[Greenbuilding] ERV strategy

Sun Oct 24 20:59:24 CDT 2010

I am getting ready to install my first ERV as my house step by step gets tighter.  It is next on my list of steps before I seal the deal (my occupied house).  I have been reviewing last month's thread regarding ERV's.  

I am still a little confused regarding the dehumidification affect of ERV's.  

Steven Tjiang writes:

A running ERV will still remove moisture from a house, just not as fast as an HRV or an exhaust fan.  ERVs slow down the increase in humidity in a well sealed house when the exterior humidity exceeds that of the interior; and decrease in humidity when interior humidity exceeds that of the exterior. 

My comments:

Dehumidifying is primarily a benefit in the summer.  We night flush our house in the summer.  Night times have a much higher relative humidity as cool air holds less water.  During the day when outdoor air temperature rises, the outside relative humidity goes down.  However, we close up the windows to ventilation during the day which does for the most part keep the house cooler.  However, the relative humidity is higher than the outside air.  If outdoor air is cooled to indoor temperature, such as an ERV or HRV, it will have a higher relative humidity.  These ERV's are transferring real latent energy (water vapor) and not just relative humidity.  If you mechanically cool your house (AC), the air is cooled to below dew point when vapor condenses and physically drains away.  At that point it has 100% relative humidity.  However, the heat of house you are cooling heats that air which reduces the relative humidity to achieve comfort.  A dehumidifier reheats the air directly at the unit with the compressor heat.  Therefore the indoor air is literally dryer in # vapor/unit volume, not just relative humidity affected by air temperature.  Then that actually dryer indoor air regardless of temperature can dry the incoming fresh air by latent transfer.  However, if the air has not gone through a process of cooling to dew point where the water can be physically removed from the air, the cooler high relative humidity indoor air being exhausted will cool the hotter outside air to a higher relative humidity.  Without an AC or dehumidifier process removing the water from the air, can an ERV reduce humidity?  Does it require a vapor condensing equipment to dry the interior air to dry the exterior fresh air? If not, does it have any advantages over a HRV where no latent transfers occur?  

----- Original Message ----- 
  From: Steven Tjiang 
  To: satjiwan at alumni.brandeis.edu 
  Cc: listserv Green Building new 
  Sent: Sunday, October 03, 2010 10:02 AM
  Subject: Re: [Greenbuilding] ERV strategy

  A running ERV will still remove moisture from a house, just not as fast as an HRV or an exhaust fan.  ERVs slow down the increase in humidity in a well sealed house when the exterior humidity exceeds that of the interior; and decrease in humidity when interior humidity exceeds that of the exterior.  

  ---- Steve (KZ6LSD)

  On Sun, Oct 3, 2010 at 6:44 AM, sat jiwan ikle-khalsa <satjiwan_khalsa at hotmail.com> wrote:

    > From: Alan Abrams <alan at abramsdesignbuild.com>
    > To: ArchiLogic at chaffyahoo.ca

    > > Don't know nuttin' about nuttin' about ERVs (because MooseLand is HRV
    > > Country but...
    > >
    > > Assuming that the ERV is functioning similarly to an HRV in MooseLand
    >
    > NO MOOSE IN SLIGO CREEK, BUT HERE WHERE THE DEER SWARM LIKE STARLINGS, AND
    > SUMMER R/H AND TEMPS EXCEED THE 90'S, THE ERV REMOVES SOME WATER VAPOR FROM
    > INCOMING FRESH AIR
    >
    >
    > BUT EVEN DURING THE SHOULDER SEASONS YOU WOULD WANT TO MECHANICALLY
    > DEHUMIDIFY THE BATHROOMS
    >
    > > ie the heating and cooling elements would cycle on and off as heating or
    > > cooling demand dictates but wouldn't the ventilation air demand remain
    > > somewhat constant (ie except for those times when the fans kick into high
    > > speed during a moisture or odour event) ?
    > >
    >
    > JUST TO CLARIFY, THE ERV'S UNDER CONSIDERATION ARE TWO STAGE.  DURING TIMES
    > WHEN THE MAIN SYSTEMS ARE REQUIRED, THE ERV WOULD RUN AT LOW SPEED, AND
    > WOULD BE KICKED UP BY A TIMER SWITCH IN EACH BATHROOM.  DURING SPRING AND
    > AUTUMN, THE ERV WOULD BE RESET TO OPERATE MANUALLY ONLY, USING THE TIMER
    > SWITCHES.

    aren't we getting caught up in the fallacy that ERV's dehumidify?  they (at least mine) don't have a condensate drain to "remove" humidity.

    - my understanding (and please correctmr  if wrong) is that the HRV does fresh air exchange while maintaining conditioned heat (at about 75% efficiency, usually in colder, dryer climates).
    - the ERV does fresh air exchange while maintaining whatever conditioned temperatures (heat and cool) AND also doing something with latent heat in the humidity differential inside and out.  (i.e. it helps maintain whatever humidity you have inside - achieved with separate air conditioners or dehumidifiers-- but does not actually change the humidity of the indoor air.  just does it's best to make the outside air just as humid as the indoor air).

    so putting them on a bathroom doesn't make sense to me.  extra humid air in there will be equalized with incoming fresh air and tend to KEEP that humidity inside the house.  why is it that people use them there?

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