A Guide to Heat Recovery Ventilator Sizing

Sizing a heat recovery ventilator (HRV) is a crucial step in ensuring it performs efficiently and effectively. A well-sized HRV can significantly reduce energy costs and improve indoor air quality.

The first step in sizing an HRV is to determine the ventilation needs of your space. According to the article, a typical residential home requires 0.35 air changes per hour (ACH) for a single-story home and 0.50 ACH for a multi-story home.

To calculate the ventilation rate, you'll need to know the square footage of your home. For a single-story home with a square footage of 1,000 square feet, the ventilation rate would be 350 CFM (cubic feet per minute).

The ventilation rate also depends on the number of occupants and their activity levels. For example, a home with four occupants and a moderate activity level would require a ventilation rate of 450 CFM.

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Sizing an ERV or HRV for your home can be a bit tricky, but it's essential to get it right. Traditionally, a home's square footage was used to size an ERV or HRV, but there are simpler methods to size the unit you need.

Both the CFM (cubic feet per minute) rating of the ventilator system and the airflow capacity of the unit are crucial to consider. For instance, the Honeywell HR150B heat recovery ventilator is rated 150 CFM.

To size an MVHR unit correctly, you need to know the required airflow. The general ventilation rate is 0.3 litres per second per metre squared of floor area, as per Part F of the UK and Irish building regulations.

In Ireland, there's a further requirement to provide a unit capacity of 25% above the general/trickle ventilation rate. This ensures that the unit can handle the airflow requirements of the property.

Here's a simple way to calculate the required airflow for your home:

Remember, these are minimum target values to maintain an acceptable indoor air quality. Selecting an ERV or HRV with a higher airflow capacity is usually an advantage, providing more flexibility to deal with different situations.

It's also essential to consider the type of ERV or HRV you need. If you have areas with frequent environmental events that affect outdoor air quality, an Energy Recovery Ventilator (ERV) might be a better choice.

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Traditionally, a home's square footage was used to size an ERV or HRV. However, there are simpler methods to size the ERV or HRV you need.

You can use the square footage of your home to determine the lowest CFM requirements. To do this, multiply the square footage by the ceiling's height, then divide the result by 60 and multiply by 0.35.

Alternatively, you can use the room count method, where every room in your house is allocated a particular number of CFMs. Here's a rough guide to help you estimate the CFMs required for each room:

Remember, it's essential to choose an ERV or HRV that can meet the required airflow for your property, and to consider factors like energy efficiency and indoor air quality.

Sizing an MVHR unit can be a bit tricky, but it's crucial to get it right. Selecting an appropriately sized unit is of utmost importance to the success of an MVHR installation, as stated in Example 1.

In fact, undersizing an MVHR unit can lead to serious issues like noncompliance with Part F, ineffective humidity control, poor indoor air quality, and even mould or damp issues. Units may also be oversized, which can be financially wasteful.

If a unit is running at near capacity, occupants will often just turn it off, which can lead to mould and condensation issues in airtight properties. Noise is one of the biggest issues that arises from undersized units.

But what about oversizing? Should you size to the high side, like you would with an air conditioner or heat pump? Fortunately, we have some guidance on this.

According to Example 3, oversizing an MVHR unit is not the same as oversizing a heat pump or AC. While oversizing a heat pump or AC can have negative consequences on comfort and humidity control, oversizing an MVHR unit can actually provide extra ventilation when needed, making it a good option for eliminating exhaust fans from bathrooms.

Here's a rough guide to help you decide:

So, to summarize, sizing an MVHR unit to the high side can provide extra ventilation and improve efficiency, but it's essential to follow the correct sizing methods to avoid issues like noncompliance and poor indoor air quality.

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The Room Count Method is a simpler way to size an ERV or HRV for your home, especially if you don't know the square footage. This method assigns a certain number of CFMs to each room in your house.

To use this method, you'll need to refer to the table below, which shows the number of CFMs required for each type of room. The table is based on the information provided in Example 3.

For example, if you have 3 bedrooms, you would multiply 3 by 10 CFM (f l/s) to get 30 CFM.

Getting a ventilation system that's too small can be inefficient and may even overload the unit.

You don't want to get a ventilation system that's rated at the same capacity as your needs and run it at the highest capacity all the time.

A larger ERV or HRV allows you to boost it to a higher rate when needed, which is especially useful for moisture removal in kitchens and bathrooms.

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If you have a sick person at home or are hosting a party, a boost mode gives you more fresh air when you need it.

You can't boost when you're already running the ventilation system at maximum speed, so it's essential to have a higher capacity unit.

If you ever add on to your home, a larger ERV or HRV may still be within the recommended size range, saving you from having to replace it with a larger unit.

MVHR sizing is crucial to ensure the success of an MVHR installation. In fact, selecting an appropriately sized MVHR unit is of utmost importance, as undersized units can lead to issues like noncompliance with Part F, ineffective humidity control, and poor indoor air quality.

The required airflow for a property is determined by Part F of the UK and Irish building regulations, which requires 0.3 litres per second per metre squared of floor area as a general ventilation rate. This means that a unit's size should be chosen based on the required airflow for that specific property.

If a unit is undersized, it may need to run at near capacity, which can cause noise issues and lead occupants to turn it off, resulting in mould and condensation problems. Oversized units, on the other hand, may be financially wasteful.

A well-designed residential ventilation ducting system will have a duct pressure of around 150PA, which can impact how much air a unit can move. To size a unit correctly, you should consider the duct pressure and the fan curve of the unit.

Here's an example of how to size an MVHR unit using the Brookvent Aircycle 3.1. If you take a static pressure of 150PA and cross-reference it with the fan curve, you can achieve around 100 l/s. However, it's recommended to specify at least 25% capacity above the general ventilation rate to avoid systems being specified to run at 100%.

To achieve the required airflows and avoid issues like noise and system failure, it's essential to size the unit correctly. This can be done by considering the required airflow, duct pressure, and fan curve of the unit.

By following these guidelines, you can ensure that your MVHR installation is successful and provides efficient heat recovery and effective humidity control.

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Choosing the right heat recovery ventilator (MVHR) unit is crucial for a successful installation. An MVHR unit's performance data is essential for making the correct unit selection.

A well-designed residential ventilation ducting system should have a duct pressure of approximately 150PA. This can vary depending on the size, complexity, and quality of the ducting system.

To correctly size a unit, you need to consider the static pressure of the system. For example, the Brookvent Aircycle 3.1 unit can achieve approximately 100 l/s at 150PA.

However, it's not recommended to specify a unit that runs at 100% capacity to achieve the required airflows. This can lead to issues such as noise and system failure.

Here are some key considerations when selecting an MVHR unit:

The Irish building regulations require at least 25% capacity above the general ventilation rate to avoid systems being specified to run at 100%. This means that the Aircycle 3.1 unit should not be specified for a house that requires 100 l/s in general ventilation.

In fact, with the Aircycle 3.1 unit, you would cap the size of the install to 69 l/s for a house of 230m2.

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