After installing a reco ZD30 in my GU3 I filled the cooling system and ran motor to get any air out of system. Turned heater on, even removed fan, and even though the temp gauge showed it was at operating temp the bottom return pipe to the radiator from thermostat housing still felt cool to touch. Top radiator hose was too hot to hold for long. I got mechanic to swap thermostat for genuine one. Same result. Has never over-heated. Had it checked by radiator shop and they couldn't find a fault. I still can't get my head around the what logic tells me: If the return pipe's cold, there's no flow through that pip. Can anyone shed some light and maybe put my mind at ease?