Trying to sell my house and an inspector said my furnace flue is over sized. If so, why would the furnace installer have done it this way?

the inspector missed that the flue is not sealed. oversized and unsealed means cold air will force carbon monoxide in the house.

backing up... this is obviously a gas furnace, because there is no "flapper" flue vent in the pipe. in gas furnaces, there is a "cap" with an opening between pipes that does the same thing... draw extra air up with the exhaust to insure the combustion gases go outside with gusto.

this exhaust pipe from the outside is the correct diameter it would appear for an oil furnace. methinks a half-donkey installer did a cut-rate conversion here without resizing the flue up through the chimney.

could have killed you.

a potential buyer could call out the inspector and I bet he'll red-tag the house until this is fixed. a grand, maybe. maybe a few hundred to fix. depends on labor in your area and whether they have to do anything expensive in the chimney cap to accomodate the smaller flue.

This message has been edited. Last edited by: swschrad,

Can't they just taper down the large pipe to fit around the small exhaust pipe coming off the furnace?

you lose two things if you do that.

(1) wind gusts at the chimney might backflow the exhaust. the airgap helps insulate against this.

(2) you probably won't have enough draft out of the furnace. oil and gas furnaces both have different means of doing this, but extra room air is supposed to be caught up with the hot exhaust gas to increase flow -- and draft -- to insure all the exhaust goes out the roof. sounds crazy, but it works.

has to be redone to code. has to. it's just one of those things, like broken drain stacks and missing structural supports, that is not supposed to be sprung on the unwary buyer.

//edit// (3) the oversized flue will get too cold to draw properly, because there is too much area to heat up first.

ever get choked out of the house because a fireplace chimney was plugged with a birds nest or whatever, or because it was 20 below? same thing. until you warm up the flue, the exhaust doesn't much go up. Nebraska, you might get away with this most winters. further north, nope.

if there was a CO monitor, I bet it would go off and not reset for a couple minutes every time the furnace kicks on. that means poison in the house all winter.

oil is a larger molecule, produces almost three times the heat of natural gas, but needs more air to do it. the stack gets hot faster. all good reasons the oil exhaust is a larger pipe than a gas one.

another good reason why you don't pay the neighbor in beer and a tankful of gas to install a furnace, you call the pro.

This message has been edited. Last edited by: swschrad,

I appreciate your help on this issue.

Are you saying that tapering the vent is not adequate but a 4" pipe (which is the size pipe coming off the furnace leading into a 6" pipe)from the furnace to the chimney is what's required?

If I'm still wrong, how would you fix it? Thanks.

After further research, you're saying the the pipe from the furnace should be one continuous 4" pipe that goes into the chimney and a 4" chimney liner that goes the height of the chimney?

the exhaust from the furnace needs to go through a "gas vent" adapter, then straight 4-inch up to the chimney. code might require type II in your area, that's a pipe inside a pipe... more of a pain to work with... depends on the furnace setup.

now, whether you can get away with a 6 to 4 reducer at the chimney cap and use the existing feedthrough and cap is dicey... in any event, you can't get to it to put a reducer on it. something about all that blinkin' brick and concrete and a clay liner in the way.

this is life-safety equipment, and I'm going to flat-out declare this is NOT DIY STUFF.

assuming that you have a clay tile chimney all the way up, the mortar cap, and a clay outflow at the top to exhaust the old oil burner's gases, a proper job would require sliding a 4 inch pipe all the way down, elbow out at the bottom instead of using a typical mortared-in connection to the tile liner, and a modified cap with a rainproof, critter-screened gas exhaust topper. I have no idea what they do inside the clay for support or whether there is insulation. that's because I let The Man deal with that and get it inspected.

but it needs to be right, and it needs to be bid out. good news is, the furnace itself isn't failed.

I appreciate the thorough explanation swschrad. I live in Chicago so it wasn't an oil burning furnace but regardless, it still isn't to code. Seems like a time consuming PITA that I don't want to deal with so I'll probably give the buyer a credit and be done with it. Thanks again for you input. I just looked through the inspection I had done when I purchased the house and the inspector was perplexed at the layout of the exhaust but I never felt like it was a big priority so I left it alone.