Combi boiler - lack of heat problem

[David W.E. Roberts]

Hi,

just had a wonderful week in a holiday cottage in Devon, with a slightly nervous start after I tried to fill the bath :-)

Sequence of events; arrive at cottage and turn up wall mounted thermostat (which is set to 'always on' and a low value as frost protection). Radiators warm up and after a while all is toasty. Hot water for washing dishes comes through fine.

I start to run a bath. The water comes out piping hot but after a while I come back and check and the water is running cold. I turn the bath tap off, and seek out the boiler to investigate.

The combi boiler seems to be working, so I leave the front flap open so I can see all the lights and stuff and turn on a hot tap. Water runs hot for a bit, then runs cold. Too much cold water flow? However, the boiler is not running flat out but is clicking on and off. The burner is off when the hot tap is on and running cold water out. I notice after a while that the radiators are cooling down as well. Panic slightly. Go away for a restorative and a think.

After a while the radiators warm up again. Further investigation reveals that once the radiators are warm then I can run about 1/3 of a bath before the water runs cold. Wait for 15-20 minutes and run some more hot water into the bath. The water is hot enough that you eventually get an acceptable bath after about an hour.

Now if I hadn't been watching the flame in the boiler, it would all have seemed very simple. Underpowered boiler can't heat radiators and bath water at the same time. Running cold mains through the boiler cools the whole thing down resulting in cold bath and cold radiators.

However I was watching the flame and it was off most of the time. This is the bit I don't understand. If there is cold water running through the boiler, surely the flame should be on all the time?

Could some kind person suggest what the flock is going on?

Cheers Dave R

[Old Bill]

Most combis devote all their power output to the hot water when its demanded. I think we would need to know your make/model of boiler to comment futher.

[David W.E. Roberts]

Unfortunately I didn't take down the details. Following this up now for interest and because we vaguely know the owners.

I can understand 50% of the symptoms if the boiler switches to hot water only when there is a demand. This would explain the radiators cooling down.

What I still don't understand is why the boiler doesn't run continuously when cold water is flowing through. If the boiler was just underpowered then it should run all the time and deliver warm water, not turn off and deliver cold water.

Also don't understand why it works for a while if the central heating is up to temperature.

I suspect some kind of mis-connection or fault in the monitoring system somewhere.

Cheers Dave R

[Ed Sirett]

Without further investigations it's hard to pin down the precise reason for poor HW.

However scaling of the secondary heat exchanger on the secondary side is centre stage.

Also are poor water pressure/flow, poor gas supply, faulty boiler...