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The following guide is 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.
Make sure that you chimney flue diameter is correct; sizing 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:
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 like the 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 performing your chimney flue sizing. But, using the tables above will at least prevent many wild, way off the chart installations. Always have Carbon Monoxide detectors installed according your local jurisdiction requirements!
If you’re looking for a gas water heater and furnace chimney flue sizing tables / guides covering most of the possibilities, try this publication by Hart & Cooley.
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Todd Beaulieu
2 months ago
Thank you for this useful information!
Sadly, I recently discovered that our water heater was (apparently) incorrectly installed, venting directly into the old chimney, w/o a liner of any kind. We’ve been struggling to keep the pilot lit. Paid over $600 so far in the past year on “warrantee” replacement parts from Sears. Their most recent visit led to this discovery (after how many visits???). The worst part is that we purchased the home like this and don’t have the receipts. I’ll bet anything that Sears installed it. And we’re left stuck!
I asked the guy if we could vent the heater out the side of the house, as our Trane heating unit does. He said no, that we’d need a heater designed for that. At this point it occured to me that it might be cheaper to buy a new heater that vents directly, than to pay to have the chimney fixed.
What do you think? Thank you!
Bill Moss
1 year ago
My hot water heater exhaust vent (3″) is connected to the exhaust vent of the furnance. When hot water heater is heating water, and furnance comes on everything is fine.. but if the pilot lite is only on the moment the heater fan comes on the lite goes out. If you try reset it while the blower and heater is activated the match goes out, no draft seems like a lack of oxygen. What can I do, it was installed before I bought the house by a lic professional?
thank you for any suggestions
admin checkthishouse.com
1 year ago
A picture of the vent pipe setup would be helpful
. Assuming that you have an induced draft motor equipped furnace, both vent pipes (from furnace and WH) must be connected together with a WYE type connector – check this post Chimney Flue and Furnace / Water Heater Venting. Otherwise, furnace induced draft motor might be forcing air / exhaust gases into the water heater vent pipe. Whenever the water heater burner starts operating first and heats up the chimney, the furnace exhaust gases have some extra draft help from already warm chimney and instead of flowing back into the WH vent pipe, they continue into the chimney. If you have a WYE type connector properly installed (I’ve seen them up side down), section of your chimney might be partially blocked, and before it picks up proper draft, it spills some of the exhaust gases back into the WH vent pipe. It might be also an oversized, cold chimney that causes back-draft. One other thing that I can think of is a missing or collapsed WH draft hood… ? Please, let me know.