Mechanicals-Heat Recovery Ventilators

I’ve had several discussions on this blog about the importance of a tight home, “built it tight and ventilate it right”.  A tight home with its reduced natural ventilation will have two problems, the first is poor indoor air quality and the second is elevated humidity levels.  Replacing natural air leaks with mechanical ventilation will gain control of the amount of outside air entering the home along with where that air is coming from.  The “ventilate it right” is a good thing!  Mechanical ventilation can be accomplished in a few ways.  I will be discussing a couple of the options in this blog posting.

Minnesota’s adoption of the 2012 IRC codes (the code currently in force) includes a change by the state to incorporate their own language regarding ventilation, the change requires all new residential construction to include balanced mechanical ventilation.  The code requirement reads:

          R403.5 Mechanical Ventilation (mandatory).  The building shall be provided with a balanced mechanical ventilation system that is +/- 10 percent of the system’s design capacity and meets the requirements of section R403.5.5, which establishes the continuous and total mechanical ventilation requirements for dwelling unit ventilation.  All conditioned unfinished basements, conditioned crawl spaces and conditioned levels shall be provided with at minimum ventilation rate of 0.02 cfm per square foot or a minimum of 1 supply duct and 1 return duct.  The supply and return ducts shall be separated by ½ the diagonal dimension of the basement to avoid a short circuit of the air circulation.  Outdoor air intakes and exhausts shall have automatic or gravity dampers that close when the ventilation system is not operating.

Some of Minnesota’s code language can be confusing.  What if I don’t have a basement, can I then put my supply and return duct right next to each other?  Never been a fan of code language…  The simplest way to satisfy the code requirement would be to take two bath fans sized to the code requirements, have one intake fresh air and the second exhaust stale air.  The problem with this set-up is the fresh air intake will be the temperature of the outside.  If the outdoor temperature is -20 degrees, that is the temperature of the incoming air.  A better way is to utilize a mechanical ventilation unit.  Two common system are Energy Recovery Ventilation (ERV) and Heat Recovery Ventilation (HRV).  They both are similar in how they exchange air in a home, but the HRV is the unit most often installed in a northern climate.

A heat recovery ventilator. The two upper insulated ducts are fresh air from the outside and stale air to the outside. The lower left duct is fresh air to the home and the lower right duct is stale air from the home.

An HRV system will include an outside fresh air duct, one or more fresh air supply duct to the home, one or more stale duct removing air from the living spaces and an exhaust duct to the outdoors.  The system will also include the HRV air handler and some sort of wall unit or push buttons to control the unit.
HRV’s are designed to transfer heat exhausted to the outside with the incoming fresh air.  This transfer happens without any of the air intermingling, accomplished with a central core called an air to air heat exchanger.  The efficiency of the heat transfer will depend the core’s configuration, or how the air moves through the core, typically ranging from 60% to 85%.  The difference in temperature between inside and outside air, called a delta T, will also affect the incoming air temperature.

Inside an HRV unit. The white center piece is the heat exchange core.

In a northern climate, the intake and exhaust ducts that lead to the outside need to be insulated, and sometimes the supply air duct to the home will also include some insulation.  At low outdoor temperatures, the ducts will act as dehumidifiers and form condensation or frost on the duct piping. Proper R-value insulation and a good air sealing strategy for the ducts will eliminate the condensation issue.  There will also be frosting inside the HRV air handling unit.  All HRV’s designed for cold weather should include a defrost cycle.  This melting of frost causes water to accumulate within the unit, which must be allowed to drain out of the system. Some sort of connection to the home’s plumbing system will be required.

Proper placement of the supply and exhaust vents within the home is also important.  The supply air can be ducted directly into the home or be installed in a forced air ducting system.  If available, I prefer the latter.  Even thought he outdoor air has been tempered, the temperatures may be uncomfortable if allowed to blow directly on a person.  Allowing the air to be mixed with the air within a forced air duct system will further increase the fresh air temperatures.  If the home does not have a ducted forced air system, the fresh air will have to be ducted directly into the home. Giving some thought to the locations of these supply ducts will help to reduce comfort complaints from drafts.  Dumping cool air directly into a bedroom or above where a person may sit in a living room will make a homeowner unhappy.

