Rust/Condensation on water heater exhaust vent hood

Can anyone tell me why this hood on water heater exhaust vent pipe is so rusty and what the solution might be. Thanks!

possibly to many elbows in vent pipe causing moisture to condensate and fall back down vent

Poor drafting possibly. Or, what is above that? Could be a blockage, causing condensate in there as well.

Missing rain cap?

How was the top of the other water heater?

I agree. Usually when you see rust on top of the water heater like that it indicates that the water heater is not drafting properly and allowing exhaust gases to backdraft into the house.

Did you try to confirm backdrafting? You can use a smoke pen to test for backdrafting.

Did you take a carbon monoxide reading?

Ultimately - not matter what the exact cause - “Heavy rust and corrosion noted on the top of the water heater and the exhaust vent for the water heater. This condition can indicate poor appliance drafting, although other or multiple problems may exist. Poor drafting can allow dangerous gases to enter the house. Safety concern. Recommend further evaluation by a licensed plumbing professional or other qualified professional prior to close of escrow.”

As the warm moist air rises inside the metal flue it cools (especially in winter), particularly if there are a lot of bends and elbows (or restrictions) or long runs of flue. The moisture condenses on the inside of the flue, collects along the interior sides and runs back down onto the top of the appliance. The flue gases also contains acid as a result of incomplete combustion, that too etches the metal, adds to the corrosion and significantly shortening the life of the unit. In short, sucky installations can and do damage to equipment, so don’t do it.

Thanks to all for the quick and useful responses.

Improper draft, lack of combustion air, mechanical room depressurizing need to get it checked out by someone that knows what they are doing.

I agree with the others; poor ventilation causing coorsive condensation of combustion gases to form and travel back down the vent pipe.

What is the diameter of the flue pipe, looks large, but it may just be the photo. Sometimes too large an exhaust flue will cause poor drafting.

Jeff Tatlock
BEACHSIDE Home Inspection
Serving Satellite Beach, Indian Harbour Beach, Indialantic, Melbourne Beach, Cocoa Beach, Melbourne, Merritt Island, Brevard County, FL

That’s insulated vent pipe.

The large amount of rusting is from vent gas spillage. As said get it checked out.


Insufficient draft.

Combustion gas is exiting outside the inner flue and condensing on the outer flue that is below the dew point temp of the combustion gasses.

If I knew where you were from, I’d offer to swing by and do a draft test…

I’d even tell you why it was back-drafting…

I’m with Michael on this one, although a draft test with a match is definitely in order.

Match smoke is not a proper test. A draft gauge during system operation and operation of other devices such as furnace, kitchen and bathroom exhaust fans, clothes dryers etc. is the correct procedure

Agreed, but do you use a draft gauge during a standard home inspection?

What was condition of the rain cap and the rest of the venting?