B-Vent on the outside

Randy Navarro · 05-01-2009, 01:18 PM

This issue isn't going away. Furnace guy and water heater guy say there aren't any problems.

Only written spec. I can find is from Simpson's B-vent install guide where outside venting is "not recommended."

Does anyone have a credible reference, spec., or other standard that speaks to the problem this is?

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Craig Ervin · 05-01-2009, 04:36 PM

Humm where to start. It looks like the flue gases are condensing in the pipe and running back down, as shown in the outside pipe. ( see the rusty pipe)

You forgot to show the furnace!!! You have a gravity water heater, you would need a very old furnace to also be built the same way.

Look at the water heater, see the little hat that lets air mix in with the flue gasses. Does the heater have the same setup? Most furnaces all have a draft blower which "could" push CO out the water heater flue when both are on. You would need a meter and run both to know. I just never ever, no not ever mix two flues!! Yes it can be done, but the liability is just to high.

Post pic of furnace flue connection or with the panel open

Randy Navarro · 05-01-2009, 06:00 PM

You have a gravity water heater, you would need a very old furnace to also be built the same way.

Hi Craig: Why would that be?

Look at the water heater, see the little hat that lets air mix in with the flue gasses. Does the heater have the same setup? Most furnaces all have a draft blower which "could" push CO out the water heater flue when both are on.

This furnace is a typical 80% AFUE with a draft inducer.

I just never ever, no not ever mix two flues!! Yes it can be done, but the liability is just to high.

It is a common installation here to vent the water heater and furnace through a common vent.

What is not so common is to run that vent on the outside of the building. Typically, it's run through the home and up and out the attic.

This home used to have the common vent running through the house and was changed to vent outside for some unknown reason.

**Scott Patterson** · 05-02-2009, 09:27 AM

About the only thing I can find about water heaters, is in the States WH install instructions. It says that that the vent connector pipe can not exceed 75% of the vertical height of the flue pipe.

I also found in the Goodman, Payne, Bryant and Trane CAT I install booklets the following verbaige, word for word in all: Vertical outdoor runs of Type-B or any single wall vent pipe below the roofline are not permited.

Randy Navarro · 05-02-2009, 02:02 PM

Thanks, Scott.

The specs from those furnace makers might help me out quite a bit.

I'll follow up with them on Monday to see if I can get a copy.

Anyone else reading this . . .

Are outside b-vent runs common in your area?

And, are you a heating climate, cooling climate, or a bit of both?

**Markus Keller** · 05-02-2009, 07:32 PM

Whether it is permitted or not is an ongoing debate. Some city inspectors flag it, others don't. I've looked for solid paper work on this issue for awhile and haven't been able to come up with anything rock solid.
However ...
- when I see the water tank tied into the run with a 'T' instead of a 'Y', I write it as D&H every time. Hold a match to the draft hood without the draft induced hvac running and then turn the hvac on and hold the match again and see what happens. Usually I get enough backdraft for the client to understand.
- From my experience two things usually happen with B-vent outside. The pipe rusts out really fast. People complain it smells funny in the basement sometimes (back draft).
- I've seen fireplace manuals that state the B-vent should be enclosed.
Sorry, wish I could provide something rock solid.

**Jerry Peck** · 05-02-2009, 08:13 PM

AmeriVent states:

2. AmeriVent Gas Vent is suitable for installation inside or outside. However,the sizing tables in NFPA 54 are for vents not exposed to the outdoors below the roof line. Outside vents could reduce venting action; therefore, such installations are not recommended. In the event outside venting is necessary, vents should be sized as close to maximum capacity as possible. Whenever possible, outside vents should be enclosed inside a chase that maintains the minimum one (1) inch clearance to combustibles. Appliances served by an outside vent must have an adequate air supply to balance inside and outside air pressure to reduce the possibility of reverse venting action.

Another AmeriVent states:

9. Tables 1 and 2 shall be used for chimneys and vents not exposed to the outdoors below the roof line. A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors.

(Jerry's Note: Tables 1 and 2 are single appliance vent sizing tables.)

Hart&Cooley states:

Outside Vents
The sizing tables are not applicable to outside (exposed) chimneys or vents per NFPA 54. A Type B vent lining and exposed masonry chimney is considered to be an enclosed vent system, and these tables may be used.

(Jerry's Note: That means their Type B gas vents installed outside would have to be engineered.)

Metal-Fab states:
10. When installing exterior vent, not enclosed by the structure or a chase, consult local gas utility, appliance manufacturer, and/or authority having jurisdiction.

Metalbestos states:
5. Vent Location
Seilkirk, Inc. Type B Gas Vents are recommended to be installed within the heater portion of the structure whenever possible. If an exterior location is necessary it is highly recommended to enclose the exposed portions (especially in cold climates) to reduce heat loss which may in turn lead to poor draft and / or condensation / icing problems. Appliances served by an exterior gas vent must have an air supply to the appliance room adequate to balance indoor and outdoor pressures. Otherwise "stack action" of the heated building can cause reverse venting action when the appliance is off, or operating only on its pilot.

~~~~~

However, they all also stated to install in accordance with the codes and the appliances manufacturer's installation instructions, which, as Scott pointed out, do not allow exterior vents (some of them).

Craig Ervin · 05-02-2009, 09:22 PM

Originally Posted by Markus Keller

Whether it is permitted or not is an ongoing debate. Some city inspectors flag it, others don't. I've looked for solid paper work on this issue for awhile and haven't been able to come up with anything rock solid.
However ...
- when I see the water tank tied into the run with a 'T' instead of a 'Y', I write it as D&H every time. Hold a match to the draft hood without the draft induced hvac running and then turn the hvac on and hold the match again and see what happens. Usually I get enough backdraft for the client to understand.
- From my experience two things usually happen with B-vent outside. The pipe rusts out really fast. People complain it smells funny in the basement sometimes (back draft).
- I've seen fireplace manuals that state the B-vent should be enclosed.
Sorry, wish I could provide something rock solid.

x2

I do installs and the liability is just more than I'm will to accept. If the furnace has an exhaust blower then you will have CO leakage.

Listen as the furnace is turn on you can tell when you hear a noise before the furnace fires, it will be the draft induced blower. If the furnace is 40 years old your fine.

Jerry said it you can get condensation freezing in cold temps too.
They would make me enclose the pipe to keep the gases warm and not allow condensation, but each local is a bit different.

**Matt Fellman** · 05-02-2009, 09:23 PM

I've never (that I can remember) seen a B-vent run outside like that. Whether it's permitted or not those pictures should be the poster child for why it's a bad idea. It's surprising anyone would take the other side of that argument.

As for the induced draft furnace on a natural draft water heater, that's another debate all together... one that's been tossed around here a few times before. Basically, it's allowed in most areas but not necissarily a good idea for the reasons mentioned. However, it's hard to say it's 'wrong'.... at least for now. Many think the code will change someday and disallow it.