Reuben's Home Inspection Blog


Brown Stains on Boilers are Bad News

The biggest problem I find with boilers on old Minneapolis and Saint Paul homes is leaking exhaust gas.  While every heating contractor knows to look for backdrafting at the vent while performing maintenance checkups on boilers, I've found that many contractors don't bother to check for exhaust gas leaking around the jacket of the boiler, which can be just as serious of an issue.

To identify these exhaust gas leaks, I start by just looking at the jacket, or 'cover' of the boiler.  If I find black or brown scorch marks around openings in the jacket, it's usually a dead giveaway that the boiler is leaking exhaust gas.

Scorching at boiler

Another obvious sign of exhaust gas leakage at a boiler is heavy rusting in one particular area on the jacket; if you see rusting like the kind pictured below, it's bad news for the boiler.

Rust hole in boiler jacket

To confirm these exhaust gas leaks, I use a combustible gas detector.  While the most common use for a combustible gas detector is to detect natural gas leaks, they can also be used to detect exhaust gas leakage.  When I inspect boilers, I check around the entire boiler for exhaust gas leaks while it's running.  A combustion gas analyzer can also be used to confirm exhaust gas leaks, but this method takes much more time, because there is no instant audible feedback.  The video below shows me finding just such a leak with my combustible gas detector - a TIF8800A.

If you don't own a combustible gas detector or a combustion gas analyzer, we can't hang... but you can still check for exhaust gas leakage on your boiler.  Simply hold a room-temperature mirror above the suspected areas.  If there is exhaust gas leaking out, it will fog up the mirror.  It probably won't be as dramatic as the fogged mirror shown below, but it's the same principal.

Backdrafting water heater

Exhaust gas leakage is a problem because it could contain carbon monoxide, which can be deadly in high enough concentrations.  Even if the exhaust gas leaking out of a boiler has a very low level of carbon monoxide, this is still a potential safety hazard that needs repair.  There is no acceptable amount of exhaust gas leakage.

The repair for leaking exhaust gas at a boiler is often to have the boiler replaced, which is an expensive repair.  In some cases, the exhaust gas leakage can be repaired by a qualified heating contractor who specializes in boiler repairs, but this is usually an expensive repair.


Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 7 commentsReuben Saltzman • July 26 2011 06:02AM


"Could" contain carbon monoxide?  We have neighborhoods here with old boilers almost all of which need serious care.  I find that if a unit is cleaned and "serviced" the contractors do not go about checking all the many, many small parts associated to see what needs replacement.

I suggested you, again.  You big baby.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Thanks Jay :)  Yeah, I suppose I could say the exhaust gas "will" contain carbon monoxide... 

I found this same thing during yesterday's inspection, and the boiler had service tags from the gas company for every fall.  They don't check this stuff!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Of course they don't check this stuff!  That's like the builder rep saying that the County inspects the house many times.  Really?  Like we do?

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Both of you are right. Most (inspectors for the county) are not trained in all aspects of inspecting like we are.

Clint mcKie

Posted by Clint Mckie, Desert Sun Home, Comm. Inspection 1-575-706-5586 (Desert Sun Home, commercial Inspections) over 7 years ago

Thanks for the tip, Reuben, the glass idea could be made part of routine home inspection...

Posted by Chris Smith, South Simcoe, Caledon, King, Orangeville Real Esta (Re/Max Chay Realty Inc., Brokerage) over 7 years ago


Great post. One way I keep up on gas fired appliances is reading. Love the mirror test. I live in the land of electricity and I must say that I do not see gas appliance as often I need to keep sharp.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 7 years ago

Jay - heck no, not like we do!  

Clint - Interesting side note on the topic of city / county inspectors: I recently called out a boiler for leaking exhaust gas.  The HVAC contractor went out, charged the client some money, said everything was fine.  I went back out to check on the work, and the boiler was still leaking.  I called the company, spoke with the tech, and was told that it was ok because it was only a small amount of CO leaking out.  He insisted it was safe.  I called the chief mechanical inspector for Minneapolis, and he backed me up.  At least they know the rules :)

Chris - and anyone can do it.

Donald - I've found electric heat much easier to inspect.  What do you have to deal with, other than installation defects?  Oh wait, I suppose you have heat pumps...


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago