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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3629
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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My hot water is ok, but central heating is not working. I

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My hot water and but central heating is not working. The boiler does not cut in. I have tried bleeding the radiators and some expel water where as others neither air or water. I have changed both the pump and three port valve but has not solved the problem. It is a gravity system. The header tank does not drain.
Hi my name is XXXXX XXXXX lets see if we can get you sorted in the shortest possible time. The header tank does not drain? How did you refill the system?
Customer: replied 5 years ago.
Not sure I have. As air does not expel from radiators I am not certain that the system has been filled or how to do this. I tried using a plunger on the outlets of the header tank (in case of blockage) but this did not help. How (other than bleding the radiators) should I have filled the system please? Three radiators downstairs spurt out water from bleed valve but other radiators in the rest of house do not have air or water spurting from them.
The system has not been filled unless you get water from the radiators. It sounds like you cold feed is blocked. You could try tapping the 15mm pipe from the tank where it joins the larger pipe, this is usually where it gets blocked
Customer: replied 5 years ago.
Thanks for that. I agree that it is possibly a blockage of the cold feed. However there are three outlets pipes from the tank (all 25mm). One is the power shower feed and the other two I cannot see where they join a larger pipe as this is not nearby in the loft. Also just to complicate things , why do three radiators appear to have water come out of the bleed nipple and not the others? Does this not suggest a blockage in the radiator pipework somewhere? Thanks. P.S I have tried tapping the tank outlet pipes (plus a few radiator pipes) to no avail. The boiler (when turned on) cuts in for about 30 seconds and then cuts out .
Your boiler is just cutting out because it is reaching temperature. The radiators may have water in as they were never drained when you did the work. The tank you are talking about sounds like the hot water tank. It will feed the hot water cylinder, you may also have a feed to a power shower and also a low pressure cold feed to the bath taps. Do you have a small tank too that usually feeds the central heating?
Customer: replied 5 years ago.

Okay I found the small tank (quite grotty and brown in there) and the 15mm pipe which leads out to the larger pipe (then tees off to the pump)

I banged it hard and even tried a plastic electricians draw wire down the pipe entrance from the tank, to see if any blickages could be felt but it went clean through and then after about 7 ft of draw wire it stopped dead. I looked in the airing cupboard where the 15mm pipe joins the larger one and it its a tee junction. So this may account for the stop dead draw wire. But still the problem persists and the blockage is not resolved. Any other ideas please?

It is usually that tee that gets blocked over time. If you cannot get it clear with a couple of taps of the hammer then I would suggest either cutting it out yourself and replacing that bit, or calling someone to do it for you. You could try filling the system backwards to get you going until you get it sorted. A hose pipe with a jubilee clip on one of your drain points and the other on an outside tap or similar and slowly fill the system backwards. Vent all your radiators one at a time and then disconnect once full - your system should run but you will have to do the other part to enable you to refill if you ever drain down again. I hope this helps.
Customer: replied 5 years ago.
Okay Andrew, I have called someone in to tackle this as the tank looks like it needs draining and cleaning as well. I shall leave this open teill they have been in case I need your advice again over this matter. Thanks for now
No problem
Customer: replied 5 years ago.

Hello Andrew,

Just one last thing before I accept the answer if you don't mind.

I have a company coming round on Friday to change the pipe where the blockage is (he checked it with a magnet and suspects a problem on the next joint of the 22mm pipe before the pump). I also asked him about the poor water quality and sludge in the feed tank and he recomended a powerflush to the whole system (11 radiators) at £350. Do you recommend this and do you feel it is necessary on the information you have(as it is quite expensive)?

Thanks again for your help


Just run some water out of one of your drain taps and see what the colour is like after 30 seconds, that will be the quality of your water. It would be very doubtful if they clean your tank also during a power flush?
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