I have a neighbor that installed an ERV but didn’t hook up the exhaust. He just lets it use the air in his furnace room. It’s been working fine for him for 20 years and the Ecobee thermostat thinks the air quality is fine now. Obviously this is a retrofit to a forced air system. Previously it had a fresh air connection into the furnace, so he just now runs that through the ERV.

This seems clever on two fronts
1) providing balance to an otherwise exhaust-only ventilation system
2) using waste heat/cold from the furnace

Heat

This setup is stealing heat from the furnace room where there is already lots of waste heat. Keeping the temperature down in the furnace room can help with the longevity and operation of a furnace. The furnace is in the basement. A significant amount of this waste heat will likely end up leaking from the basement to outside the house rather than migrating to the rest of the house. In the summer the basement will have cool air even without AC waste cold- in some ways this setup is exchanging outdoor air temperature against in ground temperature.

A downside to the heat stealing is it would compete with a heat pump water heater in the winter, although it would help in the summer.

When the furnace is not running and generating free waste heat, and the outdoor temperature is the same as the ground’s, this will be the equivalent of having just a fan. But if we assume a cold climate, the ERV is providing most of its energy savings at the extremes of the year when there is the most waste heat and the largest difference with ground temperature.

Balance

Setting up the ERV conventionally means exhaust fans in the house both remove conditioned air and pull in unconditioned air.
In this setup the ERV is helping to balance out the exhaust. Since the exhaust ductwork is not running through the ERV, there is no energy recovered from the exhaust. However, exhausting no longer implies an additional energy loss of pulling unconditioned air into the house.

Installation

A minimal ERV installation cost from an HVAC company into existing duct work is $4k when the ERV costs around $1k. My neighbor installed this himself. If he keeps his blower running 24/7 then there is little to mess up in this installation. He does have it connected to an Ecobee now.

Your opinion

In theory this doesn’t work, but in practice at least a gas furnace in the winter seems to generate lots of waste heat and in the summer the basement stays cool. This setup might be pragmatic.

What do you think of this setup?