Reader Frank recently commented:
“Another strategy that I’ve found useful is the proper use of the bathroom vent fan. I know the cfm rating of it and after figuring the size of the room determined that the fan is capable of completely exhanging ALL the air in the room in under two minutes! Excess humidity is no good either; the windows throughout the house frost up noticeably more when the fan doesn’t get used and interior humidity is too high. Anyway~ fan stays off during shower operations and gets flipped on for 1-2 minutes right after, and that’s it. Most of the humidity goes out, most of the heat stays in.”
This reminded me that I’ve been meaning to to discuss bathroom vents. To start with my conclusion: unless you use them as Frank does (i.e., most judiciously) I think they are a horrible idea in winter. They are the equivalent of cutting a hole in the wall and blowing cold air in through it at somewhere between 50 and 200 cubic feet per minute. Oh, I know– they don’t blow air in, they blow air out. But that air is replaced with air from some other part of your house… which is replaced with air straight from outdoors. There’s no way around this fact. You don’t really notice it, probably, because it’s sneaking in through a thousand cracks, and you’re likely in the shower while it’s happening. But it’s a travesty.
In fact, it’s a double travesty, because besides replacing warm air with cold air, you are losing the (much larger) amount of heat contained in the water vapor. One of my Cold House mottos is that water should leave the house in the same phase of matter that it came in. This is because of the enormous heat that it takes to melt or vaporize water. If water comes into the house as liquid, but leaves as vapor, you’ve put in a lot of heat that you aren’t getting back.
But of course you can’t just let the bathroom get and remain sopping wet all winter. My solution at the Cold House is controversial, but I’m prepared to defend it vigorously: we use a very small dehumidifier. It sits in the corner of our very small bathroom.Here is the routine: In the morning, I switch it on just before showering, set at 50% humidity. It stays on steadily through a shower, and for perhaps 15 minutes after, as it dries out the air. Then over the course of the day it will briefly re-activate now and then, as the air becomes a bit humid again from wet towels, etc. By bedtime things are quite dry in there, and we turn it off altogether overnight. Next day, repeat. With two of us showering, I estimate the thing actually runs a total of 1.5 hours a day. It condenses about a quart of water a day into its bucket.
The dehumidifier (running) uses about 400 watts to power its fan and compressor. All of that winds up as heat in the bathroom. Moreover, it claims to condense 1.6 liters of water for each 1 kWh of electricity it uses– which about agrees with what I’ve observed. Now, each liter of steam condensed back to liquid releases a further 3,600 BTU of heat, which equals roughly 1 kWh also. So, for each kWh of electricity used to run the dehumidifier, we get back 2 kWh of heat in the bathroom. It is, effectively, like a 200% efficient space heater. Plus, we get a quart of distilled water every day (it’s enough to “small flush” the toilet, saving yet more energy!)
Let’s compare to a vent fan. That might use 50W to run its motor. All that turns to heat as well, but it goes out the vent– no use. All the steam goes out the vent– no use. Furthermore, in running 1kWh of electricty through a 50W, 50 cu. ft. per minute fan, you will vent out 60,000 cu. ft. of air. If the air in your house is 70º, and outside is 20º, you’ve blown out 54,000 BTU of heat (= 15.8 kWh).
So, assuming they serve the same function, about equally well, and you’re happy to have extra warmth in your house, which appliance would you rather use: the one that gives back 2 kWh of heat for each 1 kWh you put in, or the one that removes 15.8 kWh of heat for each 1 kWh you put in?
One final item: the vent-user will get out of the shower into bathroom air that is either warm but 100% humid (if the fan is used the way Frank does) or relatively dry but no warmer than it was to begin with (if a high-power vent is running while showering). The dehumidifier user, however, steps out into a bathroom environment that is warmer than it started, yet not saturated with humidity. And when the rest of the house is 45º, this is quite a pleasant bonus.
I conclude that the only rational reason for using a vent fan instead of a dehumidifier is if your bathroom smells so bad that you actually need to replace all the air in it eight times an hour. But if this is so, you may have other problems.
[Addendum: some will question whether the capital cost of buying the dehumidifier is worth going this this route. I agree, this might be a rational counter-argument. However, the cost of many high-quality vent fans is actually higher than our miniature dehumidifier. If you add in the cost of vent hosing, an exhaust-side fixture, installation labor, etc., it will surely come out to more. Plus, you avoid altogether having this unnecessary 4-inch hole in the side of the house that you’re trying so hard to keep warm. You might not have a choice in the matter, of course– your fan may already be in place, and I understand these silly vents are often required by code for new construction– but if starting from scratch, I maintain: no contest.]