Make your HRV or ERV turn up when the bedroom needs air

Plenty of homes already have an HRV or ERV that runs one speed around the clock. A bedroom with two people in it can still climb past the 800 ppm goal on that speed, then sit there all night. Turning the system up at bedtime often fixes it, but few people remember to, and high speed all day is louder and dries the house in winter.

A CO₂ control does the remembering. It watches the bedroom and turns the system up when the air builds up, then back down. A night schedule does a simpler version of the same job. This guide covers whether that will work in your home, the four ways to set it up, the numbers to set, and how to check it with your own monitor.

Start here: does high speed reach 800?

A control can't do more than high speed does. So before you buy anything, find out what high speed gets you.

1. Measure two usual nights with the system on its normal setting and the bedroom door as you usually have it.
2. Then two nights on high, everything else the same. Use the system's highest steady speed. A boost button that runs for 20 minutes and stops won't tell you much.
3. Compare. In the HouseExhale app this is the fix called "Turn up your ventilation system", and the app compares the nights for you.

Turning up a system that supplies or pulls air in the bedroom typically lowers overnight CO₂ by roughly 40–55% (free fixes has the evidence). In a Belgian study where researchers switched whole-house systems to their high setting, bedrooms averaged about 810 to 860 ppm: close to 800, not comfortably under it.

If high gets you to 800 or less, it's worth making automatic. Read on.

If it doesn't, the bedroom isn't getting enough outdoor air even on high, and no setting will change that. Ask for a balancing check: about 17 cfm of outdoor air per person sleeping there should reach the room on high. Check that the room has a fresh-air supply grille at all, and that air has a way out when the door is shut. Getting a bedroom to 800 has the numbers, and what to do if the bedroom still reads over 800 has the checks.

The whole system turns up, not just the bedroom

Most homes have one ventilator with ducts to every room. A sensor in the bedroom can tell it to speed up, but the extra air is shared out the same way as before: the bedroom gets its own share of the total. If the main bedroom gets a fifth of the supply, getting 34 cfm to two sleepers takes about 170 cfm from the unit. Many home units can't do that, or can't do it quietly.

Two things follow:

• Put the sensor in the bedroom that gets stuffiest. Some systems take several sensors and follow the highest reading.
• A sensor in the return duct or inside the unit sees the whole house mixed together, empty rooms included. In a modelling study of a small apartment, most single-sensor strategies in the shared exhaust did worse than leaving the system on a constant speed when one bedroom sat empty. A bedroom sensor is better. If the duct is your only option, set it lower (about 650 on, 550 off) and check the bedroom with your monitor.

Four ways to make it turn up by itself

Each control we read about has its own page, with how to set it under 800 and what it's documented with: CO₂ controls.

Its maker's CO₂ control

The most straightforward choice when your maker sells one, because the maker documents how it connects and how the unit responds. RenewAire's CO2-W wall control switches its units to high above a trip point you can set anywhere from 500 to 1,500 ppm. Zehnder's CO₂ sensor connects to its ComfoAir Q units through an Option Box and raises the airflow gradually.

Check what it's set to before you rely on it. Zehnder's control works to hold the room at its setting rather than switching at it, and Zehnder suggests 1,000 ppm; its sensor's built-in controller mode holds 1,050. Ask your installer to use sensor mode with the Option Box set to 700 ppm (3.5 volts). The CERV2's guide recommends 900 to 1,100; set its CO₂ setting to 750. On a RenewAire EV Premium, ask for the unit to go back to its normal speed when CO₂ drops, not to switch off.

Broan's Overture system says it ventilates when CO₂ climbs, but Broan doesn't publish its sensor type or the levels it uses, and one spec sheet lists eCO₂, an estimate. Check with your monitor before relying on it.

• RenewAire CO2-WWall CO₂ control
• Zehnder CO₂ sensor

A CO₂ control on its high-speed input

Many units have a pair of terminals that run them on high while they're joined, meant for a dehumidistat, a timer or a switch. A wall CO₂ controller with a built-in switch (a relay) can join them while CO₂ is above its setting. On a Fantech HERO, for example, the dehumidistat input holds maximum speed until the contact opens, and Lifebreath documents terminals 2 and 6 for continuous high speed.

