Many homeowners choose to install ventilation systems like an ERV. But before anyone jumps to do it, a common question many seem to ask is how to calculate ERV size for HVAC.
There are two ways to calculate ERV size: the square footage and room count methods. For the square footage, multiply the square footage and ceiling height of your home. Then, divide the result by 60 and multiply by 0.35. For the room count method, identify each room, then multiply by an allocated CFM count. The final result is your home’s CFM requirements.
This is all but a simple guide to help you out. If you aren’t sure what size to get, it always makes sense to seek professional help. Finding out your locale’s ventilation requirements should also help you out.
In this article, we’ll go over the ERV basics you should know. Then, we’ll jump right to the methods of calculating the right size and why you should consider oversizing.
ERV: What is It?
Daily activities inside the house like cooking, washing and drying clothes, baths and showers, and breathing produce emissions. Over time, this accumulates and a build-up of it can make indoor air stuffy, making breathing uneasy and uncomfortable.
An ERV system, or Energy Recovery Ventilator, will help you and your family.
What an ERV system does is exhaust stale, used air into the outdoors, while at the same time drawing in a new and fresh supply of air. This means fresh, clean air is constantly being distributed to the rest of your home and your family is not breathing in and out the same stale air all day.
An ERV circulates fresh air to your home but can also preheat or precool incoming air before it enters. It’s also capable of retaining some moisture and bringing it back inside your living spaces, ensuring your home is not too dry.
This is all thanks to the heat or chill and moisture coming from the outgoing air.
How does it work?
An ERV system has two air ducts – one for exhausting and venting used, stale air out and another for drawing in a new supply of fresh air from outside.
These two airstreams both pass through a heat exchange, which is the very core of an ERV system. At this heat exchange, some of the heat from the outgoing air is retained and transferred over to the incoming air. This heat exchange would preheat incoming air even before it enters your home.
During hot summers, the same thing happens – only this time, it pre-cools incoming air. Once these two airstreams are at the heat exchanger, some of the air’s coolness is retained and transferred over to the new supply of air from outside.
What this does is take some of the load off your heater or AC unit so it doesn’t work as hard and doesn’t use as much energy.
These two airstreams never meet so there’s no air contamination.
On top of regulating air temperature levels, an ERV system can also regulate humidity in indoor air.
At the heat exchange, some of the moisture from the outgoing air is also retained and transferred over to the incoming air. This ensures indoor air is never too dry.
It’s worth remembering though that ERVs are not dehumidifiers. While they can retain and recover moisture, they can’t remove humidity once it’s inside your home. What it’s capable of doing is preventing excess moisture in the incoming air from ever entering your home. This makes sure no excess moisture makes it inside.
Benefits of Installing an ERV for Indoor Air Quality
- An ERV system for your HVAC (heating, ventilation, and air conditioning) system makes sure you and your family have a consistent and continuous supply of fresh, clean air so it’s never stuffy indoors.
- ERVs can either preheat or precool incoming air before it enters your home. This provides optimal living temperatures without having your heater or air conditioner unit work as hard as it typically would. ERVs boast of their energy-efficiency.
- ERVs can maintain a balanced supply of humidity and moisture levels while preventing too much of it from entering your home.
- ERV systems come with built-in filters. These filters prevent pollutants, dirt, toxins, insects, and other contaminants from infiltrating your home’s airstream. This means more fresh air for you and your family without all these indoor pollutants.
How To Calculate ERV Size For HVAC
Calculating and actually purchasing the correct size of ERV for your home’s HVAC system is essential to ensure it has enough power to provide your home with quality, fresh air.
But first, what is CFM?
Before we move on to the methods of calculating the right ERV size your home would need, let’s get units out of the way.
In the US, CFM, or cubic feet per minute, is the unit used to refer to airflow rates.
CFM, measured in cubic feet, is the measurement of the volume of how much air a fan moves in one minute. It’s the most common measurement of airflow. Airflow volume is measured by how many cubic feet of air pass by a stationary point in a given minute.
There is another terminology that might be important for better understanding: ACH or air changes per hour.
ACH refers to the air exchange rate. This is the measurement of how many times per hour a volume of air in a given space (room) enters and exits through the HVAC system.
Now we’ve got that out of the way, let’s move on to how you can calculate the right ERV size for your home – and there are two:
Method 1: Square Footage
Most ERV (and HRV or Heat Recovery Ventilator) systems are designed to ventilate entire houses at a minimum of 0.35 ACH.
To determine the minimum CFM requirements your home needs, use this formula:
Square footage of your home x ceiling height = cubic volume
Cubic volume (result) divide by 60 and then multiply by 0.35.
The final result will give you the minimum CFM your home requires.
Method 2: Room Count
Not all homeowners know the exact square footage of their homes. And for these instances, the alternate room count method is recommended.
The advantage of using this method instead is it allocates a particular number of CFMs for every room in your home, including the basement. When all these values are added, you will then end up with the total number of CFMs your home requires.
To use the room count method in sizing the ERV your HVAC needs, refer to the table below:
|Room||Number of Rooms||CFM (L/s)||=||Required CFM|
|Master Bedroom||x 20 CFM (10 L/s)||=|
|Bedrooms||x 10 CFM (5 L/s)||=|
|Living Room||x 10 CFM (5 L/s)||=|
|Basement||Yes or No?||If yes, x 20 CFM (10 L/s)||=|
|Kitchen||x 10 CFM (5 L/s)||=|
|Bathroom||x 10 CFM (5 L/s)||=|
|Laundry Room||x 10 CFM (5 L/s)||=|
|Utility Room||x 10 CFM (5 L/s)||=|
|Other(s)||x 10 CFM (5 L/s)||=|
|Total Ventilation Required(add all results from the last column)||=|
By adding all the results on the last column (Required CFM column), you get the total value of CFMs that your home and all the rooms in it require for proper ventilation.
Why You Should Oversize an Energy Recovery Ventilator (ERV) or Heat Recovery Ventilator (HRV)
You’ve most likely heard by now that you shouldn’t oversize an air conditioning or a heater unit. You’ve also been warned that it’s not a good idea to get a unit that offers more capacity than what your home would need.
The same can’t be said for an HRV or ERV though. In fact, oversizing an ERV or an HRV might be a good and better idea.
Here are two reasons why you should consider oversizing your ERV or HRV unit:
You can always turn it down
Say your house needs to move 300 CFM. You can get an ERV or HRV unit that can move around 400 CFM or more.
Getting an oversized ventilation system means you can always turn the power down when you feel like it’s ventilating your home too much.
Another benefit of toning down a unit’s power is the fans actually have higher efficiency when set at a lower power. This means you get far better results with a lower fan speed than having it run at full speed.
It’s not recommended to have your system always running at its highest capacity.
Going back to our example above, if your indoor space needs 300 CFM of ventilation and you get a unit with only a 300 CFM capacity, that would mean the unit will run at its full capacity at all times.
This may overload the unit and it will become less efficient over time. This will have you pay for repair or a whole other unit sooner than you should.
You can boost it to a higher rate
Getting an ERV or HRV unit with a higher capacity than what your home actually requires would leave room for boosting the exhaust fan to a higher rate if needed in the future.
Simple daily tasks like cooking in the kitchen, using the bathroom, or doing your laundry can accumulate a lot of emissions. When it gets too much or when you need to remove moisture, the choice of boosting your ventilation system will make things easier.
Having guests over or hosting parties means more people in your home. In these times, you might need to boost your ventilation system to ensure there’s constant fresh air in the space.
Remember, you don’t have the luxury of boosting an ERV or HRV unit that’s already running at its full capacity.
To ensure your family has nothing but quality and clean fresh air, many households across the country are installing ERV ventilation systems. Before going ahead to buy one, many often ask how to calculate ERV size for HVAC systems.
There are two ways you can calculate this: the square footage and room count methods. For the square footage method, you’re going to need your home’s square footage and ceiling height measurements. The room count method takes the number of rooms in your home and multiplies it by an allocated CFM count.
And that has been our detailed guide on how to calculate ERV size for HVAC units. Reach out to us below if you have more questions!