Are you a subscriber but don’t have an online account?

Register for full online access.

 
 
 
 
+ Reply to Thread
Page 1 of 2 12 LastLast
Results 1 to 15 of 26
  1. #1
    Join Date
    Aug 2007
    Location
    Dallas, Texas
    Posts
    839

    Default Air changes per Second or per Day?

    Air changes per hour seems arbitrary to me.
    We pay an Energy penalty for uncontolled and poorly controlled air change.

    I believe that we should strive for Airtight construction.
    Our buildings should have only "Intentional openings".
    They should have dampereed intake valve(s) and dampered exhaust valve(s).
    If the power goes out... then the valves should spring open.
    That way at least our buildings are just as safe as conventional leaky builings.

    Next, we should consider how much outside air we REALLY need and WHEN the outside air should be brought inside.
    Arbitrary example: if a constant 60 cfm is desireable then must we really bring in 1 cubic foot every second?
    How about 120 cfm for 30 minutes and not for 30 minutes?
    How about 120 cfm for 12 hours and not for 12 hours?
    Why not choose the best time(s) of day to bring the air inside depending on conditions outside and conditions inside?
    We have the technology.

  2. #2
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by John B View Post
    Air changes per hour seems arbitrary to me.
    We pay an Energy penalty for uncontolled and poorly controlled air change. I believe that we should strive for Airtight construction.
    Who's talking about uncontrolled air exchange? Airtight construction is the industry standard. In fact, almost any house built to current energy codes will be too tight without controlled ventilation.

    Poorly controlled can also mean inconsistent airflow - too much at times, and too little at times, possibly creating drafts and cold spots when running and allowing excess moisture (and other toxic) accumulation when not.

    Our buildings should have only "Intentional openings".
    They should have dampereed intake valve(s) and dampered exhaust valve(s).
    If the power goes out... then the valves should spring open.
    That way at least our buildings are just as safe as conventional leaky builings.
    Why introduce additional mechanical complication? There are a number of air inlet vents on the market (Aldes, Therma-Stor, Panasonic), most of which are completely passive and rely on no internal mechanism to function but limit and filter incoming air depending on indoor negative pressure.

    If you like complicating the system, there are humidity-sensing and even occupant-sensing air inlets that regulate the flow depending on interior conditions and use.

    Next, we should consider how much outside air we REALLY need and WHEN the outside air should be brought inside.
    This is established by ASHRAE and Energy Code standards. It may be based on a 15 cfm/person occupancy, or number of bedrooms plus one times 15 cfm, or a target of 0.35 ACH (but not less than 15 cfm/person).

    Since I build non-toxic houses for people with non-toxic lifestyles, and design to minimize the heating load, I always aim for a target of 0.25 ACH.

    if a constant 60 cfm is desireable then must we really bring in 1 cubic foot every second? How about 120 cfm for 30 minutes and not for 30 minutes? How about 120 cfm for 12 hours and not for 12 hours? Why not choose the best time(s) of day to bring the air inside depending on conditions outside and conditions inside?
    Such control strategies are often both allowed and encouraged, but it is important to consider the effects.

    A slow, steady air flow maintains a relatively constant base-line indoor environment, but it may make sense to increase the air exchange when the house is occupied.

    Bathrooms should have a mininum intermittent exhaust capacity of 50 cfm and kitchens 100 cfm to evacuate pollutants at the source. A bath timer can be used to boost the flow rate from a central HRV. Any indoor fuel-burning appliance (other than a gas cookstove) should have a dedicated air supply.

    Local or central fan systems can be controlled with programmable time-of-day switches, or dehumidistats, or "smart controls" which combine and automate these functions.

    HRV systems that are coupled to FHA heating systems, can be regulated by any of the above along with constant airflow regulators or electronically controlled motorized dampers.

    Bottom line is: we can get as clever as we'd like, but each additional control technology complicates the system and creates more potential failure points and failure modes. As always, KISS (keep it simple and safe) is the best approach.

    - Robert

  3. #3
    Join Date
    Aug 2007
    Location
    Dallas, Texas
    Posts
    839

    Default Re: Air changes per Second or per Day?

