When I talk to people in the industry –
plumbers, designers, and engineers – I find a lot of confusion
surrounding proper venting techniques. Venting in a piping system is
based on one simple principle. If you place a drinking straw in a
glass of water, then place your finger over the end of the straw and
remove it, the straw stays filled with water. The water in the straw
cannot flow out until air is allowed in. The straw is air-locked.
In a piping system, as fluid flows
through the pipe, air is pushed ahead of the fluid and air must enter
the system behind the fluid. This creates pressure changes in the
system with pressure ahead of the flow increasing and pressure behind
the flow decreasing. In a closed system the flow would stop, as it
does in the straw. In an improperly vented plumbing system the flow
is restricted, pressure on downstream traps can cause sewer gasses to
push through, and negative pressure on upstream traps can siphon them
dry. The restricted flow can also cause stoppages.
The principles behind proper venting
allow these pressure changes to be eliminated. The most basic and
effective vent system would be to individually vent every fixture in
the system, and some engineers use this configuration for its
simplicity. But this is lazy design and adds unnecessary piping which
in turn adds cost to the customer. It is not necessary to
individually vent every fixture in order to create a fully functional
vent system. With a little thought, vents and their related cost can
be eliminated without changing the effectiveness of the system.
Two of the chief confusions I find
comes from terminology used and proper application of principles. I
will focus on three main concepts that represent the same basic
principle and seems to create the most confusion – Wet Vents,
Circuit Vents, and Loop Vents. Keep in mind our guiding principle –
fluid flowing in a pipe creates pressure fluctuations. We want to
limit those fluctuations and allow air to freely enter and exit the
system to keep the fluids flowing.
Circuit Vents and Loop Vents are in
essence the same thing. In both cases a vent is extended up from the
horizontal piping between the last two fixtures on a branch in order
to vent a group of fixtures. The only difference is where the vent
terminates. In a Loop Vent the vent loops back to the Stack Vent.
This is only possible on a stack with one branch interval or the
topmost interval of a multi-interval stack. Circuit Vents terminate
individually or tie into a vent stack or vent header.
Loop Venting has advantages over
Circuit Venting as it creates a relief vented loop with a vent before
the first fixture on the branch (the Stack Vent) and a vent before
the last fixture on the branch which are then tied together providing
circular air flow within the branch. We will discuss relief venting
later. In the Circuit vent we are limited to a single vent to provide
air to our system. Yet often that is all we need.
In a Circuit Vent, as in a Loop Vent,
the horizontal branch line is vented, providing a common vent for all
fixtures connected to the branch. This eliminates the need for
individual vents. We are limited to eight total fixtures on the
branch, and only four can be water closets unless we add another vent
before the first fixture – a Relief Vent. With the relief vent, or
with a Loop Vent (which provides its own relief vent) we can vent up
to eight water closets. That eliminates up to seven individual vents!
We can also connect multiple circuit
vented branches in series, so in essence we only need one vertical
vent for every eight fixtures on the horizontal branch (Remember:
only four can be water closets unless we relief vent). In
applications such as gang toilets or gang showers the cost savings
for the customer really add up. And we are still providing ample air
flow to our system. The principle behind this technique is based on
the physics of fluid flowing in the pipes. In a properly sized and
graded horizontal drain line (the grade on a circuit or loop vented
drain should be no more than 1:12) the fluid stays in the lower half
of the pipe, leaving air in the top half. This allows the drain
itself to partially act as a vent.
That brings us to the Wet Vent. A wet
vent is simply a vent that is also used as a waste for another
fixture – generally a lavatory or sink drain. In a bathroom group,
or two adjacent bathroom groups, all of the fixtures can be vented
through the lavatories as a wet vent. This configuration is very much
like the Circuit or Loop Vent – the fixtures on the horizontal
branch are commonly vented through the waste riser serving the
lavatories – but the wet vent can be the last fixture on the
branch. It is also permissible in Circuit Vents and Loop Vents to
utilize a Wet Vent for any or all of the required vents or relief
vents mentioned previously.
A Wet Vent can also be used to vent
another fixture as a Common Vent, such as venting a floor drain with
a lavatory drain. Also, one fixture above another on a stack, such as
a sink connected above a washing machine or a lavatory above a water
cooler, can act as a wet vent for the lower fixture. We can even
stack fixtures floor-to-floor, such as hand sinks or mop basins on
multiple floors connected to a single vented waste stack. There are
of course restrictions to what fixture drains can serve as wet vents,
and pipe sizes required, but often one vent can serve multiple
fixtures and thereby lower costs and complexity of the plumbing
system.
Proper venting is not that complicated
when we keep the basic principle of pressure changes in mind and
follow the simple sizing and maximum fixture loads spelled out (with
charts!) in the NC Plumbing Code. And when in doubt, an extra vent
here or there will not hurt anything. You cannot over vent, but you
can waste the customer's money by adding unneeded vents. The balance
we seek is adequate air flow with minimum vent piping for the most
efficient design and installation.
A few points to keep in mind:
- Eight fixtures on a single branch (no more than four water closets) can be vented with a single vent located between the last two fixtures on the branch – Circuit Vent
- More than four water closets on a branch requires a relief vent before the first fixture on the branch – Circuit Vent or Loop Vent
- A Loop Vent is, in essence, a Circuit Vent tied back into the branch's relief vent (Stack Vent)
- A Wet Vent is a vent that receives discharge from another fixture
- A Wet Vent can be used to vent two complete bathroom groups
- A Wet Vent can be used as the vent and/or the relief vent on a Circuit or Loop Vented branch
- Multiple fixtures can drain into a single, properly vented stack
A few “Gotchas” in Mecklenburg
County:
- Urinals on a Circuit or Loop Vented line should tie-in to the branch on the horizontal and cannot be used as a Wet Vent
- Fixtures downstream of a water closet must be individually vented (some inspectors allow floor drains on a Circuit Vent with water closets – but better to individual/common vent and be safe)
- Pumped lines, such as a washing machine drain, cannot be used as a wet vent. Some inspectors consider a dishwasher to be a pump discharge – or a pump from an icemaker or condensate line.
In closing; I would suggest a quick
reading of the NC Plumbing Code, Chapter 9 – Vents. It is very
clear on when and where vents should be placed. I will also suggest a
quick Google Image Search of Circuit Vent. As I said at the
beginning, much of the confusion seems to stem from the terminology.
There is very little difference between Circuit, Loop, and Wet Vents.
They are all based on the same principles.
