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New territory for ME... HRV installation in Passive House.

Bill et al,

I've been asked to participate in the construction of a brand new Passive House here in Colorado. The GC for the job spoke with me on the phone. He administers an on line course that I've taught for, and requested i be involved in the construction of this home, as well as the monitoring of same. He said that the homes energy consumption would probably fall in the range of 3 to 5 btu's per square foot.......(wait for it) per Y E A R.

This is obviously not going to require a heat source larger than a hair dryer. We are proposing the use of our radiant windows to supplement heating needs of the dwelling..

THis being new new territory for me, I could use some guidance and assistance on sizing and installing the HRV to maintain good air quality within the home.

1. How many air changes per hour are required for extremely tight construction?

2. Where should the exhaust vent terminations be installed within the home? Can I use the exhaust to exhaust the bathrooms?

3. Where should the incoming air termination be located, other than obviously equally spaced out?

4. When it gets extremely cold outside, can I vary the blower motor speed to avoid over cooling the space and avoid the need to fire up toaster elements in the discharge air side?

5. Do they make a High Velocity unit that would utilize small 2" ducts, like the Centro-Therm product?

6. Does the unit need to run 24/7/365, or can I base its operation on the accumulation of CO2?

The older I get, the more I realize, the less I know about moving air :-)

Never too old to learn, and thanks to everyone for contributing to my education.


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  • BillW
    BillW Member Posts: 198
    ER/HR Ventilators

    Hi, Mark.  ER/HR ventilators are needed in any of the super-tight construction that this type of building has.  No ventilation quickly renders them uninhabitable.  Energy recovery ventilators recover both latent and sensible heat.  They do not require a drain or a defrost cycle.  Heat recovery ventilators recover sensible heat, and must be mounted in a conditioned space, and provided with a drain to remove liquid water during the defrost process.  Either are about 80% efficient. 

    Fresh air intakes should be mounted away from any source of odors or contaminants.  That includes dryer and heating equipment exhausts, garbage cans, pet areas, carports and driveways.  ER/HR units have some basic particle filtration, but will pull in any gases or vapors that are present, so don't be surprised to smell your neighbor's BBQ during the summer, or his fireplace during the winter.

    That said, they do help lower indoor odors/gases/vapors by diluting and exhausting them.  Heat recovery ventilators tend to dry a space, and energy recovery units tend to balance humidity.  They are not substitutes for range hoods, and should never be used for that, but they are quite effective in diluting bathroom odors and removing water vapor from showers.  As whole-house units, they can be stand-alone, with their own duct work, with a central intake and a vent in each room, but most often, they are ducted into the existing HVAC duct work.  They usually run 24/7on low speed and can be put into a higher fan speed by humidistat, manual or time switches, certain thermostats and home energy management systems.  Since they are 80% efficient, it is quite possible that the air coming from them may be cold on very cold days, so the discharges are usually mounted high on the wall so they won't blow on the occupants, or you can install a small hydronic or electric reheat to temper the air.

    The number of air changes per hour is based on number of occupants, and most states have local regulations.  Most resi units have small capacity, and if this is large structure, several many be needed.  A heat recovery unit has a fan and an element made of thin metal.  The incoming air and the outgoing air never mix, but the heat transfers to the cooler side, warming the air in winter, cooling it in summer. Frost buildup can occur, and is dealt with by either recirculating air or activating an electric heater to melt the frost, which drains away as water.  The cycle is automatic.  An energy recovery unit has a fan and hygroscopic paper core, and allows both heat and humidity to transfer from the higher level to the lower, warming and re-humidifying incoming air in winter, cooling and dehumidifying air in the summer. Some ER units used a silica-gel desiccant wheel instead of the hygroscopic paper.

    I hope this helps you in your project
  • Simply Rad
    Simply Rad Member Posts: 184
    ME is smart to ask


    Hope all is well. I have not spoke to you in a while.  Hopefully you were able to get up to the lake for those few weeks of summer...hahaha.

    I have sucessfully installed 4 of these systems over the years.  First, contrary to popular belief I have only used ERVs.  I did lots of research, as always, and found a Minnesota based company called Renewaire.  Dwain is the contact and he knows his business.   Being in a dry climate I think its good to recoup heat and moisture and there units do a good job.  By no means am I an expert in this field but I found some answers. also no drain is recovered.  I have also found a tool called the "Duculator" to be very useful in sizing. 

    1.  a 1/3 charge/hr

    2.  I exhausted in the baths and kitchen.  Some people say that there is not enough suck for the stink, but I have not had a complaint.  I have used timers in the baths to mitigate this stink problem. 

    3.  You want to hide the supply air. I have found that behind the fridge works well, by cooling the exhaust from the fridge and making fridge more efficient.  Hallways work well too. 

    4.  not sure about fan speed, but you could lower the percentage timer. 

    5.  not sure, but be aware of noise

    6.  I have used a percentage timer.  This is based on space volume and the air change you are aiming for.  I have tried to size the unit to operate less, 60% or lower on the percentage timer. 

    Sounds like a fun project, have you thought about waste water recovery for the DHW. I have seen a few of them over the years.  Simple yet effective. 

    Jeffrey Campbell
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Thanks guys!

