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Sub-Zero Temps, Plumbing Vents Frosted Shut

By
Home Inspector with Structure Tech Home Inspections

When there's a long period of sub-zero temperatures in Minnesota, plumbing vents often stop working.  The problem is that plumbing vents can get completely blocked shut with frost when it's very cold outside for a long period of time.  I've seen this at a lot of home inspections this winter.  I wrote about why houses need plumbing vents last month, which should help to explain why this stuff matters.

This first photo shows how the frost begins to form.  This is a 3" steel vent pipe at an old house in Minneapolis.  Frost was just beginning to form at the top, and could actually be seen from the ground.

Plumbing vent frost forming - three inch steel

This next photo shows a 4" cast iron vent pipe at a multi-family home in Saint Paul.  It wasn't completely blocked with frost... yet.

Plumbing vent frost forming - four inch steel

As the frost keeps accumulating with very cold temperatures, eventually the vents get completely blocked with frost, which prevents them from 'venting'.   I've included several photos here to show that all types of vents can be subject to frost closure, given the right weather conditions.

Plumbing vent frosted shut - three inch ABS

This next one just looked like it could use a cherry on top.

Plumbing vent frosted shut - two inch PVC with a cherry on top

Plumbing vent frosted shut - two inch steel

Plumbing vent frosted shut - three inch steel

The way to prevent plumbing vents from getting covered over with frost is to increase the size of the vent before it protrudes through the roof.  Here in Minnesota, the smallest size vent that can penetrate the roof surface is 2", according to Minnesota Plumbing Code section 4715.2530, Subp.2.

Keep in mind, however, that this is the minimum requirement.  That's all the code is; a minimum requirement.  As you can see from the photos above, this isn't necessarily a formula for success.  Many old-school plumbers in Minneapolis and Saint Paul knew about this problem, and it was common for old houses to have vents that were significantly increased in size before penetrating the roof, just to help prevent the vents from getting blocked by frost.

Vent size increased before roof

The photos below show how this looks in real life.

Plumbing vent increased in size before roof

Plumbing vent increased in size 2

Another way to help prevent plumbing vents from getting covered over with frost is to keep them short.  The plumbing code requires plumbing vents to terminate at least 12" above the surface of the roof to help prevent the vents from getting blocked with snow, but the higher the vent the greater the potential for getting blocked with frost.

In other words, the best height for a plumbing vent is 12" above the surface of the roof, and the wider the pipe the better.  If you look up at your roof from the ground and you can see frost accumulation at the plumbing vents, don't get too worried about it.   The frost will go away as soon as it warms up a little - maybe later in the day, definitely by April.

Show All Comments Sort:
Sally K. & David L. Hanson
EXP Realty 414-525-0563 - Brookfield, WI
WI Real Estate Agents - Luxury - Divorce

Tis the season for being ever watchful of this situation...and thank you for the remedy as well !

Feb 19, 2013 07:13 PM
Clint Mckie
Desert Sun Home, commercial Inspections - Carlsbad, NM
Desert Sun Home, Comm. Inspection 1-575-706-5586

Hi Reuben,

Where in the world would you find a cherry to add to the top of all that frost. I bet you stopped by a Sonic on the way to the inspection. :-) Really cool pictures

Have a good day in Minnesota.

Best, Clint McKie

Feb 19, 2013 07:42 PM
Scott Seaton Jr. SLS Home Insp
SLS Home Inspections-Kankakee Will Iroquois Grundy Counties - Bourbonnais, IL
The Home Inspector With a Heart!

So you're telling me that it gets cold in Minnesota? Who knew? Great pictures for sure. Stay warm.

Feb 19, 2013 08:50 PM
Gita Bantwal
RE/MAX Centre Realtors - Warwick, PA
REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel

Thanks for sharing the information and the pictures. Have a good day.

Feb 19, 2013 08:55 PM
Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Reuben, good information for homeowners in your area. Come April families will wonder why their plumbing works better.

