Purpose of a condensate trap?

[Bill Gardener]

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?

Bill

[Christian McArdle]

The trap does two things.

1. It prevents a direct air path to the drain, prevent smells and draughts.

1. 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.

Christian.

[Tim Mitchell]

In article , Christian McArdle writes

Isn't that the other way round, doesn't it prevent a direct air path to the flue, which is full of nasty combustion products? Otherwise you would get flue gases out of the condensate pipe.

[Andy Hall]

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 a pump.

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. .andy

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[Tony Bryer]

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

[Christian McArdle]

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 suppose.

Christian.

[Tim Mitchell]

In article , Christian McArdle writes

Yes but some condensate pipes are internal to the house (ours is, it T-s into the washing machine drain pipe). You wouldn't want flue gases coming out of it.

[Christian McArdle]

Indeed, which is one reason why you have the trap!

Christian.