    Robert,
    Thanks for the feedback.
    When I say Airtight I am not talking about code standards or even Energy Star Standards...
    I mean as airtight as possible.....more like the German Passive Haus Concept.
    http://www.passivhaustagung.de/Passi...htness_06.html
    The Passive Haus homes are virtually airtight and almost all fresh air is directed thru an HRV.

    How tight are the homes that you build?
    What kind of blower door results do you consider good enough?
    Can a house be too tight?
    John
    Last edited by John B; 11-26-2008 at 11:58 AM. Reason: added link to passive haus...hope it works

  4. #4
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by John B View Post
    When I say Airtight I am not talking about code standards or even Energy Star Standards...I mean as airtight as possible.....more like the German Passive Haus Concept. The Passive Haus homes are virtually airtight and almost all fresh air is directed thru an HRV.
    What you're suggesting is what I call the "hermetic house", as it's hermetically sealed, like living in a plastic bag. Doesn't sound very inviting, does it? One of the best ways to achieve this is with SIPS panels, but this wouldn't meet one of the most important qualities of the Bau Biologie movement (also from Germany): that a house must be able to breathe moisture (just like our skin).

    While it's possible to achieve extremely high levels of air-tightness, and compensate for it with HEPA-filtered HRVs, this creates unintended consquences (as do all techno-fixes). One of those is low negative ion concentration, which can be alleviated by having many indoor plants or a water fountain or by cracking open windows (which, of course, undermines the air-tightness). Another problem is such a uniformity of indoor temperature that it's impossible to keep bedrooms cooler, as is considered physiologically preferable for optimum health as well as comfort. This can also be alleviated by cracking open bedroom windows (which, of course, undermines the air-tightness).

    How tight are the homes that you build?
    What kind of blower door results do you consider good enough?
    Can a house be too tight?
    I've been building houses, using the air-tight drywall system (and no vapor barrier) that are very tight: 2-3 ACH@50 pascals (equivalent to 0.11-0.15 natural ACH) but not hermetically-sealed, and I use American Aldes Airlet 100 passive make-up air inlets in combination with Panasonic bath exhaust fans on 24-hour programmable timers for 0.25 ACH mechanical exchange (with local bath timers and kitchen stove hood for boosting the exchange rate during moisture production and eliminating it at the source).

    I consider such blower door test results (2 ACH with inlets taped, 3 ACH with them open) to be a good target. My houses consume about 1.4 BTU/DD-SF, which is more than the PassivHaus average of about 1 BTU/DD-SF (though, oddly, the German requirement is 15 kWh/m² per year regardless of local DD climate, which makes them cost-prohibitive in northern Europe), but far less than the new house average.

    Can a house be too tight? The perennial question! To my way of thinking, a truly passive house, unlike the Passiv Haus, will be reasonably healthy and comfortable without energy-consuming mechanical devices (even in a power failure or mechanical breakdown). Two of my superinsulated houses are heated only with passive solar and woodstove (4/5 of a cord per year), with the woodstove as the primary whole house ventilator so that fresh air is exchanged at a reasonable rate without mechanical or operator intervention.

    My inclination is always to use the best low-tech solution, as those rarely have devastating unintended consequences as high-tech almost always does. The irony of modern life is that most technological innovations are necessitated by the problems caused by previous technological innovations. As Kirkpatrick Sale said (in his book Human Scale) "invention is the mother of necessity".
    Last edited by Riversong; 11-26-2008 at 02:04 PM.

  5. #5
    Join Date
    Jul 2008
    Location
    Suburbia (Washington, DC area)
    Posts
    1,936

    Default Re: Air changes per Second or per Day?

    One funny thing about current tight-house thinking (reflected in LEED for Homes and other similar directives) is that active participation by the occupants doesn't seem to be figured in. What I mean is, air changes are supposed to be set up by the builder to be automatic without much attention from the occupants: you're supposed to install a system that ensures .35 ACH or the ASHRAE standard takes place.
    In most climates there's a substantial part of the year when someone would just open their windows! Or if they are like many (but not all) homeowners, they'll run exhaust fans in kitchen & bath at the appropriate times, which might nearly cover daily air changes by themselves, depending.
    Seems odd to put in a dumb mechanical device when there's a smart person in the house to manage the exchanges. Hate to think of the energy spent running HRVs with the windows open.
    So, if I can hijack this thread, anyone heard about the relative merits of HRV/ERVs vs. inlet-only (dampered outside air to return duct) vs. outlet-only (timer on bath fan) type ventilation systems? A few years ago Building America & JL did a study where in a moderate climate the energy of running the HRV fan was about equal to the energy saved by the exchanger, meaning an inlet-only system with no separate fan was the same, energy-wise.

