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Another flue question


Robert Jones
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The furnace is approx 15 yrs old. The flue is vented to a masonry chimney(which is in poor shape). What would normally cause a drafting issue with this type of venting? There are no strange bends. It appears to have been going on for quite some time. Did I just ask a vague question?:)

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With older more inefficient furnaces, there was a lot of heat loss into the chimney flue, so the flue gases were able to rise and exit the chimney before losing buoyancy. Now you've got a more efficient furnace that doesn't send enough hot air into the chimney. The temp. of the exhaust drops too fast so the exhaust gases do not all vent to the exterior. Something like that anyways. Sorry, I'm not the best with explanations. Basically, the chimney flue is probably over- sized and a liner most likely needs to be installed all the way to the top of the chimney.

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That looks like a single-wall vent connector. Is the furnace in an unheated area? If so, the cold, single wall pipe might be causing extra condensation.

It also looks like the vent connector is larger than the outlet on the furnace itself. If the manual is available, I'd check the size of the vent connector. If it's too big, then you might get extra condensation.

Another possibility -- and a likely one at that -- is that the furnace is short cycling. That is, it never runs long enough for the flue to warm up and re-evaporate the condensate that formed after the initial firing. Short cycling can be caused by lots of things but the two most common are that the furnace is oversized or that the thermostat is poorly placed near a supply register.

- Jim Katen, Oregon

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Did the chimney have a rain cap? Was the cap or top of the chimney possibly letting water into the stack? I see this all the time and sometimes when its raining you can see the water running down the chimney into the flue pipe. Because their are no restrictive bends, water can make its way, all the way back into the furnace.

Does the water heater flue have and water marks on it? it may not be as bad as the furnace as it should be entering the chimney above the flue of the furnace.

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Going by what can be seen in the photos...That unit is a forced draft model isn't it? If it is, I wouldn't think that it has draft issues. Also, if it is a forced draft model, you do not want to share that flue with any natural draft, gas powered appliance.

Marc

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If the masonry flue is not tall enough it will not draw properly. I had the same set up in Kansas City, furnace and gas water heater in one metal flue and both going into a masonry flue, the only time I had any trouble with it was when the wind blew from a certain direction, raised the flue another two feet and no more back draft. The back draft was bad enough to blow the pilot out on the water heater till I raised the flue. Another thing if there are any cracks and leaks in the masonry it will cause the same thing. May not be whats going on here but is worth a look.

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Going by what can be seen in the photos...That unit is a forced draft model isn't it? If it is, I wouldn't think that it has draft issues. Also, if it is a forced draft model, you do not want to share that flue with any natural draft, gas powered appliance.

Marc

That's a category I furnace. It has negative pressure in the flue. The draft inducer fan on these units draws a regulated amount of air through the burners, but it isn't strong enough to pressurize the flue. The buoyancy of the exhaust gases is what drives the venting in these units.

- Jim Katen, Oregon

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Reading the other answers makes me realize I get TUNNEL VISION.

BTW: I now carry my camera with me during client walkthroughs once "done" with my inspections. I often notice other issues that I didn't catch during my inspection. I can't say I remember seeing anything that I've missed that was serious, but there is often something that I consider important enought to write up.

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From my report:

If you replace your old furnace, your chimney may require modifications to assure that it is sized to fit your new heating system. If your chimney is too large for your new system, it may not remove combustion gases properly from your home, possibly leading to backdrafting and allowing combustion gases such as carbon monoxide to enter your home. Your old chimney may also be prone to moisture accumulation and deterioration. Mid-efficiency furnaces (80% plus) produce a smaller volume of combustion gases than traditional furnaces, and those gases are slightly cooler and therefore condense in the chimney instead of leaving in a vapor condition. If this type of system is vented into an older, un-lined chimney, it could result in chimney deterioration or back-drafting. The best approach is to reline the existing chimney with a smaller, corrosion-resistant metal liner.

Re-lining the chimney can be expensive, but neglecting your chimney when you install a new furnace can lead to a crumbling or rusting chimney in a few years. If you plan to install a new furnace, be sure to ask your serviceman about proper chimney sizing to protect your home and your health.

Call a licensed HVAC contractor for further evaluation.

