Indoor Relative Humidity on a warm day
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  1. #1
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    Indoor Relative Humidity on a warm day

    I have a 50 year old house located in Northern NY. It has original windows and doors, I would assume its not the tightest house. To help mix the air better (old ductwork, floor supply and returns) I have increased the blower on my 2 ton AC with Comfort R from (640cfm and 800cfm) to (800cfm and 1000cfm). It makes the air feel less stagnant. I know by doing this my humidity could suffer. I do keep the fan set to Auto. On a 80F (I think design temp was 95F) day outside, with my thermostat set to 72F, I can hold humidity in the low 40s. This is with the AC cycling on and off. a few times an hour I think. When the AC is not running (at night) the humidity goes up near high 50s, which I would assume is normal.

    I was wondering what kind of relative humidity readings others get when the AC is running with similar conditions (moderate load etc.) ? Thanks

    Teddy

  2. #2
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    You might be better with the lwoer airlfow, but the fan set to circulate.

    Ultimately, if the hoem isn't sealed, you will be limited as to how much lwoer of a dewpoint you can maintain indoors compared to outdoors.

    Hers' a neat dewpoint calculator I found wiht a simple google search. It will give you an idea of how indoor RH changes with temperature and dewpoint. http://www.dpcalc.org/

    I discovered that you need to hold a very tight dewpoint to maintain ideal humidity levels. This is not easy to do in a smaller home, since just a single shower can add maybe 10 pints of water to the air even with a ventilation fan. That moisture could shift your indoor RH by almost 10%. But that's only a 5 degree change in dewpoint.

    So when it's a dewpoint approaching 70F+ outside, holding even 58F dewpoint indoors isn't so bad.

    At night, if you lower your thermostst, you may remove a little more moisture with the A.C cycling on more often, but the lower tempatures will cause the RH to rise even if the dewpoint indoors hasn't changed very much.


    For reference, in my home, on dryer, more mild days outdoor tmepratures (dewpoints of 55F-65F outdoors) and dry bulb temps around 75-85F, I'm around 45%-55% indoor RH at 72-75F depending on time of day. When it go really humid and hot (depoint of 70-80F outdoors) and dry bulb temps above 90F, I was only able to maintain around 51%-60% RH.

    50% RH is ideal, but generally anyting under 60% is still comfortable and the accptable range given by most asthma and allergy groups is 40%-60%. Now as you know, in winter, on the coldest days 25-35% is all that's possible without condensation on the windows.

    What I've been doing with my Visionpro IAQ is allowin it to overcool by 3F to reduce humidity and reach a good compromise of temprature and humidity. It seems like the controller overcools more the further you are from the humidity setpoint. So I set hte humidity setpoint for 45%, and set the tempreture ot where its comfortable at 45%. The thermostsat then will overcool in higher humidity levels and reach a good compromise, so I'm generally comfortable no matter the humidity level.

    I'm also using (based on a recommendation from Beenthere) 2 cycle per hour for hte compressors which forces longer run times. I also wired my 2nd stage cooling to the dehumidifer contacts so 2nd stage is disabled if it's above the humidity setpoint. That also forces longer run times. Os far, so good.

    For supplemental dehumidification, I'm running 2 portable dehumidifiers in my partially finished basement. I think they are both standard 40 pint/day models. One runs contantly, the other is set to cycle at 50% RH.

  3. #3
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    Quote Originally Posted by motoguy128 View Post
    You might be better with the lwoer airlfow, but the fan set to circulate.

    For supplemental dehumidification, I'm running 2 portable dehumidifiers in my partially finished basement. I think they are both standard 40 pint/day models. One runs contantly, the other is set to cycle at 50% RH.
    How many lbs./pints per day are pulling out per day with the two dehus? What is your basement temperature? How much fresh air are suppling? Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
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    Quote Originally Posted by teddy bear View Post
    How many lbs./pints per day are pulling out per day with the two dehus? What is your basement temperature? How much fresh air are suppling? Regards TB
    Hard to say. I don't measure the actual qualitity of the water comming either of the units or the A/C coil. I have them all just drining straight to a floor drain in my basement. So I'd have to rework some piping, mount the dehumidifiers up higher to have them drain into buckets.

    I haven't measured my basement temp lately. I'd estimate it's around 66-68F. So the dehumidfiers are probably well below their rated capacity due to coil icing, and resulting defrost cycles.

    The fresh air is a passive vent, with a manul damper. It's a 6" duct on my return going through about 12' of insulated flex, then a custom made filter box and leading to a 4" vent in the sill plate. I'd guess that the maximum it can supply is around 50-60 CFM. In the current damper position, it's probably closer to 40CFM.

    Actually it wouldn't really matter what the exact numbers are because it's unknown exactly how much air my homes leaks, and with no vapor barrier, I don't know home much moisture permeats through the walls themselves. I could be getting 100 CFM of air just through air leaks in the home.

    It doesn't help that I have a 35 Gallon Aquarium upstairs... and the bathroom fan is probably a little undersized so much of the moisture from showers stays indoors.

