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  1. #1
    Join Date
    May 2004
    Posts
    21
    On a heatpump, what level of relative humidity should there be in the air at the point of the unit and at the point existing registers? If the unit is operating properly, what range should the overall indoor RH be?



  2. #2
    Join Date
    Dec 2004
    Location
    Derby City
    Posts
    3,962
    Depending on your geographical area, and depending on whether or not you are talking about heating or cooling, your levels of humdity will fluctuate. Electric heat, i.e. heat pump is extremely dry, simply because you don't have the benefit of moisture content that exists in fossil fuel. You may need to add a humidifier to the system to achieve acceptable levels of humidity in the wintertime. In the summer, the heat pump acts exactly like central air conditioning and not only conditions the air but also dehumidifies. In this geographical area, i.e. the ohio valley, normal relatively humidity is around 40 - 45%. Like I said some like it higher, and some like it lower. If your system is properly sized, then humidity should not be an issue for you. If, like one of my customers, you have an indoor swimming pool that accesses the rest of the house, guess what? He can't keep the humidity level to an acceptable level, no matter what he does. He is going to have to treat the pool space independently from the rest of the house in order to dehumidify properly. Are there specific problems you are having that are the basis for your question? By knowing a little more, we can probably better answer your question.
    Everyone has a purpose in life..........even if it's to be a bad example.

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  3. #3
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    respitory doctors state that indoor RH% = 35- 55 for my ex with emphysema
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  4. #4
    Join Date
    Jun 2004
    Posts
    146
    Originally posted by John Lloyd
    Electric heat, i.e. heat pump is extremely dry, simply because you don't have the benefit of moisture content that exists in fossil fuel.
    Whatever moisture content that exists in fossil fuel should be going straight outside unless there's a crack in the furnace heat exchanger. If it was entering the home, it definitely wouldn't be a benefit to the occupants.

    If anything heat pump is less drying than fossil fuel heat because the output is not as hot and therefore the relative humidity at the supply registers is higher. Away from the supply registers, the humidity with a heat pump or fossil fuel will be about the same since neither should be adding or removing moisture while heating.

  5. #5
    Join Date
    Apr 2002
    Location
    Pottsboro TX
    Posts
    181

    RH of heat pump

    Redskins,
    I don't get it. Lower heat for heat pump might have lower RH because temp is lower. How does that work?
    Phil

  6. #6
    Join Date
    Jun 2004
    Posts
    146

    Re: RH of heat pump

    Originally posted by Pschneid
    Redskins,
    I don't get it. Lower heat for heat pump might have lower RH because temp is lower. How does that work?
    Phil
    No, I said heat pump supply air would have higher relative humidity because the temperature is lower.

    Objects/people that are close to the supply register get more dried out with higher temperature fossil fuel heat because the supply air has lower relative humidity.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,903
    It doesn't matter what the rh out of the register is.

    In the end, the homes rh will be the same, without a humidifier.

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  8. #8
    Join Date
    Apr 2002
    Location
    Pottsboro TX
    Posts
    181

    I read it correctly

    I got it. I read your explanation correctly but answered to you wrongly.
    (If) there were ANY difference in theorectical RH between the two systems would the difference be noticeable? Would it be at least a difference of 10 percentage points?
    Phil

  9. #9
    Join Date
    May 2004
    Posts
    21
    John, to clarify:

    We have a close to properly sized heatpump (Trane XL 16i), just installed last year. (Based on the load calc I did with HVAC-calc, we needed closer to a 2.5 than a 3; HVAC initially wanted to put in a 3.5 based on his calculations, so the compromise was 3 ton.)

    We've not felt uncomfortable with the humidity, but have been having some trouble getting the system balanced (85 ft' long single floor rambler, with master bedroom with high solar gain).

    We had a contractor out here for some other work, who checked humidity/temp coming out the registers, and the RH was running between 78-80%. He suggested we contact our HVAC guy, and that the system may not be performing correctly.

