Gas Water Heater and Furnace Chimney Flue Size Calculations

The basic method for the chimney flue size calculations is "The Seven Times Rule" (applies to natural draft and / or fan assisted type appliances vented into the same chimney).


The following gas water heater and furnace chimney flue size calculations are based on Peoples Gas “Construction Guide for Gas Usage” in the city of Chicago and National Fuel Gas Code. Before applying any of the following rules, you must check your local building code requirements, and appliance manufacturer installation instruction.

Chimney flue size - diameter must be properly calculated and installed - it is a serious safety issue!

  • Have you checked your house brick chimney lately… or whatever type of the chimney you have?
  • Did you have a new furnace or water heater installed / was your house chimney flue recalculated for those new appliances?

Make sure that you chimney flue size / diameter is correct; chimney flue size is one of the parameters that will help to achieve proper draft – the water heater and a furnace exhaust gases will be discharged safely to the house exterior – your life may depend on it. Some of the consequences of an excessive or insufficient chimney draft are:

  • Poor burner performance
  • Excessive combustion (burning process) noise
  • Improper combustion which could result in Carbon Monoxide spillage into the living area
  • Soot buildup inside and around the burner chamber of your appliances

The basic method for the chimney flue size calculations is “The Seven Times Rule” (applies to natural draft and / or fan assisted type appliances vented into the same chimney).

When two or more appliances are connected to a chimney, the flow area of the largest section of that chimney cannot exceed seven times the smallest flue collar, or draft hood outlet area, unless designed with approved engineering methods.

To make it simple – if you have a gas water heater with a 3” draft hood, and a furnace with a 4” flue collar connected to the same chimney flue, you’d calculate that chimney flue size based on the 3” draft hood – the smaller of two. So let’s calculate… or why don’t we skip those calculations and I’ll give you the answers:

The largest permissible flow area (in²) of the chimney flue for draft hoods or flue collars.
Diameter of Draft Hood Outlet or Flue Collar Flow Area (in²)
3″ 7.065
4″ 12.560
5″ 19.625
6″ 28.260
7″ 38.465

The Seven Times Rule application results:

The Seven Times Rule for appliances connected to a multistory common chimney
Smallest draft hood outlet or flue collar Largest section of vertical vent or chimney
3″ 7″
4″ 10″
5″ 13″
6″ 15″
7″ 18″

Q. Are those numbers sufficient for calculating you chimney flue size / diameter?

A. No, they aren’t! There are several other factors responsible for chimney flue size; chimney placement, height, lateral length of the vent pipe, type of the vent pipe, number and type of the connectors along the entire chimney flue run, size and category of the vented appliances…

So it’s always smart to have a professional calculating your chimney flue size. But, using the tables above will at least prevent many wild, way off the chart installations. Make sure that you have Carbon Monoxide detector installation performed according your local jurisdiction requirements!

If you’re looking for a gas water heater and furnace chimney flue size tables / guides covering most of the possibilities, try this publication by Hart & Cooley.

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