Can we install a tundish and route through a water free tap to relieve the pressure and temperature of a boiler PRV?

Question from R Shaw, via email:
If the pressure and temperature relief of an unvented cylinder can be routed through a water free trap to a waste pipe, why can’t we install a tundish and same arrangement for a boiler PRV? I noticed that ATAG Boilers combine its PRV and condensate internally. How come we are not allowed to achieve this externally with other boiler manufacturers?

Response from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
I agree with Mr Shaw, it is very frustrating and limiting that there doesn’t appear to be an allowance to terminate via a tundish. I think it’s one of the older parts of the standard that simply hasn’t been challenged before and at the time of writing the standard was the right thing to do.

Under the essential parts of the Gas Appliance Directive the control system on a boiler must shut the boiler down before it reaches 110ºC, therefore if you combine 110ºC with a potential pressure of 3 bar then it is virtually super-heated steam leaving the PRV, when it initially opens. This would of course melt or distort any plastic waste pipe as even the strongest of plastic pipe systems can only sustain temperatures of up to around 90ºC.

Personally I believe this is now out of date and the very precise temperature control of gas-fired boilers today means this 110ºC temperature would almost certainly not be reached. It is possible however to get temperatures up around the high 90’s so regrettably a plastic waste pipe system could be affected by the initial release of water from the PRV.

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