Author Topic: Central heater_white powder deposit-exhaust port; peeled paint in draft hood  (Read 2743 times)

Offline wizedup

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Possibly 2 separate issues on this nat. gas, forced air central heat unit in an attic.
Any ideas on causes?

* installed in attic
* in the deep, coastal south
* An old Singer, horizontal unit.

 1) See Pics. Grayish-white "dust"  deposited mostly on the interior metal surface, mostly inside the rectangular, exhaust port - venting from the burner / heat exchanger box.
Most of the white deposit is at the end of rectangular exhaust port - nearest blower.

Unit's burner & heat exchanger have been checked - OK - each of last several years (but not at winter's end - 2015).

Not sure if the whitish powder could be from
- Crack in heat exchanger
- burner defect, malfunction
- They're "Normal" deposits from nat. gas combustion on this age / type unit, in warm, humid climates.

2) See pic. Paint is curled or missing on INSIDE surface of the draft hood, that's mounted on side of main unit.
On older nat. gas heaters (in warm, humid climates), is peeling paint common - inside this style of draft hood?  Exterior paint is fine.

Apparently, this is 1st year that flakes of green paint were noticeable on the floor.


Offline Admin

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This is pretty normal in a lot of the older systems I see with metal venting.  It's usually caused by the flue gases condensing inside the venting.  The flue gases contain sulfur which is likely the cause of the white soot.  I would make sure the exhaust vent is functioning properly and is in fact venting and not spilling back into the attic.

The fact you are seeing paint beginning to peel inside the draft hood could indicate the flue products are spilling back down the venting.  How much venting is installed in the attic?  The venting is in an unconditioned space so the cold temperatures might be causing the condensation inside the venting.

If the burners are clean and the gas pressure is good then it's unlikely flame impingement is causing the problem.

Offline wizedup

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Thanks.  The vent (flue) connects to the side of the draft hood (in pic), takes a 90, then goes up at an approx. 35 - 45 deg. angle, to exit the roof.  Probably less than a 10 ft. run.  Has a pretty standard (in our area) roof jack for water heater / gas central heaters.

I can measure any of these approx. angles, lengths - if needed.
I'll check the roof jack cap openings for any obstructions (doubt that's it).
None of several techs from different HVAC companies mentioned potential problem w/ the venting setup (how installed, flue sizing, etc.).
Quote
How much venting is installed in the attic?
You mean, how much soffit & ridge venting?  (& not the length of flue pipe).
Quite a bit of attic ventilation.  Continuous ridge vents on long stretches of ridge (ridge runs right over center of the heater), along w/ balanced amount of net free area of soffit vents.  Total ridge vent NFA is plenty for this sq. ft. house, per everything I've read.

But, until we bought it in 1995, the house had only the soffit vents and a few wind driven "turbine" vents on the roof.  I assume that was for about 19 yrs.  Until now, I never closely inspected under the draft hood or the rectangular exhaust port.  Nor did any inspection tech mention them.   Some white deposit or peeling paint could've been there for yrs??

True, it's in unconditioned attic (& often humid in deep south, even in the best ventilated attics).
But it's usually not "cold" here - rarely gets < freezing.  Average cold night is more like 45 - 50 F, but only for parts of 2 - 3 months.

Is it likely, after many yrs, the moist exhaust gas w/ lots of CO2 & impurities just started breaking the paint down?
Much of the exhaust would hit the draft hood walls, before going up the flue.
Don't know if it'd made a difference if they connected the flue to top of the draft hood, instead of the side (in pic).

Offline Admin

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It's hard to see how the whole furnace is configured in the pictures.  I'd have to read the installation manual as well.  I've seen natural draft boilers with venting that is installed on the back instead of the top.  It would be a good idea to make sure that appliance is configured properly for a horizontal installation.  I don't see an optional top port.  You mentioned the vent could be installed on the top or side.  Do you have a picture of the entire furnace facing the burners?  Is there a rating plate you can take a picture of.  I've never heard of a Singer.

The venting connection at the appliance looks wrong.  Is that a 6" B-Vent elbow installed inside the appliance flue collar?  A single wall adapter should be used on the appliance draft hood.  The fitting should be installed overtop of the draft hood collar, not inside it.  They make a single wall to B-Vent adapter.

I would see how many appliances and BTU's are using the common vent and make sure the sizing is correct. 

Seeing a vent connection like the one in your picture would make me question the entire installation.  It looks like an old furnace though.  I agree the rust and corrosion must have existed much longer than last years maintenance.