Sink Pump Plumbing Questions

I'm installing a sink pump in my basement which does not have access to a drain unless I go up. After looking at several units, I'm thinking of using a Liberty 404 unit because it is compact enough to fit under a laundry sink. The document at http://www.libertypumps.com/Data/InstallationManual/7212000aWeb.pdf shows a "Typical Residential Installation" which appears to have the output of the washer going directly into the pump through a tee just above the trap. Is this going to handle the output of a modern washer or would it be better to have the washer discharge into the sink first instead. The Liberty unit has a 4.3 gallon tank. Also I have been told that I could use a Studor vent which is an AAV (Air Admittance Valve) as I don't have ready access to the vent stack in the location I want to put this. But then I read this on a site where I am thinking of purchasing the Liberty pump:

A.A.V. (Air Admittance Valve's) *** & Drain Pump - FAQ's

Q: Can I use an AAV (Automatic Air Vent) on a Reservoir Style Sink Drain Pump?

A: No - It is not recommended by the manufacturer.

We put this information about AAV's here because it is a Frequently Asked Question and people have been known to use AAV's on Drain Pumps although it is against advise.
(You ask why? OK picture the Tank of the drain pump, being both filled quickly from the inlet to the reservoir and discharged very rapidly by the pump. This causes both positive and a negative pressure effects; similar to air being both inhaled and exhaled through the vent stack. The problem is a typical AAV is designed to allow air to pass only one way. AAV's are designed to only allow air in, imagine "an inhale", an AAV does not allow an "exhale" of vent air from the Drain Pump Vessel and Drain Waste Vent System so back-pressure problems can occur. So an automatic or mechanical vent device can not be used since this type of vent typically allows air to pass only one way. Drain Pump Systems such as the one detailed on this web-page require venting in both directions. The system must be vented directly to the atmosphere in accordance with national and local plumbing codes. Using these types of vents may cause improper drainage of the fixture or affect pump cycling.)
We and the manufacturer do NOT suggest or and DO NOT recommend that an AAV be used on this application. Typically Compact Utility Drain Pumps are vented through the roof, that is the recommendation. We further refer you to your local codes for proper venting of any Compact Utility Drain Pump.

After reading this, I went back an asked the plumber I had talked to about this. He said I could still use the AAV as I am feeding into a 3" inch line which would serve as a "wet vent" which would allow air out. Is he correct in this assessment? And lastly,does anybody know if the Liberty unit is self priming? He suggested on another brand of sink pump that I put a hole in the line just above or below the pump inside the reservoir (I can't remember which right now) to release air or something.

The code requirement for a washing machine is to have a 2" standpipe, trap, and drain. The 404 only has 1-1/2" inlet and outlet and should not be used for direct drainage of a washing machine. It would be ok for use with a laundry sink with the sink basin is accepting the washer discharge, as the sink only requires a 1-1/2" trap and drain.

If you really want a direct drain for the washer, you could upgrade to the larger Liberty 405 system.

An AAV is never appropriate for a pump basin, as air must be able to flow both into and out of the basin. An AAV only allows air to flow inward. Installing a new vent in an existing home is more often than not a fairly easy task.

cacher_chick said:

The code requirement for a washing machine is to have a 2" standpipe, trap, and drain. The 404 only has 1-1/2" inlet and outlet and should not be used for direct drainage of a washing machine. It would be ok for use with a laundry sink with the sink basin is accepting the washer discharge, as the sink only requires a 1-1/2" trap and drain.

If you really want a direct drain for the washer, you could upgrade to the larger Liberty 405 system.

An AAV is never appropriate for a pump basin, as air must be able to flow both into and out of the basin. An AAV only allows air to flow inward. Installing a new vent in an existing home is more often than not a fairly easy task.

Click to expand...

Did you read what I said about a "wet vent?"

Yes, the person that told you that was mistaken.

The pump inlet is underwater. Flow of liquid into the basin must push air out of the basin. It cannot push anything out through the pump. It would be like trying to blow through a straw when the end is crimped shut.

A basin must have a real vent.