We have a tankless water heater that's mounted in an exterior north facing wall. In the past on nights with a hard freeze, the hot water to the two vanities in our master bath, also along the north wall, have frozen. Cold water fine. Hot water to the rest of the house is fine. We have PEX throughout the house. When we trickled the water to those two vanities to prevent freezing, we got an error reading on the water heater display saying essentially there was not enough water pressure to fire it up (or something like that).

Question, if I allow the water to trickle, can I turn off power to the water heater? Or am I setting myself up for damage to the heater.

Thanks for any advice!

If your demand WH has freeze protection, hitting the off button wouldn't be wise.

The facts thus far suggest that you need to add cavity wall insulation where the MBa sink HW PEX is located, as best you can from inside, then from outside if this fails. A foam pro might have other ideas, if they are repair-plumber approved.

Note that some plumbers like to clear their work area (your MBa HW pipes) of insulation and don't replace all that, perhaps forgetting after pressure checks (& blow torching copper in-the-day to avoid house fires).

A temporary quick fix could be a VERY SAFE electric space heater (Dyson?) on thermostat 80F pointed at these undersink pipes through open cabinet doors all night.

Satying tuned to discover the real answer ... in time.

Thanks for the advice about the water heater! I'll leave it on. I'll also have a plumber look at the insulation from the WH to our vanities. I'm afraid the pex from the WH is running in an uninsulated area of the attic that I unfortunately can't reach. Knowing the builder, that shouldn't have been a surprise to me.

And this has only happened twice, both times when temps were down in the single digits. Hopefully we won't see that this time around.