Trained Eye

 
 
 

High Efficiency Furnace & Fresh Air Intake

Question:

I'm forwarding this question to you in the hopes that you will be able to help us.

We own a home in southeast Winnipeg and in 2002 we replaced our old furnace with a new high efficiency furnace. At the time that the furnace was installed, we were told that since this furnace was so much more efficient than our old furnace they would take our cold air intake out. They also said that if we thought we needed it at a later time they would come back and replace it. We thought that they must know what they are talking about and when we questioned why we wouldn't need it, their explanation seemed to make sense.

We did notice that when the furnace ran last winter there seemed to be a lot more humidity in the air. There was condensation on the windows, to the point that we were wiping them down so that the windowsills would not be soaked and run down the walls. I called the company that installed and supplied the furnace to ask about this problem and I was told that there were quite a few complaints that they had been receiving about this problem. I was told that was likely because we were having a milder winter than normal and there was more humidity in the air, but they would check and call back.

This year, we are having the same problem. Since I had not heard back from the company, I called to complain about the high humidity in the house. I was immediately asked if we had a cold air intake and when I said no and that they previously had told us that we didn't need one, an appointment was made to come out and check the situation. Now I'm told that with winter, the windows are closed more, that we cook more and that we should put plastic on the windows. I think they are all blowing smoke. We have triple pane windows and put them in specifically for their energy saving features.

Is it true that a high efficiency furnace does not need a cold air intake? Is the high efficiency furnace causing all this excess humidity? There is a drain line from the furnace to the sewer and that seems to be draining properly.

Answer:

The most common questions, and complaints, that I receive from readers are related to condensation and moisture on windows in the winter. I attempt to answer this question once or more times each heating season, and your question perfectly illustrates a common problem encountered when furnaces and other components are upgraded on an older home. A home should always be thought of one large system comprised of many smaller systems or components. When one of these individual systems, in this case the heating system, is changed it can affect the performance of the entire house system.

Firstly, in reply to your final question about the new furnace being the cause of the excess humidity in the house, I will suggest that it is mostly but not totally responsible for the added humidity in the home. This can be explained by describing the major difference between your new furnace and your old furnace. Other than the obvious lower fuel consumption, the biggest difference is the method of venting exhaust.

Your old furnace was likely a natural draft appliance with a draft hood that vented combustion products to the outside of the home through your chimney. These by-products of the burning of natural gas in your furnace were automatically mixed with house air, drawn in through the draft hood on the furnace, and sent up the chimney by the heat of this exhaust. This warm house air contained a fair amount of moisture that was naturally removed from the home during the heating season. The old fresh air intake also introduced some additional outside air to the home that was relatively dry. This differs greatly from the venting method of the new furnace.

Your new high efficiency, condensing furnace will vent to the exterior of the home through a small plastic pipe, normally exiting through the foundation wall. The much cooler exhaust products are blown out this vent by a small fan, integral to the furnace. This vent will be accompanied by another plastic fresh air intake pipe that brings combustion air directly into the burner chamber of the furnace. This chamber is often partially or completely sealed from the house air. I assume this is the reason that the furnace installers insisted that a fresh air intake is not required. Indeed, the fresh air intake is not needed for the furnace combustion air, but is even more important now that the draft hood of the old furnace is gone.

The majority of the moisture that used to go up your chimney, along with your heating budget, is now staying in the home. This is compounded by the new triple pane windows, that are far tighter the older window they replaced. Some of the normal winter humidity in the home would also have leaked out through the poorly sealed old windows. Combine this with the elimination of the old fresh air intake, and the moisture level in the home will increase considerably.

Reinstalling a fresh air intake should be your first action, not only to reduce humidity, but also to provide combustion air for the natural draft hot water heater that may still vent up the chimney. Running bathroom and kitchen exhaust fans regularly may also help reduce the moisture in the home and on the windows, but these measures may not be sufficient. Installation of an HRV or ventilation system that is controlled by a humidistat may also be needed for better control of excess moisture in the home.