I had this ages ago following draining down and refilling the c/h system. I seem to remember on mine the flow pipe was roasting hot but the return pipe was stone cold and only the rad in the same room as the boiler would heat up.
Assuming its a combi, there should be an air vent either on top of, or underneath the boiler. It's usually a small valve which is similar to the one on a car tyre. Unscrew the cap or press in the vale and any excess air in the system should be expelled until water comes out. Also bleed all your rads.
Its worth a check as its relatively easy to do (consult your manual for its location) and its free
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