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White Powder around Gas Vent

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8 hours ago, Homeowner in Ohio said:

I have  white gritty sand-like debri on top of my gas hot water tank and on the floor near it.  I'm guessing it's falling from the vent which goes directly to my roof.  This is not a powder like the earlier posts.  What is this, and what should I do?

Without pictures of the stuff and the entire configuration of the installation, my best advice is to sweep up all the debris, throw it out, and go have a beer. 

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On 3/24/2020 at 4:02 PM, hausdok said:


The white powdery substance you are seeing is mineral salts caused by a chemical reaction between the acids in the exhaust gases and the zinc contained in the galvanized pipe used for the single-walled vent connector. To make that relatable, think battery post corrosion in your engine compartment.

That length of single-walled pipe from the collar allows those exhaust gases to cool, get heavier and slow down way too early. When that happens, the gas cools to dewpoint and condenses on the inner walls of the vent somewhere above the connection between the single-walled connector and double-walled vent pipes. Wherever the acids in that condensation comes into contact with galvanized pipe, that reaction occurs. Then, when all of that excess condensation drains back to the joint between the vent and connector and leaks out of the joint, the water evaporates and leaves the salts behind.

That's why gas exhaust vents and connectors should be double-walled all the way from the collar of the appliance to terminus. The fuel-gas code used to require only B vents to the collar in attics and concealed spaces and anywhere considered to be cold, but it seemed like the majority of HVAC installers ignored that rule anyway if the furnace or water heater was installed where it's readily viewable in unheated areas like garages. Somewhere back around the 2003 - 2005 timeframe that requirement disappeared from the code and they allowed installation of single-walled connectors in areas other than the attic or concealed spaces as long as the area wasn't any colder than 5-deg. F. in winter. This despite the fact that a lot of condensation can occur even in vents that are luke-warm to the touch in such areas like garages. I'll bet you a box of donuts that most HVAC installers still couldn't care less and are still using as much single-walled material as they can, in order to squeeze as much profit as possible out of every job.

Too much salt indicates an issue with the exhaust vent system. A little bit of salts isn't really too much of a concern at a single-walled to double-walled connection, if there isn't any issue at the vent terminus (Excessive rusting, cracks, holes due to condensation and acidic vapor lingering around the vent once it leaves the pipe), but a lot of salt is telling you there's an issue with exhaust gases cooling too rapidly - especially if there is a lot of salt sifting back down from the vent connector at the collar and ending up in/on the furnace. Cause could be the single-walled connector from the collar, if it's too long and it's in an unheated (cold) area. The fix could be to replace the single-walled section with a double-walled section, but a lot of the time that doesn't necessarily stop it if the installer didn't properly calculate the vent size requirements using the vent sizing tables in the code or if he/she didn't pay attention to the rule about limits on number and degree of bends in the vent between collar and terminus - bends that violate the 'no bends greater than 45-deg but one bend of no more than 60-deg. rule, slow those gases. The more bends, the slower the gas moves - even if the furnace has an exhaust gas inducer. Around here those numbskulls will use two to four 90-deg. bends to detour around furnace filter access doors instead of using 45-deg. vents to go around. Another thing to look at is whether or not the appliance has enough makeup air. If they've installed it in confined space with insufficient makeup air, or in a laundry room where there's a clothes dryer and no makeup air openings to the outside, those gases will stall - even with an inducer integral to the appliance. There have been thousands of times I've stepped out of my vehicle, glanced up at a rooftop, saw a heavily-rusted vent terminus and knew right away that as soon as I checked out the furnace I was going to find issues with stalling exhaust gases. Sometimes, the issue was so bad that condensation would literally leak out of the vent connector connection at the collar and drip onto components in the controller bay - eating holes through the floor of the bay and sometimes causing electrical connections to severely corrode. So, when someone tells you, like the gent above, that the salts mean nothing, take it with a grain of salt and thoroughly inspect that vent installation anyway.

It's good that you're asking questions about this. I've always said that inspectors need, more than anything else, to understand basic building sciences, and need to look at the whole picture, not just the tick marks their report format asks them to check, that are taught by a lot of so-called inspection schools, in order to really do this job well and be able to make the client fully comprehend the implications of issues found.



This is all spot on, but Mike forgot to mention one of the most common problems that causes excessive build-up of this salt: short cycling. If the furnace is over sized, if the thermostat is poorly located near a supply register, if the burners are over-firing, if the high-temperature limit switch is failing, the furnace might be shutting down before the flue comes up to a steady-state temperature, allowing lots of condensation to build up in there. 

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