I have a tankless water heater and the water pressure is not enough to turn the tank on demand causing the water in the shower to be just barely warm. I purchase a Kolerflo 120W to boost the pressure, should I install it before enters a tankless water heater or after?

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Welcome! I would install it before the tankless water heater for several reasons. Putting it on the outlet would tend to lower the pressure in the tank. Also if it is on the output it will need to operate with the hot water internally causing it to get hotter. To do this it must be rated for that temperature. Putting it if front of the heater will raise the pressure in the tankless water heater hopefully accomplishing what you need. Be sure the pump can run a short while with its outlet blocked as that will happen when shutting the shower off.

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