So, a while back we replaced an old furnace with a heat exchanger heated by a tankless hot water system. Becky noticed that the air seemed not quite as dry as with the old furnace.

My initial thought, was that the heat exchanger in the old furnace would get hotter, and therefore burn off more moisture, while the hot water powered solution would limit how much the heat exchanger would heat the air. However, during today’s hike, Peter pointed out that the moisture would presumably find its way to the rest of the house. Grrrrr.

The straight dope forum mostly blames the air expansion for the drop in humidity, and this affect alone can be extreme – see the psychrometric chart. For example, 30ºF at 10% RH which is heated to 70ºF will be at 2% RH. With the new system, we did tend to keep the house a little cooler. To continue the example, if we only heated the 30ºF at 10% RH air to 60ºF (probably an exaggerated delta), shows essentially the same 2% RH result on the psychrometric chart. So, I doubt that was the difference. Grr.

There is another potential cause described in a post by Allison Bailes PhD – dry outside air entering the house. According to the post, either a leaky house, or a leaky return vent on a furnace in an unconditioned space can be the culprit according to the article, and this article that describes how the cold dry air mixing with the interior air is the problem. While we likely have both of these problems, I doubt either changed in the transition to the newer heating system. Following the reasoning in the article, if your furnace runs more, with a leaky system, then the air will get drier. However, our newer system would typically run the blower (but not necessarily the tankless hot water) much longer than the older system, but provided moister home air. Hmmm.

I’m not sure I buy the mixing argument of the 2nd Dr Bailes article, since where did the air in the house come from – outside, and got heated up. So, wouldn’t the outside air actually be moister? Mike Rodgers makes a better case for the leaky house argument by saying that houses provide plenty of moister through occupant perspiration, showers, boiling water on the stove, etc; and that a leaky house essentially looses this moisture, to be replaced by dry winter air. This explanation I understand better. It still doesn’t explain what we’ve observed.

Maybe the years with the new system were just moister? The newer system is almost 8 years old at this point. So, doubtful.

Comments and thoughts welcome on FB or via email.