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Old 03-28-2011, 06:39 PM   #1
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combining a water heater and furnace Both natural gas


How do I configure what size B-VENT do I need when combining two appliances to vent out roof?
A water heater and furnace.
Is it a formula with the btu's or should I just go oversized and be safe.

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Old 03-28-2011, 07:19 PM   #2
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combining a water heater and furnace Both natural gas


It is a formula with BTU's and length/rise/height of chimney from furnace to where it exits roof and must meet the gas code specs. If it is oversized then moisture can condense inside and damage it and may not pass inspection either. Post the BTUs of the furnace and water heater and where you live and vertical rise of venting in feet here and some nice tech may look it up for you in his code book. Not sure if the US and Cdn codes are the same.

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Old 03-28-2011, 07:38 PM   #3
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The water heater btuh 34000 Well let me tell you that I install fireplaces for a living and my friend just bought a house and her chimney was falling off her house and asked me to help her. So i went into basement and saw the furnace and water heater hooked into the old masonary chimney that was falling off the house. So i told her to go down to township and get a permit to take down chimney and see if she can just run b-vent up the side of house. So i asked her the btu's of each appliance and she sends me Water heater input btuh 34000; natural gas boiler=csa input 90000/doe capasity 76000/net1=b=r water 66000. To be perfectly honest i have no clue what thaT MEANS. wHEN HER BROTHER IN LAW TOOK DOWN THE OLD CHIMNEY there was seven inch pipe ran into old chimney.
New Jersey, Jackson
Is state and town. I was trying to do reasearch online and i think I came up with seven inch b-vent
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Old 03-28-2011, 08:02 PM   #4
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combining a water heater and furnace Both natural gas


You add the inputs 34,000 and 90,000 to get 124,000. 7" sounds good to me. 6" may be borderline. Up to 120,000 on 6" but if it is 2 story that may increase. Need to get a proper ruling from the authorities AND AGA code book. Has to be done right or the insurance co may be a problem also. Hang in there I am pretty sure one of our guys will look it up for you.

There may also be code issues with running it outside the house up a wall. I have heard that they want it enclosed in a chase (boxed in). Usually it goes thru the house and won`t get cold and have condensation and possible freezeup problems. More things to check B4 doing it. Local codes may apply there also.

We need to know the vertical rise in feet from the top of the boiler to the roof line also.
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Old 03-28-2011, 08:17 PM   #5
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I was there today and came up with about 23 feet from appliances to top of vent cap. Give or take a couple feet
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Old 03-28-2011, 08:39 PM   #6
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combining a water heater and furnace Both natural gas


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You add the inputs 34,000 and 90,000 to get 124,000. 7" sounds good to me. 6" may be borderline. Up to 120,000 on 6" but if it is 2 story that may increase.
What table are you using there yuri? C3 and C4 indicate 5" is the proper size.
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Old 03-28-2011, 08:49 PM   #7
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combining a water heater and furnace Both natural gas


Have not looked it up. Old 120,000 BTU furnaces had 6" venting when natural draft. Sounds like both his appliances are natural draft.
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Old 03-28-2011, 09:00 PM   #8
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Yep, I also assumed both appliances are natural draft.
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Old 03-28-2011, 09:12 PM   #9
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combining a water heater and furnace Both natural gas


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What table are you using there yuri? C3 and C4 indicate 5" is the proper size.

My book list 5'' if both are nat draft. I didn't look up fan / nat

Like Yuri stated. Around here you have to build a insulated chase if the dw vent runs up the side of the house
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Old 03-28-2011, 09:16 PM   #10
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combining a water heater and furnace Both natural gas


both appliances are nat draft. I dont know the codes but her brother in law is a framer by trade so if she has to box it in she will find out i guess. So seven inch is to big and i should go with five inch. It is roughly about 23 feet of pipe
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Old 03-28-2011, 09:23 PM   #11
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4- 5' sections & 1 - 3' section


Get to work
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Old 03-28-2011, 09:28 PM   #12
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combining a water heater and furnace Both natural gas


My old school bean wuz thinking single wall venting/liner and my code book is not close by. B vent may be smaller.

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