Moisture inside air intake venturi = flue gas recirculation?

sunlight33

The air intake venturi (black plastic tube) on my Viessmann 200-W sometimes has moisture inside the opening, is this normal or a sign of flue gas recirculation?

EBEBRATT-Ed

Maybe, maybe not. If warm air from outside migrates down the intake into the boiler compartment and the boiler area is cooler you could get condensation. Probably unlikely

Are the flue outlet and intake properly separated and not in an inside corner?

sunlight33

It's single pipe venting, flue goes out the side of the house, intake is simply the air inside the house. It's likely the pressure of the first floor house where the boiler is located is lower than outside, worse during winter, then the negative pressure can suck the flue back in once the boiler burner is off.
Any easier way to resolve this other than convert to two pipe venting?

EBEBRATT-Ed

not that I can think of. In theory hooked up 2 pipe both pipes inside and out are under the same pressure on both ends so you shouldn't get much flow through them when off....in theory

sunlight33

My exhaust pipe is 2 inches (15 feet long), intake vent pipe would be slightly longer (18-20 feet), is there any benefit of using 3 inches for intake as opposed to 2?

mattmia2

Look in the manual, it will give you the maximum lengths. Be sure to add in the equivalent length of the fittings.

sunlight33

Yes I will.
If it's very windy outside, will there be issue preventing intake pipe from drawing air in?