When ever I go into an older home and see un-lined chimney with a newer furnace which includes a draft inducer I always write this up. Your picture is a good example of what happens when they forget to install a chimney liner.

Keep an eye out ofr orphaned hot water tanks too. We discussed this a few weeks back.

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That's a category I furnace. It has negative pressure in the flue. The draft inducer fan on these units draws a regulated amount of air through the burners, but it isn't strong enough to pressurize the flue. The buoyancy of the exhaust gases is what drives the venting in these units.

IRC 2006, G2427.3.3 (503.3.3) MECHANICAL DRAFT SYSTEMS

4. Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under pressure.

How do you know that the forced draft furnace is not operating under pressure on this particular unit? It might be at times, but you cannot be certain that it is always so. The wind could come up and the draft could even reverse. That's a purpose of the draft fan...to insure adequate combustion air under less than ideal draft conditions.

Marc

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While the chimney liner is probably too big, other factors come into play.

Is the chimney interior or exterior?

Is there a natural draft water heater attached to it?

How long is the furnace vent connector? How many bends?

How long is the WH vent connector? How many bends does it have?

Your best bet is to call out a problem and recommend a certified F.I.R.E. inspector conduct further review.

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Marc

You've made a good observation but the furnace in question is a draft induced exhaust system.

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Although I would agree by the presence of the fan, that a slight positive pressure exists at the outlet side (flue connector), the flue itself is considered at atmospheric pressure. That's the reason we can combine natural draft appliances with draft induced ones.

Forced draft fans push air through the furnace while induced draft fans pull.

All gas burning appliances require a correctly sized vent as previously stated but sometimes that may not be enough.

As Darren mentioned, it's location is equally important.

Here is a recent inspection.

This is a two storey built in 2006 with the vent located on an outside wall.

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How do you fix this?! ... Move the vent.

I'll be inspecting for these folks on Monday, I suspect the vendor didn't like my solution.

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Marc,

You are looking at the chapter for Mechanically-drafted systems; Category I induced-draft furnaces are not considered mechanically drafted and same way that the Category IV furnaces are.

Out this way, nearly all of the non-induced-draft Category 1 furnaces are gone and on probably 90% of all forced-warm-air systems we see the furnaces are induced-draft, located in garages and are sharing the same vent with a gravity-vented gas water heater.

This does not violate the prohibition against connecting mechanically-vented and gravity vented systems on the same flue because these inducer fans just don't have that kind of power. The scenario you envision just doesn't happen unless there is a blocked flue.

This is from the 2003 IRC. The chapter and verse number has probably changed in the 2006 and 2009 codes.

G2427.10.4 (503.10.4) Two or more appliances connected to a single vent. Where two or more vent connectors enter a common gas vent, chimney flue or single-wall metal pipe, the smaller connector shall enter at the highest level consistent with the available headroom or clerance to combustible material. Vent connectors serving Category I appliances shall not be vented to any portion of a mechanical draft system operating under positive staitc pressure, such as those serving Category III or IV appliances.

ONE TEAM - ONE FIGHT!!!

Mike

Edit: Looks like RobC got there before I did.

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That's a category I furnace. It has negative pressure in the flue. The draft inducer fan on these units draws a regulated amount of air through the burners, but it isn't strong enough to pressurize the flue. The buoyancy of the exhaust gases is what drives the venting in these units.

IRC 2006, G2427.3.3 (503.3.3) MECHANICAL DRAFT SYSTEMS

4. Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under pressure.

How do you know that the forced draft furnace is not operating under pressure on this particular unit?

Because that's not a forced draft furnace. It's a fan-assisted natural draft furnace. If you read the label, it will say Category I. By definition, this furnace vents by negative pressure in the flue.

It might be at times, but you cannot be certain that it is always so. The wind could come up and the draft could even reverse. That's a purpose of the draft fan...to insure adequate combustion air under less than ideal draft conditions.

The furnace vents through negative pressure in the flue. If the wind prevents that from happening properly, a pressure sensor shuts the unit down. You are correct that the purpose of the draft fan is to ensure adequate combustion air, but it doesn't affect the draft in the flue. That fan doesn't have the power to do that.

The code section that you cited, above, is referring to Category III and Category IV appliances. These are designed to have positive pressure in the flue and shouldn't be combined with appliances that vent via natural draft.

- Jim Katen, Oregon

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