Tag Archives: HRV

How do I keep dust, smoke and other particulates out of the house?

A house should manage indoor air quality by regularly exchanging stale “used” indoor air with fresh outdoor air. You also can improve indoor air quality by avoiding unnecessary sources of contamination, such as restricting smoking to outdoors, storing fuels outside, and selecting low-VOC paints and furnishings. During the year, the air in the Interior can contain particulates from wildfires, wood smoke, dust, pollen, car exhaust and other sources that cause you to shut the windows. That’s where filtration systems can help.

Air filtration options

When it comes to indoor air filtration, the best choice for you depends on many factors, including the size and tightness of your house, your existing ventilation system, your sensitivity, and the amount of particulates and other contaminants in the air.  Be aware that irritating and harmful particulates don’t just come from outside but also inside — sources like tobacco smoke, animal dander and mold spores. Other contaminants include gases in paints, carpets, cleaners and other household products. The most common filtration systems are mechanical and target particulate matter. Prices range from about $200-$300 for a one-room portable filter to $6,000-$8,000 for a heat recovery ventilator installation with filtration.

Standalone filtration

The simplest system is a standalone air purifier, which contains a fan and filter elements all in one unit and can be plugged into the wall. These systems are designed to be portable and recirculate air in a single space, and will reduce pollutants like allergens, pet dander and dust from that space. These work well in homes where air quality problems are isolated to one or two areas.

Multiple room air cleaners

Air filtration systems that can serve multiple rooms or even the whole house typically cost more and will require an in-line fan and ductwork, but tend to be more effective.

Keep in mind that whether large or small, filtration systems by themselves don’t introduce fresh outdoor air, but they can provide air cleaning and heat distribution. Whole house systems may be a good option for those with bad allergies or respiratory problems.

Many homeowners who heat primarily with wood install small circulation systems, with an in-line fan and ductwork in just a few rooms to move heat around the house, said Richard Musick, of Ventilation Solutions LLC. The size of the fan is based on how much air you want to circulate.

“If it’s only a couple of rooms, you can get away with a 200 cfm (cubic feet per minute) fan. Big houses can require up to 900-1,500 cfm,” Musick said.

Heat recovery ventilator filtration

While new HRV systems often have high levels of built-in filtration, older models are generally only equipped with coarse debris filters whose primary purpose is to keep the core and motors clean. To help ensure good air quality, a simple filtration system can be attached separately in line with the warm-side supply port on the HRV. All the HRVs at CCHRC have a prefilter to catch the big particles, a main particle filter to catch small particles, and a carbon filter to remove odors, aerosols and VOCs. These filters can be found at HVAC and hardware stores, and are inexpensive and easy to replace. Note that the carbon filters typically need to be replaced more frequently than other air filters.


Filtration systems are measured by a MERV rating — or minimum efficiency reporting value — which goes from 1 (traps bigger particles) to 20 (traps the smallest particles). You pick a MERV rating based on what you’re trying to filter. For example, MERV 1-4 will take care of pollen, dust mites, and most animal dander, while you’ll need at least MERV 13-16 to filter out smoke particles. HEPA (high efficiency particulate arresting) is in the 17-20 range, removing more than 99 percent of tiny particulates such as carbon dust from the air.

Typically MERV 15 represents the upper limit for residential HRV systems as anything finer may restrict too much airflow. The EPA Office of Radiation and Indoor Air notes that filters with MERV ratings between 7 and 13 are capable of reducing unhealthy particulate matter almost as well as HEPA filters. Additionally, activated carbon filters can be used to neutralize smoke and VOCs.

House tightness

Homes built today are more energy efficient with better insulation and higher levels of air tightness than many of the homes built in previous decades. Building codes now require mechanical ventilation systems for all new residential construction in most if not all northern states. This is simply because uncontrolled air leakage can no longer be counted on to provide the fresh air needed to keep a home healthy. Generally speaking, the highest performing ventilation systems available today will include balanced and regulated fresh air exchanges, in combination with air filtration.

No matter what system you get, check to see what type of replacement filters are required.  Some models may use proprietary filters that are more expensive to replace or have more limited filtration capacity.

How does the recirculation mode on an HRV work, and is it safe in a cold climate?

We often stress proper ventilation as the key to maintaining a healthy indoor environment in a home, and promote heat recovery ventilators (or HRVs) as the best option for energy efficient ventilation in a cold climate.

HRVs exchange stale indoor air with fresh outdoor air, capturing heat from the outgoing air to pre-heat incoming air. They exhaust excess humidity, carbon dioxide, and indoor pollutants from pet dander, cleaning supplies, offgassing furniture, and other sources. The role of the HRV becomes increasingly important as homes are built tighter to save energy, which cuts down on passive air exchange.

To maximize the benefits of having an HRV, it helps to understand the different operation modes. One of the often-debated modes included in most HRVs in the United States is the recirculation mode. This mode is not often used in Europe because it is believed that the health risks outweigh the energy benefits. This article provides a description of the recirculation mode and gives pros and cons for the house and its occupants.

Under normal operation, the HRV replaces moist indoor air with fresh outdoor air. While HRVs recover much of the energy from the heated air during winter months, a considerable amount of heat is still lost due to the frigid temperatures in the Interior Alaska. In addition, extremely cold outdoor air contains virtually no moisture, which can result in very low humidity levels indoors—a negative for some homeowners.

In recirculation mode, the unit closes the connection to the outside and brings the exhaust air back into the rooms. This saves a lot of energy, since there is no cold air coming in from outside. On the other hand, moisture and indoor pollutants are no longer being flushed out of the home, and their concentration will continue to rise and can eventually reach harmful levels. Recirculation can also spread unwanted smells from more to less polluted areas, such as from the bathroom to the living room.

