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I have a tankless water heater similar to this one, which I'd like to setup at a campsite where I'll have water but not from a pressurized source (e.g. a bucket of filtered river water). The heater I have has a hookup for a standard garden hose. What should I look for in a pump? Do I need to match (or exceed) the GPM rating of the heater? Will using a pump force water through the heater faster than it can heat it, or would the heater regular how fast water can flow through it?

5 Answers 5

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I'd use something like that:

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It's a pump with an output pressure of around 4 bar (60 PSI), a pressure tank, and a pressure switch. When pressure in the tank drops, it starts. When pressure is restored, it stops. This is what you use to supply water from a well to a house.

The tank provides several advantages:

  • Pressure is regulated.

  • The pump won't run without flow. It's important to not run these pumps without flow, because the power from the motor heats the water instead of pumping it. Then it boils, and something will burst, for example a plastic pipe. Then it runs without water and fries the watertight bearings. That's what happened when a contractor used my well pump to power his high pressure washer and forgot to add a tee to let some water spill so the pump would always get flow.

  • Instant start: there is pressure, no need to wait for the pump to start. It will start if needed.

  • The pump doesn't do very short cycles every time someone washes their hands.

Then you connect the tank to the heater and to the faucet, with a tee so the faucet gets both hot and cold water.

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If you exceed the GPM rating of the heater, the water will not get the specified temperature rise that you are probably counting on.

In the listing your provided it says:

A 2GPM portable tankless water heater at a 35F rise in temperature can easily provide enough hot water for your campsite or RV

So at 2 GPM flow you can expect 35F temperature rise. At a higher rate you will get less than 35F rise. From the 37500 BTU rating you can calculate the temperature rise at different flow rates although I'd be surprised of the installation manual doesn't have a chart for this.

The formula needed is:

BTU = Flow Rate In GPM (of water) x (Temperature Rise) x 500.4

Since the BTUs are fixed at 37500 and the GPM is 2 you can write this as:

Flow Rate = 37500/(Temperature Rise * 500.4)

or if you want to calculate Temp Rise for various flow rates:

Temperature Rise = 37500/(Flow Rate * 500.4)

Find a pump that will operate from your power source (120VAC or 12VDC) and will provide something line 2 GPM.

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Using the information from the unit similar to the one you have, your pump shouldn't exceed 1.4 gallons per minute or you won't get the hot water you're setting the unit to provide. There are pressure switches in the heater so the heater won't turn on unless there's flow. The heater does not regulate your water flow, that will be the pump's job. Take a pressure gauge and get a reading from your garden hose and look for a pump that delivers a similar reading.

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Most likely, the heater has no ability whatsoever to regulate flow.

The way these heaters work, they are injecting heat into the water at whatever rate they're capable of. If you flow water through the unit faster, then each ounce/CC of water gets less heat. Get it?

We can quantify that, if you're willing to do the math.

A "BTU" is the energy needed to heat 1 pound of water 1 degree F.

Your heater says "BTUs" but anytime you see that on HVAC equipment, it really means "BTUs per hour". It might be constructive to divide by 60 to get BTUs per minute.

1 pound of water is very nearly a pint.

The shower flow rate British people are accustomed to is 1 gallon (8 pints) per minute. Americans closer to 2.

So, suppose your heater has 200 BTUs per minute. You are flowing 8 pints per minute. That means in 1 minute it will put 200 BTUs into 8 pounds of water. So 25 degree F temperature rise on the water. (200/8).

If you slow rate down to 4 pints per minute, you get 200/4 = 50 degrees F temperature rise in the water.

That's how you do that math.

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I think most answers and comments are off base here. A tankless WH is normally supplied by pressurized water (municipal or well) at somewhere between 30-60 PSI. The actual flow is controlled by the faucet/dishwasher/shower on the receiving end of the tankless, not the supply.

I'm concerned about what I consider an overly simplistic setup. What turns on the pump? What are the controls? Directly limiting the flow rate from a pump without an intermediate pressure tank is tough on the pump, generally speaking, but since the OP said it's being pumped out of a bucket, I would guess it's all manually controlled. Just don't throttle down the pump too much, in this case, smaller might be better, IMHO.

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