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  1. #1
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    Absolute water content, not relative?

    Has anyone got knowledge of using sensors to measure or calculate absolute humidity in air, and use that info to control outside fresh air intake?

    There is a product called "Smartvent Hi-IQ" which apparently does that. Honestly, their website is vague enough that I am still struggling to figure out exactly what it might do:
    http://www.smartvent.net/

    They have one product to ventilate crawlspaces, and another to ventilate houses. FWIW the words "smartvent" and "Hi-IQ" are generic enough that a Google search is near useless. The BBB seems to have no info on this company either. There don't seem to be any published articles, with one exception: a professional looking paper on crawl space moisture on the site.

    So... anybody know anything? -- P.Student

    [Edited by perpetual_student on 12-08-2004 at 12:30 PM]

  2. #2
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    Hmm Air Exchange

    My understanding of this product and other similar ones are to have a mechanical air exchange in a space if the outdoor conditions (RH, Temp or both) are better for the space then currently present. If the outdoor RH is lower then indoors, the fan can operate and the same can be true for temperature etc. Unlike an ERV or HRV, there is no exchange of energy from the air going out to the air coming in. Some of these systems can pressurize the space and I never recommend a pressurized living space (only in factories i.e. a clean room).

    My question is always the same, what is the ROI?


    The quality of my performance, sometimes depends on the quality of my audience.
    Imitation (Plagiarism) is the best compliment one can get -- "Open A Window"

    To improve Indoor Air Quality: Control Indoor Air QUANTITY = "I.A.Q.Q."

  3. #3
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    Different forms of humidity

    Xavier, I think your understanding of it is correct with one exception:

    >> If the outdoor RH is lower then indoors, the fan can operate...

    I believe the operative concept is ABSOLUTE humidity, rather than relative. Relative humidity at a different temperature, will become better or worse (vs. a reasonable target of 48%RH, 72 degrees F) when temperature changes. For example if the outdoor air is 51.3 degrees and 100%RH, it will become just perfect when warmed up to 72 degrees.

    This I think is the smart idea behind Smartvent. I understand you live in the North and this product may not have any appeal to you because of the large temperature differences between indoors and outdoors. On the other hand we in the Gulf Coast area have small temperature differences most of the year, combined with large differences in absolute humidity. Over the decades mankind has gotten pretty good at heating and cooling, but humidity control is the more difficult problem.

    >>My question is always the same, what is the ROI?

    That's the most logical way to speak the same language. But in the HVAC field we have to understand the laws of physics too, in order for someone in the Dry North and the Humid South to exchange ideas. The head of Building Sciences Corporation, Joseph Lstiburek, is good at that.

    It's hard to justify something like ERV in the hot-humid South, the ROI isn't there. Replacing that with a form of supply ventilation seems to be the smart thing to do. Since Smartvent is much cheaper, it has a lower ROI hurdle to justify itself. One might compare the capabilities, and I would not assume ERV would do more than Smartvent in a humid climate.

    Hope this helps -- P.Student

    P.S. Xavier, you have some consolation prizes living in the North. HRVs and ERVs actually do more up there. You might have to add humidity most of the year, but that's far easier than bringing down humidity. Many rules of thumb you are wise to follow, are the opposite of our rules of thumb e.g. your exhaust ventilation is harmless, in our region it can lead to structural moisture damage. For you supply ventilation is capable of causing moisture damage, for us it is incapable of doing so.

    [Edited by perpetual_student on 12-09-2004 at 06:59 PM]

  4. #4
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    Hmm Changes in outdoor temperatures and humidity

    I disagree with one of your statements. It is more difficult to control the IAQ in a home in the North because of the significant Temperatures swings of over 100 degrees F and swings of Relative Humidity of over 80%.

    The best way to control the IAQ in a dwelling in all locations is to condition the air (Heat/Cool and RH) at the lowest cost and then reduce uncontrolled indoor air changes.

    You are correct the logic of the system should consider the RH based on the Delta T and then determine if the fan will operate. I should have been more specific, but I assumed you would understand my point.
    The quality of my performance, sometimes depends on the quality of my audience.
    Imitation (Plagiarism) is the best compliment one can get -- "Open A Window"

    To improve Indoor Air Quality: Control Indoor Air QUANTITY = "I.A.Q.Q."

  5. #5
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    IAQ

    I gotta admit that all I know about IAQ in the North, has come from reading and not experience (except for living 3 years in St. Louis). And while trying to give myself a crash course on all HVAC recently, I've deliberately put off topics focusing on heating climates. There are so many subtleties with hot-humid climates, that alone is like hard to cover.

    Yes, you *do* have those large ranges of temperature and humidity. In a small way I envy you because things like ERVs have been developed to work best for your climate, plus the most well respected engineers are more likely to have come from universities with cold weather. More of the world understands your problems vs. those problems unique to the hot-humid climate.

