When it comes to sizing an air conditioner, you have probably heard that it’s not a good idea to install a unit that has more capacity than what your house requires.
However, that isn’t true when sizing a heat recovery ventilator (HRV) or energy-recovery ventilator (ERV). As a matter of fact, oversizing can be a good thing.
But before we talk about oversizing an HRV or ERV, we first need to have a reference point—a size above which the HRV or ERV may be considered oversized. Thus, we must start by talking about what the correct sizing means in the context of HRVs or ERVs.
An HRV or ERV is one of many systems that ventilate homes by bringing the outside air inside. Since they have exhaust airflows, a balanced supply, and recover heat, they also become homes’ most efficient ventilation system.
In this article, we will look at how to size an ERV or HRV for your home. The correct size will ensure your system works efficiently and doesn’t make your home too dry or humid.
Ways to Size ERV or HRV for Your Home
Traditionally, a home’s square footage was used to size an ERV or HRV. However, if you do not know your house’s square footage, there are simpler methods to size the ERV or HRV you need.
Both the methods detailed below produce a result related to the CFMs (cubic feet per minute) that the ventilator system can move with its internal fans. For instance, the Honeywell HR150B heat recovery ventilator is rated 150 CFM. On the other hand, the Fantech SHR2005R is rated 168 CFM.
Method #1 – Square Footage (Traditional Method)
HRVs and ERVs are generally sized to ventilate the entire house at a minimum of 0.35 air changes per hour. To determine the lowest CFM requirements, take the square footage of your home and multiply it by the ceiling’s height. This figure is the cubic volume. Next, divide the result by 60 and then multiply by 0.35.
Method #2 – Room Count (Alternate Method)
Using this method, every room in your house (including the basement) is allocated a particular number of CFMs. When you add all the values from the rooms, you get the total number of CFMs required.
You can use the table below to determine the number of CFMs required to size your ERV or HRV correctly.
|Room||Number of Rooms||CFM (L/s)||Required CFM|
|Master Bedroom||X 20 cfm (10 l/s)|
|Basement||Yes or No?||If yes, add 20 cfm (10 l/s)|
|Bedrooms||x 10 cfm (f l/s)|
|Living Room||x 10 cfm (f l/s)|
|Bathroom||x 10 cfm (f l/s)|
|Kitchen||x 10 cfm (f l/s)|
|Utility Room||x 10 cfm (f l/s)|
|Laundry Room||x 10 cfm (f l/s)|
|Other||x 10 cfm (f l/s)|
|Total Ventilation Needed (add results of the last column)|
Important Note: Parameters for building requirements and government regulations can also drive initial HRV or ERV sizing and specification to ensure the system is big enough to satisfy energy efficiency and air movement standards. For instance, the ASHRAE Standard 62.1 “Ventilation for Acceptable Indoor Air Quality” delineates the minimum requirements for the amount of fresh air that needs to be brought inside a building.
In addition, the ASHRAE Standard 90.1, “Energy Standard for Buildings except for Low-Rise Residential Buildings,” restricts the amount of energy a building can use. Rightly-sized ventilation systems can effectively fulfill both requirements for efficiency and air movement.
Reasons to Oversize an ERV or HRV
Once you choose an ERV or HRV, you can always turn it down if you think it is ventilating too much. The best thing about reducing power is that fans have higher efficiency when running at a lower speed.
And that’s the first reason to oversize your ventilation system. If you wish to supply 200 cfm of ventilation to the house, you can get an ERV or HRV that can move 300 cfm or more.
One thing you don’t want to do is get a ventilation system that’s rated at 200 cfm and run it at the highest capacity all the time. It is less efficient and may overload the unit.
The second reason to opt for a larger ERV or HRV is so that you can boost it to a higher rate. If you are setting up the HRV or ERV to exhaust from a kitchen and bathrooms, you will need a ventilation system with a boost mode, which allows inhabitants to increase the fan speed from a switch when moisture removal is required.
Similarly, if you have a sick person at home or having a party, the boost gives you more fresh air when you need it, meaning you need a ventilation system with a capacity higher than your constant ventilation rate. You cannot boost when you are already running it at maximum speed.
Can You Over-Ventilate Your Home?
You indeed can create humidity issues (too humid during summers, too dry during winter) with ventilated air. As a result, you can cause comfort problems and high electricity bills.
In fact, you can even damage your house by sucking moisture into spaces where it is not needed. However, that risk is significantly lowered with a balanced system that’s neither positively nor negatively pressuring the indoor space.
This balance is why you need to choose an ERV rather than supply-only or exhaust-only ventilation. Also, make sure you pick one that offers high recovery efficiency and electronically commutated motors for the fans.
A higher amount of fresh air is better for your health. It reduces the effects of asthma and hay fever and decreases the concentration of indoor contaminants and pollutants. However, you do not want to compromise on indoor air quality, so never settle with your ventilation system.
Other Factors to Consider Before Purchasing an HRV or ERV
There are many factors, apart from the size that you need to account for before purchasing an ERV or HRV. For instance, you need to look at the ductwork arrangement and the compatibility of other equipment, like the HVAC unit and air handling units, and ensure they’re working in tandem with the ventilation system you choose.
Here is a list of other factors you need to consider before purchasing an ERV or HRV.
1. Ease of Installation
Ventilation systems can be installed either during new construction or retrofitted into an existing HVAC system. The degree of installation difficulty depends on the type of system, the building’s configuration, and the ductwork location.
Professional installers can instantly carry out diagnostic testing to ensure the unit is installed and calibrated correctly by connecting with the ERV’s communication controls.
Unitized ERVs are an excellent choice for fitting into an existing HVAC system as they have adjustable legs to allow for easy and quick installation.
Most ERVs and HRVs are designed to last two decades or more. Nevertheless, certain construction features might help the system exceed that period.
For instance, ERVs with double-wall construction provide enhanced thermal protection and durability. Therefore, less energy is lost during operation. Additionally, such design features also make the unit quieter. Plenty of advancements have been made to ventilation systems, making contemporary versions highly dependable and requiring very little maintenance.
For external use in high-wind areas, consider getting an ERV with hurricane-proof roof curbs for extra protection. Also, make sure the electrical components in the system are secured from sparks that might happen when connected to a natural gas HVAC.
Last Few Words
When you purchase an HRV or ERV for your home, make sure to look for these features to ensure you select a unit that serves you well:
- A maximum rate that’s almost twice as high as you plan to operate it constantly.
- The ability to change the rate so you can operate it at a lower rate.
- The ability to boost to a higher rate when you require higher ventilation.
- Electronically commutated motors.
- A core with a high recovery efficiency for moisture (ERV) and heat (HRV and ERV). The best systems offer approximately 70 percent and 95 percent, respectively.
Also, make sure to look at the specifications of the models you plan to purchase. There are plenty of low-quality, low-cost ERVs and HRVs available on the market that you need to steer clear of!