Is a pump needed for boiler auto fill line?

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swtools

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I am installing a Navien CH240 tankless water heater which supplies domestic hot water and is the radiant heat source. It has two circuits and is designed for the task. The tankless heater heats the radiant heat water, and it has a heat exchanger to heat the domestic hot water. It has an internal circulation pump for the radiant heat loop, and maintains the pressure at 15 psi.

My question is about how to connect my makeup water source to the auto fill connection on the tankless water heater's radiant heat loop.

I want to use a Reverse Osmosis (RO) system to provide the makeup water. We have very hard water, and our previous heaters have all failed due to excessive scaling/build up of precipitates, even with a water softener on the incoming water line.

So the question is how to overcome the 15 psi head at the autofill valve. Do I need a pump for that, or will the bladder tank on the outlet side of a household RO system easily be able to work at a higher pressure than 15 psi, so no pump is needed? Our water pressure here is about 65 PSI. The amount of makeup water is small, and the tankless heater shuts down if the water level drops too low.

If I do need a pump, what kind? How would it be set up so that it does not run continuously, but there is always an input line pressure above 15 psi to the water heater?

-Dave
 
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