Replies

1. 
   DanH | Dec 30, 2011 11:13pm | #1

   To function properly, a standard humidifier must blow air through a "medium" (not a Gypsy fortune teller, but a sort of air filter like thing generally made of a sort of plastic sponge, soaked in water).  Some humidifiers (particularly the stand-alone "room" units) have their own fan to do this, but a "bypass" unit installed on a furnace makes use of the furnace's fan to move air through the medium.

   So air moves through the bypass tube from the supply duct to the return duct, under the force of the furnace fan.  Most such units "don't care" which way the air moves, so the actual humidifier unit may be mounted on the supply duct or the return duct -- whichever is more convenient -- and the air may flow either way through it.

   Some sort of sensor/relay determines when the furnace fan is running and activates the water flow through the unit.  Most modern units run a trickle of water continuously through the medium and out a drain while the furnace fan is running and the humidistat is "calling" for added humidity.  Older units used a tank and a fan-driven water wheel to keep the medium wet (and the 400 probably still does).  This style must be cleaned several times a season, especially if the water is hard or contains a lot of sediment, while the type with a drain only needs to have its medium replaced occasionally.

   The main advantage of the powered unit (one that contains its own fan) is that it doesn't need a bypass connection and thus can be installed in a few situations where the bypass style would be impractical.  (We have a 700 for this reason -- there was simply no way to get a bypass unit to fit.)  It also is likely more powerful in terms of gallons per hour.  In terms of "efficiency" there's probably little difference -- a humidifier itself doesn't really use energy to speak of, and doesn't significantly affect the efficiency of the furnace.

   The moist air does not cause any damage to the furnace or ductwork -- the relative humidity never reaches 100% and so condensation does not occur.  The only units apt to cause damage are the spray or steam units, not those that employ a medium.

2. 
   kurt99 | Jan 01, 2012 12:03pm | #2

   Yes, that is RH

   Your thermometer/humidity meter is giving you the relative humidity, the same measure as the RH given by Aprilaire.  Those dial humidity meters are notoriously inaccurate.  You might want to try a sling psychrometer to get a better reading.  These use two thermometers, one measuring the normal dry bulb temperature and another with a wet sock on it.  There are commercial units with a built in fan or you can make one yourself with two thermometers, a mounting board, some cotton to make the sock, and a string to swing it on.  Or, you could hold the thermometers in front of a fan if you don't want to risk hitting something as you swing it around.  There are psychrometric charts on the web.  They look very complicated, but you find your dry bulb temperature on one side and the wet bulb temperature on the other, and where the lines meet, it gives you the relative humidity.

   If the humidity meter is correct, you don't want to significantly increase the humidity as it will cause condensation on the windows and inside the walls where you can't see it, causing wood to rot, mold to grow, and paint to peel.  If the meter is wrong and you have 10% to 20% RH, you will benefit from a humidifier.  An older, drafty house will tend to have lower humidity as the air that flows though the house not only take the heat out, the moisture goes with it too.  A well build modern house will have very low air leakage and care must be taken to remove the moisture put into the house by its occupants, showers, cooking, house plants, etc.

   There are many things that contribute to your comfort or lack there of.  Humidity is just one of them.  Other things to consider are radiant cooling when near windows or cold walls, drafts and air movement, and uneven heating.  The 75 degree thermostat may be on the warmest wall in the house.  The place you want to sit and be comfortable may be much different from that.

3. 
   DanH | Jan 01, 2012 03:12pm | #3

   The humidistats that come with Aprilaire units generally include a temperature sensor that is placed outside, away from any heat sources such as furnace exhaust pipe or kitchen/bath exhaust.  This temperature sensor "turns down" the humidity setting of the humidistat when it's colder outside, in an attempt to maintain the best humidity possible given outside temps.  This minimizes window condensation and also minimizes the risk of condensation in the walls and other such problems.

   So in theory the humidistat is a "set and forget" unit that doesn't need to be adjusted up/down with the weather.  In practice this isn't quite so -- the temperature sensor doesn't "remember" how cold it was last night, so the humidity may be run up too high during the day, and the sensor obviously doesn't read weather forecasts, so a sudden cold snap can cause problems.  So some manual adjustment of the humidistat is necessary with the seasons and with sudden changes in the weather.  The general goal would be to minimize window condensation (though eliminating it entirely is probably impractical, with standard double-glazed windows).

4. 
   emaxxman00 | Jan 03, 2012 12:38pm | #4

   Thanks guys for your comments.  I'm getting a reading of ~26% RH today.  The heat is running more often now since it's colder outside.  We had 50F temps the last few days of the 2011 year.  

   I'll keep an eye of the condensation as well.  It seems if I can just keep the house at 35-40% RH, it may be more comfortable.  I do have some duct sealing to do first.  I did an inspection of the return system and can see where there are some bad gaps. I'm hoping with some sealing, insulation on the ducts, and the humidifier, things will improve.