condensation

drhvac

On a job today doing a service on an American Standard 90% efficient single stage furnace. I put this in about 8 years ago. This is a sealed burner assembly. I took the burner cover off to inspect and noticed signs of condensation. Inside the heat exchanger just pass the burneris rusty, as well as some some other spots in the chamber. Nothing crazy, but it was there. Why? Gas pressure was perfect at 3.5". temp difference across the furnace was 50 which is right in the middle of manufacturers specs between 35 - 65. Its direct vent so it is taking its combustion air from outside, not drawing in any dirty air from the utility room. Combustion test came out as follows :

stack temp - 126

co2 - 6.39

co air free - 6ppm

exair - 73.9

efficiency - 88.7

draft - .047

Steve Whitbeck

Reversion

Is the furnace vented with a concentric vent?

If the exhaust gasses get sucked back into the intake pipe you will rust out the burners and heat exchanger.

drhvac

no

Is that a problem with concentric kits? I rarely use them. The vent pipe goes straight out about 12" - 14" and stops, and the fresh air elbows down close to the house. So I don't think that is happening.

icesailor

Condensation/Restriction:

I've yet to see this problem mentioned here but there is another thing to add to this mix.

Some homeowners don't like that ugly pipe(s) sticking out from the side of their house and spoiling the aesthetic beauty of their home. So, they plant bushes as a screen to hide the exhaust. The bush WILL grow and it WILL cause exhaust to hang around the opening and get sucked back into the intake. Where I work, Designers and architects are notorious for having the landscapers plant vegetation to hide these ugly and necessary things.

I once had a customer place something like those plywood panels painted to look like a larger woman working in a garden, bent over only it was a cast iron plate painted with flowers or something like a fashion/garden statement.

It will be worse on a day when it isn't that windy, regardless if you use a concentric or dual outlet. If it is making a lot of condensation, you will see the condensate/vapor, stay up against the foundation and travel along to be sucked up by the intake. There is ALWAYS soime wind movement, even when it is dead calm. Just the warm exhaust air will cause a wind pattern.

When sailing on a lake, it may be dead calm in the middle but go to the windward shore and sail along  by the trees, you will get thermal convections from the dark trees from the sun.

You have to understand about the wind. It does strange things.

JStar

RE

I'm always suspicious of outside combustion air. What happens when it rains or snows? All that humidity is going to be sucked in to the furnace. I've also had a manufacturer's rep tell our company that when the A/C is running, the furnace will become cooler, and outdoor air will travel into the combustion box and condense. In any case, it's a "necessary" design flaw to use outside combustion air. You start to realize how important the maintenance of these 90% systems really is. And how terribly difficult most of them are to maintain. Some of those burner boxes are just down right impossible to access.

Henry

Vents and combustion air

We have a lot of problems with the manufacturers supplied combined vent/combustion air pieces. With our cold and humid air, often the combustion air inlet will freeze shut. IE it is swallowing wet combustion gases.

Then you have a condo association that does not like the pipes hanging out of the side of the building in the rear courtyard and the noise. The installer could of used the old incinerator chimney to vent!

 

SWEI

equal pressure zone

I don't recommend or install anything that requires intake and flue to be in the same pressure zone any more.  There are alternatives, and they work better.