P trap in exhaust system.

[Joe Hillmann]

I am remodeling my shop and that means moving my exhaust system.

Sometimes I forget to close all the blast gates on my exhaust system as soon as I am done running the lasers and during the winter I assume a lot of warm air finds its way out of the building because of open blast gates.

So my question is would putting some type of P trap on in my system prevent the warm air from leaving the building? Or will the air keep right on going as if it wasn't there?

[Scott Shepherd]

Or will the air keep right on going as if it wasn't there?

That's what would happen.

[Dan Hintz]

would putting some type of P trap on in my system prevent the warm air from leaving the building? Or will the air keep right on going as if it wasn't there?


That's what would happen.

I disagree. A water trap would be no different (actually, better) than the thin metal plate on a dryer vent. It will prevent the vast majority of warm air from escaping your room when the DC isn't running (actually, it will prevent the heavier cold air from pushing its way in).

But you'll likely have to use a longer P-trap than one off of the shelf. With fast-moving air, the water will quickly bubble up the side of the trap and flow out. So, you'll need to create your own trap with longer vertical sections. In addition, you'll probably want more water than usual in there to take care of losses, but not so much it comes to the top of the trap wall, so you'll need some way to look at how much water is in there and top it off from time to time.

Personally, for the trouble and minor efficiency gain, I'd probably just stick with a sealed blast gate.

[Joe Hillmann]

I should have been more specific. I mean just a P trap filled with air, not water.

[Richard Rumancik]

Joe, the P trap won't help. Dan is suggesting that cold air ingress is predominant over warm air escaping. I'm really not sure - I think you might see both happening depending on wind action and geometry of the exit port. A passive (one-way) damper in the loop will prevent cold air coming back in, but won't prevent warm air from escaping.

(You might be able to come up with an exhaust port on the roof or wherever that is less likely to cause a draft to be created in the exhaust system when the wind blows.)

Not sure how much money you want to spend to solve this - if you are willing to spend $50 -$100 you could get an electric damper on a 6" duct that is normally closed; wire it so it opens when the blower is turned on. This would prevent air movement both ways. If you don't want to spend that much then perhaps you could just make a reminder sign for the door . . .

[Joe Pelonio]

Wouldn't it be easier to put a sign near the power switch that says "Close Blast Gates" ? I have a hair-dryer vent on the end and the little flaps close when I turn off the fan.

[Mike Lassiter]

Joe a P trap in plumping works by blocking the flow of gases in the pipe using the liquid in the dip to make a seal that keeps smells from your DWV system from coming back into your house. If you install a P trap in you exhaust vent system it will not do anything as there is no liquid in the dip to do what you are thinking. It would merely be a bend in the piping which might hurt your air flow slightly on your exhaust system. Any bends create loss of flow when compared to a straight pipe.

I recirculate the air in my shop and avoid the whole issue.

[Dennis Rech]

I am remodeling my shop and that means moving my exhaust system.

Sometimes I forget to close all the blast gates on my exhaust system as soon as I am done running the lasers and during the winter I assume a lot of warm air finds its way out of the building because of open blast gates.

So my question is would putting some type of P trap on in my system prevent the warm air from leaving the building? Or will the air keep right on going as if it wasn't there?

A standard water filled P-trap has about 2 inches of water column in it.
This adds 2 inches of static loss to your exhaust fan.
Most fans on lasers have less than 4 inches of total static and likely only an inch or so of total static.
If you add a P-trap, you will likely have NO airflow through the exhaust system.
The correct method of closing off the vent when the system is not running is to add a simple back draft damper.
These can be either motorized (sort of expensive) or counterbalanced (rather cheap).
A counterbalanced back draft damper installed just prior to the exit point is the most common means of blocking drafts.
If you place the damper some distance from the discharge point the duct will become very cold and should be insulated to avoid duct heat loss.
Also, in very cold weather, your P-trap full of water will freeze.
Dennis

[Jeff Belany]

Penn State (and I assume others) have electric blast gates. You could have the switch right next to your laser switch.

Jeff in northern Wisconsin