Steven L. Smith, Bellingham, WA Home Inspector (King of the House)


Plumbing Trap Air Gap (Bellingham Home Inspection) King of the House

I remember riding the subway in London many years ago. Over and over on the PA system you would hear the words -- "mind the gap." This warning referenced safety when getting on or off the train.

There is another kind of gap that enhances safety. At a potable water system, you do not want the sewer system, or gray water for that matter, contaminating the fresh water supply. It is necessary to avoid cross-connections.

The cross connection that I see most frequently in my area involves water heater temperature-pressure relief valve drain lines that are routed down below the flood rim of a plumbing trap that connects to the sewer line.  This is, also, seen at trap seal primers that are installed to keep infrequently used traps from going dry. Below is an unsanitary cross connection.

That piping, with the yellow on it, comes from the water heater.....potable water. The pure white drain line goes back to a furnace and different less strict, but specific, rules apply to HVAC condensate drains that discharge into plumbing traps.

To reduce the probability of cross-connections, "air gaps" (physical separation, 1" minimum space) are required between drain trap flood rims and lines that connect to the potable water system. If a trap is a receptor for discharge condensate from HVAC equipment, those drain lines must be installed with "air breaks" (lines may be below flood level of receptor but must be above trap seal).

I see this type of installation done wrong more often than not. Frankly, I do not know if HVAC technicians and plumbers are not well informed of this air gap requirement or if builders are skimping on costs and having their own workers go about this procedure in a haphazard manner.

The good news is that, regardless of the type of pipe used for the line, it is not too difficult to trim the pipe or otherwise modify the arrangement in a manner that establishes a proper air gap.

Steven L. Smith

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Comment balloon 7 commentsSteven L. Smith • February 15 2014 05:15PM


Hi Steven,

I write this up as well. The HVAC guys say I am over picky and everything is O.K. The JHA was there and passed the job. But the local JHA has no clue as to codes. IMHO.

Good post and good to see you back in the Rain.

Have a good evening in Bellingham.

Best, Clint McKie

Posted by Clint Mckie, Desert Sun Home, Comm. Inspection 1-575-706-5586 (Desert Sun Home, commercial Inspections) over 6 years ago

Steve, it does seem to happen more often than not for sure

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

You are right Steve, it can be an easy fix. I'm sure that I would have been done correctly if the installer were aware of the purpose.

Posted by Tom Arstingstall, General Contractor, Dry Rot, Water Damage Sacramento, El Dorado County - (916) 765-5366, General Contractor, Dry Rot and Water Damage (Dry Rot and Water Damage Mobile - 916-765-5366) over 6 years ago

I was going to say something about wanting to know the size of the pot that should have been used for the potable water, but it was too silly even for me, and I decided not to.

Say hi to Grandpa Steve for me Mason!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Clint, thanks for stopping by. I keep planning to get more active again and then my schedule interferes with plans.

Charlie, time and time again.

Tom, seems that would be the case.

Jay, even you ran out of quips...I do not believe it.


Posted by Steven L. Smith, Bellingham WA Home Inspector (King of the House Home Inspection, Inc.) over 6 years ago

Steve, Those simple little rules are just so complicated ;) Lucky it is a very easy correction.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago

Mason, I quipped without quipping!  That is pretty slick.  Even grandpa, as old as he is, would get that.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago