A breath of fresh air

Your green certification program (Energy Star, Greenbuilt NC, LEED Homes) requires a whole house ventilation system.  What does that mean?  What are your choices, and which one is best for you?

There are three common strategies for whole house ventilation – I’ll offer a short description of each below and try to summarize the pros and cons that I most often discuss with my clients. While I would love to see everyone install a balanced system with energy recovery, I understand that people have budgets and it isn’t always feasible – and in many cases another system can be a very good option.   If you’re not located in Western NC, please be aware that some of my comments (especially on energy use and humidity) are based on local climate, and I might have a different opinion in a hotter/colder/wetter/drier location.

1) Balanced system with heat recovery: This is a device that has its own fan that brings in fresh outside air and exhausts some air from the home. It can be tied into the home’s HVAC ductwork, but it doesn’t have to be. An ERV (energy recovery ventilator) or HRV (heat recovery ventilator) will each use about the same amount of energy in WNC.  The ERV is a slightly better choice if you’re most concerned with reducing indoor humidity in the summer, but it’s not a dehumidifier.  The HRV is usually better for homes without air conditioning or when winter moisture problems are the bigger risk – usually super-tight homes (<1 ACH50) or passive solar.   If you’re picking up your exhaust air from bathrooms then you don’t want to recapture that humidity, so that’s another reason to go with the HRV. PROS:

• Pressure-balanced system. This is mainly a concern for super-tight homes.
• You know where the air is coming from (because you choose the location of the outside air intake).
• Energy recovery recaptures some energy from the outgoing air.
• The ERV/HRV fan is smaller than the air handler (usually about 100W, but some new models are as low as 40 W).
• Can work with non-ducted systems (like radiant floor).
• Versions available with high-efficiency filters.

CONS:

• Most expensive option – probably $1500-$2000 installed.
• Some less expensive versions don’t have timers or speed controls and may give you more air than you want, adding to energy use.
• The smaller fan means it’s not an air handler – you can’t put too much ductwork on it.

2) Supply air only – This system usually involves a small outside air intake duct that connects to the return side of the air handling unit. To qualify as a ventilation system, there MUST be an automatic damper in the duct and a controller. The controller is set to operate for a specified number of minutes each hour, and will cycle the air handler fan in mild weather to get enough run time. If the fan runs longer than it needs to in extreme weather the damper will close to avoid over-ventilation. The “air cycler” and a similar controller by Honeywell are the most commonly installed versions of this system locally.

Local note:  a few years ago many systems were installed locally with no controller.  These don’t meet ASHRAE Standard 62.2 for indoor air quality because they are uncontrolled and result over-ventilation in very cold or hot weather and under-ventilation when the weather is mild.  So, not only is the ventilation unreliable, but they are very wasteful of energy.  This approach is no longer allowed by Energy Star, Greenbuilt NC or LEED for Homes.

PROS:

• You know where the air is coming from (because you choose the location of the outside air intake).
• Intake air passes through the filter in the air handling unit, which could (should) be high-efficiency. It’s also a very good idea to have at least a coarse filter right at the intake duct.
• Since it uses the air handling fan, this also mixes air in the home to dilute pollutants. This can reduce occupant exposure to pollutants emitted in frequently occupied rooms
• Less expensive to install (~$300-400 typically).
• Puts house under positive pressure. Some people feel that this is better than negative pressure, but the pressure is fairly small, and it’s not clear to me that this is improving the durability of homes here.
• Can be used with a whole-house dehumidifier in humid climates.

CONS:

• No energy is recovered from the outgoing air.
• Air handling fans can use a lot of power (~300-400W), so it doesn’t need to cycle for much extra time before a lot of energy is used. There is little additional cycling in many climates, but Asheville has extended periods of mild weather. In energy models, this system uses the most overall energy of the three options.
• Cannot be easily be used with non-ducted HVAC systems.

3) Exhaust only – This system involves using an exhaust fan that either runs continuously or on a timer to exhaust air from the home. There are bath fans available that do “double duty” as this type of fan – they run on a low, continuous setting all the time and then kick up to the normal flow rate when in use as a bath fan. The fans have to be super quiet (less than one sone). Specialty fans sold for this purpose include the Panasonic Whisper Green and the Broan Ultra. There is also a switch called the “Smart exhaust” that will convert any low-sone bath fan to this function.

PROS:

• Does not require the home to have a ducted HVAC system.
• Very low installation cost. For a smaller home using a single fan, this adds about $50-$100 to the cost of construction.
• Overall operating cost in Western NC is relatively low. Although there is no heat recovery, the fan wattage is typically very low (10-20 Watts), so the overall energy consumption is almost as good as most ERVs.
• Usually involves a bath fan upgrade, which increases the likelihood that the bath fan will function well as a bath fan and remove moisture from the home. This system is actually one of the best at reducing winter humidity problems since it removes more moisture from the bathroom.

CONS:

• Incoming air is not filtered (except through your building envelope).
• Puts house under negative pressure. Some people don’t want to do this, but the pressure is fairly small (unless your home is ultra-tight), and most building envelopes will experience similar negative pressures due to wind and other forces. There are limits to how much air you can move at a time with this system in hot/humid climates, but it’s usually possible to comply.  Asheville is not considered a hot/humid climate (it’s mixed/humid).
• No energy recovery.
• You can’t control where the air is coming from, so it is important to seal the house well from garages, crawlspaces, etc. A house with a  problem crawlspace might not be a good candidate for this system.
• The system does not mix the indoor air.

These are the major system types.  Some other specialty approaches and different flavors exist, but these options account for 99% of the systems being installed locally.

So what system do I have in my own house?  We actually have a Venmar Eko ERV.  This is a balanced system with energy recovery that has a very low Wattage fan.  In the winter, we switch out the energy recovery core with the one from the Venmar Eko HRV.  This gives us better energy performance in the winter and helps dry the house out a bit more.  Our house is both super tight and passive solar, so controlling winter humidity is something we need to pay a little more attention to than the average family.

Copyright 2013, Amy Musser