  6. #6
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by ThingOfBeauty View Post
    One funny thing about current tight-house thinking (reflected in LEED for Homes and other similar directives) is that active participation by the occupants doesn't seem to be figured in.

    Seems odd to put in a dumb mechanical device when there's a smart person in the house to manage the exchanges. Hate to think of the energy spent running HRVs with the windows open.
    Of course the problem is that most homeowners know as little about operating their houses as they do about the mechanics of the vehicles they drive. Even if every homeowner got a thorough "owner's manual" with a new house, most would not read it and the rest would not follow its procedures faithfully.

    Which is why I advocate truly "passive" homes that require as little occupant intervention as possible.

    anyone heard about the relative merits of HRV/ERVs vs. inlet-only (dampered outside air to return duct) vs. outlet-only (timer on bath fan) type ventilation systems?
    One study in northern US cities showed an annual savings of approximately $100 over non-heat-recovery ventilation. With installation costs of $1-$3 thousand, and adding in semi-annual maintenance costs (particularly if you're replacing a HEPA filter every year, which also increases electrical consumption), the payback is not as good as it might seem. If the unit includes a resistance defrost cycle for cold weather, than the payback is extended even more.

    Given that code (and common sense) requires the installation and use of bathroom exhaust fans, my approach is to use an efficient bath fan as the central exhaust unit and provide fresh air through strategically-placed passive make-up air inlets (American Aldes Airlet 100). Of course, to meet standards requires a programmable 24-hour timer connected to the fan (I run them in parallel with a short-term timer in the bathroom), and this can be reprogrammed or over-ridden by the occupant.

  7. #7
    Join Date
    Aug 2007
    Location
    Dallas, Texas
    Posts
    839

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by Riversong View Post


    I've been building houses, using the air-tight drywall system (and no vapor barrier) that are very tight: 2-3 ACH@50 pascals (equivalent to 0.11-0.15 natural ACH) but not hermetically-sealed,
    Robert,
    I think that your performance target for uncontrollable air changes is excellent.
    The performance of your homes is near that of the Passive Haus standards.
    I think that there is room for improvement with the controllable air changes.
    Heat recovery and ventilation time management do not have to be complicated or high maintenance.
    We don't need to live in plastic bags or add Hepa filters.
    Exhaust air could be "ducted" through the intake air stream by passive or mechanical means.
    Time management can be as simple as your 24 hour timer as the default.
    An intelligent being could over-ride the timer with a programmable device.
    The device could keep track of on demand ventilation(showers) at the least and then coordinate with fresh air timing.
    A more sophisticated device could compare inside conditions to outside conditions and then optimize the ventilation timing.
    Our automoblies are far more sophisticated than our homes.
    The addition of a radiator to the auto did not overcomplicate things.
    John

  8. #8
    Join Date
    Aug 2007
    Location
    Dallas, Texas
    Posts
    839

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by Riversong View Post


    Two of my superinsulated houses are heated only with passive solar and woodstove (4/5 of a cord per year), with the woodstove as the primary whole house ventilator so that fresh air is exchanged at a reasonable rate without mechanical or operator intervention.
    Robert,
    I admit that I do not know much about wood stoves or cold climates.
    Are you saying that the woodstove is operating 24/7/52?
    What is the secondary ventilation?
    Who decides when it is time to activate the secondary ventilation?
    Is the stove sealed combustion?
    If it is "sealed"how does it ventilate the house?
    If it is not "sealed" how can you use it with the Aldes inlets?
    John

  9. #9
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by John B View Post
    I think that there is room for improvement with the controllable air changes.

    Heat recovery and ventilation time management do not have to be complicated or high maintenance.
    And then you list all kinds of sophisticated control devices to "improve" the system.

    "Room for improvement" depends, of course, on how you defne "better".