Three things to check first:

• Does the input stay on high, or run a timer? A 20-minute boost input runs its time and stops, whatever CO₂ does.
• Can the controller's switch point be set to 750? Several commercial ones can be programmed from 500 ppm. A few come set to 800 and only go up from there, and some have a fixed 1,000, which won't do.
• Is it the right input? On a Broan AI unit, use OVR, not VENT: a contact on VENT drops the unit to standby every time it opens, so ventilation would stop whenever the bedroom is fine.
• Is the input low-voltage? Most are. Panasonic's Intelli-Balance 100 boost switch carries 120 volts, which a low-voltage relay isn't made for, and Broan warns that 24 volts on its AI terminal block can ruin the board.

• Fantech CO₂ sensor 99315Shown with Systemair branding
• Greystone CD2RMCRoom CO₂ transmitter

Most makers don't document this pairing, so it's your installer's call, and worth asking the maker. The CO₂ relay controllers we priced in October 2026 cost about $290 to $440, before wiring. Most run on 24 volts, and makers don't document powering one from the unit, so plan on its own small power supply. We didn't find a published price for the whole job.

Your monitor and a smart-home relay

If you run Home Assistant, it can read a CO₂ monitor you already own and switch a small relay wired to the unit's high-speed input: on above 750 ppm, staying on at least 15 minutes, and off below 650. It reads Aranet4, SwitchBot Meter Pro CO₂, Qingping and INKBIRD monitors over Bluetooth on your own network; an Aranet4 needs firmware 1.2.0 or later with its Smart Home integration turned on. It's the most do-it-yourself of the four.

• Use a true dry-contact relay. The Shelly 1 Gen3 and Zooz ZEN17 are; the Shelly Plus 1PM isn't. The Sonoff MINI-D switches only DC, and some units' inputs are AC.
• Low-voltage inputs only. Switch the unit off at its plug or breaker before you wire anything. Anything at 120 volts is an electrician's job.
• Plan for the hub or the power to stop. The unit should simply carry on at its normal speed. Some relays switch back on after a power cut, so set yours to come back off, or to switch itself off after a while. Unplug the hub once and check.
• Read your manual and warranty first. No maker documents this.

There's also a no-wiring version: SwitchBot's app can have a SwitchBot Bot press the unit's boost button when the Meter Pro CO₂ passes a level. It needs a SwitchBot hub, and the monitor on USB power so it reports often enough. A boost button usually runs a timer rather than holding high speed.

A night schedule

No sensor at all: the system runs high from a little before bedtime until you get up. Use the unit's own schedule if its control has one, or a timer on its high-speed input. Some smart thermostats run a ventilator too: ecobee can run one a set number of minutes an hour, and Aprilaire's can follow a timer. None we found switches it by a measured CO₂ level. Starting 30 to 60 minutes before bed gives the room a head start on the climb. It runs high whether or not anyone's home, and on dry winter or smoky nights too, but there's nothing to calibrate and nothing to drift.

The numbers to set: 800 or lower

• A control that switches: high at 750 ppm, back to normal at 650, and staying on at least 15 minutes at a time so it doesn't click up and down all night.
• A control that ramps: start rising at 600, full speed by 750.
• A control that holds a setting (Zehnder's works this way): 700.
• A sensor in the return duct: about 650 on and 550 off, then check the bedroom.

Why not just set 800? The goal is an average across the whole night, including the climb after bedtime, and good home CO₂ sensors are rated within about 55 to 90 ppm at 800 (how accurate monitors are). In a Danish study, a bedroom fan that came on at about 900 ppm still left the nights averaging 835. Starting at 750 leaves room for both. No study has yet tested a control against an 800 night average, so these are HouseExhale's numbers built on that evidence, and your own nights are the real test.

Some controls can't go this low: one European unit's top setting can't go below 1,000. If yours can't, use its lowest setting with a night schedule, and let your nights tell you whether it's enough.

Keep it running on low

Let CO₂ add high speed on top of the normal speed. Don't let it switch the system off when the bedroom is fine.

