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Old 04-29-2013, 07:00 PM   #16
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Furnace/water heater vent pipe??


Historically furnaces were oversized because "bigger wuz better" gas wuz cheaper, contractors did not have heat load calculators and were too lazy too use them. If you are getting a newer furnace you should do your own heat load calc. I think Beenthere is implying you may have an oversized, in-efficient furnace/system and would benefit from a high efficient furnace and not doing your chimney etc.

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Old 05-07-2013, 01:30 PM   #17
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Furnace/water heater vent pipe??


Ok, I wish I could capitalize on it being oversized if it is in fact, as my 2nd floor doesn't have central heat/air, but that's a different thread!

For now, being as my furnace is 100,000 BTU input and the water heater is 40,000 BTU, am I ok if I combine them just before the chimney stack and run stainless flex liner up the chimney? Post #8 has all that info, in case you missed it. Looks like I'd have to increase to 5" diameter pipe instead of the 4". On that note, can someone please explain why you can run more BTU through the vent as it gets longer? ....or am I reading that incorrectly?

It's possible, I'll just leave well enough alone for now and keep them seperate and/or just run two seperate 4" pipes up the stack. When the time comes to replace the furnace, get one that can vent out the wall and at that point convert it's flex liner into one for a gas insert for the fireplace.
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Old 05-07-2013, 01:53 PM   #18
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Furnace/water heater vent pipe??


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Originally Posted by Master Brian View Post
On that note, can someone please explain why you can run more BTU through the vent as it gets longer? ....or am I reading that incorrectly?
As a chimney gets taller vertically, the "draft" increases, sucking combustion gasses outside better.

As a chimney gets longer horizontally, the opposite happens (less sucking action), which is why you should keep natural draft chimney runs as vertical as possible, building structure allowing.

As for a natural draft vent being like an open window 24/7, yeah, you're going to lose conditioned air if your builder has foolishly placed your gas appliance in the wrong place -- inside your house. Furnaces and water heaters etc all belong in the attached garage! (Or at the very least, in an unheated basement airsealed off from the rest of the house.) Less CO concern too, since most people don't sleep in their garage.
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Old 05-07-2013, 02:54 PM   #19
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Furnace/water heater vent pipe??


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As a chimney gets taller vertically, the "draft" increases, sucking combustion gasses outside better.

As a chimney gets longer horizontally, the opposite happens (less sucking action), which is why you should keep natural draft chimney runs as vertical as possible, building structure allowing.

As for a natural draft vent being like an open window 24/7, yeah, you're going to lose conditioned air if your builder has foolishly placed your gas appliance in the wrong place -- inside your house. Furnaces and water heaters etc all belong in the attached garage! (Or at the very least, in an unheated basement airsealed off from the rest of the house.) Less CO concern too, since most people don't sleep in their garage.
So, it looks like I'd likely need to go with a 5" pipe, if combined. Will the 5" pipe work ok, if/when the time comes for the water heater to be the only item venting into it?

As for the placement, water heater & furnace are both located in the center of the basement. The basement is in process of being well insulated and with and without the insulation it stays a pretty consistant 65-70* year round. There are some vents down there, but they are rarely used. I am no expert on this by any means, but in the grand scheme of things, that flue pipe on my water heater is the least of my worries when it comes to unconditoned air coming in and conditioned air going out on a house built in 1915. They were designed to be somewhat drafty, I've done a lot to combat that, but there is still a long way to go and my understanding through it all is that I don't want to make it air tight as it wasn't designed to be that way! With that, I do enjoy learning and all of this helps going forward!! So I appreciate all of the feedback!
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Old 05-07-2013, 03:26 PM   #20
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Furnace/water heater vent pipe??


I may be wrong but I was under the impression that you were never to have a forced draft vent and an atmospheric vent vented into the same chimney.

Chimneys are touchy and I wouldn't suggest doing this job yourself.

If you are replacing the units then look into the condensing 90+ efficiency units that can vent outside with PVC pipe.
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Old 05-07-2013, 08:55 PM   #21
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Furnace/water heater vent pipe??


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I may be wrong but I was under the impression that you were never to have a forced draft vent and an atmospheric vent vented into the same chimney.

Chimneys are touchy and I wouldn't suggest doing this job yourself.

If you are replacing the units then look into the condensing 90+ efficiency units that can vent outside with PVC pipe.
An 80% induced draft furnace is permitted by code to be vented with a natural draft appliance. Not the best thing to do, but allowed by code.
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Old 05-07-2013, 10:06 PM   #22
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Originally Posted by beenthere

An 80% induced draft furnace is permitted by code to be vented with a natural draft appliance. Not the best thing to do, but allowed by code.
And this is because an 80% induced draft furnace is not a forced draft. Of course, a forced draft furnace isnt allowed to be vented with a natural draft appliance, or another forced draft furnace unless allowed by the manufacturer.

This isn't a reply to you Beenthere, as I know you already know this. It's for the benefit of others who don't, just easier to quote you.
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