Somewhere in your furnace room there is a second metal box — roughly the size of a carry-on suitcase, usually hanging from the joists, with two insulated ducts running out through the wall. If your house was built around Edmonton after the mid–1990s, you almost certainly have one. A remarkable number of homeowners have never opened it. It is a heat recovery ventilator, an HRV, or as most builders here call it, the air exchanger. In July it is the most misunderstood appliance in the house: people expect it to cool, dry or purify the air, and it does none of those three. Here is what it genuinely does, what it does not, and the ten-minute summer cleaning that stops it from quietly turning into a dust dispenser.

65%
minimum heat an ENERGY STAR certified HRV recovers at 0°C (NRCan)
2 mo
how often CMHC says to clean or replace HRV filters
<50%
indoor relative humidity Health Canada recommends in summer
90%
of their time Canadians spend indoors (Health Canada)

What An HRV Actually Is — And Four Things It Is Not

Modern Alberta houses are built tight. That is a good thing for heating bills in January, but a sealed house has no way to shed the moisture, cooking smells, cleaning-product fumes and carbon dioxide that four people generate every day. The HRV is the deliberate, controlled leak: a small box with two fans that continuously pushes stale indoor air outside and pulls the same volume of outdoor air in to replace it.

The clever part is the core in the middle. The two airstreams run through it in separate, sealed channels — picture a stack of thin plates with the outgoing air flowing one direction and the incoming air flowing across it at ninety degrees. Heat conducts through the plate walls from the warm stream to the cold one, but the air itself never mixes. In winter, that means −25°C outdoor air arrives in your living room already partly warmed by the 21°C air on its way out. To carry the ENERGY STAR label in Canada, a residential HRV has to recover at least 65% of that sensible heat at 0°C, and at least 60% at −25°C, tested under CAN/CSA C439–18.

That is the whole job. Which means it is worth being blunt about what your HRV is not:

Set against those four, what an HRV does do is genuinely valuable. Health Canada estimates Canadians spend about 90% of their time indoors, most of it at home. A steady supply of outdoor air is the single most effective lever on almost every indoor pollutant that does not have a dedicated filter.

Why Summer Turns The HRV Into A Humidity Problem

Here is the seasonal trap. Alberta summers are dry by national standards, but a July thunderstorm week, a stretch of hot muggy weather, or the days after a heavy rain can push outdoor humidity well past what is comfortable indoors. Because the HRV pulls in unconditioned outdoor air, whatever moisture is outside comes in with it — and if you are running the HRV on high because the basement feels stuffy, you may be actively making the basement damper.

Health Canada recommends keeping indoor relative humidity below 50% in summer, and notes that persistently high humidity above roughly 55% is usually a sign of inadequate ventilation or a moisture source you have not found. Both halves of that matter. If your indoor humidity is high because of a moisture source — a damp crawlspace, a bathroom fan that is not moving air, laundry drying indoors, a dryer venting into the basement — then ventilation genuinely helps. If it is high because it is muggy outside, more ventilation makes it worse, and a dehumidifier or your air conditioner is the right tool.

A ten-dollar hygrometer settles the argument in a day. Put one in the basement and one on the main floor, check them morning and evening for a few days, and compare against the outdoor humidity in the forecast. If indoors is consistently wetter than outdoors, ventilate. If outdoors is wetter, dial the HRV back to its minimum continuous setting and let the A/C do the drying — central air conditioning removes moisture as a side effect of cooling, which is why a properly running system makes a house feel less clammy, not just cooler.

The Filters Nobody Has Ever Cleaned

This is the part of the article that will actually change something in your house. An HRV has two filters — one on the incoming outdoor-air side, one on the outgoing house-air side. They are usually simple washable screens or thin foam panels sitting in slots just inside the door. CMHC’s guidance is to clean or replace them every two months. In practice, in a home with pets or a gravel road nearby, the house-side filter fills with lint and pet hair astonishingly fast.

What happens when they clog is not dramatic, which is exactly why it goes unnoticed for years. The fans keep spinning and the unit keeps humming, but airflow through both sides drops. Ventilation quietly falls below what the house needs, the balance between the two streams drifts, and the debris that gets past a saturated filter starts loading the core itself. Nothing beeps. The house just gets stuffier by degrees.

The cleaning itself takes ten minutes:

The Core: The Part That Actually Does The Work

Behind the filters sits the core — a block of aluminum or plastic plates, or on some units a slowly rotating wheel. It slides out on rails. This is where the heat exchange happens, and it is the component that suffers when the filters have been ignored: dust bridges across the narrow channels, the passages tighten, and both efficiency and airflow fall away.

