ERV vs. HRV: Does Your Tight, Modern Home Need a Fresh-Air System?

Today’s homes keep out the weather better than any homes in history. Tight construction, good windows, modern insulation and careful air sealing keep heat in, cold out and bills down. That’s all good. It also changes what happens to the air inside.

An older, drafty house swapped a fair amount of air on its own through cracks and gaps. Nobody planned it, but it happened. A modern house doesn’t do much of that. Whatever gets generated inside, from moisture and cooking fumes to chemicals off new furniture and the CO2 you breathe out, stays inside longer than most people realize.

That’s no reason to go back to a drafty house. It’s a reason to manage your air on purpose. In a tight home the same air circulates through the house again and again unless you’ve given stale air a way out and fresh air a way in.

I think about healthy air in three parts. Source control means putting less into the air to begin with. Filtration catches what’s floating around. Ventilation swaps stale indoor air for fresh outdoor air. You need all three, sized to work together. Filtration without ventilation just runs stale air through a clean filter. That’s the gap an ERV or HRV fills.

Both are balanced ventilation systems. Each one has two fans and a core. One fan pulls stale air out of the house, the other brings fresh air in, and the two airstreams pass each other through the core without mixing. As they pass, the core transfers energy from one stream to the other.

• An HRV, or heat recovery ventilator, transfers heat. In winter the warm air going out pre-warms the cold air coming in, so you aren’t heating raw outdoor air from scratch.
• An ERV, or energy recovery ventilator, transfers heat and moisture. In summer it keeps much of the outdoor humidity from coming in with the fresh air. In winter it holds on to some of the indoor moisture you’d otherwise lose.

Around here we get humid summers and dry winters, so an ERV is often the better fit. An HRV tends to make more sense in colder, drier climates or in homes that run damp inside during the winter. Which one is right depends on the house, how it’s used and what the indoor humidity looks like, which is why we measure first.

Either way, it needs proper sizing, installation and balancing, with the intake placed so it isn’t pulling in exhaust from a dryer vent or the garage. And it has to be maintained. Cores need cleaning and filters need changing, or the system slowly stops doing its job.

The industry standard for home ventilation is ASHRAE 62.2. The current version works out to 3 cubic feet per minute (CFM) of fresh air for every 100 square feet of floor area, plus 7.5 CFM per occupant, counting occupants as the number of bedrooms plus one.

That’s a steady, modest flow, not a gale. The standard also lets a home take credit for its natural air leakage when that leakage has been measured with a blower door test, so an older house may need less mechanical ventilation than a brand-new one. The point is that the number is calculated, not guessed.

CO2 readings are the easiest way to see whether your home is getting enough. If bedrooms climb well past 1,000 ppm overnight, the house isn’t getting enough fresh air where it matters most.

A lot of homeowners tell me they handle air quality by opening the windows. I get the instinct. But it isn’t a system. It depends on the weather, pollen season, outdoor air quality, whether anybody’s home, and whether it’s too hot, too cold or too humid to leave a window open in the first place.

Around here that rules out a lot of the year. Open windows on a Chester County August afternoon let in heat and humidity your air conditioner then has to fight. Open them in January and you’re heating the neighborhood. Spring pollen and the occasional smoky day make it worse.

Opening windows can be part of a plan. It can’t be the plan. Real ventilation works every day with the windows shut, whatever the season.