    Overall, looking at a RH/dewpoint chart, it's not bad. I can't really complain with a 55F dewpoint indoors when there's a 80F dewpoint outside. With more direct sun like they have in Florida, I'd probably have a large enough heat load to pull that down a little more.


    I might be able to get a long shallow pan and use that to measure the water removed over an afternoon.

  5. #5
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    Quote Originally Posted by teddy bear View Post
    How many lbs./pints per day are pulling out per day with the two dehus? What is your basement temperature? How much fresh air are suppling? Regards TB
    Hard to say. I don't measure the actual qualitity of the water comming either of the units or the A/C coil. I have them all just drining straight to a floor drain in my basement. So I'd have to rework some piping, mount the dehumidifiers up higher to have them drain into buckets.

    I haven't measured my basement temp lately. I'd estimate it's around 66-68F. So the dehumidfiers are probably well below their rated capacity due to coil icing, and resulting defrost cycles.

    The fresh air is a passive vent, with a manul damper. It's a 6" duct on my return going through about 12' of insulated flex, then a custom made filter box and leading to a 4" vent in the sill plate. I'd guess that the maximum it can supply is around 50-60 CFM. In the current damper position, it's probably closer to 40CFM.

    Actually it wouldn't really matter what the exact numbers are because it's unknown exactly how much air my homes leaks, and with no vapor barrier, I don't know home much moisture permeats through the walls themselves. I could be getting 100 CFM of air just through air leaks in the home.

    It doesn't help that I have a 35 Gallon Aquarium upstairs... and the bathroom fan is probably a little undersized so much of the moisture from showers stays indoors.

    Overall, looking at a RH/dewpoint chart, it's not bad. I can't really complain with a 55F dewpoint indoors when there's a 80F dewpoint outside. With more direct sun like they have in Florida, I'd probably have a large enough heat load to pull that down a little more.


    I might be able to get a long shallow pan and use that to measure the water removed over an afternoon.

  6. #6
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    Mar 2008
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    Thanks Motoguy128. Is your AC sized properly? I was wondering if that is why your need to set the AC to 2 CPH?

    I noticed today the RH in my house was around 47%. So I guess low 40s was a bad estimate. A week ago I started holding the temp at 72F instead of using a setback like I have in the past. So today I set the blower to 640 and 800 cfm, which is the default. Now that I am not using a setback, I wondering if the humiditiy will be even lower. I have been hung up on having a quick recovery, I just said what the hey and I am going to just set it and forget it.

    Teddy

  7. #7
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    Quote Originally Posted by motoguy128 View Post

    Hers' a neat dewpoint calculator I found wiht a simple google search. It will give you an idea of how indoor RH changes with temperature and dewpoint. http://www.dpcalc.org/
    That's a neat calculator--will come in handy for 'what if' scenarios.

    I could not find any explanation of PI and %ECM. Do you have any info on them? Thanks.

    Amp

  8. #8
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    Quote Originally Posted by teddyc27 View Post
    Thanks Motoguy128. Is your AC sized properly? I was wondering if that is why your need to set the AC to 2 CPH?


    Teddy

    No. It was intentionally oversized for heating capacity with the idea that on low stage it would be sized OK. That would be OK if it was 60% on low speed. A load calc was never done, but some rough calculations as well as obervation when at design temp (93F) tells me that 2 Tons is all I need. On low stage its about 2.3 Tons.

    If I had the communicating furnace, I would probably be OK, because it could dramatically slow the fan speed for better dehumidification on 1st stage to get the coil temp down. Just lookin at the data sheets, I think the coil temp only gets down to around 50F since sensible capacity equals latent at just 75F DB and 59F WB...even with 350CFM/ton.

    Latent capacity is better in 2nd stage... but then run times will be a little shorter.


    Ultimately... as I've said before, I desperately wish I had just gotten a 2 ton a XL15i. With a 2.5 Ton coil it meets the tax credit as well. I'm tempted to ask my installer if he'd swap out the unit for a reasonable restocking fee along with time and materials. I'd have 30% of the unit's installed price to play with. Maybe I can offer him 15%, and I keep the other 15%. When he quoted the equipment, a XL15i heat pump was quoted at almost the same price. After all... the duel fuel was my suggestion...but he suggested using a oversized 2 stage unit, not me.

    I'm just not all that happy with how it's performing overall. It's a little noiser that I expected too. I'd have the XL15i mounted without snow feet to get it a little lower so the compressor is lower than the subfloor of the house. The pad still sits a few inches off the ground, and we don't get snow accumulation over 4" very often.

  9. #9
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    Quote Originally Posted by ampulman View Post
    That's a neat calculator--will come in handy for 'what if' scenarios.

    I could not find any explanation of PI and %ECM. Do you have any info on them? Thanks.

    Amp
    The site is related to preservation of photographs I believe. So ECM I think it related to some type of corrosive factor related to air being either too dry or too humid. PI might be some sort of preservation index.

    I just thought the RH/Dewpoint calculator proortion was handy. I think the thresholds for corrosion and mold are useful.

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