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Since the annual service of my traditional gas boiler the house takes considerably longer to heat up. The boiler flame no longer seems as "fierce" as previously. I'm told the burner pressure and temperature flow rates on the boiler are correct and that the input gas pressure is just about within the expected range. However it was observed (in passing) that the input gas pressure does fall "significantly" when the boiler calls for heat compared with the gas pressure being delivered to the boiler when it's off.

Could this be causing the problem? Is this something that can be fixed? Does it mean an adjustment, a new part or a complete new boiler?

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When the boiler is not heating, no gas is flowing, and you will measure the static input pressure of the gas supply line. The input pressure at the boiler drops when the boiler calls for heat because the gas is flowing during heating, and pressure drops occur in the gas piping upstream from your boiler due to the gas flow.

Without knowing the flowing gas input pressure before the annual service, it's not possible to say whether the present situation is abnormal or the service is the cause of less fierce flame. But an increased input pressure drop compared to before could be caused by increased obstruction in the upstream piping (flexible gas pipe being partially crushed, corrosion or debris and narrowing of rigid pipe, etc.) or decreased gas flow resistance in the boiler after servicing (removal of build-up from gas orifices and plenum in the boiler?). Another cause of could be a new gas leak in the boiler after the gas valve, however I would think you would smell gas if this were the case.

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