Highflow 400 RSF flue getting very hot

21 Nov 2008
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United Kingdom
Help -I've got a Highflow 400 RSF with vertical flue. It recently started getting very hot especially when running hot water. This actually caused the venturi to melt.

I've had the gas to water and water to water heat exchangers replace which has stopped the venturi melting. However, the boiler flue still gets very hot after the hot water has been running for several minutes (i.e. too hot to touch). If hot water is run after the central heating has been running for a while then the whole boiler seems to get even hotter and there is a hot plastic type smell.
The flue does get quite hot on central heating after a much longer period.
The hot water thermostat has also been replaced.

I've had the flue completely disassembled and put back together with no evidence of blockage or crossflow. There is a restrictor ring in the flue inlet stack and this is the correct size for the flue length.

Any suggestions on what the problem might be?

It seems that on this boiler the gas valve runs at full tilt for hot water regardless of the flow rate of the hot water tap. I assume it relies on the heatbank stat to control it. I've never had the gas switch off on hot water demand once started under the control of the hot water stat until you switch off the tap.

Could it be the pump running too slowly? The pump is set to full speed - 3.

Any advice appreciated
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