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Corroded vent pipe  Rating:  Rating
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 Posted: Tue Oct 20th, 2009 02:07 pm
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GSlahor
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I'm new to the forum, so please forgive if the question has been asked before.

My gas hot water heater flue piping is constantly rusting. New piping lasts no more than 3 years before it's rusted completely through.  I don't think it's normal ( OR IS IT ?)

House was built new in 1997. Vent piping is 4" galvanized. Hot water heater flue is approx 6' long and is routed uphill and over to combine with gas furnace flue (8" double wall pipe), which runs vertical to roof. It's open (not blocked) all the way to the roof, and has a good natural draft.

I'm on my 2nd hot water heater (both professionally installed), and both units experienced the same accellerated flue vent pipe rusting, so I assume it's not a problem with the heater or it's burner, but rather something corrosive in the flue gas(?)

Any help would be appreciated. The corrosion is spreading to the main flue pipe running up through the house, which could get dangerous if it rusts through (as well as expensive to replace)

Gary

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 Posted: Tue Oct 20th, 2009 05:13 pm
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eleent
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Hello:  Two questions to start; is the furnace power vented and are you able to post a photo of the vent set-up?  Also, is it a bigger heater, so four inch vent is the right size?  You may be getting cooler fumes from the furnace blowing back through the water heater vent.  Rusting through so fast is not normal.

Yours,  Larry

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 Posted: Tue Oct 20th, 2009 08:17 pm
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GSlahor
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Larry.....Thanks for the feedback. It's nice to know that it's not unreasonable to expect longer life out of the flue.

Furnace is natural draft - not power vented.... and water heater was replaced with same size.  I believe 60 gal...but I can check.  Are you suggesting that maybe original 4" flue was too small ? ?

I'll try and post a pic, if I can figure out how...

Gary

 

 

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 Posted: Wed Oct 21st, 2009 03:54 am
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eleent
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Hello  Here is a simple, but telling test.  When the water heater first fires up, a little heat will spill from the draft hood.  It should only spill for a second or two under normal circumstances.  So, with the furnace off, turn the temperature on your heater up to cause it to fire up (you might make a mark on the thermostat dial so it can be put back to the same place).  Quickly, put the back of your hand to the draft hood to feel any heat spilling out.  Count the seconds.  Return the heater to normal.  Now wait a bit and fire up the furnace.  Feel for any spillage at the water heater draft hood with the water heater off.  Count seconds.  Now fire up the water heater (furnace running) and check for spillage again.  Count seconds.  Do let us know what you find out ;)

Yours,  Larry

ps.  If you want to be really thorough, do the same tests with every exhaust fan in the house running and windows closed.

Last edited on Wed Oct 21st, 2009 03:55 am by eleent

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 Posted: Wed Oct 21st, 2009 02:42 pm
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GSlahor
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If I did it right, there should be a pic of the water heater vent piping attached.

Pipe coming up in lower right is furnace flue.

Larry - I can certainly do the experiments you suggested - but what are we trying to prove ?

Gary

Attachment: Water heater flue1a.jpg (Downloaded 48 times)

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 Posted: Wed Oct 21st, 2009 05:49 pm
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eleent
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Hello:  Thanks for the photo.  I think it shows streams of water running back down the inside of the pipe!  Of course it raises more questions :cool:  Is the draft hood for the water heater three or four inch?  (you'll need to lift up the vent pipe or feel up from inside to see if the draft hood is made for smaller pipe)  Is there evidence of backdrafting (rusting etc) on top of the heater, by the draft hood? Is the furnace room particularly cold or wet? (this would speed up condensation in the flue pipe)  You say the natural draft is good.  What is the evidence for that?  The main vent pipe is is eight inch, but the two vents going to it are both four inch.  That bigger vent might be oversized, (hard to warm properly for good drafting).  Do we know there is a good cap on the vent termination at the roof? (rain is not our friend) :P

All those tests I suggest are to learn how well the vent really works under different conditions.  I do see there is some rusting in the furnace vent as well, so it wouldn't be bad to figure this out.

