Venting Question and I need answer for Monday

[scarey8]

I have a 35 000 btuh tortilla baking machines, natural draft appliance, unit has a 6" breach on it, placed approx 4 feet from the chimney, total chimney height of 60+ feet, its kind of a temporary emergency install for a restaurant, chimney has an existing 4" liner in good condition.  gas code states venting and vent connectors of 12" diameter and less shall not be reduced more than one table size (eg: 12" to 10" is one table size)   i went with this information, hooked existing 6" B vent to 4" liner with 6" run from appliance to the chimney connection, testing found i had a draft issue with spillage.  like the oil code would all reductions in vent connector sizing, should the reduction take place as close to the appliance as possible?   any suggestions to correct this issue?

Scarey

[Admin]

(8.18.5) When a vent connector is required to have a size other then that of either the appliance flue collar or draft-hood outlet, the change in size shall be made as follows:
(a) If the size is increased, the change in size shall be made at either the appliance flue collar or draft-hood outlet, except that when either the flue collar or draft-hood outlet is inside the casing of the appliance, the increase shall be made immediately external to the appliance casing.
(b) If the size is reduced, the change in size shall be made at the appliance flue collar.

(8.18.8) When either a chimney or a vent has been sized in accordance with the requirements of Clauses 8.13.1 and 8.13.2, and the flue area is less than the area of the vent connector, the change in size shall be made where the vent connector is attached to either the chimney or the vent.

I'm assuming there is only a single 35,000 btu appliance, with a 6" flue collar.

You're reffering to C.2.3, which allows us to use a smaller vent connector.

Also C.2.16 states,

When the vent table permits more than one diameter of pipe to be used for a vent connector or vent, the smallest permitted diameter shall be preferred.

I would install a 6" to 4" Bvent adapter at the appliance, and use a 4" Bvent vent connector and adapt it to the 4" liner.

Using Table C.1, a 4" liner at 50' and 5' lateral with a 20% reduction for the liner, would have a capacity of 96,000 btu.
At 100' it's not recommended to vent a natural draft appliance.  I'm not sure exactly how high your chimney is.  Table C.1 only shows data for 50' and 100', so you would have to interpolate to find the exact capacity for your chimney height.  It looks like capacity isin't the problem though.

Your problem might be caused by lack of combustion air.  Is the structure energy star or R2000?  They may have exhaust fans running in the restaurant, that could cause the negative air problem in the chimney.

[scarey8]

Thanks for the quick reply, thats what I was thinking about doing to alter the venting connection.  it is a single appliance connected to the unit.  chimney height is around 60 feet, the building has been unused for around a year now so i'm going to check on the roof to find out if a wasp or any insect has made a nest in my liner.  everyone has told me you cannot go down two sizes in venting but the code states that you can, i will be doing more testing on monday and will post how the outcome is.  it is a restaurant and i am new to that area of the hvac field, so i will investigate the depressurization issue, its definately not a r2000 structure.  is there anywhere in the code book that states where the reduction in venting sizes is to take place, or is at the appliance one of those many unwritten rules in the code book. 

[Admin]

If you use a 6" Bvent connector to the chimney liner follow Clause 8.18.8;

(8.18.8) When either a chimney or a vent has been sized in accordance with the requirements of Clauses 8.13.1 and 8.13.2, and the flue area is less than the area of the vent connector, the change in size shall be made where the vent connector is attached to either the chimney or the vent.

Reduce your 6" Bvent connector at the liner.

OR;

C.2.3 allows us to use a 4" vent connector and you would follow Clause 8.18.5(b);

(C.2.3) If the vent connector size, determined from the tables, is smaller than the appliance draft-hood outlet or flue collar, the smaller size may be used, provided that
(a) the total vent height (H) is at least 10 ft (3 m);
(b) vents or vent connectors for appliance draft-hood outlets or flue collars 12 in (305mm) in diameter or smaller are not reduced more than one table size (e.g., 12 in (305 mm) to 10 in (254 mm) is a one-size reduction);
(c) vents or vent connectors for appliance draft-hood outlets or flue collars above 12 in (305 mm) in diameter are not reduced more than two table sizes (e.g., 24 in (607 mm) to 20 in (508 mm) is a two-size reduction);
(d) the maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10% (calculated as 0.90 x maximum capacity); and
(e) the draft-hood outlet is greater than 4 in (102 mm) in diameter.  For example, a 3 in (76 mm) diameter vent or vent connector shall not be connected to a 4 in (102 mm) diameter draft-hood outlet.  Tis provision shall not apply to fan-assisted appliances.

(8.18.5) When a vent connector is required to have a size other then that of either the appliance flue collar or draft-hood outlet, the change in size shall be made as follows:

(b) If the size is reduced, the change in size shall be made at the appliance flue collar.