I was up on my roof today and noticed rust on the furnace vent. The vent is a typical 4″ galvinized pipe with a cap to keep the weather out. Its was rusting just below the cap. Im wondering if the cap is trapping the heat, and causing the zinc coating to fail, then rust.
Any Ideas on how to solve this? would a high temp or rust stop paint help?
thanks
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Replies
It's not the heat; heat won't accelerate the corrosion. Most likely it is the vapors of combustion products. They are acidic, and over time degrade the zinc. Even though the fittings are galvanized, the galvanizing is done to the sheet steel, so all the cut edges are exposed steel, which will corrode first, then migrating to the zinc.
They all do it. Paint may buy you some time, but around the acidic vapors corrosion is inevitable.
"It's not the heat; heat won't accelerate the corrosion. "Actually heat does accelerate any chemical reaction including corrosion.In fact heat is one of the methods used in accelerated life testing.I think that it work be more appropriate to say that heat does not cause the corrosion.
True, in general chemical reactions progress faster in warmer temperatures, but this rust requires moisture, which is dried off at the high temperatures. For this poster's particular situation there are corrosive combustion products at the root of the problem, not the high temperature degrading the metal in some way.
>>They all do it. No, they don't. I look at this stuff every day. The rust usually indicates a venting/draft problem. (At least in the climate - NW OH) I'm in.)I believe one should assume that rust indicates a problem unless shown otherwise - don't assume it is normal.
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Sojourners: Christians for Justice and Peace
wire brush any rust to eliminate any loose stuff. Degrease with mineral spirits as those heater exhausts sometimes are a bit oily. Don't get the solvent down the flue.
Give it two or three light coats of any of the better 'zinc rich cold galvanizing' spray paints. CRC makes a good one I use a lot of. Comes in the standard GRC blue label and is named, if I remember correctly, 'ZincIt' or something similar. Home Despot and most electrical supply houses carry it.
Good stuff. This is essentially the same stuff they use on guard rails and bridges where the zinc gets scratched or after welding. Supposed to last as long as the factory applied galvanizing.
As noted it won't last forever, what does, but after five years or so, if the rust returns, you can repeat the procedure.
THanks a lot for the help,
Is that zinc paint available at lowes or HD? in the spray paint Ilse?
My propane fired boiler corroded the galvanized flue pipe between the boiler and masonry chimney, in two years time. I just replaced it with 6 inch stainless steel pipe. It was way more than galvanized but it won't rot away again.
>>My propane fired boiler corroded the galvanized flue pipe between the boiler and masonry chimney, in two years time.You've got a significant combustion problem: putting in the stainless just treated the syptom - you need a good heating contractor, ASAP!
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Sojourners: Christians for Justice and Peace
Bob I had a factory rep for viessman out when I first noticed the condensation. At the time the house was still under construction and not occupied. He did test for CO the levels were ok. I think the problem was the boiler water would get cold in between minimal calls for heat and domestic hot water tank was not being used yet.
When boiler fired, the heat hit the cold cast iron jacket and would create a lot of condensation, the initial condensation before we moved in caused the most damage to the flue pipe. After the domestic hot water was hooked up and I turned up the heat the boiler stays warmer and the large amounts of condensation have been eliminated.
I used stainless the second time to avoid replacement in my lifetime.
Ahh.That's why you're not supposed to continue use the heat source used during construction.(At least, that's what the 80+ furnaces say - but that requirement is rarely followed.)
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Sojourners: Christians for Justice and Peace
Bob walker,
Thanks for the pics, thoes are all A LOT worse than mine. Im looking at the flue off the HWH and there is a few rust particles near the bonnet. The flue pipe for the furnace and HWH are both in great condidtion. There are no signs of deteriotation even at joints. The Furnace vent and HWH vent meet at a "Y" fitting then go up through the attic and roof. The rust is only at the very top of the vent pipe. Its is maybe only 25% of the area of the pipe. Its about 9 years old.