Most manufacturers want the core vacuumed gently and washed with warm soapy water once or twice a year, and summer is the natural time to do it because the unit is not fighting a −30°C night while you have it apart. Two cautions worth taking seriously: check your owner’s manual first, because a few cores — particularly the enthalpy cores in ERVs — must never be submerged or washed at all, and never use a pressure washer or a stiff brush on aluminum plates. They bend, and a bent plate is a permanently reduced core.

If you open your unit and find the core matted grey and solid, do not be discouraged. That is the most common finding in a house where nobody knew the box was there, and it is usually recoverable with a careful wash. What is not recoverable is a core that has been repeatedly frozen and thawed with a failed defrost damper, or one that has been physically deformed — those get replaced.

Two Hoods Outside, And The Screen Mistake

Walk around the outside of your house and find the pair of hoods, usually within a metre or two of each other, often on the same wall as the furnace room. One is the fresh-air intake, one is the exhaust. They are typically 150 mm hoods with a coarse mesh or louvres.

Three things go wrong out there, and all three are visible from the ground. Cottonwood fluff, grass clippings and dust mat the intake screen until it is a felt pad. Wasps and mud daubers build in the hood. And in spring, small birds find the exhaust hood an appealing cavity — the same problem that plagues dryer vents. Clear all of it with a gloved hand and a vacuum, and check that shrubs, fences and stacked firewood are not crowding the hoods.

The mistake to avoid: do not upgrade the coarse factory mesh to fine window screen to keep bugs out. Fine mesh on a ventilation hood loads with fluff and lint in a season and chokes the airflow you installed the unit to get. If insects are a real problem, the answer is a proper insect-rated hood designed for the airflow, not screen door mesh stapled over the opening.

While you are in the furnace room, one safety note that has nothing to do with the HRV but everything to do with where you are standing. If you ever smell rotten eggs or sulphur near the furnace or water heater, do not investigate, do not flip switches, and do not go looking for the source. Leave the house and call ATCO’s 24-hour emergency line at 1-800-511-3447, or 911, from outside.

An honest note about what duct cleaning does and does not fix. If your HRV core is caked and its filters are grey, having your ducts cleaned will not fix that — they are separate systems, and a duct crew that never opens the air exchanger has not touched your ventilation problem. The two do work together, though: the HRV on most Alberta installs dumps its fresh air into the furnace return, so the ductwork is the delivery route for everything the HRV brings in. Dirty ducts distribute; a dirty HRV under-delivers. If you want the whole air path handled, ask specifically whether the HRV is included — and if a contractor tells you a duct clean will sort out your air exchanger, get a second opinion.

Wildfire Smoke: The One Stretch You Might Want It Off

Everything above assumes the outdoor air is worth bringing in. During a heavy smoke event, that assumption breaks — and this matters here, because Alberta summers now routinely include days when the outdoor air is the problem.

Health Canada’s guidance for cleaner air spaces is more nuanced than “shut it off,” and the nuance is worth understanding. Sealing a house completely is not a free win: mechanical systems need enough intake air to avoid pulling the house into negative pressure, which drags outdoor air in through every gap you cannot control instead of through the filter you can. The recommended approach is to alternate — reduce or recirculate during the worst of the smoke, then ventilate during the clearer stretches to flush out what has accumulated indoors — and to lean on high-efficiency filtration in the furnace and on portable HEPA units in the rooms you use most.

In practical terms for a home HRV: on a heavy smoke day, turn the unit down to its lowest setting or off for a period rather than running it on high, keep windows closed, run your furnace fan continuously with a good pleated filter, and put a portable HEPA cleaner in the bedroom. When the smoke clears — and Edmonton’s smoke often lifts overnight — put the HRV back on. Leaving it off for the whole summer is not the answer either; you will simply trade smoke for stale.

When To Stop And Call Someone

Filters, hoods and a core wash are homeowner territory. A few things are not. Call an HVAC technician if the unit is silent with power on, if it trips its breaker, if you find standing water in the cabinet after clearing the drain, if the defrost cycle runs constantly or the unit ices up, or if the balancing dampers have never been set — a badly unbalanced HRV can pressurize or depressurize a house enough to affect how other combustion appliances vent. If your HRV has no controller anywhere in the house, or the wall control is a mystery dial nobody has ever moved, ask to have it commissioned and labelled. It is a short visit and it makes the difference between owning a ventilation system and owning a box that hums.

