Combi hot water issue. Worcester Junior 28i

Hi, my first post on this forum so here goes-

My combi boiler hot water goes hot then after a while runs cold - then about a minute or more later starts getting hot again - this cycle happens in a cycle

I have been searching the site and found it it could be the secondary heat exchanger, flow rate issue, gas setting issue or even a flow switch issue.

My boiler doesn't have a secondary heat exchanger, so I ruled that out.

The flow rate at the tap after measuring is approx 9 litres a minute.

I have not checked gas settings for obvious reasons

I guess the flow swith (this boiler has a turbine, which is similar I believe or does same job could be faulty, but am not sure how to check this?

But how can I test the hot water thermistor? I know you have to check resistance across its terminals, but I have heard that when cold they can be ok and when hot go faulty - is there a way I can check the thermistor?


Thanks for reading my post and I'd be grateful for any help offered

SJ

As soon as the boiler cuts out, shut off the power and the hot tap...then immediately measure the resistance, you may find the sensor has gone open circuit.

There are other reasons for the cycling buit they are related to flueing etc.

a few questions pal

1) is it the greenstar model or standard efficiency
2) as it cycles as you say, have you watched the flame light, does it remain constant or go on and off
3) have you checked for cross leaking? turn off cold under boiler 2nd pipe on right 15mm, turn hot tap on does anything come out of tap

I fix a lot of wb boilers I change very very few thermistors!

I recently had to replace TWO thermistors on Worcester 240s fitted in the upstairs/downstairs flats which both failed at about the same age within a few months of each other.

Unlikely in this case but still worth checking early on.

A little tip is to measure the applied voltage as it warms up. It should fall as it heats up.

Tony

boilerdoktor said:

a few questions pal

1) is it the greenstar model or standard efficiency
2) as it cycles as you say, have you watched the flame light, does it remain constant or go on and off
3) have you checked for cross leaking? turn off cold under boiler 2nd pipe on right 15mm, turn hot tap on does anything come out of tap

I fix a lot of wb boilers I change very very few thermistors!

Click to expand...

Thanks for replying

I have watched it and the flame goes out - would that mean it could be a lot of things at fault though?

What is cross leaking? I am wary about turning valves off under boilers as I have heard they leak and cause problems after!

Any ideas ? I'm thinking possibly sticking flow turbine but not sure?