I live outside Boston. In fall 2003 we had our basement finished. Two gas-fired furnace/AC units and hot water heater were enclosed in a large "mechanical closet" separated from the finished part of the basement with double French doors. The HVAC contractors were not sure the mechanical closet required a combustion air system (CAS) to provide fresh air. To be safe, one was installed anyway -- the Field Controls fan-in-the-can model CAS-4. The exterior air intake vent hood is on the side of the house that gets a lot of wind at it.

For the first big snow storm, our furnace wouldn't turn on. Discovered a lot of snow and ice build up in the CAS motorized unit and the duct pipe was frosted on the outside. The ice inside the CAS unit prevented the motor from coming on, preventing the furnace from turning on.

HVAC contractor came out and knocked out the ice in the CAS unit so it would go on. On the outside of the house, I put a large plastic bin in front of the vent hood to prevent snow from going into the vent hood, but still able to draw in fresh air.

The rest of that winter (2003-2004) and this winter (2004-2005), the CAS unit has never been blocked with ice again, but when temperatures are cold outside (i.e., most of the winter), the duct pipe is frosted, there is a layer of frost/snow/ice around the CAS unit in the basement, and the concrete basement floor has a large puddle of water around the CAS unit (reaching near and under the furnaces and hot water heater -- fortunately not reaching the carpet or anything else in the finished area of the basement).

The interior air in the mechanical closet in and around the duct pipe and CAS unit must be getting so cold that the moisture in the air is condensing and freezing. The air temp in the mechanical closet is not that cold (guessing 45-55 Fahrenheit?), just the CAS unit itself and duct pipe are very cold.

Is this the way it's supposed to be in the winter time? If not, is it dangerous and how do I prevent it from happening? (Seems like basic physics/thermodynamics at work here that would be difficult to overcome.) Also, I don't have a drain in the concrete floor. I could check with the town building inspector to see if it's okay to put a drain in the concrete floor to at least have the puddle drain somewhere.

Thanks for any thoughts and suggestions.