Potterton combi, intermittent stop of DHW

[NickB_99]

Hi,

Hoping for a bit of advice with a Potterton promax combi HE plus.
When using the shower, the hot water seems to stop for a short period then restarts after a minute or so. A few minutes later, same cycle.

Watching the boiler indicators, when running normally, the two LEDs are lit for the tap indicator and also that it's burning. When the water runs cold, both lights go out and water continues to flow, then it sorts itself and both lights start again and hot water returns.

The boiler was last serviced in Sept 2013, so will get it done again.

Any ideas if this is a common fault?

I don't know many local plumbers and I can imagine a service and diagnosing/repair is not a massively attractive job. I'd like to avoid being ripped off so any advice would be useful. I would use someone gas safe of course.

Thanks

 

[ScottishGasMan]

Firstly is the boiler cycling on and off noticable in the shower? If you run a hot tap for a while at a moderate rate does it do the same?

Showes dont always pull that much hot water through, and as such if its using a low flow rate of hot the boiler may need to shut off once the whater has reached temp for a moment till it cools and starts up again, something that you likley wouldnt notice in the shower too much.

If its cyling on and off while running a tap at a decent rate, the most common reason would be the plate heat exchangers choking up and not getting the heat from the boiler into the mains water efficiently causing the boiler to cycle on and off similar to above, but would normally be noticable with hot/cool fluctuations at the tap/shower.

 

[NickB_99]

Firstly is the boiler cycling on and off noticable in the shower? If you run a hot tap for a while at a moderate rate does it do the same?

Yes, very noticeable in the shower. Steadily goes from hot to v cold. Stays this way for about a min, then steadily increases back to properly hot.

I will go and test with the bath tap now.

 

[ScottishGasMan]

Yea, try a tap first and see the response, its not uncommon to have a faulty shower mixer (assuming its a thermostatic mixter?) restricting the HW flow too much

 

[NickB_99]

Yea, try a tap first and see the response, its not uncommon to have a faulty shower mixer (assuming its a thermostatic mixter?) restricting the HW flow too much

Didn't seem to have a problem with either the basin mixer tap or the hw tap for the bath on this test.
Running at full flow of bath tap, temp would rise to about 72degsC. Running with lower flow rate, seemed to be slightly lower temp at 65degs, but couldn't get it to fail.
Yes, shower is thermostatic and when running at full, boiler temp shows about 64degs. When shower failed, the burner light went off first, then the one next to the hw tap. Temp started to fall to about 55degs and boiler kicked in again and both lights came back on and the temp climbed back up.

Will take a pic of shower mixer. To follow....

 

[gigz]

Take the shower head off and run it like that, if it stays on either clean the head or change it.

 

[ScottishGasMan]

Yea, try a tap first and see the response, its not uncommon to have a faulty shower mixer (assuming its a thermostatic mixter?) restricting the HW flow too much

Didn't seem to have a problem with either the basin mixer tap or the hw tap for the bath on this test.
Running at full flow of bath tap, temp would rise to about 72degsC. Running with lower flow rate, seemed to be slightly lower temp at 65degs, but couldn't get it to fail.
Yes, shower is thermostatic and when running at full, boiler temp shows about 64degs. When shower failed, the burner light went off first, then the one next to the hw tap. Temp started to fall to about 55degs and boiler kicked in again and both lights came back on and the temp climbed back up.

Will take a pic of shower mixer. To follow....

HW demand light has gone off indicating no boiler fault, a low flow caused by the shower more likley.

 

[NickB_99]

Ran it with the shower head off, didn't fail after a few minutes.

The head was replaced about 6 months ago. The original square 'rain effect' one had got v mouldy so I bought a new one from screwfix. Had been fine until a month ago.
Just dismantled it and cleaned it (tho it didn't seem mucky) and put it back. Running for a few minutes it was fine.

Will see how I go tomorrow.

Thanks ScottishGasMan and gigz. Many thanks for the help this eve.

 

[Agile]

A more simple test to see if the flow rate is not adequate is to run a hot tap half on together with the shower.

Another later test is to SLOWLY turn up the flow on a hot tap and see at what flow rate the boiler fires up.

[ You stop increasing the flow and measure it in litres per minute! ]

Tony