Where should I set the humidiy in a home
I was in my sister’s home over the holidays. She has forced air gas heat with a humidifier control at the furnace. It was set at about 35%, which I thought was low for the winter months (suburbs of Chicago). I noticed some longitudinal gaps in her hardwood flooring, and some very slight separation of casing around some windows. These gaps are acceptable in my 95 yr old boiler heated home, but I think the humidifier set point needs to be higher on her 3 yr old home. By the way, these gaps do not exist in higher humidity months (Spring, Summer, Fall). Isn’t the point of modern HVAC units to maintain a pretty constant relative humidity in a home for 12 months a year (i.e. so you never have separation ikn woodwork, etc.)?
So, what should it be set st for this time of year? How about the summer months?
Thanks, one and all!
Replies
30-50% is the range to keep RH. Below 30 uncomfortably dry, above 50 supports mold.
40% is ideal.
"some" gaps could indicate too dry, or also areas of flooring directly above a heat duct.
I rest easy at 35%. There's a small window above too dry and sweating windows.
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"Isn't the point of modern HVAC units to maintain a pretty constant relative humidity in a home for 12 months a year ..."
No. The failure, BTW is with the building envelop more often than with the HVAC system. Few residential systems even barely address humidity much less completely or properly. The modern residential HVAC system is designed to maintain a relativeley reasonable range of comfortable temperatures only.
"To maintain a pretty constant relative humidity in a home for 12 months a year" would be quite expensive, involving significant summertime dehumidification. Believe it or not, "to maintain a pretty constant relative humidity for 12 months a year" is rare even in most museums; it just costs too much, except for special collections.
The appropriate setting for a humidifier depends on the outside temp and the construction of the house. For your average 70s home (insulated glass but not real tight) you'd set it at about 35% for temps above 40F and reduce the setting about 5% for each 10 degrees below 40F.
(Note that some humidistats incorporate an outside thermocouple so that they can make this adjustment for outdoor temp automatically.)
"Tune" this algorithm by observing window condensation: Below 10F or so you should see some slight condensation in window corners, but it should never be so heavy as to cause runoff onto the woodwork, except when the temp drops so rapidly that inside humidity can't adjust.
Tighter homes will generally permit you to set humidity 5-10% higher. In homes with non-insulated glass you need to set the humidity lower. And in some cases where attic ventillation is poor you may need to reduce humidity to prevent condensation staining of the ceilings near the eaves.
In warmer weather it may be necessary to turn the furnace fan on to allow the humidifier to run when the furnace isn't -- otherwise the furnace doesn't run long enough to "satisfy" the humidifier. Plumbing the humidifier with hot water (if the manufacturer permits this) improves function when running in this mode.
Too high and you can expect to be feeding mold growth and causing other problems with the house.
How high that is varies according to the rest of the insulation, venting, and vapour control package.
generally 40% is a max. Some can handle up to 50% with all systems functioning and designed right.
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You're going to get seasonal changes in wood dimensions regardless.
A house prefers relative humidity in the range of 20-40%. The human body prefers it in the range of 30-70%. So the best compromise is 30-40%.
At 40% you may get some condensation on double-glazed windows in very cold weather. LowE windows can normally tolerate 50% except in extreme cold.
40% RH is a good set point for winter.