Thread: Flex Aluminum At Forced Furnace Flue Vent

I've never encountered this before. I've happened upon a house with a gas fired furnace accessible in an exterior closet. It has a forced flue vent. A significant portion of the vent is flex aluminum. It appears to be that because I was able to easily dent the flex material with my finger. It was far thicker than your typical clothes dryer vent, but.... A licensed plumber was at the house doing a hydrostatic test. I asked him if this was kosher. He said, "Only if it is stainless steel." Is that correct? I can't find anything in the code book about it and there was no labeling on the vent pipe itself. Anyone have any idea?

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What did that label on the connector in the first picture say?

Originally Posted by Michael Thomas

What did that label on the connector and the first picture say?

It was a double wall metal pipe so I didn't even read it. Probably should have, but after the plumber said what he did - I just accepted his comments. It was only later that I began to question what he said.

Originally Posted by Michael Thomas

What did that label on the connector and the first picture say?

I've uploaded a higher resolution image (if that might help).

check attachment.
.

Here in a cold climate we use this stainless chimney liner to help a Category I furnace (negative flue pressure, non-condensing) such as an induced draft (80% +/- efficient) vent up a masonry or transite chimney without condensing on cold, vulnerable clay, brick, or asbestos-composite (OK-"asbestos-like") flue surfaces.

First clarify the Class of appliance. Is this a Cat.I appliance such as Ed discussed relining for (a very correct and appropriate point Ed) or a Cat. III or IV condensing furnace, which would use venting such as in Billy's post. "forced vent" is a confusing term. I recommend just sticking to Category I, II, III, or IV for gas heaters because that's how the venting listings are broken out.

Next, read the label on the attached piece of flex. Within Cat. I, you have B-vent, masonry chimneys and masonry chimneys with liners. Sure you can legally vent these appliances into factory built chimneys, L vent for oil or as in the doc. Billy provided, even Special Gas Vent listed to UL 1738, which is intended for condensing furnaces and boilers. However, the vent connector on a Cat. I can be ordinary snaplock galvanized steel pipe or its equivalant. Now, some mfrs. have developed an aluminum flex connector with fittings to connect to B-vent and a draft hood connector to attach directly to the appliance collar. These connecters, being listed, generally carry a 1" clearance to combustibles, rather than the usual 6" clearance off single walled pipe. I have seen short pieces of flexible aluminum liner used as a vent connector to weave around pipes and obstructions in lieu of a box of elbows and cutting pipe. However, the Code sets requirements for what material is acceptable as a connector and flex generally must be upsized or derated. One of the biggest problems with these unlisted slinkies is support or lack thereof. The listed connectors are stiff and self supporting. Ordinary listed liner is not intended to be self supporting outside a flue and thus sags and is more vulnerable to damage. Also, unlisted connector carries the 6" clearance while listed carries the 1" usually.

Recently, a flexible B-vent has been listed for use anywhere in the system except penetrations. They still want rigid pipe penetrating firestops and wall thimbles. Metalbestos comes to mind.

Before you make a call that flex on a vent is inappropriate, you need to identify the appliance and venting then do the research. Don't forget sizing. Generally, when you see flex the same size as the appliance collar, you know it is probably not properly derated and thus undersized even if it is listed for the application you found it in.

HTH,
Bob

Originally Posted by Bob Harper

Before you make a call that flex on a vent is inappropriate, you need to identify the appliance and venting then do the research. Don't forget sizing. Generally, when you see flex the same size as the appliance collar, you know it is probably not properly derated and thus undersized even if it is listed for the application you found it in.

Thanks for this info Bob. I see that I need to learn much more about this issue. Prior to this inspection I'd never seen this material before. In the end, I choose not to call out this vent as deficient. Given your description my intuition tells me I was right in the call. The flex was VERY stiff. It had a couple of bend with one short segment running on the horizontal with no sagging at all. And, as you can see in the photo, it was over sized. I know these observations are far from a proper way to make such a call, but there were other issues at the furnace that caused me to recommend further review by a technician.

I resolve to do better next time. I resolve to not allow the comments of a licensed plumber to enable me to become complacent.

You are on the right track Robert, which is learning, as we all hopefully are. Just because someone holds a license doesn't mean they have any sense or can remember things more than an hour or two. Technology changes and we have to stay up on it.

In the meantime, I suggest you along with all inspectors and technicians copy down or photograph the rating plate data of every appliance and vent you can. You can never take too many photos but you can always take too few. Every job I touch has photos recording the rating plate and layout of every appliance and vent. It's always on my harddrive should an issue arise.

In this case for example, you can document in the field then later do some reearch back at the office if need be. As to what the material is, you can always state that you were unable to ascertain if it was approved for that application while you were on site (if you write reports on site) or upon basic research into the matter back at your office ( you are not performing a forensic investigation so there is a limit to the amt. of research that can be expected of you), you cannot make a determination so you recommend a qualified professional be consulted.

Bob

Originally Posted by Bob Harper

In the meantime, I suggest you along with all inspectors and technicians copy down or photograph the rating plate data of every appliance and vent you can. You can never take too many photos but you can always take too few. Every job I touch has photos recording the rating plate and layout of every appliance and vent. It's always on my harddrive should an issue arise.

I normally do photograph labels and tags as well. I guess for some reason I let the talkative plumber throw me off my game? When I couldn't see a label on the flex I didn't even photograph the label on the connector. It never even occurred to me that they might have been a single assembly. Thanks again.