Its odd because almost everyone eles in our subdivision has very similar looking vents(rusting)
We have a 80% carrier furnace if that matters.
Yeah, as I noted the pics I posted are extreme."25%" can still indicate problems - they may or may not show up under the bonnet depending on the stack configuration.I've tested some situations with that amount of rust where there were venting and CO problems, so that can't be ruled out, but the odds are lower.And the drafting problems will often show up only with winds from a certain direction.>>Its odd because almost everyone eles in our subdivision has very similar looking ventsBetcha they are only a foot or so above the ridge and within, oh, 3-4' horizontally.And I'll bet the rust is different depending on whichh way the houses face!And it is entirely possible everyone has the same venting problems!
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Sojourners: Christians for Justice and Peace
When they checked the boiler out the efficiency was about 89%. Viessman also started to recommend smaller size main flues due to ice formation near the top of chimney. Of course that came out after my chimney was all done. I have not had any icing problems, chimney is not on an exterior wall which helps.
This can indicate serious problems with you combustion appliances.
In my experience as a home inspector and combustion analyst, that rust often is a symptom of poor venting. (The more the rust, the higher the chances of a venting problem. The flues in the picture are pretty rusty, but I've seen problems with much less rust than those.) And poor venting can cause high CO levels.
The next place to look is at the top of your water heater, under the draft bonnet, looking for rust particles (see pic), or for significant rust inside the furnace draft hood (if atmospheric) or condensate marks on the bottom of, or under the draft inducer or excessive rust in the heat exchanger (or "leaking" out of the tubes if it is an inshot burner.)
Also look for condensate marks and deterioration of the flue in the furnace room at the flue joints.
One common cause (at least in my area) is the top of the flue too close to the roof, especially if it's near the ridge; or too long of a flue sticking above the roof.
Combustion problems with the furnace and/or water heater can also cause that rust.
And it is often an indicator of a carbon monoxide problem.
(I haven't done an exact study, but based on my experience since being trained in these issues, the odds are maybe 10-30% that noticable rust at the top of the flue indicates an immediate CO problem.)
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Sojourners: Christians for Justice and Peace
Read all the good advice given...Would like to add one thing.
The cap on my furnace flue was blown off in a wind storm last summer. The cap was covered with rust and the base strap had cracked from the corrosion. Obvisiously why it blew off. The old cap appeared to only be untreated iron.I replaced it with a higher quality Stainless cap. I noticed rust at the tob of the vent also, but I'm convinced it was caused from the original cap that failed. I was able to wipe most of it off. You might want to check the cap througly and determine were your rust is coming from.
Edited 4/6/2005 3:05 pm ET by hammer-n
thanks all for your help,
Im wondering if having the flue pipe below the ridgling is part of the problem. the rusting is facing the West and so is our neighbors. I know the builders had the same HVAC guys do all the buildings here. I believe they wanted the flue pipe to be below the ridge for asthectic reasons. I think they did put one about the ridge, and they complained that it looked bad.
So all I need to do is buy another piece of flue pipe and add it on? the taller pipe will draft bettter?
should I measure the pipe to make sure its 2ft above anything within 10ft?
Edited 4/6/2005 3:41 pm ET by LEDEBUHR1
The hate the thought that a contractor would weight aestetics over safety.
"To bad the family suffered from CO exposure, but their house has great curb appeal"
Do the measurements and add the extension is needed. Not sure if it's 2' over 10' or 6'. Maybe someone else can clarify.
Depending on the cap, 1' or 2' above any vertical surface (sidewal) withing 10'. Pluse the height above the roof itself depends on the pitch of the roof:flat to 7/12 1.0 ft
9/12 to 10/12 2.5 feet
11/12 to 12/12 4 feet.If it gets too long though, above the roof, it has to be boxed in.
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Sojourners: Christians for Justice and Peace