Sunday, June 17, 2012

Heat Recovery Ventilator (HRV)

When you build a tight home in a cold climate it is recommended that you have some way for the house to allow fresh clean air into the house so that moisture and pollution do not build up in your house.  This could easily and cheaply be done by opening a window for a couple of minutes during the winter to let fresh air in.  The obvious problem is that the incoming air will be very cold and you will have to use more energy to heat that cold air.

This can be solved by a heat recovery ventilator. (HRV).  The basic idea is that the HRV unit has two fans in  it, one that pulls stale air out of your house and one that pushes fresh air into your house.  The process is improved by the use of a heat exchanger core.  As the warm air is sucked out of your house it goes through the core at the same time as the cold air from outside is travelling through the core.  The two streams never come in contact but they travel through little pathways that have a small amount of material between the two streams that allows heat from the warm air to warm the incoming cold air.  These systems claim up to 84% efficiency.  Meaning there is only a 16% loss in heat to the new air entering verses the exiting air.

Our HRV is made by Bryant.

So on the surface it appears to be a great idea but there are a few things that you need to watch for or it is only a good idea.

The first is cost.  Our system cost around $5,000 dollars professionally installed.  This included the HRV unit as well as all the ducting.  Since we do not use a forced air system our ducting is only for the HRV.  Ours draws air out of the house from all three bathrooms and the kitchen.  It returns fresh air into all three bedrooms and the living room.  We use our HRV in place of bathroom fans.  When we take a shower we can push a button on the wall that runs the HRV for 15 minutes to help remove moisture out of the bathroom.  When you push the button and turn the system on it pulls air through all the bathrooms and not just the one where you pushed the button.  The "suction" of an HRV vent is not as strong as a good bathroom fan but it is quieter.

The second issue is the wall controller.  I mentioned the 15 minute bathroom wall timer, this is a great idea and we use it all the time.  You will have one main wall control unit that will give you more options.  Ours allows us to run the HRV on high speed or low speed, run automatically based on the humidity in the house or to recirculate air within the house without bringing in outside air.  All of which are nice features.  The main problem with our control unit is the humidity sensor.  It is so inaccurate as to be useless.  I would not recommend buying an HRV from bryant because their humidity sensors are junk.  I went through three different units looking for one that worked and none of them did.  Even the installers used their high tech humidity sensor to determine the humidity was 12 %  but the controller activated at 55%.  The sensor is made of a nylon coil that expands and contracts at different humidities.  I would only recommend digital sensors they are more accurate and more sensitive.  If the controller did work I could set it to 45% and if people were cooking and taking showers then the HRV would come on and stay on until it removed enough moisture from the house to get below 45%.  If our worked like that it would be great.

After further research I would look at going with a Venmar HRV.  I have never used one but their controllers look to be much better designed.

The third issue is easily solvable but you need to mount the control unit with a humidity sensor close to where most of your humidity will be.  In our house all the showers and laundry room are upstairs but the controller is downstairs.  Most days the upstairs feels much more humid than the downstairs but the sensor (if it worked) would not be responsive to the moisture upstairs.  So figure out where most of your moisture will be and wire the controller close to that location.

The fourth issue is the length of time that the unit must run to bring the humidity levels down.  It takes hours to see a change in the humidity levels in your house.  This also depends on the humidity of the incoming outside air.  The amount of electricity required to run this fan for hours just does not make sense verses opening a window.  When we take a shower in the winter we crack the window because it is a much faster way to get the humidity out of the house.  If the humidity gets too high in the house during winter and the wind is blowing I just open the windows for five minutes and the humidity drops rapidly.  I have an indoor digital humidity sensor so I can watch it drop by the minute.  Yes the incoming air is cold but after closing the windows the house warms right back up again due to the heat trapped in the objects of the house.

Overall the idea is good and the device works like it should.