Wall CO2 controllers with a relayGreystone Energy Systems

Greystone CDD4A

A room CO2 sensor whose relay can be programmed from 500 to 2,000 ppm on its keypad.

No price found for the CDD4A. The dual-channel CDD4B101R (relay and display) was $434.07 at e-controls on October 17, 2026; confirm its relay range.

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At a glance

  • Can be set under 800
  • Relay programmable from 500 to 2,000 ppmOn its keypad, so 750 is in range, with a 25–200 ppm gap before it switches back. The datasheet states this for the A model; for the dual-channel B model, confirm it.
  • A normally open relay, 2 A at 140 V AC or 30 V DCSuits a unit’s low-voltage dry-contact input.
  • Its factory setting isn’t stated
  • Plan on its own 24 V supplyVentilator makers don’t document powering a sensor from the unit.

Setting it for a bedroom

HouseExhale’s setting: switch up at 750 ppm and back at 650, or ramp from 600 to full by 750. Never above 800.

  1. Set the relay to close at 750 ppm with a 100 ppm gap, so it opens at 650.
  2. Wire it to the unit’s input that holds high speed while closed (the manual names it), with its own 24 V supply.

A Danish study ran a bedroom fan from about 900 ppm and the nights still averaged 835, so a control has to start below the goal. Good home CO2 sensors are rated within about 55 to 90 ppm at 800, and the night average includes the climb after bedtime, so 750 on and 650 off leaves room for both. A control in the return duct mixes the bedroom with empty rooms, so it needs lower settings still (about 650 on, 550 off), and your monitor in the bedroom has the final word. These are HouseExhale’s numbers: no study has tested a control against an 800 night average yet.

What it works with

Any ventilator with a low-voltage input that holds high speed while two terminals are joined. Ventilator makers don’t name third-party controls: check your unit’s manual and ask your installer first.

How to add CO₂ control to an HRV or ERV · HRV and ERV units

Specs

Sensor
True CO2 (NDIR), single or dual channel, with self-calibration
Relay
Normally open, 2 A at 140 V AC / 30 V DC
Power
20–28 V AC or DC
Display
Optional two-line LCD

From the maker’s documents, read October 17, 2026. Prices are before tax and shipping and change often.

Wall CO2 controllers with a relay

A sensor that closes a switch above its setting. Wired to a unit’s high-speed input, it runs the unit on high until CO2 drops. Commercial products; an installer fits them.

Sources