.

This is going to be an interesting problem with heat pump water heaters as they become more popular, forced by government regulations. Heat pump HVAC systems are fantastic because the air that they are heating (when you are cooling your house) or cooling (when you are heating your house) is outside. But heat pump water heaters, at least the ones I am aware of, use the air inside your house to do their magic. Which makes them super-efficient at heating the water, but a good part of that efficiency is lost in the winter because you now have to heat that air to be comfortable.

But back to your idea. It simply isn't a practical solution. The more effective the pre-heating is (e.g., hypothetically if you were to run the water through lots of thin pipe along your basement walls before it gets , making your basement a giant heat exchanger), the more heat you have to add back in to your house to be comfortable and the harder your heating system has to work.

Since you have gas (I wouldn't recommend on-demand heating at all if you didn't have gas), see if you can find an on-demand system that (a) doesn't need a larger gas supply than you have (because upgrading that would be expensive) and (b) provides the desired combination of flow rate and temperature rise. Alternatively, if the core problem is simply not enough hot water for typical usage (e.g., everyone wants to bathe around the same time + run the dishwasher, etc.) then add a second 50 gallon natural gas tank heater.

  • @isherwood I won't disagree with that being the root source. But I'd argue that at this time, most people who install heat pump water heaters do so because they are forced to do so by government regulation, not out of any sense of environmental duty. That will change over time. Arguably, heat pump HVAC has reached that level - i.e., the cost savings are significant enough for the average person to be willing to spend the extra up-front cost (which is relatively little if you are replacing air conditioning and not just heating) because of the energy savings. Very different (for now) for Jan 6, 2023 at 17:10
  • water heaters because (a) the overall energy cost is lower so the savings is lower, (b) the difference in initial cost is significant, (c) the huge problem of it cooling your basement in winter. Once these things get worked out due to (a) increasing energy costs overall, (b) lower cost of equipment as it gets more mass produced, (c) someone figures out a cost-effective way to split the system like with HVAC, that will change. Similar to electric cars - early adopters mostly people interested in the environmental aspect. Tesla (and now many others following along) made electric cars something Jan 6, 2023 at 17:12
  • people really like for non-environmental reasons - better acceleration, lower noise, (steps towards) autonomous driving, etc. Arguably government tax incentives helped too. But forcing everyone too soon would have been a disaster. Jan 6, 2023 at 17:14
  • 1
    Chiming in here: Living in a northern climate, where most of my energy use is for space heating, a heat pump water heater would be a disaster, they are much more complex, much more expensive, slow to heat water and probably have a much more limited life span. They also suck heat out of the house (as others have mentioned) so your heating system needs to work harder. If you live in a southern clime, yeah, you can consider it "free A/C" but not up north., Electric or gas tank style WHs are dead simple and last a long time. Jan 6, 2023 at 17:54

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