    HVAC technician came out, checked the system and the readings (still high RH), and said the charge was off. He fixed that, and initially was still getting the same high RH readings. He checked for leaks in the return (visual inspection of part of the system), then checked registers again. This time he said the "average RH was 53%."

    This seems high to me, given that it was a relatively dry day outside (52% RH in am, dropping throughout the day). I also got to see the first set of readings, but he did not show me the second.

    I'm thinking of getting a tool to measure the RH at registers and indoors, just to keep an eye on this. I want to have some ballpark benchmark of whether or not to go back to the HVAC about this. Don't want to bug them if system is performing right. On the other hand, we've had some disappointing experiences with them lately.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,903
    In cooling mode, the rh coming out the supplies will be high.

    Thats normal.

    It is close to the dewpoint temp, so it is high, as that air mixes with the room air, the rh will go down.

    At the coil, in cooling mode the rh will be near 100%, if its operating properly.

    Its the rh of the house you need to be concerned about.

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  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,973
    This seems high to me, given that it was a relatively dry day outside (52% RH in am, dropping throughout the day). I also got to see the first set of readings, but he did not show me the second. - bbqjason
    ===============
    Any existing humidity above the level of 52% in the home is being released from all the materials in the conditioned space. Also, there are other things in the home adding humidity, cooking, showering, etc. - Darrell
    ============
    I'm thinking of getting a tool to measure the RH at registers and indoors, just to keep an eye on this. I want to have some ballpark benchmark of whether or not to go back to the HVAC about this. Don't want to bug them if system is performing right. On the other hand, we've had some disappointing experiences with them lately. - bbqjason
    ============================
    At the Supply Air diffusers, as the return air is heated its %RH should be lower than the conmditioned space. When return air is cooled the %RH should be higher at the registers.
    - Darrell

    http://www.udarrell.com/latent_heat_condensation.html

  12. #12
    Join Date
    Dec 2004
    Location
    Derby City
    Posts
    3,962
    kokopup: does the system 'short-cycle' at all? In other words, on and off more than constant running? This would be an indication of possible oversizing which could adversely affect the dehumidification.

    I am disappointed to hear you have had problems and the contractor did not show you the second rh readings. I would still recommend going back to them to 'get it right' and if you still don't have a properly operating system, I would ask them to get a factory service rep involved.

    If your expectations of the operation of the system are reasonable, then the contractor should be willing to do what is reasonable to achieve those expectations.

    I am also curious, did he perform his own load calculations or did you both go by yours and then agree to the compromise.
    Everyone has a purpose in life..........even if it's to be a bad example.

    Seek first to understand, before seeking to be understood.

  13. #13
    Join Date
    May 2004
    Posts
    21
    No, we've haven't had short cycling with AC, (although this was a real problem this winter with heat. I think my post was something like "my heatpump has gremlins." HVAC guy bit by bit replaced blower, motor, circuit boards and thermostat and the problem finally went away.)And comfort level has felt fine re: humidity, just this other person told us we should get the unit checked because of the high humidity levels at the registers. (And the charge was low.)

    We both did separate load calcs. The HVAC guy's own load calc indicated at best a 3 ton, but he wanted to go "a little bigger." Because of length of house, he also wanted me to think about going with two units, but the high efficiency units only go down to two tons, which would have been too much for our house.

    Our two disappointments with HVAC guy, are 1) lack of balance in the system--one end of house is 5 degrees warmer than the other; and 2) (and it's a biggy!) the condensation from the AC supply lines in the crawl is so bad that we are going to have get all the (brand new this winter) insulation ripped out; mold removed with HEPA vac, sanding, scraping; biocide applied; new insulation installed--and find a solution to the condensation. May have to rip out some of the supply lines as well, which are quite moldy on the outside.

    HVAC guy is washing his hands of it, saying its the moisture in the air, not the supply lines. Our position is either more insulation should have been used on the supply lines, or the HVAC guy should not have designed system to put lines in an unconditioned space. He knew the space the lines were going into.

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