CH Working but no HW

[Rossco]

I have an old Gravity fed system using a Gloworm back boilder. Basically the CH works fine but two days ago the HW stopped working.

I checked the hot water feed to the heat exchanger at the top and bottom of the HW cylinder and both seem to be cold. One is showing faint signs of heat but not enough to heat the water.

When I flick the boiler onto HW only I find that the boiler kicks in for a short time but then stops. The pipes leaving the boiler to the HW cylinder are very hot but the heat is not reaching the cylinder. Could this be a blockage or could it be a valve problem preventing the cold water circulating back down to the boiler thus causing the boiler to trip out?

There seems to be no motorised switch over valve and everything seems to be controlled by the Gloworm back boiler.

 

[oilman]

It is a blockage, whatever causes it. It might be scale, though unlikely, and it might be a stuck non-return valve in the flow pipe from the boiler. Do you have one of these?

 

[Rossco]

I don't believe I have an non-return valve on there. It could be scale as I'm in a hard water area and some of the pipes used to make a good old racket before this happened.

Can you recommend a way to clear this? Would some sort of CH cleaner work? Would I have to drain down the system then add the cleaner and re-fill for best results? Whats the best way to do this?

 

[marios_kin]

If you do not have a non return valve, try tracing how far up the pipes to the cylinder heat is traveling.
A descaler chemical would work but i am unsure, with a gravity system, wether or not it would circulate into the blocked area.
I have in the past got a pumped system working again prior to using descaler by tapping around sharp elbows. This should be done with care as old pipes can be fragile and may fracture.

 

[galwag]

Have you checked the F&E tank - ballvalve may be stuck.

If ok look on return pipe of cylinder for a cyltrol valve (has blue cap) may need opening bit more

Or could be blockage

 

[Rossco]

Right, I'm going to drain down the system, add in the descaler and refill it. Hopefully that will ensure the chemical mixes properly. Is this the best approach?

 

[oilman]

Is this the best approach?

It will be unlikely to work if you have a blockage, because the chemical will not flow.

 

[sterose]

I have an old Gravity fed system using a Gloworm back boilder. Basically the CH works fine but two days ago the HW stopped working.

I checked the hot water feed to the heat exchanger at the top and bottom of the HW cylinder and both seem to be cold. One is showing faint signs of heat but not enough to heat the water.

When I flick the boiler onto HW only I find that the boiler kicks in for a short time but then stops. The pipes leaving the boiler to the HW cylinder are very hot but the heat is not reaching the cylinder. Could this be a blockage or could it be a valve problem preventing the cold water circulating back down to the boiler thus causing the boiler to trip out?

There seems to be no motorised switch over valve and everything seems to be controlled by the Gloworm back boiler.

Can you detect exactly where the pipes stop getting hot and go cold (i.e. if a blockage, the pipes should go from steaming hot, almost instantly to cold[er]), because, obviously, the source of the problem will be around that point.

But if there is no point where it goes straight from hot to cold, the problem could be anywhere in the circuit.

If you dont know, drain the system and try checking check valves [pun not intended] and any other devices that control water flow.