I want to put an extension to the back of my house and our combi boiler blow off pipe and vent will come into the equation.
Can I run the blow off pipe down the wall and under the floor or into the soil pipe? Also can I put the vent up through the new roof?

John Belshaw, September 2014

You can run the discharge pipe outside the new extension but it must terminate 100mm above the ground (so that you can see if there's a problem) but the flue is a different matter,the air intake is balanced to the length of pipe so it will have to be re-balanced otherwise boiler will not burn correctly

toptrish, September 2014