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  1. #1
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    Originally posted by Carnak
    Originally posted by mjballweg
    "Commercial buildings can typically have ventialtion systems that are slightly negative in the winter and slightly positive in the summer."

    I've been wondering whether it would be practical trying to accomplish the same in a residence in a mixed-humid climate. It seems to me that there is plenty of info on maintaining a slight positive pressure during the summer but there doesn't seem to be much info on creating a slight negative in the winter. The potential consequences of being too positive in the summer don't seem to be nearly as bad as being too negative in the winter. It seems that ventilating with fresh air through the winter would create an undesirable positive pressure that you would try and control. Or, is this something that nobody should worry about? Any thoughts/ideas?
    You want to hear a long winded one from a guy who spent about 37 years in a cold dry climate and six years and change in a tropical climate?

    Maybe if I average them I can come up with mixed humid.

    I will certainly take you up on your offer to provide a long winded one, Carnak. You may think I'm off the wall, but I do enjoy the lessons you provide. The mid-Atlantic mixed-humid climate allows us to reap the "benefits" of multiple climates, depending on the time of year. While the forecast high on Monday is for the upper 20's, it was only a few months ago that we experienced the humidity of several sub-tropical storm systems. Don't ask what its like trying to grow grass here - that would require it's own discussion group.

    [Edited by mjballweg on 12-18-2004 at 01:22 AM]

  2. #2
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    Ditto

    I too would like to hear Carnak give a long winded lecture on this subject.

    Thanks in advance -- P.Student

    P.S. Lstiburek has written 4 related books titled "BUILDER'S GUIDE FOR xyz CLIMATES". Mine is for "hot-humid" climates and is very insightful, yours would I think be "mixed" climates but take my word for it that Lstiburek knows the subject for all the different climates. About $35.

    [Edited by perpetual_student on 12-17-2004 at 11:08 PM]

  3. #3
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    Seems like we are running parrallel thought paths here, P.Student. I've been trying to read as much of the stuff written by Lstiburek as I can find. The age old problem is too much stuff with not enough time to read. I've been thinking about buying the Mixed-Humid Builder's Guide but am a little trigger shy as it is the oldest of the four and would appear to be the next to be revised. However, I don't know if or when that is planned.

    Another of many things I don't quite know how to reconcile are Joe's Top Ten lists for the South and North. Where does the Mixed-Humid/mid-Atlantic region fit in this otherwise bright line scheme of Top Ten Dumb Things?

    And yes, thanks in advance Carnak.


  4. #4
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    Re: Ditto

    Originally posted by perpetual_student
    I too would like to hear Carnak give a long winded lecture on this subject.

    Thanks in advance -- P.Student

    P.S. Lstiburek has written 4 related books titled "BUILDER'S GUIDE FOR xyz CLIMATES". Mine is for "hot-humid" climates and is very insightful, yours would I think be "mixed" climates but take my word for it that Lstiburek knows the subject for all the different climates. About $35.

    [Edited by perpetual_student on 12-17-2004 at 11:08 PM]
    Dr Joe is good, have the hot humid book myself, sort of figured out the extreme cold one myself. Just about through exhaling.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  5. #5
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    I have gone from a cold dry climate to a tropical climate, I have good first hand experience of these two climates, however I have only read about the problems of a mixed humid climate. So I will give you my ‘take on this climate’ and I am sure it will stir up a lot including comments that “I don’t think or I am uneducated or I just guess”. I also expect comments from some of the informed posters in here as well.

    Here is the first comment and it is really to make sure the home you build is right for your climate.

    Building Science has a good book on building in hot humid climates which I have had for sometime now and I am slowly implementing topics in this book. P-Student has quoted this book as well and I agree it is a good one. My problem is that I have control over mechanical systems and not the actual building systems.

    Building Science has a book on the Mixed Humid Climate as well, and I would GUESS that it is also an excellent book. There is a good article by Dr. Joe Lstiburek, P.Eng in the August 2004 ASHRAE JOURNAL magazine. If you cannot find it in the library, non members can get a copy of this article for $8 at

    http://resourcecenter.ashrae.org/sto...oginid=1459217

    It details wall and vapour barrier construction techniques for the various ‘Hygrothermal Regions’, including a mixed humid climate.

    The definition of mixed humid climate is long but it notes that you can have average monthly outdoor temperatures below 45F. So, if you consider keeping a home at 70F and 40% RH in the winter, the indoor air can have a dewpoint of about 44.6F so in your coldest weather, there is a risk of some wall cavity condensation.

    One thing to be noted is that there are vapour barriers like poly, and then vapour retarders like Kraft papers.

    The walls recommended for the mixed humid climates seem to only use a vapour retarder, and provide a means to let water trapped in walls have an escape path out of the wall either through the inside walls or through drainage planes.

    With your mixed humid climate you would have good stretches of hot humid weather, periods of in between weather where you would not be running heating and cooling, plus a definite streak of heating required. So probably once the outdoor temperature drops to the point that you do not have to run air conditioning until it falls into the low 40s, you will not have to worry about condensation inside your wall cavities. You will still need to control humidity.

    To avoid condensation inside of building cavities you have the problem when the dew point of air is higher than the temperatures inside of the building cavities.

    So when your outdoor dewpoint is higher than what you maintain your home at there is a danger and when the room air in your home has a dewpoint is higher than the outside air temp you also have a danger.

    The danger of building cavity condensation would not be there during the "in between times" when you are not running heating or cooling.

    Building Science is still recommending positive pressure ventilation for the mixed humid climate with the fresh air typically brought in through the central air conditioning system, (meaning it conditions the air either by heating or cooling) plus using a dedicated independent dehumidifier to dry out room air only.

    http://www.buildingscience.com/house...id/atlanta.htm

    This scenario also seems to opt for intermittent ventilation. With this set up, when the system is facing near design cooling and heating situations, you would be getting a lot of fresh air. As heating/cooling equipments run time was decreased as in the "in between times", you could use something like an air cycler to make sure you get a little fresh air when there are prolong calls of no heating or cooling.

    I am a firm believer in intermittent ventilation and will address this in my next deep breathe.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  6. #6
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    Here is the last of my hot air and it is on ventilation rates, steady and intermittent ventilation etc.

    Most of the posters here from the USA advocate 15 CFM per person per ASHRAE, and a typical 3 bedroom house would then require 60 CFM of fresh air. If the ratio of house volume per occupant is not out of the ordinary this should give very good IAQ, and if the house is tight enough, a lopsided ventilation rate of 60 CFM could create a slight positive or negative pressure differential. As a fresh air intake to create positive pressure or a point exhaust to create negative pressure.

    If a house is leaky and would require 400 plus CFM to create a pressure differential, then money could be best spent, increasing the air tightness of the home.

    If the house is spacious with great rooms, large family rooms, high vaulted ceilings then 15 CFM may be on the low side, as the fresh air would be more dilute.

    Canadian ventilation rates are based on 'habitable rooms' or the 'house volume' and typically could be equal to 0.3 air changes per hour.

    Example A

    A three bedroom house 1050 square feet on main floor with a 1050 sq foot basement.

    ASHRAE would say 60 CFM and 0.3 air changes with 9 foot ceilings would require 95 CFM.

    Example B

    Three bedroom house 1600 sq ft, 1600 sq ft basement. Vaulted ceilings give an 'average flat ceiling height' of 10' upstairs with 9 ft ceiling in basement

    ASHRAE would say 60 CFM and 0.3 air changes would say 152 CFM.


    Dr. Joes Very Cold Climate Scenario

    Dr Joe classifies most of Canada, Michigan’s UP, Northern Minnesota, North Dakota, Maine etc as very cold.

    Tight homes need mechanical ventilation to avoid excessive humidity and window condensation in the WINTER.

    Consider Example A as a new construction home. An area WITHOUT a Radon problem. A HRV is installed and BALANCED for 95 CFM on high speed and low speed may give closer to the ASHRAE ventilation level of 60 CFM. Fuel burning appliances are direct vent. The house can easily have well sealed vapour barriers, which although are meant to stop moisture, but will also stop air. The building construction limits drafts, however in the winter the home will be slightly negative due to natural forces and there will be intermittent exhausts running.

    Typically the HRV installed could run constantly on low speed (60 CFM) and have the capability to go to high speed (95 CFM) on a rise in humidity.

    The first winter after that home is built, like one month after the people move in, that HRV is going to run steady on high speed to get rid of the moisture of new construction. By the second winter, with that HRV running constantly on low speed (closer to ASHRAE Levels), the house will be so dry that lips will be splitting, and people end up installing humidifiers. The outdoor air is initially below freezing and very dry.

    Ventilate to dry the air out, then add moisture to it. Defeats the purpose.

    What causes bad IAQ? Too much humidity, too much CO2, maybe gases released by building materials. I am not sure if building materials are like time release capsules or not but maybe after the first year its released what ever it is going to release.

    So we are looking at CO2 and H20, both by-products of people breathing.

