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Humidity pre and post AH...

I knew there was a simple explanation for the change in humidity... and if I had used that void between my ears, perhaps I would have figured it out on my own! :-P

I don't quite agree with Don though on the evils of controlling infiltration and/or air quality issues. An HRV/Vent/whatever combined with a tight envelope gives you the kind of control that older houses with very infiltration loads do not. I would argue that a home where you can determine where and when air is let in will be potentially safer than one where you hope that the proper amount of air will reach a combustion appliance, that the draft hood will not spill backwards, etc.

What bugs me more than anything is that most AC systems/thermostats do not offer their users the ability to move air during the winter season on their schedule... fans are either on or off continuously.

Comments

  • Constantin
    Constantin Member Posts: 3,782
    Hi Professor!

    I hope classes are treating you well. I have something here that puzzled me a bit and I thought you'd perhaps have the answer.

    I recently hooked up RH and Temp sensors in the supply and return of my AC system. The results of which have me puzzled. Specifically, while the temperature of the air passing through the HX drops 20°F, the RH also goes up 20%... Presumably, this has to do more with the measure being relative than anything else, but if you have a minute to spare, please explain!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Humidity...

    Here we go...

    As we know, the cooling coil (evaporator) in an air conditioning system is designed to do two things: dehumidify and cool. When dehumidification is taking place we get condensate which, in turn, drips from the surface of the evaporator coil.

    As air passes through the cooling coil a certain amount of the moisture is picked up in the air stream and carried from the coil. So, as strange as it may seem, the relative humidity of the air immediately after the cooling coil is about 95%.

    Now, your next question is going to be, "If the air conditioning system is intended to dehumidify and cool, how can we be dehumidifying the air if it is at a relative humidity of 95%?". Glad you asked. As air is cooled, the ability of the air to hold moisture decreases so, at a given moisture content, the relative humidity of the air will increase. Now, when the cooled air is introduced to the air in the occupied space, it is warmed as it mixes with the air in the space and its ability to hold moisure increases. This causes the relative humidity measurement to fall.

    For more on this, check out the following article that was written by a guy who I see every morning as soon as I look in the mirror.

    http://www.heatinghelp.com/newsletter.cfm?Id=170

    Enjoy!
  • don_156
    don_156 Member Posts: 87
    Bravo!

    Well said Professor.I can no longer count how many customer ask me this question when they walk around the house with their new meter.

    I tell them wait till winter arrives and go looking for your highest humidity reading.This is where you will find your greatest heatloss in the envelope.

    Not only do you need to know the refrig system but the airside become just important when trouble shooting.

    Good Stuff!

  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    Why its dry and other cool things

    I have something to add to this, and some cool things to consider.

    When it is wintertime, we are using gas appliances that require air for combustion and flue gas dilution. This air comes from inside for the most part (older appliances) and it must be replaced with outdoor air. (infiltration air) When you start to consider, to burn 1 cf of gas requires 15 cf of air, (10 cf to burn and 5 cf excess air) and if you have a draft hood an additional 15 cf is required (dilution air) you are talking a lot of air.

    Consider 100,000-btuh boilers at 75% AFUE efficient

    1 cf gas has about 1000 btu of heat energy

    So the furnace requires 100 cf/hr of gas per hour

    30cf x 100 cf/hr = 3000 cf/hr of air to burn and for dilution

    In a 24 hr continuous on period this would mean 72,000 cf of air just to burn and for dilution.

    This would mean all of the air in a 20x40 house with 8’ ceilings would be changed 15 times every 24 hours with new air. (This air would have to come from outside.)

    If the outdoor air was 20 degrees at 95% RH and you warmed it to 75 degrees the RH would be 11% RH. (See attached chart) That’s why the house gets dry!

    If you consider the density is .0824 lbs/cf, for air at 20 degrees and 95%RH and the specific heat of air is .24 btu/lb on average:

    The heat required is Q=Weight x Specific Heat x Temperature Difference)

    72,000cf x .0824cf/lb x .24Btu/lb/°F x (75-20) = 78,312 btu/24 hour period just to heat the infiltration air that is required by the old boiler.

