I know this is a weird place to post this, but 2 other plumbers websites gave me no response, and this section seems pretty active/versatile Any feedback would be greatly appreciated...
My central heating system is vented and very simple (pretty old setup, probably 20 years old) - no 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 (I have experience with fitting radiators etc). 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. The pump is a grundfos 15-50.
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). Since the gravity domestic hot water system works fine, this makes me feel that there's no problem with trapped air (since it runs on the same circuit)?
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). I have bleeded the pump (a little air comes out, not much) a few times. At one point, I tapped on the pump and surrounding pipes and the pump and the pipe going into it got hot all of a sudden then went cold again. The motor's working fine on the pump (tried stopping the spinning with a screwdriver and it didn't stop so there's enough force there).
Through some research I've found it's:
1) The pump? (e.g. impellers worn?)
2) Blocked? (some pipes blocked especially around pump?)
3) Airlocked? (have bleed the system and pump, however, - not much air comes out)
Any feedback would be great, thanks in advance!