Biasi M90.28S Intermittent Hot Water

[IKM]

This boiler was run for several years with a leak in the underfloor pipework and DHW was operating intermittently. The leak was fixed and the system flushed several times before adding inhibitor and all worked OK for about 6 months. Now the intermittent DHW fault has returned. The DHW will usually work if the CH is run for a few seconds first. I think there is a partial blockage in the DHW circuit which is causing the bypass to operate which in turn causes the main flow switch pin to retract, shutting down the boiler. If the CH is on first the pin is already pushed out and there seems to be enough force with DHW operating to keep the pin out. I've removed the diverter valve and DHW heat exchanger and cleaned both internally. When flushed under a tap there is good flow through both sides of the heat exchanger. I did this until the water ran clear of black particles. The fault persists after reassembling. I think that the only other place for a blockage is in the brass return group housing. I'm planning to remove the DHW heat exchanger and pump to check if there is a free passage within the brass housing for the DHW return water. I'd be grateful if anyone has any comments on my theory and approach.
Thanks.

 

[Agile]

You have correctly identified the problem. Except that the problem is in the flow block.

But to cure it you need to use acidic chemicals and not just washing specs out.

When the problem first occurred it should have been properly treated with the right chemicals and power flushing followed by the installation of a magnetic filter.

It sounds as if this is someone else's boiler that you are working on but without the full experience that a proper boiler engineer would have.

Tony

 

[IKM]

Thanks for quick response. Am I right in thinking that the flow block you refer to is the dhw heat exchanger? I could fit a new one at reasonable cost to get the dhw working reliably then my daughter can think about a replacement boiler, power flush and filter. The current boiler is 13 years old. Does this sound OK.
Thanks again.

 

[Agile]

No, its the brass block on the left.

Although the blockage could be anywhere in the system including a weak pump or partially blocked impeller.

Tony

 

[Agile]

Although getting on a bit, they are quite reasonable boiler if properly maintained by someone who understands them.

No immediate need to replace it.

But a magnetic filter is a good idea now to catch circulating dirt.

Tony

 

[IKM]

OK thanks again Tony. I've already given the left hand block a good clean but I'll have another look at the weekend.
Iain

 

[Agile]

The only way to clean them is with acidic chemicals.

There are lots of hidden drillings inside.

Tony

 

[IKM]

Had another try at fixing the boiler today. Stripped the diverter valve and fitted repair kit BI1141501 and new diaphragm BI1011103. Also fitted new dhw heat exchanger. The boiler now works ok on CH and HW modes but I'm not convinced it's fully fixed. In CH mode the flow switch pin pushes out to its fullest extent. In HW mode it pushes out significantly less but just enough to operate the microswitch. While the diverter valve was out I checked for correct operation of the valves and that all waterways including the two leading to the diaphragm chamber were absolutely clear. I also check there was no blockage in the return block. I'm thinking that the bush for the flow switch pin may be a bit tight. I can replace this easily enough but I'm wondering if I'm missing something?
Thanks again for any advice.

 

[Agile]

You seem to be missing most of the advice that I gave you!

And the pump impeller?

I use chemicals to clean the whole boiler simply because I cannot see inside it and cleaning everything simply cleans everything!

Only you can judge the gland tightness. Unlikely to be a problem in my view but when apart should move with a female small finger pressure.

Tony

 

[IKM]

Ok, thanks for reply. I'll leave the boiler for now as it's currently working ok but in case the problem recurs I was interested to know if it's normal for the flow switch pin to push less for HW than for CH.
I didn't investigate the pump impeller because it's common to both CH and HW and the CH has always been ok.
Thanks again.

 

[Agile]

The hydraulic circuits are different and so the pressure applied to the diaphragm is different.

Its not a case of the pump pressure "working" on both. Its a question of "is the pump pushing as hard as it could" !

You need it to push the pin out fully on both!

Tony

 

[IKM]

Thanks Tony. Your advice has been very helpful.
Iain