    I was a stickler on the absolute humidity vs. relative humidity (RH) concept, and hope I did not cause any offense there. It was only recently that I learned about that part of psychrometrics (many of you may have learned that long ago), and boy is it an eye-opener!

    It is so easy to find a gauge that tells me RH outdoors, but that is misleading unless one understands just how much RH will change as the air is warmed/cooled to room temperature. I would very much like to find an instrument that measures absolute humidity or something equivalent to that.

    Best wishes -- P.Student

    [Edited by perpetual_student on 12-10-2004 at 10:28 AM]

  6. #6
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    Hmm Humidity

    P-Student, I agree that there is a lot to learn and undertand on what is the best way to condition the air in a home no matter where it is located.

    Here are links that may help you:

    http://apollo.lsc.vsc.edu/classes/me...nd_parcel.html

    http://www.skuttle.com/humid.html
    This is the reason homes have low "Humidity" in the North in the Winter, but not many people understand it!

    http://www.eustis.army.mil/weather/w...s.htm#Humidity

    I have a question on the system, does it temper the incoming air? If so how and at what cost? If not then it will affect/change the temperature and RH of the space, correct?

    Good luck and continue your reading.
    The quality of my performance, sometimes depends on the quality of my audience.
    Imitation (Plagiarism) is the best compliment one can get -- "Open A Window"

    To improve Indoor Air Quality: Control Indoor Air QUANTITY = "I.A.Q.Q."

  7. #7
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    Madison, WI/Cape Coral, FL
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    Re: IAQ

    Originally posted by perpetual_student
    It is so easy to find a gauge that tells me RH outdoors, but that is misleading unless one understands just how much RH will change as the air is warmed/cooled to room temperature. I would very much like to find an instrument that measures absolute humidity or something equivalent to that.

    Best wishes -- P.Student

    [Edited by perpetual_student on 12-10-2004 at 10:28 AM]
    Currently the lowest cost dew point meter I know about is the Kestral 3000 pocket weather meter. It in includes air flow and several other functions. It's little over hundred dollars. nkhome.com or thermastor.com

    The Hobo line of data loggers track dew point but no readout, less than hundred dollars.

  8. #8
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    Jan 2004
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    Sounds to me like your original request was for an economizer with enthalpy control. An economizer brings in 100% outside air when you want cooling AND when the outside air is cooler than the return air. An enthalpy sensor allows the economizer to use enthalpy instead of dry bulb temperature to control the dampers.

    Google "economizer enthalpy control" (without the quotes) for some good information.

  9. #9
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    Wow thanks. Several rich sources of info to follow up on. I really appreciate that!

    -- P.Student

  10. #10
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    Xavier asked:

    >>I have a question on the system, does it temper the incoming air? If so how and at what cost?
    >>If not then it will affect/change the temperature and RH of the space, correct?

    Received the "Smartvent" literature recently in the mail and studied it. Unless I greatly misunderstand, it does not temper incoming air -- unless you consider feeding air to the air handler return to be conditioning it. Pictures of the unit appear to have only the space for a couple ducts and a damper, it's not big enough to do much else.

    The gadget seems to focus its intelligence on determining whether the humidity outside, would improve the humidity level inside once the air temperature is changed. So it has a valve and acts mainly as a gatekeeper in its automatic mode. There are 3 levels of automatic control with varying bands of tolerance, plus a couple of manual override options. Again this is a supply-only type of ventilation system, air will exit through the random holes in the structure. This is fine in a humid environment, has virtues for keeping outside pollutants from coming in, but I would never want to use supply-only in certain climates.

    This product is sold alongside a crawlspace ventilator which apparently has great success reducing crawlspace humidity. Sounds like the basic concept is sound for local applications at least. The company is located in Arkansas which has similar enough climate that I am highly interested. For someone whose main concern is a cold climate, it probably would seem alien and not at all attractive.

    Regards -- P.Student

  11. #11
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    It has been 10 years since I worked with those type of sensors.

    A simple hand held meter is relatively cheap, but would not do your job. Back then the cheaper products with a mV output had a drift problem. That is something to look at in the product data. They like to gloss over the "less than flattering" details.
    Once you get the senser you need a controller too.

    Do you know anything about wheatstone bridges?
    That was the controller technology used back then. It was the first thing I learned about with my radio shack electronic project board. It has been used since electronics began.
    If the superheat ain't right it ain't charged right.

  12. #12
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    So drift is a problem

    >>the cheaper products with a mV output had a drift problem. That is something to look at in the product data. They like to gloss over the "less than flattering" details.

    Thanks, I will look warily for that.

    >>Do you know anything about wheatstone bridges?
    That was the controller technology used back then.

    At present I don't know anything about them. But I could learn if that's what it takes.

    Best wishes -- P.Student

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