Boiler condensate & pipe discharge points for new builds?

4 Jan 2009
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True North
United Kingdom
A couple of questions here as I'm trying to plan out drains required & how many outlets from a plant room:

1. If fitting a gas boiler on a ground floor plant room external wall, where is it best to discharge the condensate and the pressure relief valve to, should they be through the external wall & into a gully drain directly outside, or should both or one of them be internally discharged to a pipe in the floor that then passes to outside system?

2. Should the condensate be connected to the foul drain or can it be surface water drain?

3. Does an unvented cylinder have it's own pressure relief discharge pipe?
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1. Internal for condensate and more manufactures are also allowing discharge of prv internally as well.

2. Definitely not surface water

3. Yes and it will be a larger size than the boilers
Thanks razor, is a system boiler PRV the same as a combi being 15mm? What size is the cylinder PRV? I don't really want to add a drain inside and another directly outside as that would be two 110mm drains unless the inside condensate one is different. If they both can't be inside, I'd rather have just one outside but there literally would be minimal pipe on show. Internal just looks nicer.
Unvented has 2 relief valves, one for the expansion and one for the temp and pressure relief, they then combine into one larger 22mm pipe (D2) that runs to a waste/drain or external.
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Thanks madrab. So with a combi or system boiler going to drain it will be a boiler condensate 21.5mm, boiler prv 15mm & a 2 into 1 22mm unvented pipe.
Worst case yes, the 2 into1 usually happens internally at the unvented cylinder before the tundish and then it's a 22mm out of that to the drain.

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