Michael P

On each manifold, under the pump there are two gauges. Under the left hand gauge, one of the manifolds has a white cap, on the other manifold the cap is missing. The caps shut a valve off. The caps should be removed at installation and replaced with a thermostat. Do you have any spare thermostats knocking about looking for a home ? Is the manifold with the cap getting hot? If not, remove the cap (turn anticlockwise) and see if that manifold warms up.