Spot HRV’s have a place–but do they belong in your place?

Have you heard of decentralized ventilation?  It’s a simple solution to getting more “fresh air” in your home.  The simplest version is a bathroom fan, that’s either turned on and off by a switch, or automatically by a “humidistat” (a device, like a thermostat, that activates the fan when the humidity reaches a pre-set level).  This kind of ventilation is always pulling air out of your bathroom or kitchen, which is replaced by air from some other source.  Decentralized ventilation is great for the spaces that really need to exhaust humid or odorous air, like the bathroom or kitchen, but they have some drawbacks: you need to leave a window open on the other side of the home, and even with this open pathway, they still tend to create negative air pressure within the home.  

Enter “spot” ventilation heat recovery units (HRV’s).  Despite the name “spot”, these are designed to work in pairs rather than alone.   They are paired physically on opposite sides of a home or large room, and paired electronically by being able to speak to one another and sync their activity so that one supplies air and one exhausts air, making a light flow of fresh air into the home.  The flow will run one way for 60-70 seconds, then reverse.  Most recover heat by exhausting warm inside air through a ceramic core, which heats up, and then when the flow switches, cool fresh air is in turn heated as it’s brought inside. 

Let’s talk about the usefulness of spot HRV’s.  When seeking to upgrade a small or older existing home, often you don’t have the space or accessibility to run new ductwork for a ducted HRV or ERV.  These spot units can be useful to get ventilation.  Also, some heat recovery is better than no heat recovery, right?  In colder climates, you don’t want to be losing all your costly heat out the vent just to get some stale or humid air out and fresh air in.  A DIY’er in the UK showed how he installed a BSK Zephyr unit and programmed it in his older “cottage” in this video and was quite happy with it except that the high fan speed was louder than he liked.  It reduced his humidity approximately 8% in his home and gave approximately 3 deg C of “uplift” when supplying air from the cooler outside into his home.   He did mention that it would be even more advantageous when paired with another unit to cooperatively ventilate.

Corbett Lunsford is a building science expert and does not generally recommend spot HRV’s.  He  reviewed the Lunos products in this video.   He explains some of their downsides:

• The price climbs very quickly when you specify “noise reducing” covers and MERV-13 filters, which you’ll absolutely want.

• These units might seem like an upgrade to your bathroom fan, but because they switch between exhausting and supplying air, they won’t continually exhaust steamy air when you’re taking a shower, for example.  If you program it or buy one that only exhausts, then you’re pulling negative air pressure, as mentioned before.  

• Depending on the size of the space you’re trying to ventilate, will 60-70 seconds of low (35cfm on average) airflow really penetrate the space with fresh air?  

• Units like the AIO all-in-one wall-mounted unit combine heating, cooling, ventilation and energy recovery.  These are great for maximizing wall space, but the relatively short distance between inlet and exhaust (for balanced ventilation) may cause the ventilation to be short-circuited around the unit, not penetrating into the room.  Also, they are quite expensive compared to traditional ERVs, which he prefers to specify. 

• For warmer, more humid climates, most “spot” units are HRVs, not ERVs.  This means they are not de-humidifying the outside air as it comes in, so by running one in a humid climate, you’re bringing humidity in that your HVAC or dehumidifier must then remove.  An ERV would be the better choice.

Here’s another reason a spot HRV may not work for you.  Take for example the chemist/building science expert Alex Kessler who built his own HRV for under $200, which turned out to be 85% efficient!  However, after using it for a few seasons in the northeast US, he discovered that when the late spring and summer came, his HRV was bringing in too much humidity into his home, causing mold to grow in his bathroom, kitchen and other places.  He ended up buying an ERV, which also removes humidity from the incoming air.   HRVs can be useful for those living in dry areas like Arizona, but if you live in a humid climate at least part of the air, you’ll want an ERV.

Every home is different, so depending on space, location, climate, air pollution outdoors, ability to modify your home, etc., you may want a spot HRV or may decide against it altogether.  For apartment dwellers, you might want to check out a new product in design: a window-mounted ERV called SWERV.  It’s still in BETA testing at the moment but this product is a good idea for dorm and apartment dwellers and we hope it will be available retail soon.

Photo by Lukáš Lehotský on Unsplash