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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 biasi boiler is working fine domestic hot water but

Customer Question

My biasi boiler is working fine for the domestic hot water but for the central heating it kicks in, runs for about 30 seconds and then cuts out (doesn't actually lock out). The temperature gauge also rises quite quickly. After about 1 to 2 minutes it fires up again and just keeps repeating the cycle.
Submitted: 1 year ago.
Category: UK Plumbing
Expert:  Brian replied 1 year ago.

I would normally suggest this is the circulating pump but as you have hot water its not so likely, it definitely sounds like a restriction of some kind, no one has turned the valves off under the boiler by any chance? If the circulating pump has speeds on it turn it up to number 3.

Customer: replied 1 year ago.
Hi Brian, thanks for your response. No all valves open, no speed on the pump. I took the small brass screw off the pump and turned the impeller shaft and it turns freely. The hot water is fine. With the heating I click the wall thermostat on and it fires up normally, it will then heat up but when the temperature gauge on the boiler gets to about 60 degrees it then rises to 90 degrees very quickly and then cuts out. It doesn't go into lock out or anything it's as if the boiler thinks its hot enough. It then waits for about 5 minutes and fires up again doing the same thing, the rads upstairs are hot but downstairs are just warm (no cold spots at the top). Ive seen comments on thermistors, would that cause anything. Thanks again
Expert:  Brian replied 1 year ago.

Hi yes it could be a thermistor. I'm going to opt out for open the question up to everyone else. they may be able to pinpoint the fault now we have more information.


Customer: replied 1 year ago.
Thanks Brian.
Expert:  Andrew Smith replied 1 year ago.

To rule out the pump can you put your hands on the flow and return pipework and tell me how many pipes get hot and how far the heat gets?

Customer: replied 1 year ago.
Hi Andrew, the flow and return pipes are hot as far as I can reach them before they disappear into the ceiling.
Expert:  Andrew Smith replied 1 year ago.

No circulation then.

If you had circulation when the boiler was firing then the flow would be hot and the return would be cold (coming back from the system)

Customer: replied 1 year ago.
Oh okay. I changed the thermistor today (I had ordered it anyway so thought I may as well) but no difference. I have ordered a pump so will let you know how I get on. I guess that would answer why the radiators at the extremes of the house are not getting hot. The air relief valve on top of the pump has been leaking for a while and I bought a new one but the clip on the old one is corroded into the plastic body and I'm afraid it will cracking the pump housing if I force it. At least the new pump will solve that. Would the pump cause the boiler to shut down after a few seconds. Do you think that's why the temperature rises quickly on the gauge?
Thanks for your help. Dave
Expert:  Andrew Smith replied 1 year ago.

Yes, all symptoms of poor circulation.

It could be worth daring the whole system when you do change it and check the water quality at the same time.

It would definitely worth spending another £15 on some inhibitor at the same time to prevent further corrosion.

Customer: replied 1 year ago.
Will do. I will let you know how I get on. Probably be the weekend now.
Expert:  Andrew Smith replied 1 year ago.

No problem - just reply once you have done it

Customer: replied 1 year ago.
No not the pump. Just put a brand new one on and still the same. It's as if the boiler thinks it's hotter than it is so switches the gas off. The temperature on the front gauge rises quite quickly up to 90 degrees.
Expert:  Andrew Smith replied 1 year ago.

can you disconnect the flow and return pipes and check for filter near the valves. Some boilers have the, and can block over time

Customer: replied 1 year ago.
Oh will look at that now. Is that the pipes going into either side of the heat exchanger (is that what it's called) above the flame.

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