    If "better" means more energy-conservative, then there are any number of increasingly sophisticated methods of getting that next incremental margin (with increasing investment and diminishing returns).

    But if "better" means simpler, more user-friendly, less lifetime maintenance and more truly passive, then sophisticated technological "solutions" would not make sense.

    Our automoblies are far more sophisticated than our homes.
    The addition of a radiator to the auto did not overcomplicate things.
    The Model T came equiped with a radiator, but - necessary as that might have been - it certainly created a whole additional set of failure modes and maintenance requirements.

    What DID make our cars unncessesarily complicated, however, was the attempt to make a fundamentally inefficient and polluting engine more so and less so, and the eventual computerization of all functions. It's almost impossible for the typical car owner to repair their own vehicles today. When I was a teenager, we all fixed our own cars.

    We've forgotten that often simple is better.

  10. #10
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by John B View Post
    Are you saying that the woodstove is operating 24/7/52?
    No. These houses have such a low supplemental heat requirement that even a small woodstove would have to be run only part of the day, and then only when it was below about 50° outside.

    What is the secondary ventilation?
    Who decides when it is time to activate the secondary ventilation?
    Secondary ventilation is source removal: kitchen exhaust hood and bathroom exhaust fan. Obviously the occupants decide when to use them (though they are advised to use them every time they cook or shower when the house is closed up for heating season). In one house, I also installed a dehumidistat (located in the living room) in parallel with the short-term timer for the bath exhaust fan, so that it could automatically maintain a safe indoor RH (which is also a good measure of indoor air quality).

    Is the stove sealed combustion?
    If it is "sealed"how does it ventilate the house?
    If it is not "sealed" how can you use it with the Aldes inlets?
    No, the woodstove is close-coupled to an outside air supply (a register near the stove) so that it can depressurize the house to provide make-up air through the passive inlet vents (which take air from a south facade overhang where it's solar heated).

    When there is a clothes dryer, I install it in an air-sealed utility room with its own make-up air supply. With a good chimney draft and woodstoves not running tightly damped, I've never seen a backdraft problem in these homes.

    The occupants are organically-living very health-conscious folks and there has been no reported problem with indoor air quality.

  11. #11
    Join Date
    Aug 2007
    Location
    Dallas, Texas
    Posts
    839

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by Riversong View Post
    Secondary ventilation is source removal: kitchen exhaust hood and bathroom exhaust fan.
    ............
    No, the woodstove is close-coupled to an outside air supply (a register near the stove) so that it can depressurize the house to provide make-up air through the passive inlet vents
    I still don't understand how the woodstove provides a reliable amount of fresh air during the shoulder season and the summer. Perhaps you are relying on the occupants to open the windows when they sense that they need fresh air?
    I also think that there is still a risk of back draft when the homeowner operates the kitchen and bath exhaust at the same time.

    following is a quote from the Aldes (inlet vent)website

    Q. CAN WOOD STOVES OR FIREPLACES BE USED WITH INLETS AND
    CENTRAL EXHAUST VENTILATION SYSTEMS?
    A. Inlets have little if any effect on a woodstove or fireplace; the exhaust fan
    with which they are intended to be used is however a real source of concern.
    An exhaust fan will compete with a woodstove or fireplace for makeup air;
    the result may be the backdrafting of the appliance. Woodstoves and
    fireplaces (and their exhaust flues) used under these conditions must be
    certified to operate safely in a negative pressure environment. It is not
    sufficient to supply makeup combustion air for the stove or fireplace
    with air ducted to a grille near the appliance. Naturally drafted equipment
    must have airtight doors, and combustion air supplied directly into the
    combustion chamber from outdoors. AIRLET's are not intended to supply
    combustion air to any appliance.

    END quote from Aldes website

  12. #12
    Join Date
    Oct 2008
    Location
    Rumson, NJ
    Posts
    379

    Default Re: Air changes per Second or per Day?

    The houses built by Riversong operate at a negative pressure pretty much whenever they are closed up. That might suffice in his region, but they would be a nightmare in a cooling driven climate, because negative pressure tends to pull moisture laden air into the wall cavities where it condenses against the chilled interior walls. Positive pressure is more appropriate for warm-humid climates. For those of us in mixed-humid climates, we should be looking either at balanced (push-pull) ventilation, or even changing the house balance from negative pressure in winter to positive in summer.

