Boiler cutting out after a while

Hi,

Have a Potterton Suprima 40 which has worked fine for 5 years. The system has cold water storage in roof along with expansion tank - both OK. 2 motorised valves (water/heating) in airing cupboard with Grundfos pump - all OK.

When boiler fires up, everything is OK for about 30 mins. Hot water fine, all radiators heat up but boiler cuts out well before room stat temperature is reached and pump stops.

When boiler cuts out, there is a great rumbling and banging/gurgling from both the boiler area (not sure quite where but maybe the flu) and around the pump pipework.

The boiler can be re-started but the pump does not cut in and the boiler cuts out again within a couple of minutes - with the same banging, rumbling and gurgling.

Leave the system for about an hour, fire it up and the sequence starts over again.

Have thought of pump (but that is obviously working at least initially) and airlock (but all the rads are warming up - no cold spots and hot water is fine) can't get to the bottom of it. Any help or ideas would be great.

Best regards,

It does sound as if the oump is failing after your 30 minutes. The boiler is then left without any water flow and seriously overheats.

Based on your description I would just replace the pump and expect that to cure the problem.

Its just possible that if its correctly connected with the boiler supplying power to the pump that there is a fault on the boiler and its cutting power to the pump. Thats unusual but anything is possible. To check it you would have to monitor the voltage on the pump when the overheating occurs.

Tony Glazier

Tony,

Many thanks for the reply. Have just attacked the box with the electrical connections and it would seem you are spot on.

No demand for heat.................. 0V across L & N to pump

Demand for heat...................... 234V across L & N to pump

No demand for heat.................. 234V across L & N to pump for about 90 seconds then down to 0V across L & N to pump.

I guess the overrun is a safety measure or something but what I don't inderstand (and perhaps shouldn't try!!) is that when cool, the pump seems to behave just fine and is absolutely quiet. Why should it just fail after about 60 mins use from cold?

Best regards

The pump over run is controlled by the boiler to remove latent heat from the boiler and cool it at the end of a demand for heat.

You have proved the pump is correctly wired and under the control of the boiler.

You are wasting your time asking why the pump should fail. Thats not relevant! Just accept that parts do fail sometimes.

What you need to find out is if there is still 230 v on the pump at the time that the overheating occurs!

Tony Glazier

Tony,

Thanks for the information. I get the same figures once the boiler has overheated. 233V just before it overheats, 233V as the boiler shuts down, 233 V for about 90 secs after shut down and then 0V.

Looks like order a new pump time. I understand your sentiments about not worrying about why the pump should fail but it's that thing called curiosity!

Thanks again for your help.

Best regards,

Tony

I'm probably not allowed to post here as I'm not at all qualified, but thought I would post my thoughts.....
My initial thoughts on pump failure after getting hot, would simply by expansion, causing the jamming up of sludgy bearings etc.
But, as I say, I have no right to give an opinion.

Good luck.
Flash.

MOD

this is a DIY forum anybody can post

You can post a suggestion, though perhaps not a professional opinion if you are not a professional.

I did not want to have to do this but as you have raised the issue!!!

The real answer in this particular case may never be known and it totally irrelevant as the only significant factor is that it is failed. Out of curiosity I always try to find the cause of failure on pumps, valves and pcbs etc. but I am aware that its not relevant to anything and the manufacturers are only interested in keeping their products running through the warranty period.

There are several causes of pump failure after a significant running time. One is electrical disconnection due to expansion movement effects. Another is bearing seisure sometimes due to attraction of magnetic sludge. Another could be due to expansion of bearings with heat but I do have to say I cannot see any great support for that theory as the inner would expand at a similar rate to the outer and maintain the status quo. Another could be dry bearings due to the boiling of the cooling and lubricating water.

Its all irrelevant anyway! He just needs a new pump!

Tony Glazier