    If an HRV ends up getting set to run intermittently such as a rise in humidity it is basically indirectly controlling CO2 as well. Some HRVs also come with a CO2 sensor in addition to the dehumidistat.

    I found it happening over and over again, people installing humidifiers because the HRVs were over drying their homes. Even with them run on the lower speeds closer to the ASHRAE levels.

    So what I found is to run the HRVs intermittent. They are off until there was a rise in humidity. There was no longer the constant supply of fresh air, but compared to ASHRAE, there was a significantly larger intermittent rate of ventilation.

    The total number of cubic feet per day of fresh air would be similar. None of my most allergy prone or hypochondriac customers in Canada had complaints with the intermittent ventilation.

    So I apply intermittent ventilation here in the tropics, as a fresh air intake to the return side of the air handlers. Intermittent meaning the air handler is in AUTO mode, running only when the compressor runs. The intermittent rate is generally higher than the 15 CFM per person, and my design starting point is based on what the infiltration from WIND EFFECT as there is no noticeable stack effect in Summer Conditions.

    Stack effect is temperature driven and in the winter you could have a subzero ambient temperature meaning the difference in temperature between indoors and outdoors is 70 degrees and even more. In the summer, the temperature difference, is maybe 20 degrees, and the stack is inverted. Other websites RECKON that there are no summer drafts because people open windows, but in fact it is the low temperature differential that would allow one to ignore this in a summer time calculation. As well people think of a draft as being cold and you would therefore not feel cold drafts in the summer to begin with.

    These Residential intermittent ventilation rates are much higher than the 15 CFM per person dictated by ASHRAE and they will work out as to being upwards to 10% of the airflow of an air conditioning system. Without having a blower door test at my disposal, the final test is to close off the fresh air BALANCING damper so that I feel a slight cool draft exiting when I crack a door or window. My best compliment is when people tell me how dry their homes are.

    The key in the COOLING season is to dry out the fresh air before it gets delivered to the space.

    In the tropics, 80% of the ventilation load is LATENT.

    Using a dehumidifier to bring in a steady, lower (ASHRAE) rate of fresh air all the time is great when the compressor is running, but when it is off it is pumping humidity into the space and it forces the dehumidifier compressor back on to remove this humidity.

    You get a steady supply of fresh air this way, but it is my OPINION that it wastes energy.

    You could ventilate intermittently at a higher rate in the hot humid or mixed humid climates. Use something to cycle the air handler like an Air Cycler in the in between seasons and have a dehumidifier standing by.

    In another thread it was being discussed on what payback there would be on a residential ERV in the South and the big problem was if the ERV is being used as a neutral pressure system, it did not generate positive pressure so you still had to deal with infiltration. You save money on dehumidifying the ventilation air, but because you do not develop positive pressure you also have to deal with the air that infiltrates in.

    ERVs could be run out of balance but with only 60 CFM to get any positive pressure, it might as well be just a fresh air intake as running them as 60 supply and 50 exhaust is not going to do much for you for positive pressure.

    When ERVs are run out of balance such as in a Commercial application, it reduces the amount of sensible/latent heat that can be transferred from the outside air. I ran Greenhecks selection program for a classroom, 450 CFM supply air, 350 exhaust for Houston and even unbalanced it reduces the outside air cooling load from 2.58 tons to 1.07 tons. Normally Greenheck would only look for a 10% imbalance, however this seems to transfer 75% of the heat in the ventilation air.

    With my office library being wiped out from a four foot sea water flood, I am forced to look at residential ERVs I can find on the internet, and in looking at ‘renewaire’ they look less efficient than the commercial ones more like 50% as other posters claim.

    A comment that outside air filters on ERVs will clog and make it into a negative pressure device in another thread has merit. However that is a maintenance issue plus with them run out of balance on the positive side reduces the risk of a negative caused by a clogged fitler.

    When I rebuild my office, I am going to add a larger residential sized ERV, something in the 180 CFM range and run it out of balance. I will try it steady and intermittent as well.

    So far what has worked in my small humble office down here has worked well in homes.




    [Edited by Carnak on 12-19-2004 at 02:08 PM]
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
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    Stirling Technology's ERV

    >>...residential ERVs I can find on the internet...

    I'm still very inexperienced at sizing up ERVs, but got stuck on one which sounds very whiz-bang with technology. I wonder if you would see any standout qualities with the UltimateAir product line here:
    http://www.ultimateair.com/
    http://rs6.net/tn.jsp?t=v99chzaab.0....paerators.html

    They make claims about being the most sophisticated, most efficient ERV in its class but I cannot be sure whether that is substance, or merely sales hype.

