Venting Question

Could someone explain to me why the water heater flue needs to be above the larger furnace flue as illistrated.

If there is a refernce code that someone could provide?

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The reason for this is that if both appliances are firing at the same time the lager unit can choke off the exhaust of the smaller unit if the larger unit is on top. Thus causing backdrafting of the smaller unit.

The way Douglas Hanson (CodeCheck) explains it, when a flue/vent is cold there is a column of cold air that must be heated and displaced before a good draft is established.

In a properly sized dual appliance vent, the vent above the junction of the two appliances is larger than would be required for either appliance connected to the vent by itself, so the column of cold air is even larger than at a single appliance vent.

Thus, this smaller appliance, especially, needs to be in the most advantageous position to displace the cold air.

If the smaller vent is the higher connection, there is less cold air to displace.

Hi all,

I think you will find this web site very useful on venting done right from the
Gas Appliance Mfg. association.

They have a really nice video explaining venting principals.

Dang! I meant venting “Priniciples”…

Venting principals is illegal in this country…