    1 hour of run time is required just to heat the infiltration air require for combustion!

    100,000 btuh x 75%

    Talk about a good reason for sealed combustion!

    Now consider the dryer vents, bathroom exhausts vent, kitchen hood vents, and other ways air escapes the building and you have a lot of wasted energy.

    But you cannot seal the house without provisions for combustion and dilution air.
  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    Why the air is dry and other cool stuff

    I have something to add to this, and some cool things to consider.

    When it is wintertime, we are using gas appliances that require air for combustion and flue gas dilution. This air comes from inside for the most part (older appliances) and it must be replaced with outdoor air. (infiltration air) When you start to consider, to burn 1 cf of gas requires 15 cf of air, (10 cf to burn and 5 cf excess air) and if you have a draft hood an additional 15 cf is required (dilution air) you are talking a lot of air.

    Consider 100,000-btuh boilers at 75% AFUE efficient

    1 cf gas has about 1000 btu of heat energy

    So the furnace requires 100 cf/hr of gas per hour

    30cf x 100 cf/hr = 3000 cf/hr of air to burn and for dilution

    In a 24 hr continuous on period this would mean 72,000 cf of air just to burn and for dilution.

    This would mean all of the air in a 20x40 house with 8’ ceilings would be changed 15 times every 24 hours with new air. (This air would have to come from outside.)

    If the outdoor air was 20 degrees at 95% RH and you warmed it to 75 degrees the RH would be 11% RH. (See attached psychrometric chart) That’s why the house gets dry!

    If you consider the density is .0824 lbs/cf, for air at 20 degrees and 95%RH and the specific heat of air is .24 btu/lb on average:

    The heat required is Q=Weight x Specific Heat x Temperature Difference)

    72,000cf x .0824cf/lb x .24Btu/lb/°F x (75-20) = 78,312 btu/24 hour period just to heat the infiltration air that is required by the old boiler.

    1 hour of run time is required just to heat the infiltration air require for combustion!

    100,000 btuh x 75%

    Talk about a good reason for sealed combustion!

    Now consider the dryer vents, bathroom exhausts vent, kitchen hood vents, and other ways air escapes the building and you have a lot of wasted energy.

    But you cannot seal the house without provisions for combustion and dilution air.
  • don_182
    don_182 Member Posts: 69
    Good stuff Jim.

    However, I always wonder were we not heathier with some of that infiltration?

    I have a house I've been working on the last two years and its so tight that we had to add a lifebreath that was properly more then the cost of ten years of leaky envelope.

    You know that old saying for every issue address another one pops up.
  • don_182
    don_182 Member Posts: 69
    Its ok Constantin

    to disagree.However,you read more into my post then what was
    actually said.The part about wonder was key.

    But thats ok I still love ya.

    Personally we no longer had that issue because,we try to push nothing but seal combustion.Also if the house is so tight that all these issue that you speak of was that big of a concern then I would push nothing but electric.LOL.

    And about the fan issue.I'm going to assume here its not that much of a issue because during the summer your ac is running and that should be enough run time for good filtration in the summer months.But being you have hot water
    heat and are using base/floor emitters then the ah are not running.So you are not getting any filtration in the winter month.Did I assume correctly?

    May I suggest a timer for the winters month.Bypass the stat all together.











  • Constantin
    Constantin Member Posts: 3,782
    You're right....

    ... and I apologize for the inference where there shouldn't have been one.

    ... you are correct, the cooling is via air, the heating via hot water. There is still enough infiltration in the winter to make the place rather dry, so we have a humidifier that runs from one humidistat on the ground floor. That might be the guinea pig for the Signaturestat, since installing a Signaturestat there would allow me to control humidity in that zone and replace two stats with one.

    Regarding circulating air in the wintertime: Your timer idea might be the KISS solution. I'll have to check into the zone dampers though to make sure that they're normally open when unenergized. I think that's the case...
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