View Poll Results: Flow direction

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  • "normal" flow (supply in bedrooms)

    1 100.00%
  • "reversed flow" (extract in bedrooms)

    0 0%
Results 1 to 5 of 5
  1. #1

    Question Balanced heat recovery ventilation - the case for "reversed" flow

    We moved into our present home 3 years ago and have a counterflow heat recovery system. I am not happy so started to study this subject and our installation issues because we want to have a better setup in a new and smaller house that we are building.

    The documentation that I've come accross recommends is an extract of the stale air in the living areas (sometimes bathrooms, toilet(s) and laundry get included but normally not done here) then goes to a heat exchanger and next gets discarded outside. Similarly the fresh air goes through the heat exchanger and gets supplied to bedrooms and living areas.

    The problem with this (imho) is several fold:
    - if the HRV is situation in the ceiling on a single level building (as normal here in New Zealand) and there is a fire then smoke gets distributed to all rooms in the building and the inhabitants of the home are quickly overcome
    - colder air is supplied in different places, often exactly in a spot where one can feel it (above the bed, above a desk in front of the window etc)
    - if there is a fireplace in the house and the doors to the other rooms are closed and there is only a single stale air extraction (often in the living area close to the heater / stove) then the living area will experience negative pressure resulting in backdraft through the fireplace (even more so with backdraft dampers in the kitchen extract fan)
    - the polluted air is not extracted most of the time at the place of pollution (!) Normally inhabitants of a house spend most of their time ... in bed (!)

    If the flow is reversed, i.e. have a single supply of fresh air in the living/kitchen area and the stale air is extracted from living area (furthest from kitchen) and bedrooms then surely this would result in better air quality? Especially if one is to leave either the bedroom doors open (I am retired and only live with my wife in the 3 bedroom home where one spare bedroom will be a home office and the other is for the occasional guests) or when bedroom doors are closed have the window in the bedrooms slightly open. The overpressure in the living /kitchen will at such time that the doors are closed simply escape through the kitchen extract fan (backdraft damper will be opened by the over pressure).

    I am making the installation in our new building in such a way that I can just swap two ducts and reverse the flow. The fans are external to the heat exchanger and we'll be using S&P silent fans with an oversized aluminum counterflow core and I will be balancing the flows. The total thoughput will be controlled by a timer, 20 min on, 40min off and have boost on at dinner times. I intend to tailor the 20/40 min to an optimum flow by measuring the CO2 levels in the home.

    Am I missing something in this reasoning , please comment since New Zealand has only 4 Million people and there is just not enough knowledge to go around and have a decent discussion on this interesting subject.

    Many thanks in advance, AM.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,364
    Like fresh air in the bedrooms and exhaust in baths/center of home better. Having 3 supplies decreases the amount of tempered air in any single spot.
    If you have any open combustion devices and/or clothes drier, fireplace exhausting from the space, suggest make air only fresh air ventilation. Monitoring with CO2 meter will show that your home gets enough fresh air during cold windy weather without mechanical fresh air. Mechanical ventilation is needed when the wind is calm and the temperatures are moderate. If so, filtered makeup air is prefered. Exhaust the baths/kitchen hoood/clothes drier.
    Ultra air tight home in cold climates with high heating cost are good for HRV. Maintaining +30%RH--<50%RH are critical to comfort and indoor air quality.

    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  3. #3
    Teddy Bear - thanks for the input. Our climate is humid and moderate. We don't have air conditioning, when the temperature gets too high we sit outside in the shade.

    Present home with HRV has dropped humidity from 70+ % with dehumidifier year round to 40 - 60% without dehumidifier. Heating costs have dropped a lot too but CO2 indicates we're ventilating on the high side - outside CO2 approx 410 ~ 420 ppm, inside 520 ~ 540 ppm climbing to approx 590 ~ 620 when cooking dinner. Some advice given was that it should climb at times up to 750 ~ 800 ppm but stays most definitely below 1000 and preferably below the 800 ppm. Did not find much collaboration for this.

    Unfortunately I am extreemly sensitive to draft due to my illness (over exited sensitivity to the point of having my immune system shut down) and wanted to have the "colder fresh air" entering the home in the living area at it's warmest point (close to an open hallway) where any draft would not be felt since we are not normally hanging out in that area. I will use one of the diffusers that forces the air to mix rather than using a simple one of those that are prevalent here which has a built in damper and which results in a jet stream. A damper will be in the supply line of the fresh air since that will supply more than is extracted. Once the supply is balanced to the extracted amount I'll then use a timer to reduce the total amount of ventilation. The house is new built and will be relatively air tight (will do a doorblower test) and the area has relatively low wind. Offgassing is an issue for me and there is no way around it to ventilate year round. Am considering to switch off supply in summer and ventilate (extraction) only during some hours of the night - outside doors being open during the day and at night have windows partially open. It is all a matter of compromises but since I am not totally sure I am making it in such a way that I will be able to change direction easily by just changing two ducts.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,364
    Quote Originally Posted by AmadeusMozart View Post
    Teddy Bear - thanks for the input. Our climate is humid and moderate. We don't have air conditioning, when the temperature gets too high we sit outside in the shade.

    Present home with HRV has dropped humidity from 70+ % with dehumidifier year round to 40 - 60% without dehumidifier. Heating costs have dropped a lot too but CO2 indicates we're ventilating on the high side - outside CO2 approx 410 ~ 420 ppm, inside 520 ~ 540 ppm climbing to approx 590 ~ 620 when cooking dinner. Some advice given was that it should climb at times up to 750 ~ 800 ppm but stays most definitely below 1000 and preferably below the 800 ppm. Did not find much collaboration for this.

    .
    Comparing the outside to the inside CO2 levels indicates you are get 30-50 cfm of fresh air per person. The is about the right amount of fresh air for a 1,500-2,000 sqft home. You are concerned about air change rates not high CO2 levels. Do everything you can to keep your home <60%RH. This reduces mold/dust mites/bateria. You would be better off with an erv than a hrv. Either way, not much benefit in your climate. I like pressurization with air filtering and dehumidification in your climate. Not to be concerned, though.
    You did well on figuring out the big things.
    Setting the shad sounds good.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    With the central supply it is easier to over pressurise (less ducting and wider ducting from exchanger to supply grille in order to minimize "jet effect" and wind noise) and as long is remembered that when bedroom doors are closed which should only happen sporadically when we have once or twice a year visitors) the windows need to be slightly opened then all should be well. Most people, even when having a balanced ventilation system, will still open windows so I am not too much worried about that. The new home will be 112m2 (about 1200 sqft), big enough for us - we brought our three children up in a 90m2 place. (a bit too small but what can one do if that's all one can afford. At present we are living in a place of approx 2500 sqft and it is too big for the two of us. It does not help that the garden has become a problem to maintain.)

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