Yours,  Larry

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 Posted: Wed Oct 21st, 2009 06:04 pm
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GSlahor
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A few questions I can answer now:

Basement is conditioned space, so same temps/humidity as house. There are several supply registers discharging conditioned supply air into basement.

Rain cap at roof just recently rusted off, due, I beleive, to this same condition.....but has been in place up till then. I climbed up on roof....trimmed back end of stack with tin snips and re-attached rain cap w/ zap screws. Looking down stack appears clear.

When cleaning out furnace for fall check-up...I can feel a pull on the stack....thus my thinking that stack is clear.  The water heater stack has been rusting since house was built, and stack was new, so unless it was blocked from the beginning....I can't fault it.

Tonight I'll try and do some of the testing you suggested and let you kow the results.

Gary

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 Posted: Thu Oct 22nd, 2009 12:51 pm
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GSlahor
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OK...I did some testing last night.

1. I must appologize for mis-representing duct sizes. When actually measuring with tape, furnace and heater duct is 3" (not 4")...and combined flue is 5" (not 8")

2. I removed ductwork from heater hood and measured size of connection at 3"

3. Furnace and heater off. Turned heater on and felt for draft at top hood as heater starts up. No apparent leakage outward. Waited 3-5 min...stil no outward leakage.

4. Heater off.  Started furnace. Significant outward flow of air at heater hood for approx 1 minute during furnace purge cycle, but it stops quickly after furnace burner lights. No further outward leakage.

5. Repeated above, but left water heater running when starting furnace. Same results, but this time the significat outward flow during purge cycle was hot air rather than cold air.

6. No visible rusting of top of water heater, but there is significant amounts of rust flakes all over everything from ductwork corroding. I'm getting close to needing to replace all the heater duct once again ! !

I was thinking of wrapping the water heater duct with insullation to keep the flue gas from condensing on it's way to the combined stack.....but was afraid that all I'd do was move  the condensing into the combined stack where it might do worse damage.

Thanks for the assistance so far, and let me know what you think of the above.

Gary

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 Posted: Thu Oct 22nd, 2009 10:18 pm
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eleent
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Hello:  I think the problem is identified.  You remember early on I asked if the furnace was power vent.  It sounds like there is some power venting happening with the purge cycle.  (Does it vent without fan assist when firing?)  To make a broad statement; any fan will overpower any natural draft.  Having the vents combined must cause problems.  Likely the right fix is to run two, separate three inch vents.  I suppose you could play with how the vents combine into one pipe, but success that way may be hard to find :?

Yours,  Larry

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 Posted: Fri Oct 23rd, 2009 05:01 pm
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GSlahor
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The fact that the backflow stops after the furnace burner lights would seem to indicate that the power fan does not keep running, once the burner lights. Otherwise the backflow out the heater's draft hood would continue.

Do you think that his whole problem exists because the purge fan causes backflow for 60 sec each time the furnace burner lights ? Furnace is in use only during the winter months, and water heater would have to be firing to create products of combustion. Otherwise backflow is only cool air....

Would there be any merit to inserting an in-line fan into the 3" water heater vent ?

Gary

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 Posted: Sat Oct 24th, 2009 04:13 am
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eleent
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Hello:  I am a bit confused about the power venting.  Usually I'd expect to see the fan continue to run when the burner fires.  Also, they can be noisy, so should be easy to hear operating, even if at a lower speed.

In the owner's book, there should be a description of the operating sequence for the furnace. 

The other thing was to check draft while any exhaust fans in the house are running.  This could give you more time backdrafting than the furnace alone seems to be creating.

If it's a wild goose chase, at least it's an interesting one :cool:

Yours,  Larry

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 Posted: Sat Oct 24th, 2009 04:42 pm
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energyexpert
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Larry,
Looking at the picture, the furnace vent terminates in the flare.  This exposes the WH vent to the furnace fan discharge pressure.  What if the furnace vent did not terminate at the flare but continued through the flare at 3" (3" pipe inside a 5" pipe) until just above the top side of the flare (5" side) of the WH vent?  When the gases leave the 3" furnace vent they will create a low pressure volume immediately around the outside of the 3" furnace vent.  This will then be above the WH flare and tend to draw on the WH vent rather than create a back pressure situation.
David

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 Posted: Sat Oct 24th, 2009 05:02 pm
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eleent
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Hello:  I agree that configuration would certainly work better than the current setup.  I'd like feedback on the other questions about drafting with fans on and when the exhaust blower is really running. 

