Plumbing – the function of a check valve on the inlet of a household macerator pump

Disconnecting the cold water flow sensor certainly sounds like a reasonable idea. Try it and see if you get the desired result. Do use care when disconnecting as the sensor wiring may not be isolated low voltage circuitry.

When the pump is deactivated you will want to evaluate fully whether the upstairs toilets and sink faucet water flow is adequate when only cold water is used.

You also have the option to add in a timer circuit device that could cut off power to the pump motor during certain hours of the day/night. The timer could, if the proper type, simply be used to cut off the cold water sensor to the pump as opposed to just simply turning off the AC power to the pump motor.

If you are (as is common) "returning" via a cold water supply line, rather than a dedicated recirculation line, I think you need a check valve where the hot recirculation joins the cold line, not just at the pump.

Otherwise the cold can backfeed from the cold water line to the hot loop when the hot water is run, without the check valve at the pump being involved. All the system designs which use the cold water line as a return typically have the pump located at the point of joining the hot to the cold, NOT at the water heater, so that the check valve in the pump works for this application.

If you have a dedicated return line, and it has a check valve in the pump, that should be sufficient, and you should complain to the supplier of the pump if the check valve is not working.