Humidifier Effectiveness

MarkC

Hi all,



I have an Aprilaire model 350 whole house humidifier which seems to be ineffective.  It runs almost constantly with the humidistat set at only 25% and the indoor humidity level measured by a separate hygrometer rarely measures higher than 32%...right next to the humidistat (inaccuracies of both devices assumed).



The humidifier is located in the basement and controlled by an Aprilaire humidistat in the living space on the first floor.  The humidifier is not mounted on ductwork - it is a standalone type and draws intake air from the basement. The outlet (1) is a floor register on the first floor.  The entering water temp is about 103 degrees and the intake air (basement ambient) is about 64 degrees.



My question - would the humidity output be increased if the intake air was higher?  Is the 64 degree intake air 'cooling' the water in the humidifier's water panel and defeating evaporation?



Plus, becasue the humidifier is moving about 150 CFM of 64 degree air into a 69 degree living space, it counts as a heatloss.  Low humidity output and heatloss is a double whammy.

Brad White

Effectiveness

As a statement, you are correct, the warmer the air moving over the humidifier, the greater its ability to evaporate and absorb humidity.



I like to start at the basics and work my way up though. Do you know approximately the ACH rate of your house? And the size/volume? In other words, typically, your infiltration rate is what drives your load.  Yes, there is permeance through walls and roof, but that is relatively small, often less than 10 percent of your load in most homes.



Say you have a 2000 SF house with 9 foot ceilings, 18,000 CF. Say also you have an infiltration rate of 0.75 ACH average. (It can vary up or down 50% or more, but just a place to start.) If 0.75 ACH, that is 225 cfm of cold outside air. At a median of 12.9 cubic feet per lb. your mass flow, weight of air through your house, is 1047 lbs. per hour. Half a ton of air per hour. Bet that makes you feel good right about now!



For humidification purposes I assume zero grains humidity. (Even if 100% RH outside, the actual specific humidity is rather low, so say zero.) Indoor target let's call that 30% at 69 degrees. This corresponds to 32 grains per lb. at sea level. Our "delta-Grains" is thus 32 grains. Easy, going from zero..



So, 1047 PPH of air, raised 32 grains per, means you have a humidification load of 4.78 call it 5 PPH of moisture.



(How I got that: (1047x32 gr)/7000 grains per lb.)



Now, you say you have 150 cfm at 64 degrees going into the humidifier. That is, at 13.3 CF per lb., a mass flow of 676.7 lbs. per hour of air.



If I am to absorb or transport 5 lbs. of moisture per hour using that 150 cfm, I need to carry 51.7 call it 52 grains per lb. of air, to my space. Starting at 64 degrees and say 20% RH to start and 18 grains per lb., raised by 52 grains per lb. brings us to 70 grains per lb.



Question for you: What is the RH of the humidified air? Can you measure that?

 If you are approaching this specific humidity, your RH for that humidified air should be nearly 80 percent.  You could theoretically carry up to almost 90 grains per lb. at 64 degrees. You can humidify satisfactorily with the assumed loads, at much below this.



This also answers the question about heating the air to amplify the carrying capacity. Unless you are getting to saturation (and dropping humidity off by condensation), there isn't a need to heat the air more than it is. The warm water has a higher vapor pressure which will drive humidity always to the lower vapor pressure, the 64 degree air.

MarkC

I See

Hi Brad,



Good to 'see' you here again...its been a while!  As always, you've provided some great info.



To your questions:

ACH - no, don't have that measurement.

CF - 14,800

Humidified air RH - 61%



You commented that indoor target could be 30% @ 69 degrees.  I thought 50% was closer to ideal?  I'm achieving about 32% but with almost 100% humidifier runtime.  The other issue is the humidifier is a bit noisy...sort of why I'm looking to increase humidity output so I can decrease runtime and hotwater consumption.  But if 30% is apprx target than its not falling nearly as short as I thought.

Brad White

Hi Mark!

Sorry, I did not recognize your handle! Good to 'see' you too.



50% RH is too high in winter for all but archival space. Especially in most homes.



The real limitation is on how well your building enclosure is sealed and insulated, including vapor permeance.  In other words, if your house has discernible cracks and gaps, low insulation levels, any single-pane or sub-standard glass, those all go to keeping low RH levels. To humidify too much means a higher probability of condensation, both where you can see it (glass), and where you cannot, (deep in your walls somewhere).



It is rather a paradox that a tighter shell with better walls, glass and permeance can tolerate a higher degree of humidification. Normally 30-35 percent RH works for most people. If there are health issues, dry sinuses, static electricity, dry skin conditions, etc. then I would focus on reducing the air leakage as the largest humidity loss, first. I hope that makes sense.



Your humidified air at 61% and, I presume, 64 degrees, would contain 52 grains per lb. moisture. Oddly and coincidentally, that is exactly what I had in my example, my last post but within your smaller volume. And you are maintaining 32 percent, so I think you do not have a deficiency to speak of, but are doing just fine!



Still, you can air seal the place, every bit helps. Start at the attic, around plumbing stacks and wiring, more as your patience allows.  But worry not, your system is doing exactly what it should!



Brad

MarkC

Relief

Brad,



Thanks so much.  It's relieving to know the humidifier is more or less performing as it should.

Now I can focus on just reducing the noise level.