Weir-d Science, how a P-trap works

This picture post will attempt to show how a P-trap functions, or more importantly what makes them function.

Notice how the weir can see the vent opening?

P-traps have for the most part replaced most other kinds of traps—notably the S-trap.  The toilet is one notable exception—it is designed to be siphoning—the more siphoning the better.  You want to hear that glug glug glug.

S-trap

Many inspectors are under the false impression that merely lengthening the trap arm will eliminate the S-trap, but what this actually does is increase the potential for siphoning of the trap as the slug of water attempts to go down the drain.  It is a similar principle to how a toilet siphons.

Still an S-trap

The configuration still makes an “S,” just an odd shaped S is all.

What makes the P-trap function is the vent, and the rules that guide where that vent has to be. 

Essentially the vent has to be at least two pipe diameters away from the Weir.  The Weir is where water either starts to go down the drain after the trap or stops going down the drain.  So in a typical 1-1/2″ kitchen sink drain that would be a minimum of 3″ away.

The P-trap rules

We also have to be careful to not get the vent too far away from the weir as the trap arm could flood and block the vent–essentially returning the assembly to more of an S-trap. 

The rules of how far away it can be has to do with the diameter of the pipe (trap arm).  The bigger the diameter the longer the weir can “see” the vent.

Charles Buell, real estate inspections in Seattle

The Range Hood Exhaust–as Air Intake

Modern tight houses can easily become depressurized when exhaust fans are turned on. What this means is there is no place for the air to come into the home to replace the air that is trying to leave. If there are gaps around door weather-stripping, or gaps around window sashes or similar locations, the air will come into the home at these locations.  Sometimes even chimneys might be the path for this air.

If we operate and exhaust fan in a bathroom the house becomes depressurized, or an area of “lower pressure.” Areas of higher pressure will tend to make balance with areas of lower pressure, so the air outside the building literally “pushes” its way into the area of lower pressure.

Most houses are not tight enough for the air to not find its way in somewhere, and general infiltration was once allowed to be the source of this air replacement.

This small condo unit was too tight for general infiltration to be the source of make-up air, as was evidenced by its finding a path through the range hood exhaust.

Most range hoods have a back-draft damper in them, but there should also be one in the cap at the exterior of the building as well. You can see in this picture there is no damper—but there is a screen.

Exhaust fan vent termination with no back-draft damper

With two bathroom exhaust fans and the laundry exhaust fan running, the purple/violet colors of the thermal image of the chase and microwave/hood shows cold air cooling the chase and the area around the microwave.

  

The screen at the exterior cap location did hold a tissue paper to show that indeed air was pushing its way through the microwave/hood.

So, let’s say we “fix” the cap at the exterior with a proper back-draft damper. Where will replacement air come from? General infiltration may still be adequate, it is just easier coming from where it is now. If it is not adequate, the functionality of the exhaust fans will be reduced. In other words, they will make noise but not exhaust enough air from the room. It is like turning a 100-cfm fan into a 50-cfm fan.

For exhaust fans to do their job, replacement air is necessary and is required by modern codes when houses get to a certain point of air-tightness. This one may be at that point, even though it is an older home in that respect.

Some “positive” means of allowing exterior air to enter the home may be indicated if exhaust fans do not function properly after the exterior cap is repaired and its back-draft damper installed.

By Charles Buell, Real Estate Inspections in Seattle

If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email! 🙂

Telling the difference between a gate and a wall

There are lots of metaphors related to the paths our lives can take.  There are even metaphors of metaphors.

nowheregateWe like to think that we can pretty much do whatever we want.  The reality, for most of us, is that our choices are limited by where we have been–where we have come from–and the many restrictions our culture places on us.  Most of the time it is difficult to tell, except in retrospect, that we have truly chosen.  If the outcome turns out “OK” we take credit.  If the outcome is less desirable we are not quite so willing to take responsibility for what has happened.

Our “choices” more closely resembles our being driven–as opposed to our choosing.

Sometimes the road blocks are obvious—sometimes not.  They are mostly of our own making but often there are limits based on the culture we grow up in.

Freedom beyond being a “state of mind” is very difficult to pin down.

Ultimately, that is all it can ever be.

Some paths appear blocked, we know they are blocked.  We may even know they are supposed to be blocked and yet we still look for a way around or through them.

Such is the case with the gate in the above picture.

Why do we waste time–sometimes our whole lives–on paths that are obviously blocked?  Sometimes we have to look very closely to tell the difference between a gate and a wall.

While some obstacles along the way are meant to be pushed out of the way, climbed over or even ignored–sometimes we need to learn when it is more efficient to go around.

By Charles Buell, Real Estate Inspections in Seattle

If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email! 🙂