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Getting noise from an Aztec heater. Where should the expansion vessel be fitted?

Question from Donald bryars, via email:
I have a Trianco Aztec 12 kw electric heater, which supplies hot water to 5 radiators andhot water to coil in anunvented hot water cylinder. The unvented cylinder is is fitted upstairs and has a pump fitted at the inputend of the heater.

Should there be an expansion vesselfitted before or after thepump? AsI am getting a noise coming from the Aztec heater and it is switching on/off.It runs for a while then does it again.

Could this be that air is getting into Aztec heater?

Regards.

Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
It is preferable to have the expansion vessel fitted to the inlet pipe of the pump as the charge vessel within the expansion vessel will add to the pump pressure being generated.

This could be one of the reasons the system is operating as you describe.

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