And if you open the door, take one look, and decide this is not your kind of job — that is a completely reasonable call too. The important part was knowing the box exists and that something inside it needs cleaning.

HRVs In Summer: Quick Answers

Should I run my HRV in the summer?

Usually yes, but on a low or intermittent setting rather than high. An HRV does not cool or dry the air — it brings in unconditioned outdoor air — so running it hard on a hot, muggy day makes the house warmer and damper, not fresher. Run it enough to control indoor moisture, cooking smells and stuffiness, and turn it down when the outdoor air is more humid than the indoor air. The exceptions where you should reduce it further are heavy wildfire smoke days.

How often should HRV filters be cleaned?

CMHC recommends cleaning or replacing HRV filters every two months, and the heat-exchange core once or twice a year according to the owner’s manual. Homes with pets, nearby gravel roads, or ongoing construction should check more often. Most HRV filters are washable: vacuum them, wash with warm water and mild soap, and let them dry fully before reinstalling.

Will my HRV cool the house or lower the humidity?

No to both. The core transfers heat between the outgoing and incoming airstreams; it does not remove heat from the house, so it cannot cool. And a standard HRV transfers heat but not moisture, so if it is humid outside, humid air comes in. An ERV moves some moisture between the streams, which moderates the effect, but it is still not a dehumidifier. For summer humidity, the tools are a dehumidifier, your air conditioner, and finding the moisture source.

Is an HRV the same thing as an air exchanger?

In everyday Edmonton usage, yes — “air exchanger” is what most builders and homeowners call the HRV. Technically the family includes both HRVs, which recover heat only, and ERVs, which also transfer some moisture between the airstreams. Look at the label on the box: the model number and manual will tell you which one you have, and that determines whether the core can be washed.

Should I turn the HRV off during wildfire smoke?

Turning it down or off during the worst of a smoke event is reasonable, but do not simply seal the house and leave it that way. Health Canada’s cleaner-air-space guidance recommends alternating — minimize outdoor air during heavy smoke, then ventilate when the air improves — while relying on good furnace filtration and portable HEPA cleaners indoors. Starving a house of intake air can pull it into negative pressure, which draws unfiltered outdoor air in through gaps instead.

Does duct cleaning include the HRV?

Not automatically, and it is worth asking directly. Duct cleaning addresses the supply and return ductwork, the furnace and the blower. The HRV is a separate appliance with its own filters, core and drain, and cleaning ducts does nothing for a clogged core. On most Alberta installs the HRV feeds into the furnace return, so the two systems are connected — but connected is not the same as cleaned. Ask whether the air exchanger is included in the scope before you book.

Verified Sources

Trusted Sources

CMHC — How To Maintain A Heat Recovery Ventilator

Canada Mortgage and Housing Corporation’s maintenance guidance: clean or replace filters every two months, keep outdoor hoods clear, and follow the manual for the core.

Read the page

Natural Resources Canada — ENERGY STAR Specs For HRVs & ERVs

The federal efficiency thresholds: at least 65% sensible heat recovery at 0°C and 60% at −25°C, tested to CAN/CSA C439–18.

Read the page

Health Canada — Relative Humidity Indoors Factsheet

The official indoor humidity targets: below 50% in summer, and why sustained high humidity points to inadequate ventilation or a moisture source.

Read the page

Health Canada — Cleaner Air Spaces During Wildfire Smoke Events

Federal guidance on ventilation, filtration and outdoor-air intakes when the air outside is the problem.

Read the page

Links open official Canadian government housing, energy and health pages in a new tab. Home Pros Group isn’t affiliated with these organizations — we cite them so you can verify the facts yourself.

Fresh Air Only Helps If It Can Actually Get Through

Home Pros Group has cleaned furnaces, ducts, blowers, A/C coils and dryer vents across Spruce Grove, Stony Plain and the Greater Edmonton Area since 2002. If you have opened the air exchanger and would rather someone else deal with what you found — or you want the whole air path looked at before the fall — the quote is free.

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This article is general home-maintenance information, not a service manual for your specific equipment. Always check your HRV’s owner’s manual before washing a core, and cut power before opening the cabinet. Balancing, electrical work and combustion-appliance venting must be handled by qualified professionals. If you smell natural gas, leave the building and call ATCO at 1-800-511-3447 or 911 from outside.