Combi boiler woes - DHW etc.

[A.Lee]

Before I bite the bullet, and get a new boiler, maybe someone could suggest something that I havent done to try to repair this one. DHW (and CH) comes on for 20 seconds, cuts out, comes on after 10-20 secs, then repeats.

This points to a blockage in the DHW heat exchanger. Flushed it out. Still the same. Fitted new DHW heat exchanger. Improved things for a week or so.

Went back to its old stop/start routine. So filled up with cleaning fluid (Fernox iirc) left in for a week, flushed through etc, made little difference, and the exiting water didnt seem particulalry dirty. So, last gasp attempt, filled with more cleaning fluid, left in for 2 weeks, flushed through thoroughly. This also made no difference. DHW H/E off again - clear, no blockage.

This now points to a primary heat exchanger blockage. To try to discount other possiblilties, I have cleaned the flue, the diverter valve insides, and other accessible parts. Still playing up.

A new primary H/E is around £300 iirc. Half the price of a new boiler, and it looks like it will be a swine to fit, as all the joints are seized.

So, have I missed anything before I scrap this one? Thanks Alan.

[Andrew Gabriel]

It doesn't. If it was that, it would only happen when combi fires up for DHW, but you indicate it happens with CH too.

Most likely, it means the system can't get the heat out past some point in the system, and the part before that overheats. You need to trace how far the heat is getting through the system to find the point where it gets no further, and identify the point where the fault is. One possibility that spings to mind is the pump. A friend had one which somehow lost most of its impellers and resulted in very sluggish flow.

The other possibility is that nothing is overheating but a sensor has gone bad. Does it look or sound like the system is overheating when it goes off?

[YAPH]

Is the pump working properly?

If not it won't shift hot water away from the heat exchanger fast enough.

In CH mode when the boiler runs (starting from cold is easier to tell) you should get the flow pipe getting almost evenly hot for a good distance (further along the longer it runs). If it's localised suggests the pump's not working well.

[Cicero]

========================================= On one occasion recently I found a large accumulation of debris almost blocking the pipe immediately before the pump. Cleaning and flushing had obviously not shifted it. It might be worth removing your pump and valves to see if a trap has been created there similar to mine.

Cic.

[dennis]

Cavitation does that to impellers. I had one go on mine once when I fitted a pump that needed a higher static head than I had. Serves me right for buying a special offer without reading the spec.

[A.Lee]

The CH cutting out didnt happen until quite recently, I had a 'qualified' CORGI man come round when it first started doing it, he immediately said DHW exchanger, then said I could DIY as it was so easy. It did sure it for 6 months, but then started to get worse.

...

The pump may well be the cause. I took it off, and it looked in not too bad condition, but it is pretty difficult to test its flow rate, so I bit the bullet, and went down to Screwfix to buy a new one, but, as the one I have has non-std. 1 1/4" outlets, they didnt have any, called into B+Q, they only had 2". Bugger. So put back the old one, and checked the attached air siphon, which was blocked up - this may well have been a cause, as if air was in there, when the pump was going round, it would be compressing the air, as well as trying to pump water, hence the efficiency of the pump would be lowered. Tested it once before posting this, and it has improved matters, though it still cuts out after 30 seconds or so, which is a good improvement, though obviously not cured. Is there any way of testing pump flow rate apart from fitting a new pump?

Both sensors were replaced a couple of months back, before the DHW H/E was replaced, I had already been onto Alpha to ask their opinion, they told me sensors first, good flush etc, then new H/E's. Thanks Alan.

[A.Lee]

Possibly, see other post, though, apart from fitting a new pump, how can the flow rate be checked? Thanks Alan.

[A.Lee]

No grunge build up anywhere that I have seen. The pump innards were quite clean. Thanks Alan.