Flue Pipe Sizing

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Re: Flue Pipe Sizing

Postby MITTS » Mon Feb 08, 2016 2:44 pm

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Recent-New Measures-1.jpg
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Re: Flue Pipe Sizing

Postby 3rdcoast » Mon Feb 08, 2016 6:20 pm

If you reconfigure your vent to the following configuration and the common vent is vertical with no offsets

vent.jpg
vent.jpg (5.6 KiB) Viewed 257 times


then the reduction goes away, and you get back to a 26 foot tall vent for 4" pipe to get to 128k BTU. The tee needs to be 4X4X4 installed as shown in the line drawing.

water heater connector at 3 inch only makes the cut if it has 3 foot of rise. You will need to check the lengths.

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Re: Flue Pipe Sizing

Postby CEO1 » Mon Feb 08, 2016 6:58 pm

3rdcoast wrote:If you reconfigure your vent to the following configuration and the common vent is vertical with no offsets

vent.jpg


then the reduction goes away, and you get back to a 26 foot tall vent for 4" pipe to get to 128k BTU. The tee needs to be 4X4X4 installed as shown in the line drawing.


You must have run out of something to do... :mrgreen:
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Re: Flue Pipe Sizing

Postby MITTS » Mon Feb 08, 2016 7:11 pm

Game on!

Thankyou

Is this due to the induced flue causing something of a bernoulli effect on the tee connection?
Also is too much pitch on the horizontal sections an issue with this configuration?
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Re: Flue Pipe Sizing

Postby 3rdcoast » Mon Feb 08, 2016 7:13 pm

CEO1 wrote:You must have run out of something to do...


Had a long wait at the doctors office, time to kill.

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Re: Flue Pipe Sizing

Postby 3rdcoast » Mon Feb 08, 2016 8:41 pm

MITTS wrote:Is this due to the induced flue causing something of a bernoulli effect on the tee connection?

I have no idea, I just go by what the book says, and in the field I have seen many tees that are the connector size, not the common vent size. For example a 6 inch common vent that is fed by two 4 inch connectors will have a 4 inch tee tying the two 4 inch pipes together and then a 4 to 6 inch adapter to hook to the 6 inch pipe. So the 2 four inch pipes are restricted at the 4 inch tee.

MITTS wrote:Also is too much pitch on the horizontal sections an issue with this configuration?
not that I am aware.

You may run into problems using the 3 inch on the water heater. If you look at the diagram i posted one appliance is hooked into the bottom of the tee with a total of two 90 degree fittings from the appliance. The other appliance is connected into the side of the tee using one 90 degree fitting, the side entry of the tee counts as a 90 degree fitting.

The 90 degree total for one appliance and the 180 degree for the other is the total that cannot be exceeded on that connector without reducing the capacity of the vent connector in the table.

Your water heater using the 3 inch connector needs at least a 3 foot rise off of the vent connector to meet 40k btu.

The horizontal offset is limited to 54 inches on 3 inch pipe, and 72 inches on 4 inch pipe.

I normally run horizontal pipes the same as plumbing drains with 1/4 inch drop per foot back towards the appliance.

All of the bends and offsets have to be counted on vent pipe, they reduce capacity of the vent. To many people think they can run vent pipe like flex duct.

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Re: Flue Pipe Sizing

Postby MITTS » Mon Feb 08, 2016 10:15 pm

Gonna crunch the numbers and will ask for a double check but was looking to lower the furnace connection to make room for a smoother transition on the supply and make room for some takeoffs. Need to work on the return also and maybe add some fresh air. Getting the bathroom wall buttoned up has been at the top of my list of things to do around the house for too long lol
Thanks again
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Re: Flue Pipe Sizing

Postby CEO1 » Mon Feb 08, 2016 11:44 pm

To many people think they can run vent pipe like flex duct.


I think he's talking about me... :mrgreen:

I never worked towards minimizing the common vent size with the relatively few systems I "designed"...always played it safe, choosing the "excess capacity" column value. If I were in your shoes, I'd take 3rdcoast's earlier suggestion and run two pipes to the attic, or buy the power venter. :)
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Re: Flue Pipe Sizing

Postby MITTS » Mon Feb 08, 2016 11:58 pm

With the flat roof over the bathroom there basically is no attic here. Any tee would be in the bathroom plumbing wall right behind where the med. cab. wants to call home. I'm not opposed to a fan. Do they make one that small ? :?
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Re: Flue Pipe Sizing

Postby CEO1 » Tue Feb 09, 2016 9:09 am

MITTS wrote:With the flat roof over the bathroom there basically is no attic here.


Hmmm...guess I missed that somewhere earlier. :oops:

MITTS wrote: I'm not opposed to a fan. Do they make one that small ? :?


I don't have any experience with/knowledge of, power venters, beyond replacing one recently. Furnace system in a crawl space, with a 20'-30' run of horizontal B-vent. I suppose the installer opted for the power venter over running 10's of feet of vertical B-vent.

But, the power venter is located at the discharge end of the furnace vent system, I assume, to maintain a negative pressure in all the vent pipe. With no attic, you'd have to mount it on the roof.

On top of all that, you got the wiring "kit" to power the venter and interlock the appliances with it...and I don't know how the water heater ties into that. And you'd probably need a barometric damper somewhere to stabilize the draft on the appliances...

Hmmm...guess a power venter's not such a viable idea either.

I'm thankful to live far enough South, where "total electric" is feasible... ;)
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