Feb 19, 2013 08:55 PM
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

That's fascinating Reubs.  I don't think I have ever heard of that happening here.  And I love a maraschino cherry on my plumbing vent.  It looks great seeing as it's so cold there.

Who talked that last photo into that foam stuff?

Feb 19, 2013 10:09 PM
Linda Edelwich
William Raveis Real Estate - Glastonbury, CT
Serving Glastonbury & Beyond | 218 New London Trnk

simple solution to a costly repair potential! Great info. Love the cherry on top picture!

Feb 19, 2013 10:13 PM
Tom Jansson
Acuity Home Inspections - Chicago, IL
Chicago Area Home Inspector - InterNACHI Certified

Amazing!  I've always wondered if something like this could happen.  Thanks for sharing!

Feb 19, 2013 10:18 PM
Edward Gilmartin
CRE - Boston, MA

One would hope that contractors would have this information.

Feb 19, 2013 10:24 PM
Adam Tarr
MavRealty - Phoenix, AZ
PC -GRI, ABR, CDPE, RSPS, ePro - Designated Broker

Time to move to Arizona!  We don't have that problem in Phoenix!

Feb 19, 2013 10:59 PM
Richie Alan Naggar
people first...then business Ran Right Realty - Riverside, CA
agent & author

Fascinating subject matter professionally presented ...good post and thank you with a cherry on top too

Feb 19, 2013 11:41 PM
Charlie Dresen
The Group, Inc - Steamboat Springs, CO
Steamboat Springs, CO e-Pro

Thanks. 

Since we too spend a lot of time in certain areas around Steamboat Springs wayyyy below zero, your post was informative. Love the marachino cherry too-a little humor helps all posts!

Feb 20, 2013 01:08 AM
Steven Cook
No Longer Processing Mortgages. - Tacoma, WA

Reuben -- it would seem that there might be more chance of the pipes icing over, if there is lack of snow cover - at least gauging by your photos.  Do you know if that is the case?

Feb 20, 2013 03:52 AM
Robert Butler
Aspect Inspection - Montreal West Island, QC
Montreal Home Inspector | Aspect Inspection

Here, because of snow depth, the recommended height above the roof surface is 18". Don't always see it though.

I would have three questions about what is seen in the last photo.

 

Feb 20, 2013 10:05 PM
Donald Hester
NCW Home Inspections, LLC - Wenatchee, WA
NCW Home Inspections, LLC

Reuben  Awesome post. Rarely do I see that type of frost build up on the vents out here but you guys get longer cold spells in your neck of the woods than I do. The cherry was a nice touch ; )

Feb 20, 2013 10:59 PM
James Quarello
JRV Home Inspection Services, LLC - Wallingford, CT
Connecticut Home Inspector

I have never seen that happen here. While it gets cold, almost never below zero and if it does, it doesn't lasts long. 

Feb 22, 2013 07:29 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Sally & David - Thanks!

Clint - I had to leave the inspection, go to a Sonic, then come back to take the picture!  :-)

Scott - yeah, it's gets a little chilly around here... mostly between October and April.  It's not too bad for the other six months. 

Gita - thanks, you too.

Michael - definitely. 

Feb 24, 2013 03:30 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Jay - yeah, the frosted shut plumbing vent thing is a super-cold-climate phenomenon, for sure.   As for the foam, that stuff is getting more and more popular around here.  You'll be seeing more of it :)

Linda - thanks.

Tom - not often, but it certainly happens.  

Edward - they know, but most don't do anything to fix it.

Adam - I don't want to hear about it ;-)

 

Feb 24, 2013 03:33 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Richie - thanks!

Charlie - I couldn't help but add that cherry :)

Steven - I've never considered that possibility, but you're probably right.  It makes perfect sense.

Robert - what questions would you have with that last photo?  I might know who insulated that attic.  I could probably have them answer any questions you might have.

Donald - count your blessings.

James - see my comment above to Donald.  Same to you ;-)

Feb 24, 2013 03:39 AM