    I will digest it and get back to you with questions.



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  • njwebdevguy
    njwebdevguy Member Posts: 33
    edited November 2011
    My experience with my HRV

    I hope that my input is helpful. My perspective is that of an HRV owner, not installer. My wife and I purchased an HRV because we want ventilation year round. We think that its been a very good purchase.. We have a Fantech unit that is around 65% efficiency. Its solid, quiet and has three speeds. Ive never seen an HRV with a variable speed moor, although I dont see any reason why its not possible. Actually, that would be a very nice feature to have, because you could theoretically turn the speed way down, lower than the lowest speed..(which we dont use much because it doesn't seem to be properly balanced, we usually use the middle speed, and vary its duty cycle)

    We also have the top-end Fantech controller that has the option to run it at 25%, 33% or 50% duty cycle - in addition to continuously. We find that we usually keep it on continuously in the late spring and early fall, and on the 25% duty cycle in the winter.

    Its nice to have fresh air in the winter. It does warm up outside air substantially - much more than I expected. However, we only have a one core unit.

    There are more expensive units that have two cores, which are around 80% efficient. They are also twice as expensive.

    We have an HRV with an aluminum corrugated core. I personally would not buy an ERV because the ability to not clean an enthalpic core as completely is really important to me. The filters and cores do pick up a lot of gunk which needs to be cleaned out every month or so..

    I hink the HRV has paid for itself in savings on AC alone, as we use AC much less.

    I have read a lot of material on the passive house concept and I have a few thoughts. One is that one undiscussed reason why I think people need an HRV in them is OSB. Houses that use a lot of OSB or any fiberboard products, I think need powered ventilation. Most American houses would qualify. (Newer homes often smell like formaldehyde to me.)

    I don't think putting a return in the kitchen is smart unless its at the opposite end of a large kitchen from the stove, an HRV is not a range hood.

    (Range hoods are also problematic because the depressurize houses)

    I also wouldn't put an HRV fresh air supply behind a refrigerator because in my experience, refrigeraor coils are often dusty and worse, sometimes they can get moldy. We tried to put our supplies near windows (and above radiators) and our returns nearer the interior. he air is often cool but really rarely is it really cold.

    An HRV return does make sense in a bathroom, as. Its preferable to an exhaust only fan. Make sure there is clearance under the bathroom door and that that HRV return is just one of many, not the only return. Try to have a pair in rooms, not just one or the other.

    I also think its super important to put the outdoor air intake where its going to get freshest air possible. Which usually means up and away on the side of a house, away from garbage storage areas and driveways where cars may idle. Putting it near the ground will also pick up more dust, pollen and bugs. For various reasons I think putting it high as possible on the side of the house with a bit of clearance away from the wall itself is best. The exhausts should be as far away as is practicable. And they should be wide enough. 6 inch is a minimum size, in retrospect, because the hood restricts airflow, even more is better if possible.  Make sure all the vents are accessible and cleanable from both directions. The manufacturers recommend it, but I would avoid flexi duct esp. fiberglass insulated duct. Also slant the beginning of the duct at the intake duct downward and outward inside so that any moisture flows outside (you can also use four very short piece of mylar flexiduct to mechanically isolate the HRV which is best hung from the rafters with cable or chain to reduce noise. Straight metal ducting with access for cleaning, with the 2 legs between the HRV and the outside short as possible and wrapped with nonfiberglass insulation I think is best. I am currently using flexiduct for some of my system and it has to be cleaned at least once a year.. the grooves catch dust- it gets dusty. Straight metal is easy to clean with a brush.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    we do ventilation regularly

    I almost never use anything but an ERV to avoid overdrying in the winter. We like the UltimateAir ERV... best stats available, and it provides MERV12 filtration while it's at is. 50 watts for 50 CFM continuous, 95% recovery, etc. Does need preheat on very cold days but that's a tiny amount of energy use. ECM, boostable to 200 CFM.

    1. we exhaust bathrooms. watch your CFMs, you want at least 50 in a bath. I've seen arguments against this. I think those arguments are dumb, at least if you are using a high efficiency unit like this.

    2. supply IN EVERY LIVING SPACE. Bedrooms, living rooms, anywhere people hang out that isn't being exhausted. Fresh air does not magically blend. Moisture will move, but pollutants may not. do not skimp on this.

    3. some passivehouses just put a duct heater in the ventilation. at 200 cfm that's over 8,000 BTU/hr.

    4. high velocity would just make it more of an energy hog. Your ducts will be 3" to 8" max anyway. no biggie.

    5. make sure those runs to the outside are insulated and DON'T RUN DUCT IN UNCONDITIONED SPACE!!!
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • SWEI
    SWEI Member Posts: 7,356

    Not sure if the their Vari-Green motor is available in their ERVs yet, but that combo would give you complete control with high efficiency.
  • Weezbo
    Weezbo Member Posts: 6,232
    Mark, *~//: )

    may i now ask you a question or two about the experience?

    Now that you have installed this do you now have a list of questions that you wish you could have known to ask ahead of time?


    Did you roll some radiant to manouver btus from high solar gain side to cool side of building in conjunction with your install by using two smaller units? and radiant zoning with constant circulation and differential pressure controls ?
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