• Natural Resources Canada advises running an HRV on low continuously, year-round.
• Model building codes allow a whole-house system to run part-time only if it runs at least a quarter of every four hours, at a higher rate to make up for it, and they require a switch you can turn it off with. A bedroom sensor can't promise that: during an empty day it might never come on.
• ASHRAE's home ventilation standard allows ventilation that varies, within rules about averages.

This means the control adds fresh air at night rather than saving energy. That's the point: it's for the bedroom, not the bill.

Quiet enough to sleep through

A control turns the fan up at the moment you're trying to sleep. In a study of 299 Dutch homes, almost all occupants ran their system on a lower setting than recommended, and the researchers put the unused high setting down to noise. In UK studies, people complained of fans speeding up at night on a humidity sensor, and some switched their systems off.

• Test high speed at bedtime with the door shut before you set up a control.
• Aim for a speed you can't hear change. Passive House sets 25 dB(A) for living spaces, and UK guidance 30 dB(A) for bedrooms.
• If high is too loud, use a quieter middle speed and fix the airflow to the room instead. A control that ramps gently is easier to sleep through than one that jumps.

Where the sensor goes, and keeping it honest

• In the bedroom that gets stuffiest, at about head height, at least an arm's length from the bed, and away from windows, doors and the supply grille.
• True CO₂ only. The sensor that drives the system must measure CO₂ itself (NDIR or photoacoustic). Readings labelled eCO₂ or CO₂eq are estimates from other gases.
• Watch self-calibration. Many sensors treat their lowest reading of the week as fresh outdoor air. In a bedroom that never airs out, that teaches them to read low, so the system turns up late. Turn it off where you can, or give the sensor fresh air now and then.
• Check it against your monitor. Put your monitor beside the control's sensor for a night. Near 750 they should agree within about 100 ppm. Check again each season. Can I trust my CO₂ monitor? has the outdoor check.

By brand

What each maker documents, checked on October 17, 2026. "Not documented" means we didn't find it in the maker's own manuals, not that it can't work. Your unit's manual has the final word. Each unit in our list of HRVs and ERVs has a page with what its maker documents for CO₂ control.

Units sold under an HVAC maker's name. We couldn't read these makers' own manuals, but the HVI certification directory lists their units with exactly the same ratings as another maker's, which suggests the same equipment underneath: Carrier and Bryant HRVs and ERVs match Broan AI units, Lennox Healthy Climate matches Lifebreath, Trane and American Standard Envirowise matches RenewAire, and Honeywell VNT5150 and VNT5200 and Daikin Clean Comfort match Fantech units. Use that maker's row as a lead, and check that your manual's terminal names match before anything is wired.

Ask your installer

Put your numbers in the request, so you both start from the same place:

1. Which terminals on our unit can call for high speed, and does that input stay on high while it's closed, or run a timer?
2. Can a CO₂ control there be set to switch to high at 750 ppm and back at 650, or ramp from 600 to full by 750? What's its factory setting?
3. On high, how much outdoor air reaches the main bedroom, in cfm? We're aiming for about 17 cfm per person sleeping there.
4. How loud is high speed in the bedroom with the door shut?
5. Will it stay on its normal speed the rest of the time, and go back to normal if the control fails?
6. Does adding the control affect the warranty?
7. What will parts and labor cost, and will you set the levels and test that high speed comes on before you leave?

Prove it with your monitor

Once it turns up by itself, sleep three ordinary nights with it running: same monitor spot, same people, same door and window as your usual nights. If those nights average 800 or less, it works. If the control ran high all night and the room still averaged over 800, the room needs more air, not a lower setting.

In the HouseExhale app, Your system → Ventilation walks through it: your unit, two nights on high, the ways that fit your brand, and three nights to prove it.

Smoke, dry winters and filters

• Smoke: more outdoor air means more smoke on smoky nights. Turn the system back to its normal speed, or off as its manual says, while outdoor air is poor. Ventilation and wildfire smoke covers filters and when to switch off.
• Dry winters: more outdoor air in cold weather dries the house. In the Danish study, people sleeping with more ventilation reported drier mouths and skin. An ERV keeps more of the indoor moisture than an HRV; HRV vs ERV explains.
• Filters: clogged filters cut the airflow on every speed, high included. Clean or change them as the manual says.