Flex vent at furnace

**Bill Anglin** · 11-10-2011, 12:05 PM

I've never seen a vent pipe like this and am unsure if it is ok. I thought they had to be type B vent. The sticker on the return air duct shows it was inspected and approved by the municipal bldg inspector. Also wouldn't the smaller appliance vent pipe connector need to be installed on the top of the larger one? The furnace is 50K and the water heater is 40K. Your comments are appreciated.

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**Rick Cantrell** · 11-10-2011, 12:39 PM

Is this it?

http://www.mtlfab.com/media/L1940.pdf

**Bill Anglin** · 11-10-2011, 01:11 PM

Yep, that's the stuff. Looks like it is OK then. What about the two connectors. I might be backwards but I thought the smaller is supposed to be on top.

**Kristi Silber** · 11-10-2011, 03:35 PM

I think you have it backwards.

**H.G. Watson, Sr.** · 11-10-2011, 05:34 PM

There are several issues with what you have pictured. An old-pre-tinkered inspection sticker is meaningless when the vent system and connectors have been subsequently altered.

Lets begin with vent connectors in contact and without required air space with cold air return and other surfaces, combustible or not - not a part of the venting system.

No propr adapter from collar to this flex, proximity to plenum. No supports lateral and its offsets/turns/changes in direction.

Outrageous conversion to laterals of venting system and incorrect orientation now as to vent connector from water heater draft hood.

Less than sufficient rise prior to offset from water heater.

The water heater in use year-round, not so for the furnace.

Unsupported vent and vent connector laterals, offsets, changes in direction, etc. Lowerst portion of b-vent fitting wrong orientation. (gases from WH gravity trap).

Strange? orange or red/orange strap at basement or garage ceiling/exterior wall corner in contact with vent.

Doubtful sufficient rise for venting system with compromised lateral (effective) lengths and multiple offsets both connector improperly employed fitting and excessive offsets. Contact and proximity with dissimilar metals, failure to maintain 1" air-space, etc. to overcome even max capacity venting, and not near mins.

Exposed spun fiberglass or minerwool and vapor barrier/plastic in occupied mechanical space using room air for combustion and flue glass dillution.

**Bill Anglin** · 11-13-2011, 12:48 PM

Thank you all for your help. I looked up and found that the smaller vent pipe connector should be on top when installed into a chimney. I'm still unsure though what it should be when connected to another connector. I can't think of why it would be opposite of the chimney installation.

**Franz Bailey** · 11-15-2011, 07:38 AM

Supports might be an issue and clearances. Is that vent spaced properly from that pipe, assumed gas pipe? I would think there is a minimal bending radius and you would need to increase the clearance of the flexible pipe at bends when there is potential to decrease the inner gap space between the inner and outer walls.

Binford Tools · 11-25-2011, 11:01 PM

I don't like dual vents. fireup the water heater and the furnace and check for CO spillage from the water heater.
That would be my concern.

Binford Tools · 11-25-2011, 11:14 PM

I don't like dual vents. Fireup the water heater and the furnace and check for CO spillage from the water heater.
That would be my concern.

I don't see any white stuff on the pipe, but could be wrong.

**Kristi Silber** · 11-26-2011, 07:19 PM

Originally Posted by Bill Anglin

Thank you all for your help. I looked up and found that the smaller vent pipe connector should be on top when installed into a chimney. I'm still unsure though what it should be when connected to another connector. I can't think of why it would be opposite of the chimney installation.

I misunderstood your question before.

(This is my take on things just thinking of the physics; I don't know much about HVAC...but ignorance doesn't stop me from taking a stab at questions like this! If nothing else, the corrections to my theories are illuminating. )

In the case of the chimney I can see it being important because the air flow from the furnace would create greater pressure in the semi-confined space than the water heater, and if it's on top the water heater combustion products could get pushed back through the vent or the fireplace, if present. In your case it seems like the air flow of the furnace is directed straight out through the vent, and may actually help create negative pressure to draw the air from the H2O heater out.

**Bob Harper** · 11-28-2011, 07:19 AM

The code contradicts itself. In one place, it discusses putting the smaller BTU input appliance vent connector above the larger but elsewhere, it allows manifolding. With manifolding, there are no prescriptions such as one above another. The concept of one blocking another are theories and not proven. I find in general as long as the vent is properly sized, unblocked, with the appliance(s) firing properly, they will vent properly as long as the vent connetors are pitched upwards, properly screwed together and supported, sized, and have the maximum vent rise possible. I cannot over-emphasize vent rise.