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Community and Q&A

Relative humidity conundrum

ChrisJS | Posted in General Questions on

I have an Aprilaire automatic humidifier installed in the return air duct of my HVAC.
The Aprilaire displays RH% and is presently set at maximum (5,000 s.f. home) because we have had a problem with our wood floors drying out too much during the winter. The conundrum is that, for example, when the Aprilaire displays 45% RH, my hand-held RH meter may display only 30% RH. The obvious check of using the same meter in the return duct air stream confirms that the Aprilkaire reading is correct. So how can the 2 readings be so different when the Aprilaire is reading the returning air stream?

Chris — Baffled!!

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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues here.

    Concerning hygrometers: these devices are notoriously innaccurate. Large differences in readings aren't unusual. If you want an accurate reading, buy 4 or 5 different hygrometers and look for a consensus.

    Concerning humidifiers: Humidifiers can easily damage your home's thermal envelope (for example, your wall sheathing) by contributing to condensation, mold, and rot. Humidifiers are dangerous.

    If you have low indoor RH during the winter, the reason is that you have a very leaky house. Sealing your air leaks is the best way to solve the problem -- and you'll save energy, too. You should certainly disconnect your humidifier -- and stop aiming for 45% RH during the winter. That's too high.

  2. Dana1 | | #2

    The accuracy of RH sensing instrumentation is all over the place. If the humidfier is sensing the return air stream it's typically 5F colder than the average room temp too, which would lead to a higher relative humidity at the same absolute moisture levels. The "relative" of "relative humidity" is the temperature at which it is being sensed.

    But humidifiers are a lousy solution to dry winter air. Dry winter air is a symptom of excessive ventilation or excessive infiltration, either of which comes at a cost in both comfort & energy use. Humidifiers are even dangerous to a wood-framed wood sheathed house, since if the interior humidity is too high in winter it results in mold-growth inside the walls in the spring. Air sealing the house and reducing the ventilation rates during cold weather are much safer/better solutions.

  3. ChrisJS | | #3

    This is a new house rated to LEED standards, sealed real tight and with excellent insulation. Humidity was in low 20% during Winter because humidifier was set too low and resulted in slight "cupping" of new wood floors - apparently not an unusual situation. So now trying to increase humidity to assist in flattening floors as recommended by flooring company. Your point about the return air being at slightly lower temp may explain the difference in RH reading (made with same meter). I will check temperature in both positions as well as RH.

  4. GBA Editor
    Martin Holladay | | #4

    Your flooring company cares about your floors, but your flooring company doesn't have any interest whatsoever in preventing your wall sheathing from rotting.

    The only way you can tell whether you home is leaky is to perform a blower-door test. A recently built home that meets LEED requirements shouldn't need a humidifier -- unless (a) the builder made some gross errors, or (b) the builder found a way to meet LEED requirements without having to meet any airtightness target.

    Do you know whether a blower-door test was ever performed?

    Who installed the humidifier?

  5. Expert Member
    Dana Dorsett | | #5

    If it was hitting 20% RH @ 68-72F, your ventilation rates are too high. (ASHRAE 62.2 is not a necessity unless you're smokin' stogies, burning incense, and going crazy with the hair spray every day.)

    That's a common problem when the ventilation is integrated into a hot-air furnace, since the duty-cycle on the furnace (and thus the ventilation) goes up during the coldest weather, which is when the outdoor air is the driest. That may be what's going on here(?). Installing a humidifier is a band-aid, a "solution-problem", that solves one issue while risking others.

    Ideally the ventilation rates would be controlled independently of the heating or cooling equipment. When the ventilation is driven by the heating & cooling equipment, the more energy efficient & air-tight your house is, the more likely you are to suffer from inadequate ventilation during the shoulder seasons when the loads are low. It's pretty common to have make-up air ported to the outdoors on the return plenum of a hot air heating system, and even in a pretty tight house there is always some amount of parasitic air-handler driven infiltration from the room-to-room pressure differences. It's also pretty common to have ventilation air intentionally designed into the duct flows without independent ventilation control.

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