In order to maintain sufficient air exchange, HRVs offer modes where these two strategies can be combined. For example, 20/40, 30/30, or Smart Mode. In 20/40, the HRV will bring in fresh air for 20 minutes and then recirculate for 40 minutes (likewise for 30/30). Smart modes usually require some kind of sensor (humidity or carbon dioxide) to dictate when to ventilate and when to recirculate, based on which measurements the HRV controller decides is more relevant at any given time.


The major advantage of recirculation mode is that it saves energy and redistributes heat throughout the house, particularly helpful if you have a localized heat source like a woodstove. On the flip side, it can potentially transfer pollution from one room to another rather than expelling it altogether. While Smart Mode seeks a happy medium between the two, there are still times when recirculation mode should not be used at all—if someone is cooking, smoking, or during times of high occupancy. One way to override the Smart Mode during these situations is with a push-button timer, a common feature of HRV installations that temporarily ventilates the HRV during such events.

If you do use recirculation mode, here are some best practices to maintain good air quality:

–High quality filters (High Efficiency Particulate Filters, HEPA, in combination with activated carbon filters) should be added to supply duct to mitigates odor or pollution from spreading

— Constant recirculation should only be used when the building is unoccupied

–If recirculation is used during occupied periods, make sure the HRV is exchanging indoor and outdoor air for at least part of every hour

While recirculation offers the perk of saving energy, if you rely on it too much, you can undermine the benefit of having an HRV—to maintain indoor air quality that is healthy for both humans and structures.

Winter Punch List

The punch list arsenal

Winter is looming, but there is still time to attack a fall home maintenance punch list before it gets too cold.  In addition to a boiler tune-up and chimney inspections, there are a few more details worth considering.

Now is the best time to make sure your roof system is in good working order.  It’s not too late to replace shingles, add snow stops, patch leaks, or replace any missing fasteners on metal roofs.  Now that the leaves are down, the gutters are ready for inspection. In the spring, gutters clogged with frozen debris are virtually impossible to clean and can cause melt water to overflow and run down next to the foundation and into the basement or crawlspace. Check the downspouts too. They should be unobstructed, firmly attached, and pointed to direct water away from the house.  While you are walking around the house, check the grade for drainage.  Surface soils will remain unfrozen for a little longer so it’s not too late to do any last minute dirt work to ensure spring run-off is directed away from the house. Inspect any heat traces to make sure they are in good (and safe) working order.  Replacing a heat trace now is a lot less hassle than a frozen waste or supply line in the winter.  If you have any concerns about the safety of a heat trace – particularly if it’s older—consult a professional.  A malfunctioning heat trace can be a fire hazard.

If you have an HRV system, make sure all parts of the system are in good working order.  Alaskans tend to spend a lot more time indoors during the cold periods, and good ventilation is critical.  An inspection should include a look at the supply and exhaust grilles on the outside of the house – not just those belonging to the HRV, but also dryers, range vents, and bathroom fans, especially if they are close to the ground where the intake can become clogged with leaves, grass, or other debris.  If an exhaust damper is present, make sure it is operating smoothly.  Open up the HRV and examine both the filters and the core. The cores can be removed and washed out if they’re dirty. The condensate drain and drain line under the HRV should be free of obstructions; if a trap is present, it should contain water. A properly installed HRV is designed to bring in and exhaust the same amount of air.  The system should provide enough fresh air to ensure occupant health and control humidity but not over-ventilate, as excess air flow is simply wasted energy.  If you’ve never had your system professionally balanced and inspected, now is the time.

If you’re planning any air sealing with spray foam, the cut-off temperature for most expanding foams is above freezing, although a few brands may go lower. Last but certainly not least, make sure your home has operating smoke alarms and at least one operating carbon monoxide detector.

A greener home

The first of a two-part series, “A Greener Future”, from the LA Times, September 14, 2008:

Innovations in designing green chemicals are emerging in nearly every U.S. industry, from plastics and pesticides to toys and nail polish. Some manufacturers of cosmetics, household cleaners and other consumer products are leading the charge, while others are lagging behind.

For decades, many manufacturers used the most powerful weapons in their chemical arsenals, with scant attention to where they wound up or what they might have been doing to people or the planet.

Click here to read the whole article.

An HRV System Overview

BY: Ilya Benesch, Cold Climate Housing Research Center
Energy Focus: Fairbanks Daily News-Miner September 11th, 2008, Section A3

Heat Recovery Ventilation (HRV) systems are a relative newcomer to the cold climate construction scene, yet have become almost indispensible in today’s super-insulated, air tight homes. They are also becoming an increasingly common element in the current weatherization and insulation retrofitting trend. As older homes are undergoing energy facelifts, and becoming tighter and better insulated, they are also facing the same indoor air quality challenges one would see in new construction. In this article I am hoping to provide a basic understanding of how HRV’s work, their applications, and their advantages.
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The HRV Myth

BY: Thorsten Chlupp, REINA Properties Corporation an Energy Star & GREEN Builder
Fairbanks Daily News-Miner 04/16/08 Section A3

One of the main goals in building a high energy efficient home in our extreme climate is to seal the inside of the home as tight as possible. This eliminates any heat loss through air leaks and saves real money on the heating bill.

But of course that tightness comes with a considerable trade off. If no heat can escape the home then no fresh air can infiltrate into the home, and the tighter the home the more this becomes an issue. This is a very important fact which needs to be considered carefully—as we all need to breathe. Lots of fresh air, at all times.

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