Our S9V2-VS would refuse to start up in the morning after being off during a cold night (below freezing outside). After a few visits from the technician, he discovered rusting around the inlet chamber and concluded that moisture from the cool air was condensing and preventing the furnace from starting up. He recommended leaving the inlet pipe disconnected from outside air. The following day the furnace started up normally, so that would appear to be the issue.

However, our basement seems to have a slightly negative air pressure. I checked the hood on the gas hot water tank and it does draw air up, even with a little negative pressure. Given this, is the negative air pressure the likely reason why moist air was condensing in the intake air combustion area? Is there a way to prevent moisture from condensing there? I am concerned that leaving the intake air open in the basement might pull exhaust gases from the hot water tank into the room air instead of going up the b-vent.

 

What causes rusting like that is moist, corrosive exhaust gas being pulled into the intake.

The answer is to fix the exhaust/intake piping outside, not disconnect the intake.
The manual has specific instructions for venting; when the exhaust and intake terminate too close together, there will always be re-circulation.

Definitely not a good idea to leave the intake open in the basement.

Can you post a picture of the intake and exhaust where they pipes terminate outside?

 

I don't have a picture of the exterior at the moment. I could take one later on.

However, the exhaust is about 6' above the intake.

 

Distance between exhaust and intake is 53".

The Installer's Guide for this furnace states a minimum of 12 between inlet and exhaust terminations.

 

I just checked with the Installer's Guide regarding slope. I knew the exhaust needed to be sloped towards the furnace so that condensate would flow towards the floor drain.

I felt that the intake should slope downwards towards the outside, and confirmed that according to the Guide. My intake was installed sloped downwards towards the furnace. This may have been the problem.

 

So far I have no definitive answer to my problem, and there seem to be no other answers suggested on this forum either. This leaves me with possible hypotheses.

I checked the air intake pipe several cold mornings and saw no evidence of condensation. Ever since the fresh air intake pipe was removed, I've had no trouble with the furnace.

This leaves me thinking that the cold dry air may have cooled the burners sufficiently to allow the more moist air inside the furnace to condense. Water will be removed by the drain, that won't dry out the air around the burner. Incoming cold air may cool the metal sufficiently so that the more moist air will condense.

 

When the furnace is firing, the burners and faceplate are hot, basically making it impossible to get condensation there due to cold air coming in. When the furnace is not running in winter, the surfaces should be warmer than outside.

Based on the picture of the exterior venting, I don't think re-circulation is the problem.

Do you have a/c? Is your climate humid?
If yes to both, what temperature is the basement typically? What do you keep it at?
Maybe the humidity in the summer which comes in through the intake is condensing on the surface -> a possibility if the indoor temperature is lower than the dewpoint outside.

 

We typically turn the furnace down at night to 16°C, so it usually will not call for heat for the entire night until 5:30 AM. The house temperature will drop to about 17°C when the outside temperatures get to -5°C.

 

I was talking about summer.

 

Our issue is after a cold night, close to or below freezing.