    An aside: because the company is named "Stirling Technology", for a long time I was under the mis-impression the gadget was some kind of application for a Stirling engine. After I phoned them, a company rep straightened me out. It seems they *would* make Stirling engines if there was any way to do it and make a profit. But the Stirling engine seems to be forever a gadget which lives more strongly in the imagination than anyplace in the real world.

    This link describes the relationship between their Stirling Engine roots and ERV technology. I can see how heat transfer is important with both, but I wish somebody would emphasize humidity transfer instead:
    http://www.ultimateair.com/about/history.html

    Regards -- P.Student

    [Edited by perpetual_student on 12-19-2004 at 03:21 PM]

  8. #8
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    Mixed-humid

    Thanks Carnak!! That was long winded in a very good sense, unlike the single digit wind chill factor here tonight (although no where near as bad as what I saw in Wisconsin). I will digest your info as I can and try to follow up with questions when they come up.

    The builder will probably start framing in about two months. I've been trying to discuss with the builder some of the materials you reference but have not yet been very successful. One problem I have is trying to figure out how close the builder is right now. The framing crew just finished one and I walked through it this morning as well as another one that had been insulated but was pre-drywall. I've got to study the houses that work examples and decide what else I can convince them to do. I will also order the Builders Guide and try to lay my hands on the ASHRAE journal article you mention. I truly understand the significance of the overall envelope, but the contract is signed. I may have to fall back to a what do I do with what I have scenario if I am unsuccessful in getting a tighter building. Thanks again!

  9. #9
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    Stirling Technology's ERV

    That particular ERV, the RecoupAerator, was recommended to me last spring as the only one to use. It does look capable of doing a lot of different things. The guy who recommended it seemed a little unorthodox. However, I'm finding out that almost everything he recommended, down to the smallest detail, is exactly on target. I previously asked if anyone had an opinion on that ERV but no one responded. I, too, would be interested in hearing of any experiences with the unit or (dare I?) opinions.

  10. #10
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    ASHRAE Article

    Just muddling on through. The August, 2004, ASHRAE Journal article "Understanding Vapor Barriers" by Dr. Lstiburek can also be found at the Building Science website - http://www.buildingscience.com/resou...AE_2004_08.pdf - among loads of other information.

  11. #11
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    My favorite Myth is "You must avoid positive pressure ventilation in a cold climate". "Most homes" in cold climates leak enough air to keep them dry during cold weather. In WI, we add a 6" make-up air on the cold air return to assure some make-up air for combustion. These homes leak 50-100 cfm of moist air without condensation problems in the walls or attic.

    Very tight homes have .1-.2 ach on a cold windy day. This amounts to 30-60 cfm of warm moist air exfiltrating out through the top of the home and an equal amount fresh air infiltrating low in the structure. Adding 60 cfm of make-up fresh air ventilation when occupied plus enough hours to control excess humidity should have the same effect as "most homes". Exhausting 60 cfm of air has the same effect with the same level of humidity in the air leaking out the top of the home. Adding HRV/ERV balanced ventilation(BV) to get 60 cfm of real air change is a real balancing act. Turn BV "off" on a cold, windy day or over-ventilate increases heating cost. During neutral temperatures and no wind, there are no savings with BV. During hot humid weather, positive pressure as a big advantage. With a 60 cfm fresh air requirement when occupied, cold climate savings are about seventy-five dollars per year max. I suggest, its most practical to have one simple system that provides year around fresh ventilation and humidity control. In green grass/cold climates, the ventilation system must include summer dehumidification. A couple thousand dollars will provide the system. All of Joe's testing proves that BV HRV/ERV does not pay and does not control summer humidity.

  12. #12
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    >>All of Joe's testing proves that BV HRV/ERV does not pay and does not control summer humidity.

    Teddy Bear, I always want to read what you write. Do you know of a link to something published which will discuss this?

    Thanks -- P.Student

  13. #13
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    Building Science tested various ventialtion schemes P-student.

    The ERVs were balanced, therefore infiltration was cumulative.

    I looked at the Stirling ERVs you or ballweg posted and I believe they use the same type of enthalpy wheel that Greenheck uses most likely made by AirXchange. They are very pricey with the ECM motors and the built in controls to pressurize/depressurize. I would guess they cost twice as much as typical residential units.

    I think York was relabelling ERVs using the same enthalpy wheels as well.

    I plan on putting my money where my mouth is and will add a larger residential ERV balanced to be positive in my small office soon.

    Concerning positive pressure in the north, I can only comment on observing frozen locks as a result, and this was a commercial application. If it is driving 'humid' air through keyholes, it can also drive it into the walls.

    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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