The stale air ducts within the home are easier to place.  Bathrooms and kitchens (away from food frying areas) are great places to remove air from the home.  As a matter of fact, if installed correctly, a bathroom fan may be eliminated.  I recommend using an HRV unit to replace bath fans in tight homes with an inside wood burning appliance.  This type of installation will include some type of timer switch in the bathroom to increase the HRV fan speed.  I have seen woodstoves backdraft when a bath fan was started (because this type of fan only exhausts and is not a balanced system).  The balanced ventilation of an HRV will to eliminate this issue.

The outside fresh air supply duct will need to be placed away from other exhaust ducts and any exterior gas valves.  You don’t want the fresh air entering the home to contain the exhaust of a fossil fuel burning appliance or a venting gas regulator.

In short, follow the installation directions from the manufacturer of the HRV unit.

Sizing the Heat Recovery Ventilator is also important and required by the state of Minnesota.  The state specifies that the exchange rate shall be a minimum of .02 cfm per square foot.  The state has developed a formula for calculating ventilation, again, this is a Minnesota code;

Total ventilation (cfm) = (.02 x square feet of conditioned space) + (15 x (number of bedrooms + 1))

A 2000 square foot home with three people and three bedrooms will need 100 cfm of fresh air.  The system must achieve 100 cfm +/- 10% of the design. Natural air leaks are also allowed to be calculated into sizing the system.  The only way to verify these leaks is with a blower door test.  If the example home is leaking 40 cfm naturally, an additional 60 cfm of mechanical ventilation will be required.  The problem with using natural air leakage rates in the calculation is the air leakage will only be known after the home is complete and the HRV unit is installed.

The system will need to be commissioned after installation to verify the cfm rate.  Commissioning the system should also include balancing or assuring the incoming and outgoing air cfm’s equal one another.  Depressurizing a home with an HRV may allow soil gasses such as radon to enter the home. Pressurizing a home may force moist, conditioned air into to wall and roof assemblies.

One of the arguments I hear from people against mechanical ventilation is the electricity cost to operate the unit.  Their argument is the ventilation can be accomplished naturally by allowing a home to “breathe”.  First, the average electrical consumption for an HRV unit is around 100 watts.  I have tested many!  Electrical costs for an HRV at $.12 per kWh will be $8.64 per month.  There will also be an energy penalty for heating the exchanged air that is moving through the HRV.  The 60-85% heat transfer will still require some additional heat to bring it back to room temperature.  This is still much cheaper than relying on natural air changes for fresh air.  The windier it is outside, the more air moving through the home, the colder it is outside, the stronger the stack affect, more air is moving through the home.  Build it tight and ventilate it right!  Its all about control.

My suggestion for operating an HRV system in a northern climate is to operate the system from October through April.  These are the heating months when we need air exchanges the most.  Summer use will depend on how you live in your home.  Do you run an air conditioner and never open your windows?  Keep the HRV operating.  If your opening windows, at least occasionally,  you are accomplishing what the HRV unit is doing.  Turn it off.

Another recommendation is to verify your humidity level in several places around the home by using a hygrometer or home weather station.  I recommend 30% during the winter for our climate.  Never allow a home to be higher than 40% during the heating season.  Most wall controls for HRV’s will sense the humidity level and adjust the unit to maintain a humidity setting.

The last item I would like to discuss is maintaining the unit.  Heat Recovery Ventilators do not require a lot of maintainance but I often see units that have never been maintained.  Cleaning the filters and the inside of the HRV cabinet annually along with clearing debris from the outdoor intake vent is the most important maintenance.  Be sure to read the owner’s manual for any additional maintenance requirements.

HRV fresh air intake. This 8 year old unit has never been maintained.
A filter for an HRV unit, the filter has not been cleaned on this 8 year old heat recovery ventilator.

In my opinion, the mechanical ventilation requirement for new residential construction, which has been around for at least 15 years, is the reason Minnesota homes have few problems with interior moisture.  Most builders in my area are still using polyethylene sheeting as the main air/vapor barrier.  Without mechanical ventilation, many new homes would have moisture related failures and mold issues.

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