1. ## Vent too long?

Is this furnace vent too long?? It is on an induced draft furnace and runs about 12' with little rise. The natural draft water heater flue had a little more rise, but ran about 16' before going out the roof. I know they are not strapped properly, but I was wondering if they are too long. There was not any signs of condensation or such, but they just looked too long to me.

2. ## Re: Vent too long?

Some instances the maximum offset (horizontal distance) allowed is 75% of the vertical height, but in no case is the horizontal offset allowed to be greater than 100% of the vertical height.

Here is an example of how to measure it:
- First, measure the height of the draft hood the vent is connected to, say a water heater draft hood is 5 feet above the garage floor.
- Then measure the height of the lowest discharge level of the vent cap on the roof, say that is 15 feet above the ground, which happens to be the same as the height of the garage floor.
- That means the total vertical height, total vertical rise, is 10 feet.
- Which means the total allowable horizontal offset is 10 feet if 100% offset is allowed, or 7-1/2 feet if 75% offset is allowed.

3. ## Re: Vent too long?

If this is a B vent termination, the general rule of thumb is the offset should be 1 1/2 the diameter (inches to feet).
In other words, if you have a 6 inch B vent, you can off-set it by 9 feet.

4. ## Re: Vent too long?

Thanks, that is what I needed. Couldn't remember the details.

5. ## Re: Vent too long?

Originally Posted by Darren Miller
If this is a B vent termination, the general rule of thumb is the offset should be 1 1/2 the diameter (inches to feet).
In other words, if you have a 6 inch B vent, you can off-set it by 9 feet.

The horizontal offset has nothing to do with vent size, only with total vent height.

Jerry.

7. ## Re: Vent too long?

Jerry,

I believe you gave the requirements for a connector to a 'natural draft chimney' (M1803.3.2).

I could be wrong, but I believe the question was about a B vent termination.

8. ## Re: Vent too long?

From the National Fuel Gas Code:

"... the maximum horizontal length of a type B double wall connector shall be 100 percent of the height of the chimney or vent"

Darren, I believe you are reffering to the rule concerning maximum vent connector horizontal length. The maximum total vent connector horizontal length should not exceed 18in/in of connector diameter. The total horizontal length of the connector and the vent must comply with the rule stated above. Also, remember that the vent sytem has minimum and maximum input values that are dependant on the size and height of the vent system.

We are not able to see the total vent system in the pictures provided, but my guess is that the total horizontal length is much longer than the total vertical length, and therefore not permitted. Most likely, this will cause improper venting, causing operational and safety concerns.

Jerry S

9. ## Re: Vent too long?

OK, let's look at this together.

Definition of a connector- The pipe that connects an approved appliance to a chimney, flue or vent.

So, to me that says a connector is, for the most part, a 'horizontal' pipe that attaches to a vertical chimney, flue or vent.

G2428.3.2 (504.3.2) Connector length limit- The vent connector shall be routed to the vent utilizing the shortest possible route. Except as provided in section G2428.3.3, the MAXIMUM vent connector HORIZONTAL length shall be 1.5 feet for each inch of connector diameter as shown in table G2428.3.2.

Last edited by Darren Miller; 01-08-2010 at 09:28 AM.

10. ## Re: Vent too long?

Jerry;

Picture this; a two story house has a chimney that's 24 feet high.
Are you telling me I can have a vent connector 24 feet long?

11. ## Re: Vent too long?

Yes, you could. If the vent is still sized correctly. The real question is, do you actually have a vent "CONNECTOR"? The pictures shown would suggest that no vent connector is being used.

But first, let's assure that our definitions are correct. A connector is, as you have stated, "The pipe that connects an approved appliance to a chimney, flue or vent". Refer to the attached document

If you have a single appliance vent system, with "B" vent connected directly to the appliance, and have that connected directly to a "B" vent system, you have no vent connector (such as in Fig 1). The lateral length of the vent SYSTEM can be up to the vertical height of the vent system.
In this senario, the horizontal could be 24 feet with 24 feet of vertical height.
As a general rule, a vent connector length (shown in FIG 2) should be no longer than 1.5 feet per inch of diamater as you stated.
If that length must be longer, the max capacity of the vent is reduced by 10% for each additional multiple of that length.
So, for a 24 foot vent with a 24 foot vent connector, if the appliance input still falls between the new de-rated max input and minimum input rating of the vent system, yes it can be used.

12. ## Re: Vent too long?

Originally Posted by Jerry Shipman
If you have a single appliance vent system
I thought we were talking a common venting system (water heater and furnace).

If we are talking common venting, then you can't use the 'no connector' theory.
Again, in G2428.3.5 (504.3.5) Common Vertical Vent offset. Where the common vertical vent is offset, the maximum capacity of the common vent shall be reduced in accordance with section G2428.3.6. The horixontial length of the common vent offset shall not exceed 1 1/2 feet for each inch of common vent diameter.

I don't know Jerry, With a vent connector or a vertical offset, I keep coming back to the 1 1/2 ft to 1 inch rule.

13. ## Re: Vent too long?

Originally Posted by Darren Miller
Jerry.

Darren,

- G2428.3.2 (504.3.2) Connector length limit.
- - The vent connector shall be routed to the vent utilizing the shortest possible route. Except as provided in Section G2428.3.3, the maximum vent connector horizontal length shall be 1.5 feet (457 mm) for each inch (18 mm per mm) of connector diameter as shown in Table G2428.3.2.

TABLE G2428.3.2 (504.3.2)
MAXIMUM VENT CONNECTOR LENGTH

Notice that it states "CONNECTOR" length limit, not "VENT" length limit.

Frank's question was: (red text is my highlighting) "Is this furnace vent too long??"

14. ## Re: Vent too long?

Originally Posted by Darren Miller
Jerry,

I believe you gave the requirements for a connector to a 'natural draft chimney' (M1803.3.2).

I could be wrong, but I believe the question was about a B vent termination.
From the AmeriVent installation instructions ( http://www.ampcostacks.com/pdfs/bven...structions.pdf ): (underlining and bold are mine, as is red text)
- General Installation Guidelines
- - 7. Wherever possible, install vertical vents directly above appliances before beginning any lateral runs. Lateral venting should be kept to a minimum to avoid vent resistance. Horizontal runs to vents should be either in accordance with the appropriate tables in NFPA 54, or should not exceed 75% of the vertical height of the vent. Laterals should be supported. Excessive number of fittings, such as elbows and tees, should be avoided.

As I recall, Simpson allowed 100%, if not Simpson it was another manufacturer.

But they are referring to the vent and not just the connector.

15. ## Re: Vent too long?

Correct, there was not a connector. They were vents. The roof was very tall, but the vertical height of the terminations were way less than the horizontal runs. Especially on the water heater.They could have gone straight up out the roof. I have no idea why they chose to run these long horizontal runs. I guess they did not want to get on a ladder in the attic to put it together and did not have a clue that it was not correct. There was a final inspection sticker, but we know how that goes when it comes to anything in the attic.

16. ## Re: Vent too long?

That just reminded me of another I did a while back with an Energy Star Rated home that had the Passed sticker on the service panel. They never insulated the attic over the master bedroom closet or the utility toom. It passed though

17. ## Re: Vent too long?

Hi Jerry;

Skimming your attachment before my AM inspection, I understand the written instructions you refer to. Now, do me a favor and read table 5 on page 9.

No where in the table does an offset exceed 1 1/2 the diameter (in-ft) of the vent. It's kinda weird, but I don't see a table for 90 degree elbows. I'll re-read it later.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•