exhaust pipe enlarged...a text book no-no

Greetings…I have seen on numerous peeks in the furnace area that exhaust and intake pipes are being enlarged. Referring specifically to exhaust the ITA manual notes as a no no both reducing (understandable) and enlarging the pipe. I talked to a furnace expert who said that is just done when there is a ‘longer run’…I see in this case the exhaust (I’m presuming) is reduced back down. Looking for input from a wide array of experts inspectors who have probably seen this many times also.
and by the way…thanks for that!
mike in MN

Usually see a T fitting being used to terminate the exhaust flue. Serves as a rain cap.
Guess the installer forgot it and didn’t wanna run back down to his truck.

I don’t see anything “enlarged”.

my rhetoric may be flawed. I simply mean the vent connector pipe is enlarged. silly me I did not take a pic at the furnace only on the roof. However here is the page out of kaplan systems and standards which basically pronounces anathema both reducing and enlarging vent connector but has the exception where more then one appliance is vented. I did not see that to be the case…and the reducer at the top end caused me to try to concoct theories…too much space to pressurize with forced draft? maybe the reducer has a venturi speed up effect Surely others have seen similar.
mike in MN where the ice just went off the lake

Just a tad of advice…

Quoting “Kaplan” will not get you too far in this business. Find the local building standard for the area in which the home is located and refer to it as your reference. A defect is not a defect simply because it doesn’t meet the “Kaplan” standard.

You should refer to that manufacturer’s specific installation instructions for that UL listed unit which has an induced draft, not a generic source for convection draft systems. I seem to recall (this could just be a figment of my imagination) some manufacturer’s specifying to use an enlarged diameter section of PVC when the overall exhaust length was too short for their HE furnace units. In any case, the only valid reference for your answer is the manufacturer’s installation guide.

ok thanks you all…looks like what i can glean from this situation is ‘everything is reletive’…just like back in college philosophy, but seriously yes, good input (checking local standards and unit specific instructions) will be heeded. however my furnace guy did say its just ‘when the run is too far’ Kaplan of course does not claim to be absolute authority on everything and they always advise to be aware of local standards and codes which would trump the general guidelines they are teaching. so I have lots of good advice…was just looking to connect with others who may have had the same questions.
thanks from mike in MN

Why do we need a “rain cap”?

Anyone know?

Because the draft inducer impeller will be in water during the non heating season and eventually rust and die.

If the reduction is too small on a 90% or greater furnace, it will shut itself down in a few minutes.

Are you sure???

Absolutely unless a drain is drain fitting is provided.


HMMMM you never mentioned a drain line! just asking

Charley, do I have to tell you everything? :wink:

Nope! Wrong answer.
The condensation is supposed to flow back to the furnace.
There is a drain designed to take care of this stuff.
A little rain won’t matter.

That is not a 90% (+) furnace fan, which is what we are talking about here.

Or so I was lead to believe…

Dead wrong David.

Not on all systems.

A drain is required only beyond certain lengths.

If this is a condensing furnace it can handle rain…

My point of asking, was that you need a rain trap on the intake, but not the exhaust.

Is this what your talking about?

You can not flood the fan from water in the exhaust pipe.
You must drain the condensate back to the unit so that it does not freeze in the exhaust during freezing temps. Rain water added to this will make no never mind.

Nope was seeing how much rope it took to hang a Yankee:D:D. I knew where Dave was going with his question and he answered already;-):wink:

Easy big boy never say never the drain line can plug up . Whazz that term Shi! Happens