In a condenser boiler why can't the condensate go straight to the pipe that
leads to the outside drain? Why does it first need to go through a
condensate trap? What function does the trap perform?
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The trap does two things.
1. It prevents a direct air path to the drain, prevent smells and draughts.
2. A well designed syphonic one will save up condensate until there is a lot
and flush it all at once, like a urinal cistern. This helps prevent
freezing, as little drips freeze much more easily than half a litre of warm
liquid chucked down.
I meant from the boiler to the drain. The internal design of the boiler will
determine the repercussions of such a draught and its direction. I probably
shouldn't have mentioned smells, though, as they would go up the flue, I
Depending on the design of the boiler, the inside of the case will be
slightly under positive or negative pressure relative to the outside
air because of the operation of the fan.
In the better designs the case is under slight negative pressure, so
that if there is a slight leak in the sealing gasket, there will be a
tendency for air to be sucked in rather than fumes potentially being
pushed out into the room.
The condensate is run in some way initially from the heat exchanger,
then to the trap.
The trap prevents air from entering through the condensate drain
(would probably screw up the gas/air mixture and lead to incorrect
running) or fumes escaping into the room or outside through the drain
if it goes there.
The commissioning instructions sometimes call for water to be put into
the trap before firing up for the first time.
It is not assumed that condensate drains run immediately to the
outside. For example, the boiler may be installed in a basement, and
discharge condensate into a container which is emptied periodically by
Hence, there has to be a method to prevent the escape of fumes. You
might have read a recent post by Ed where he attended a boiler where
the drain cap had split. This is there to allow draining and
cleaning but must be in place during operation.
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It basically does the same as a WC trap in reverse: the
condensate can drain away but the flue gases have to go out the
flue. If you had a long flue and short condensate drain then
the exhaust would be likely to come out of the latter
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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