Central Heating System Problems

[Kevin]

Hi all, I have a continuing issue with my central heating system that is manifesting itself in two ways.

(1) When both my hot water and central heating are on, the boiler (a potterton suprima 50 litre - 3 years old) bangs and then locks out. I've had several vists by gas fitters who have recorded the temperature of the inlet and outlet who state that the boiler flow is restricted somewhere and the water is boileing in the cast iron heat exchanger. I've had countless plumbers and heating engineers who have replaced the circuit boards in the boiler, the flow restriction valve at the hot water cyclinder, the cold water inlet pipes, the flow control valve between the heating system and the hot water system etc. I've also had the system flushed with a chemical cleaner.

(2) Recently my aqualisa power shower is failing to pump water. It pumps well for the fist 5 minutes and then the water suddely dries up. You can hear the pump whining but there is no water. Neither cold or hot. Previously, after showering or running a bath, you could hear the hot water cylinder being refilled from the cold inlet. Now I cannot hear that noise.

Well, that's all the information I have. I've spent a lot of money on sepatate engineers trying to resolve these issues. I was hoping that someone could give me a few pointers to help me locate what the issue may be. Otherwise I've a feeling that I'll have to replace the whole system!

Any advice would be appreciated.

[cynic]

I once attended a boiler with similar symptoms, where the tenant had closed off "all" the radiators and there was no bypass. When the cylinder was hot and no path through it there was no flow via the radiators, the pump ran the boiler fired and the banging started until the boiler locked out on overheat. Do you by any chance have thermostatic valves on your system which are shut off or stuck closed? Ref the shower - check your ball-float valve in the head tank has not jammed through corrosion or has bebris in the cone resulting in a dribble only being allowed to enter to replace the run-off. Final tip get someone competent in who understands what they are diagnosing.

[Roger Mills]

I don't think you have a single problem which is manifesting itself in two ways - I think you have *two* distinct problems!

In the first one, water in the primary circuit (boiler, indirect coil in HW cylinder, radiators) is failing to flow through the boiler - so it understandably overheats and locks out. Obvious things to check for are failed pump, or 3-port mid-position valve (if you have one) stuck in the CH-only position, coupled with TRVs on the radiators all shut. Another distinct possibility (assuming a vented system) is that the small fill & expansion tank in the attic has run dry, allowing an air-lock to develop in the primary circuit. This has a float valve which is supposed to keep it topped up to compensate for evaporation and slight leaks but these operate so infrequently that they often sieze up. It's easy enough to check.

You say that this happen when *both* HW and CH are on. What happens if you select just HW or just CH?

The second problem concerns the secondary (domestic hot water) circuit. Either the large cold header tank -which feeds the HW cylinder and (presumably) a direct cold feed to the shower pump - is running dry or something is happening between the shower pump and shower head. What happens when you run a bath - is that ok or do the taps dry up after a while? When the shower stops in this way, get someone to look at the cold header tank to check whether it has water in it, and whether it is being replenished through its float valve.

I've been having problems recently with my power shower - which I suspect to be due to the flexible hose which connects the mixer valve to the shower head. I first noticed that it was starting to bulge when the shower was running. I effected a temporary cure by binding the affected part with Gaffer tape but shortly afterwards got similar symptoms to yours whereby after a few minutes of operation the pump would make a different noise and the flow would dramatically reduce. I suspect that my hose had delaminated, allowing water to get into unscheduled places - causing the lining to bulge and block the flow. Fingers crossed, it hasn't happened since I replaced the hose. You could check very easily to see whether that is your problem by unscrewing the hose from the mixer valve when the problem occurs. If water suddenly starts gushing out of the mixer valve, it's an indication that the hose is blocked.