It would be sad to give advice on fixing this problem just to have it fail because there's a commercial range hood in the kitchen that pulls on your hair :shock: and depressurizes the entire house.

Yours,  Larry

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 Posted: Mon Oct 26th, 2009 05:04 pm
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GSlahor
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I don't believe there is anything major in the house that will create a seriously negative pressure. Just a small range hood (bult into the microwave), and bathroom exhaust fans. Since it's just my wife and I, and we both work full time.....it would be extreemly unusual for even 2 of these to be running at the same time. We very seldom have any windows open either....just A/C in the summer and heat in the winter.

I can dig into the furace venting a bit more (power venting vs non) if it would be benificial ..but the draft at the water heater hood DEFINITELY reverses when the burner lights.... so I'm not sure it really matters (?). When the furnace is purging, air flows OUT of the water heater hood.....but as soon as the furnace lights off...the flow reverses and air is drawn INTO the water heater hood.

It doesn't seem like something that only happens for a few seconds (backflow during furnace purging)...and then only during the heating season, can be causing this problem, but maybe I'm not seeing something. At the rate this flue pipe is rotting out, something must be happening every time the water heater is firing.

Are the new water heaters so efficient that the flue gas is right at the condensing point ...and just a small drop in temp will condense out the corrosives ? Would it tell you anything if I measured the temperature of the flue gas leaving the hot water heater ? It's pretty warm though... I can't keep my hand on the flue pipe while the heater is running.

Still perplexed ....Gary

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 Posted: Tue Oct 27th, 2009 09:26 am
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eleent
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Hello:  A problem seems to be that we have pieces of evidence that seem to contradict each other, which only means we don't understand yet.  The seriously corroded vent pipe does not agree with backdrafting only happening for very short periods of time.  If it were so, the pipe wouldn't get and stay wet enough to rust.

High pressure always goes to lower pressure.  Even with a reconfigured vent pipe, there is bound to be some spillage from the furnace back down through the water heater vent.

One test to complete is to see if the fan really shuts off when the furnace fires.  Another is to sit with your favorite beverage and reading material next to the equipment and watch it through a complete cycle.  Sometimes there are surprises.

Good troubleshooting demands no assumptions.  I'll assume we all know that :cool:

Yours,  Larry

ps. Yes, newer heaters are designed to take as much heat as they can from flue gasses without allowing condensation for too long periods of time.  It's a balancing act.  Even slight negative pressure or difficulties in venting can have an effect.

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 Posted: Tue Oct 27th, 2009 12:06 pm
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GSlahor
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So.....if I understand from your 2nd sentance above..("The seriously corroded vent pipe does not agree with backdrafting only happening for very short periods of time")...you have concluded that the corrosion is due to backdrafting from the furnace ? ....thus the corrosion in the water heater flue is due to combustion products in the furnace gas making their way back down the heater flue and condensing on the walls of this duct......

If that's the case, and since I need to replace the water heater's flue once again.... I kind of like David's idea of reconfiguring the furnace flue to extend up the 5" tee past the entry point of the heater flue, thus creating a low pressure behind the furnace flue. I believe I will reconfigure the furnace and water heater flue piping to match his discription and see if it improves the situation.

Last edited on Tue Oct 27th, 2009 02:36 pm by GSlahor

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 Posted: Sat Jul 24th, 2010 11:23 pm
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sky_tech
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I realize this is an older post, but something to consider anytime you are confronted with excessive corrosion of vents, burners, or heat exchangers, is contamination of the supply air.  It turns out that traces of chlorine bleach vapors from clothes dryer vents can do considerable damage when ingested by a burner and then deposited with condensation onto metal surfaces such as vents, etc.  This turned out to be the cause of widespread failure of some home furnace heat exchanges years ago when high-efficiency furnaces first became popular.

A long-shot, but worth keeping in mind.

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