Sime super 90 – loud noise when calling DHW.

25 Nov 2005
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United Kingdom
Sime super 90 – loud noise when calling DHW.

The problem with this boiler is when the Domestic Hot Water (DHW) thermostat is set to Max, and there is a demand for DHW the boiler fires up normal, the flame keeps increasing so does the temperature of the water until it reached to 60 C. At this point the boiler starts making a lot of noise and vibrates, like it is going to explode. I believed this is coming from the circulating pump. This is not happing when the DHW thermostat at minimum or half way through the scale i.e. when the water temperature is below 60 C. the Central heating working o.k. Any idea why the boiler is behaving like this?

Many thanks.
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Dear all,
Searching this forum, I came across one post that suggests a swollen O ring in the diverter valve causing this problem . The post also suggests the water treatment may cause the O ring to swell. This does make sense as I had a leak in inaccessible place few months ago and I feed the system with sealant which seems to have worked and stopped the leak.
My question now ……., do I have to change the diverter valve ( I only replaced it summer 2006) Or replace the O ring, if that is the case, can buy the O ring individually or it comes with diverter valve service kit. Or do you think that the problem can be eliminated by cleaning the system i.e filling it and flushing it few times. Any reply would be highly appreciated.
If you have to try to fix it yourself then why not concentrate on cleaning the blocked plate heat exchanger?

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Many thanks for the reply. It sounds good place to start (i.e. cleaning the blocked plate heat exchanger) but I have not got the manual for this boiler as it was lost by the previous tenants. Is there any procedure that I can follow to clean it?. Can it be cleaned by using central heating cleaner such as Fernox. Is this the heat exchanger for both domestic hot water and Central heating?

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