Central heating water not circulating

10 Mar 2011
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United Kingdom
Hi guys, first post.

My central heating system is vented and very simple (pretty old setup, probably 20 years old) - no 3 way valves (multi-fuel boiler heats DHW through a gravity pipe leading up to coil in tank), and the CH is pumped from the boiler too (4 connectors on the multi-fuel boiler). I manually turn the pump on when there's enough heat.

There is only one feed, and one return pipe for the radiators.

Recently, I fitted a bathroom with underfloor heating (wet). The floor is less than 2 square meters. I concluded that since the piping for underfloor heating was short, I could test it out on the central heating circuit (after all, I thought the pump could handle it since the demand would be small and would be equivalent to adding another few radiators). There is only 5 radiators in the house.

I drained the system, and connected the underfloor pipes to the central heating return pipe (in series). Refilled the system (released air in radiators), put the fire on, switched pump on but none of the piping heated up (including the feed pipe). Pump SOUNDS like it's working. Gravity hot water supply is working perfectly fine as before.

I returned everything back to the way it was (took the underfloor piping out of the circuit). Refilled, put the fire on, switched pump on but still NONE of the pipes heated up (water not flowing). Pipes before and after pump aren't hot, same for feed pipe (until convection happens).

Each time I refilled, I bleed the radiators to release air. I have refilled 3-4 times now. Each time, there's plenty water in there and water is pretty clear (used to be sludgy).

Through some research I've found it's:

1) Probably the pump?
2) Airlocked?
3) Blocked?

I don't know what to do in either situation (apart from bleeding/replacing the pump).

Just wanted to have some feedback before I go under the house again to get the pump :)
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