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I’d like to increase the humidity levels in my old stone and brick house (~120 years) during the winter. Right now it’s about mid-20% and I’d like it to be 30% to 35%.

This house came with a HRV system system that I understand brings outdoor air into the house. Considering that it’s usually more than 50% RH outside, I am curious why running the system seems to dehumidify the house and make my problem worse rather than improve things. I understand that the water-carrying capacity of cold air is less than warm air, so why would bringing in 50% RH cold air into a warm space of 20% RH not increase the indoor RH? Is there something else possibly wrong that I should investigate?

Bonus details for those curious:

The house came with an evaporative flow-through bypass humidifier that pours about $25 to $60 of water down the drain each month while also not affecting the humidity. My current working strategy is run a large console humidifier next to a large return near the furnace – with this setup I can crank the house up to 40% RH easily but it requires filling up the water tanks every day or so, which isn’t sustainable while I’m away. Coming up with a better way using the HRV would be neat.

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    What kind of evaporative flow-through bypass humidifier pours about $25 to $60 of water down the drain each month while not affecting the humidity? – Jim Stewart Jan 1 '17 at 17:03
  • One that should be serviced or replaced? – Ecnerwal Jan 1 '17 at 21:45
  • Hahaha yep. I've taken it apart, replaced the calcified expanded aluminum medium the water trickles over, and have had it and the installation looked over by a pro HVAC tech. He wanted to replace it with a $500 version of the same thing but with a wicking paper pad and a more water-efficient reservoir system with floating bobs, etc. – Edward Ocampo-Gooding Jan 1 '17 at 22:30
  • More context: this is 1960s-era technology implemented in a time of non-scarce pricing on water. – Edward Ocampo-Gooding Jan 1 '17 at 22:31
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Ah, I think I’ve found the answer:

Admitting cold outdoor air into a space will lower the indoor RH. When the lower temperature outdoor air is brought indoors and heated, it loses moisture and reduces the overall RH.

For example: if the outdoor temperature is 0°F and 50% RH and the air comes indoors and is heated to 70°F, the residual moisture after heating the outdoor air will only be about 3% RH. Even on a nice, sunny 35°F and 50% RH day, the residual indoor RH will only be about 14%.

So by heating the cold, high-RH outdoor air up, the RH content drops tremendously. By introducing this extremely low-RH air into the house, you can expect the indoor RH to plummet.

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    Unless something drastic has been done to your 120 year old stone and brick house, it probably leaks a good deal of air, which is also replaced by outside air, and lowers the Relative Humidity (with relative being the important factor in how this all works - it's a percentage of "what air at this temperature can hold" not a fixed amount of water.) The air does NOT "lose moisture" - it has exactly as much water in it as it did before - but the amount of water it can hold at high temperature is much greater than at lower temperatures. Thus lower relative humidity. – Ecnerwal Jan 1 '17 at 21:50

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