Flue Pipe Diameter?

15 years ago

I was having my 2nd floor furnace (80% efficiency Tempstar) replaced with a Carrier 58CVA070 (70k BTU 80%). I was going to go with 80% efficiency because I didn't want to have to deal with any roofwork (flashing) which obviously would be required if I went with a high efficiency model and PVC flue (I have cedar shake shingles).

One factory authorized installer said he would utilize the existing 3-inch flue pipe (so no roofwork would be necessary) and another factory authorized installer said he recommends substituting a 4-inch flue pipe because that is what is recommended for the furnace model.

What are your thoughts as to flue pipe size? I'd really hate to have roofwork done in the middle of the winter...I just dread having it create a leak.

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Comments (10)

zl700
15 years ago
Installer needs to follow the manual.
As all 80% furnaces have the same 3"-4" outlet on top the venter that doesn't dictate flue size. Typically, and all manufactures vary slightly in BTU capacites,but this is a common guidline: 50K is 3", a 75K is 4", 100K, is 5", and many 125-140K require 6". This is done by installing an increaser on top the ventor assembly.
tlewis_hvac
15 years ago
The size of the flue also depends on the length of the pipe. On a 70,000 btu furnace, the 3" line can run several feet and then need to be increased to a 4". But the manual will tell the installer what is required.
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mr_havac
15 years ago
You'd best have them check out also if (1) the new system requires B vent (2)if the installation requires a seperate draft inducer. A lot of installers are under the impression that the inducer fan which is part of the burner can be used to vent flue products. In some instances yes but its purpose is to draw air through the burner for combustion not venting the by products of combustion. Read the ventings section of the manual.
baymee
15 years ago
zl700 is on the right track. If I think of it, I'll look up the standards number, post it here, and you can read up on them.

As a manufacturer, we get this question all the time because our large boilers require and 8" flue and many people don't have an 8" flue.

It depends upon YOUR firing rate of the unit whether you can go one inch less on flue size than what the manufacturer calls for. This would apply to units that have several firing rates.
il_home
Original Author
15 years ago
I remeasured the flue pipe last night and it measures 3.5 inches diameter, which I assume is pipe within a pipe.

What size flue pipe is this likely to be?

I assume the inside pipe is at least 1/2 inch smaller in diameter than the outside pipe.
il_home
Original Author
15 years ago
Also, one installer said he'd use 4 inch flue pipe up to the roof level, then downsize to the existing 3-3.5 inch flue pipe there.

Is this an acceptable compromise? I assume that the longer the run of 4 inch pipe, the better, but is the smaller 3 inch diameter at the end of the run going to significantly and detrimentally impact the airflow to a point that I should be concerned?
zl700
15 years ago
3" B-vent is close to 4" in diameter. 3-1/2" is most likely single wall. 3" b-vents aluminum inner pipe is a hair over 3", then the almost 1/2" air gap and outer galvanized pipe leading it to be just over 4" in diameter. The way to tell is if there is any embossed writing on it and what does it say?

No you can never downsize an appropriate sized chimney!!! If that installer is chosen, what else we he ignore and take shortcuts on?

Below is a link for venting guidelines, it may read like a medical journal for some, which is representative to HVAC people how important proper venting is.

Here is a link that might be useful: Venting guidline
baymee
15 years ago
The allowance for a reduced flue diameter depending upon the firing rate was derived from several sources. This is for oil heat, so the information is just general information.

National Fire Protection Agency Paragraph 6.5.6 of NFPA 31 Standard.

Section 803.0 Sizing of Category 1 Venting Systems in the Uniform Mechanical Code and Table 8-8 of the same code and then Paragraph 803.1.6 and then the values were compared against the DOE capacities for the various firing rates of the boiler.

The end result is that a one inch smaller diameter flue was allowed, based on the reduced firing rate.

Sorry if it's all mumbo-jumbo, but that's what we go through.

It may be only interesting/boring reading because it applies to oil, but I'm sure there are oil heads here.
zl700
15 years ago
Good info Baymee

So there is no misunderstanding, smaller diameter flues are allowed in some instances referenced above at the draft connector or sometimes the flue connection, however never in middle of the chimney riser.
baymee
15 years ago
In the cases that I am familiar with, and we make no recommendations, the flue is reduced at the breech. Depending upon the height of the chimney and draft, I've seen some people get away with even more reducing than that. Of course, we never recommend any deviation from what is allowed in the above references.

I don't know the answer about reducing somewhere in the chimney riser, but it doesn't sound like a good idea to me.