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I have a 20 year old Gledhill Boiler Mate 2 thermal store (BM2). It's been working with a Potterton 24kW Pro Max boiler for many years. It was recently inspected and all working well.

The Potterton boiler died and was replaced with a Vaillant 32kW open vent heat only boiler, and the whole system was given a chemical flush. The following problems occurred that evening and getting worse daily:

1. The new boiler went into fault mode f.83 (sensors detecting insufficient temperature increases, after the burner has been lit.)
2. The BM2 developed glugging, flushing, dumping, crashing and banging noises. These were intermittent, but appeared cyclical. They do not happen constantly, they happen mostly between midnight and 6am (notably when the outside temperature is at its lowest). This is definitely a new thing since the boiler was changed.

The boiler is then either reset manually, or after a while automatically. The thermal store is sometimes cold as a result, and I wonder if recovering from cold to normal storage temperatures triggers the problem. However, I've read many articles that indicate this is likely to be a blockage in the cold fill pipe which will need a new bypass installing.

I'm looking for thoughts on the following please:
1. If the cold fill pipe is blocked would the symptoms be apparent all the time, or would there be periods of normal operation (we're getting a mix of both).
2. If it is related to air in the system would it lead to these BM2 problems. If so, wouldn't that be apparent whenever the heating is on?
3. If it is a BM2 pump fault wouldn't that be apparent whenever the heating is on?
4. Other thoughts on why this might happen mostly at night but work better during the day.

Just for clarity, the boiler does go into f.83 frequently during the day. I just don't hear the BM2 noises as oftern.

Many thanks to anyone who can offer knowledgeable info. I'm am untrained homewoner just trying to figure it all out.

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