Should you run an HRV in Summer?

In this article, we will discuss whether it is a good idea to run an HRV in the summer. As houses have become more airtight, which is a good thing by the way, mechanical ventilation devices such as heat recovery ventilators (HRVs) as well as energy recovery ventilators (ERVs) have become an essential piece in the building system that is often underappreciated and misunderstood. HRVs, rather than ERVs, have been more commonly used in our cold northern climate, but there are some who now would argue that ERVs can/should be used in our climate, but that's a discussion for another time. Just know that the difference between the two is that an ERV also transfers moisture to the other air stream it comes in contact with, and is generally the more expensive technology.

What is a Heat Recovery Ventilator?

Before we delve into the benefits and drawbacks of running an HRV in the summer, it's important to understand what it is and how it works. An HRV is a ventilation system that is designed to exchange stale indoor air with fresh outdoor air. This process helps to improve indoor air quality while reducing heating and cooling costs compared to natural ventilation. An HRV typically consists of two fans that are connected by ducts. One fan draws in fresh outdoor air while the other exhausts stale indoor air. The two air streams pass through a heat-exchanger that transfers heat from one stream of air to the other, but do not directly touch. This helps to preheat the incoming air in the winter and pre-cool it in the summer.

Like many topics, you will find one group of people who say that you should never run an HRV in the summer and another group of people that say that you should run an HRV continuously regardless of the season. Let's drill down into why smart people on both sides of the argument might hold their particular opinion, and see if we can use that information to help make a decision.

Benefits of Running an HRV in the Summer

1. Improved Indoor Air Quality: As mentioned above, the primary function of an HRV is to exhaust out stale air, and to introduce fresh air into the home. This can help to reduce the concentration of indoor air pollutants such as dust, allergens, and volatile organic compounds (VOCs), all of which can create health issues for a home's occupants. So, a proponent of running an HRV in all seasons might ask why in the world would we stop being concerned with indoor air quality once the season turns to summer?

Drawbacks of Running an HRV in the Summer

1. Increased Humidity: Running an HRV in the summer can increase the humidity level in the home. This is because the incoming outdoor air is often more humid than the indoor air in the summer, and an HRV does not remove any moisture from the incoming air the way that an ERV can. In winter, this is not a concern as cold, winter air is always much drier than our indoor air in our climate zone. Bringing in humid air by running an HRV in the summer months can lead to discomfort and may cause problems such as mold growth if the humidity level is too high.

2. Increased load on A/C =: Running an HRV in the humid summer months can increase the run frequency of an A/C unit. This is because, while an air conditioning unit naturally dehumidifies indoor air, there is a good chance that it will not be able to overcome the amount of humidity being brought inside the home by running the HRV in very humid conditions. Because dryer air feels cooler to the human skin and vice versa, a homeowner will tend to drop the thermostat temperature a couple degrees to combat what they feel as warm/muggy indoor air. In short, an A/C set at 70° with 45% relative humidity will feel cooler than an A/C set at 70° with 65% relative humidity.

So, if an HRV exists to provide a home with fresh indoor air, which we want, but we also don't want to bring in excess amounts of moisture/humidity in the summer months; what should we do? In my opinion, it may depend on how closely you desire to balance the two objectives, and what is important to you. You can certainly turn off you HRV for the summer if you believe that you will have windows open a decent amount of time and/or you know that you will be going in and out of the home more in the summer compared to the winter. Many times, that alone will be enough to assure sufficient fresh air exchange. Conversely, maybe you aren't as concerned with paying an energy penalty by running your HRV during the summer and making your A/C work harder and more frequently so long as you know that you are continually exchanging stale air for fresh air; just make sure humidity levels in the home don't increase too much to present a mold problem (max of 60% relative humidity). Or, you could even try to selectively run the HRV on days throughout the summer where humidity is low. If you are looking at your weather app on your phone, a good day to run your HRV would be a day where the dew point is expected to be under 60°f.

That's it, until next time!