Condensation on windows: Furnace Dehumidifier?

[Carnak]

Originally posted by teddy bear

The quality of the window and the indoor temperature are fixed. The outdoor temperature/dew point are a varible.

How about "the quality of the window is fixed and the indoor room temperature is fixed as long as the thermostat is not set back at night or while the occupants are at work . The outdoor temperature, dewpoint and the inside surface temperature of the glass are a variable."

U-shaped condensation pattern is common in particular when there is not a good thermal break between the panes of glass. The indoor dewpoint and the inside temperature of the windows (which will be colder than the room air in the middle of the room) determines whether there will be condensation or not.

Usually you get your coldest ambient temperatures as 'the overnight low' and combine this with the thermostat being setback at night and the windows can become significantly cooler. Bay windows are prone to being cooler cause the surrounding air loses heat compared to the rest of the room. Keeping the curtains closed lets the air between the window and glass cool, and the inside surface of glass gets cold and is more prone to sweating.

Running the system on a fixed timer will probably control the condensation problem, but it could over dry or under dry the home as well. Why set it to time when you need to control RH?

Running it when humidity rises will control condensation and operate only as much as required.

Water vapour is a by product of respiration and so is CO2. If you neglect cooking or a housewife 'sweating to the oldies' humidity is being generated by people breathing. So in a lot of situations excluding hydroponic operations etc, it is possible that a ventialtion system responding to a rise in humidity is indirectly responding to a rise in CO2.

Intermittent ventilation of a high enough rate can cycle on/off as the humidity builds up, better in my humble opinion, than drying the house bone dry for 12 hours straight, then letting it get rehumidified for 12 hours.

You can use timers for minimum run times every hour, but why use time to control humidity? Control it with a dehumidistat.

[Carnak]

Good windows will allow 35% RH during some extreme weather.

[mechanicalmaster]

Thanks for all the responses. All the points are quite valid and after hearing all of the arguments I think my soloution is going to be going with a dehumidistat. Iam going to have to do a little research on the proper way to do control wiring circut on the AprilAire unit that is being used but I think this is a good soloution to the problem at hand .If anyone else has done this kind of a hookup any suggestions would be greatly appricated.

[Carnak]

Originally posted by teddy bear

In other words, you must re-adjust the indoor %RH to match the changing outside condition. Otherwise the HRV will operate all the time during high outdoor dew points or none of the time during the low dew points.

Teddy

I see what you are getting at, but in the heating season, there is no outdoor dewpoint that is going to be detrimental.

The humidity settings you mention , concerning dialing the setting up and down, remind me of those on the controls of residential humidifiers which are going extinct in new homes in Canada. People have been setting humidifiers up and down for years.

With good windows, you can maintain 35% during some pretty cold weather. The homeowner will soon learn what level avoids condensation in the coldest weather.

I was working on an automatic RH reset before I left Canada but back then it looked like I was re-inventing the wheel.

From reading this site, it seems as though that some humidifier set points are being reset automatically with respect to outdoor temp, so it makes one wonder why you could not reverse the output of these controls with a relay or two.

[Carnak]

If the dehumidistat did not turn on the HRV during low ambient dewpoints, you would almost think the house was so drafty it would need a humidifier