Hot water and heating come on but after a while heating shuts off and if increase thermostat nothing happens to rads. manually pushing mid position valve then send heat to rads. Is this thermostat problem?

Jerry Cotton, April 2011

When the heating shuts off after the short period, test for 220-240v at the diverter valve white wire, if there is power there, the most likely outcome is the actuator motor is failing.
If there is no power on the white wire, it would indeed be the room thermostat.

Carefree, April 2011