Heat recovery ventilation (HRV), is an air-to-air heat exchanger which employs a cross flow or counter-flow heat exchanger (countercurrent heat exchange) between the inbound and outbound air flow. HRV provides fresh air and improved climate control, while also saving energy by reducing heating requirements for incoming fresh air for the home.
Not to be confused with Energy recovery ventilators (ERVs), which are closely related, however ERVs also transfer the humidity level of the exhaust air to the intake air. These are not used in our colder Canadian climate, but found mostly in southern States.
Back in the day, if you were fortunate to have a new gas forced air furnace installed in your home, heating was as easy as turning up the dial on the thermostat for a cozy home. As years passed on, our heating costs climbed higher and higher, forcing us to look for ways to make our home more efficient. To some it was as simple as putting on a bigger sweater, to others it meant investing in plastic window coverings that needed to be installed with the hair dryer each year and a box of silicone for all the cracks around the home. Every year there are incentives to make our older homes more efficient and people installing new windows , doors, and insulation in the attic. With the homes prior to the 70’s, drafts were responsible for up to 40% of the heating bill! When they introduced the R2000 Home back in the 80’s, our heating bills dropped, but so did our homes quality of air. So much so that it caused lower oxygen levels in the home. Excess humidity creates a perfect environment for black mold, mildew, fungi, bacteria, dust mites, which in turn, cause tons of other symptoms and allergic reactions. Because the outside air was no longer so easily able to infiltrate through the cracks, radon gases are also more likely to seep in from the ground. Basically, the lack of fresh air makes for a habitable environment for many things, just not us.
Why did all that fresh air want to come in before? Because we ventilated. We ventilated to remove the undesirables such as smells, too much humidity on the mirror after a shower or on the living room windows from boiling vegetables on the stove. When you install a fan to remove air from a box (your home), unless air can come in to replace that air you’re removing, you will have a ‘negative pressure’ in that box. A vacuum like effect that will suck in any air to equalize those pressures. So if your home has exhausting appliances which exhaust deadly things like CO (carbon monoxide), it will want to come in to a negative pressure zone instead of a chimney as well, creating not only an undesirable situation into a deadly one. The trick is to harness that ‘effect’ and have it happen in a controlled environment, like an HRV(Heat recovery ventilator). A Lennox Pure Air filter installed in a forced-air heating system will help eliminate airborne contaminants, but not with moisture, stale air or gaseous pollutants. Opening a window is a cheap way to introduce fresh air, but we were better with a loosely constructed home where it came in evenly throughout. This all started because we wanted to be more efficient.
At the bare minimum, according to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), has set our standards for residential ventilation at a minimum of .35 air changes per hour, and not less than 15 cubic feet per minute (cfm) per person. Installing an HRV in your home provides a controlled highway for all of the undesirables to leave, and all of the fresh air to come in, while reclaiming as much of the one quality we do not want to ventilate… heat. Using two fans, one for outgoing air and one for incoming, we can balance the homes pressure and blowing the warm exhaust air through a heat exchanger which is never in direct contact with the incoming cold air, the air streams never mix but rather transfer heat to the incoming air using the highly conductive walls of the heat exchanger.
This is the ‘piece de resistance’ of an HRV. Some cheaper models use less conductive material so it’s something to watch out for when making your decision. But mostly, how it gets installed is what you need to be aware (beware) of. It’s like that for all HVAC equipment really. Statistically, 75% of new HVAC equipment is installed incorrectly to some degree, and when you’re paying for efficiency and a job well done, it’s more than just equipment specifications you need to keep watch for. Currently in Manitoba, there is no permit to pull for HRV installations. Which means no one is legislating the quality of the job. HRV’s are installed and left unbalanced all too often. Which sometimes leaves homes in a worse way, than it was without an HRV.
HRVs can recapture up to 85 % of the heat in the outgoing air stream, depending on the model you go with as some have a ‘Dual Core’ heat exchanger. This means the ventilated air is run through two different heat exchangers before it is dismissed. More of its heat can be recovered this way, making these ventilators a lot easier on your budget than opening a few windows. The heating bill still goes up slightly to pay for replacing the heat that isn’t recovered. An average HRV installation can run from $2000 to $2600, but costs obviously vary depending on many variables which may arise. You can get them in many different sizes and capacities which can serve many different situations. Square footage of your home as well as how many occupants, are variables which would affect the sizing. Two holes must be drilled through the exterior walls of the home, usually right through the foundation walls, ideally 6 ft. apart on the same wall. This can be tricky in some applications but I don’t remember an HRV I couldn’t make work in some way. Many times we will try and utilize an existing vent hole. Usually the holes are 6 inches in diameter each, but this too is dependent on the size of HRV being installed.
HRV’s are designed to remove that unwanted humidity in our homes, so it stands to reason a ‘humidistat’ is used to control them. Because they are designed to run continually on a low speed for fresh air introduction, they only kick into high speed when the humidistat senses the homes humidity levels are above the set point you have set it for, usually between 35-45% RH (relative humidity). This may seem low, but in our colder winters here in Manitoba, any higher and your windows would drip and condensation would be found throughout the home, potentially causing serious damage. The humidistat’s also have a manual control for such times you wish to exchange more air for when you burn the bacon on the stove, or you have not yet been able to identify and remove whatever it is the smell is coming from in your teenagers bedroom.;) You can also get HRV fan timers installed in the bathrooms, typically with 20-40-60 minute options on them. Wiring these in can get costly if your walls are all dry walled and finished, so if your renovating a bathroom it is a good thought to install wiring even if an HRV isn’t in the budget.
Generally speaking, an HRV is made for the months which we have our homes shut up tight for the winter season when we would not normally get that infiltration of warm air. I personally shut mine off all summer as I do not want the hot humid air coming in, causing my A/C to run overtime. Sure the temperature is transferred through the same heat exchanger, but my home gets a lot of fresh air in the summer with the windows and screen doors. Excess humidity is removed by the air conditioner and if you still have too much humidity, an HRV won’t help that in the summer months because it is no longer dry air it is pulling in from outside, its hot and humid as well. At that point, you need a Lennox ‘Healthy Climate’ whole home dehumidifier installed. We have installed many, and they work really well.
When HRV’s first came out they were considered a good alternative. Nowadays, they are becoming more of a necessity. Especially with the addition of insulation and efficient windows and doors we are installing in our renovated homes. In all new homes it is no longer a suggestion, it is mandatory.
I hope that answers all you wanted to know about HRV’s and more!
Till next time..that’s all the Weatherman has to say. But I do have one last question for you…How’s the Weather in YOUR Home?