Ideal Logic combi. The mfr's instructions show the combi boiler condensate drain running into a branch waste pipe downstream of any other appliances (bath, basin etc).

I understand this is so CO from the flue can't or is less likely to find its way into the house if there's a fault with the boiler and the U-bend of one of the appliances has evaporated no longer forming a seal.

Is this an inviolable rule or is there a way round it? A Macalpine has been mentioned, but I don't know if this is a special trap, a condensate pump, or what.

I've had previous boilers condense into the branch below the kitchen sink without comment from a Gas Safe person.

I have a long way between the boiler and the stack, and a bath, basin, sink, washing machine, and potentially a dishwasher all on that branch pipe. Getting a dedicated condensate pipe all the way to the stack is almost impossible. External condensate soakaway is not viable.

Boiler will be put in by a Gas Safe but I'm trying to anticipate difficulties rather than them happening on the day (and it is only one day allocated for a full heating system in 6 rooms, so everything's got to run smoothly!)

Owain