Flue Pipe Sizing

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Re: Flue Pipe Sizing

Postby CEO1 » Tue Feb 09, 2016 6:29 pm

MITTS wrote: looking to install a recessed med cabinet in my bathroom where a smaller flue would allow clearance.


This "project" of yours has been in and out of my thoughts all day...have you considered a different med cabinet design? Can you build an offset into the cabinet? Or, is there any way to put an offset in the existing 6" vent pipe? Is it running through an interior wall or chase?

Or have you already answered those questions somewhere? :mrgreen: I'll re-read the posts...


EDIT: You've already made some compromises. ;)
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Re: Flue Pipe Sizing

Postby MITTS » Tue Feb 09, 2016 7:58 pm

The cabinet can be mounted either recesed or surface mount. We have a light that will hang above the cabinet that has a mirrored front. The styles won't match if it gets surface mounted because the light will hang behind the front of the mirror.

I already picked up the 4"
I'll get a good CO alarm too ;)
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Re: Flue Pipe Sizing

Postby 3rdcoast » Tue Feb 09, 2016 8:09 pm

Is that tee in your original pipe single wall?

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Re: Flue Pipe Sizing

Postby CEO1 » Tue Feb 09, 2016 8:18 pm

MITTS wrote:I already picked up the 4"
I'll get a good CO alarm too ;)


That's what the typical HO would do (minus the CO alarm)...ignorance is bliss.

"Ah hell; it'll work." :mrgreen:

Install a reverse mounted vent fan in the basement to create a positive pressure... :idea:
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Re: Flue Pipe Sizing

Postby 3rdcoast » Tue Feb 09, 2016 9:12 pm

From an earlier post the height from the appliance to the top of the vent was 26 feet. He stated he was going to add a few feet as insurance. 26 feet on the table is 132k btu a couple extra feet (total of 28) at 1500 btu a foot will put him at 135k btu.

The table used for this calculation is based on using b vent connectors so they will have to be double wall, not single wall (some pipe in the picture appears to be single wall, some double, and possibly sealed with duct tape?). As stated earlier water heater connector will need to be measured to make sure 3 inch is large enough, with 26 foot height it should be.

If you look at the picture the water heater vent is jogging around the water pipe, a better installation would have rotated the water heater slightly and let the vent connector take a direct shot at the common vent. A little jog in the water pipe will have no effect on the water flow, but a jog in the vent pipe has to be taken account in the calculations.

The main problem I see that needs to be addressed is the missing cooling coil for his air conditioner!!! :lol:

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Re: Flue Pipe Sizing

Postby MITTS » Tue Feb 09, 2016 9:28 pm

3rdcoast wrote:Is that tee in your original pipe single wall?

I picked up a 4" tee single wall. The basement is conditioned space. Did I goof?
I also got 4" single wall for both the HWH and furnace. Planned on twisting the HWH also
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Re: Flue Pipe Sizing

Postby MITTS » Tue Feb 09, 2016 9:32 pm

3rdcoast wrote:
The main problem I see that needs to be addressed is the missing cooling coil for his air conditioner!!! :lol:

Copper theif from inside out?

Can you make out the pic on the plenum? Lol :lol:
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Re: Flue Pipe Sizing

Postby CEO1 » Tue Feb 09, 2016 10:35 pm

MITTS wrote:
3rdcoast wrote: The basement is conditioned space. Did I goof?


Not sure what you're referring to there...

I been dealing with residential furnace systems for some 30 years, and seen some vent "designs" you'd bet money, wouldn't work. Single wall, rusted single wall, taped up holes, downhill slopes, holes cut in one appliance connecter, with another appliance connector stuck in the hole, with foil tape, commons dumping in masonry chimneys...you name it. Like I said, ignorance is bliss...and nobody's dead yet.

H-Talk has quite a few threads come up on combustion issues...I read them all, trying to learn something. Jim Davis, AKA Captain CO, the "Combustion Guru", most often makes comments...I have yet (that I remember) to hear him say, "Well the cause of the problem is this, and the solution is that. Which is not to say I don't accept his expertise, rather, combustion and vent systems are like a lot of other things. You can do something "by the book" and it still may not work...and as we all know, you can juryrig some sh!t, and it works.

And that guy has been doing combustion analysis for 30 years. But, the more I read, the more confused I sometimes get...as previously stated, he recently poo-poo'd the GAMA tables saying they don't take air flow principles into account, leading to the "you can't oversize a common vent" statement. Far be it from me, to argue with an accepted authority, but as I understand the purpose of venting, the objective is to get the gases out of the system before they cool down and condense. Which involves some maximum obtainable velocity...and the bigger the vent, the slower the gas. :?

Put the 4" pipe in, turn on the furnace, fire up the water heater and see if it drafts...if it does, happy days. If not, then worry about it... ;)
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Re: Flue Pipe Sizing

Postby MITTS » Tue Feb 09, 2016 10:35 pm

Recent-New Measures-6.jpg
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Re: Flue Pipe Sizing

Postby 3rdcoast » Tue Feb 09, 2016 10:41 pm

MITTS wrote:I picked up a 4" tee single wall. The basement is conditioned space. Did I goof?
I also got 4" single wall for both the HWH and furnace. Planned on twisting the HWH also


Using single wall connectors raises your minimum vent height to meet 128k to 28 feet with no extra margin. The single wall connectors loose to much heat and that slows down the draft. The 4 inch would work on the water heater, but there is no size that meets the furnace capacity in single wall, 3 inch is to small, 4 inch is to large based on the fan min btu.

Your furnace OEM book should have the manufactured supplied chart, but it is usually just a copy of the GAMA table.


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