    All of this can be done with relatively low-tech ventilation equipment. Even the seasonal changes can be interlinked with the heating/cooling thermostats so that exhaust fans can run in winter and intake fans in summer. Injecting the outside air into the main return trunks in both seasons will reduce uncomfortable drafts and other local effects.

    On another note, it is useful to have full-time ventilation at a low level to deal with the natural off-gassing of VOCs from nearly every modern material. Building a VOC free house is pretty difficult (expensive). I particularly like the technique of using two-speed exhaust fans pulling their air from the kitchen and baths. As Riversong mentioned, this gives you good source management. They run at a very low (and efficient) speed 24/7, and run at higher speed when the bath fan switch is turned on. The natural inlet makeup air then goes into the HVAC return ducts, if you've got them.

    Of course, that approach is for cold climates. For warm climates, you would blow outside air into the return ducts, and provide natural relief vents from the kitchen and baths. With a tight house, blowing air into the returns and allowing it to escape from the baths is just as effective at moisture management as sucking air from the baths and allowing natural makeup returns.
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

  13. #13
    Join Date
    Oct 2008
    Location
    Rumson, NJ
    Posts
    379

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by Riversong View Post
    In one house, I also installed a dehumidistat (located in the living room) in parallel with the short-term timer for the bath exhaust fan, so that it could automatically maintain a safe indoor RH (which is also a good measure of indoor air quality).

    The occupants are organically-living very health-conscious folks and there has been no reported problem with indoor air quality.
    This seems to violate your KISS rule. I've had little luck with humidistats. Their accuracy is generally poor, and if the HO doesn't understand the thing, there's a pretty good chance someone will mess with it and set it all wrong.

    Also, indoor RH is a rather poor surrogate for IEQ. CO2 is slightly better, but it's more effective for office environments. Either way, it is very likely that there are contaminant sources that do not produce either water vapor or CO2, and then you've got a problem. This is why I much prefer the 2-speed ventilation strategy. You've always got a little bit of fresh air, and more when the bathrooms are operating.

    Organically-living health-conscious folks are probably using very low VOC cleaning solutions and furniture. Added to your careful construction techniques, this provides very effective IEQ management, but this combination of careful construction and proper living is rare in modern housing.
    Last edited by Pete Engle; 12-05-2008 at 01:35 PM. Reason: Still getting the hang of formatting posts.
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

  14. #14
    Join Date
    Jul 2008
    Location
    Suburbia (Washington, DC area)
    Posts
    1,936

    Default Re: Air changes per Second or per Day?

    Robert, I am not getting the picture of how the air gets to your wood stove. Would you mind trying to paint another verbal picture for me?
    My folks in Rutland (VT for the rest of y'all) just bought another wood stove this year--we had two when I was living there and able to move the 8 cords of wood, now their oil is so expensive they're going back to wood again.
    Thanks,

  15. #15
    Join Date
    Feb 2008
    Location
    North Central Vermont
    Posts
    1,866

    Default Re: Air changes per Second or per Day?

    Quote Originally Posted by Pete Engle View Post
    it is useful to have full-time ventilation at a low level to deal with the natural off-gassing of VOCs from nearly every modern material. Building a VOC free house is pretty difficult (expensive).
    The houses I build have vitually no indoor VOCs and the occupants are conscious of their lifestyle choices. I find it's less expensive to use natural materials which don't contribute to IAQ issues.

    Quote Originally Posted by Pete Engle View Post
    This seems to violate your KISS rule.
    Not when compared with a HRV and full ducting, or even compared to a programmable timer.

    I've had little luck with humidistats. Their accuracy is generally poor, and if the HO doesn't understand the thing, there's a pretty good chance someone will mess with it and set it all wrong.
    Homeowners who are interested in a clean living environment do understand such a simple control.

    Also, indoor RH is a rather poor surrogate for IEQ.
    I disagree. In a cleanly-built house for cleanly-living occupants, there is no better indicator if inadequate air exchange.

    ...but this combination of careful construction and proper living is rare in modern housing.
    It's going to become far more common.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts