Wall CO2 controllers with a relayGreystone Energy Systems

Greystone CD2RMC

A building-controls room sensor with a 0–10 V output and an optional relay. Its relay range wasn’t published where we looked.

$347.01 at e-controls (relay and display version, sold out); $463.32 at Kele for another version (October 17, 2026)

How to set itCompare it
Greystone CD2RMC wall-mounted room CO₂ transmitter with a small display reading in ppm

Photo: Greystone Energy Systems

At a glance

  • Setting range not published
  • Dual-channel true CO2 sensorWith Greystone’s self-calibration algorithm.
  • 0–10 V output, and an optional relay that can be adjustedHouseExhale’s research lists its 0–10 V output on a Zehnder Option Box as a controls contractor’s option. Zehnder doesn’t document it, and the sensor needs its own 24 V supply (the Option Box gives 12 V).
  • The relay’s range and factory setting weren’t foundIts datasheet couldn’t be opened. Ask before relying on it at 750.

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. On a Zehnder: a controls contractor matches its range and voltage to the Option Box input, set to hold 700 ppm.
  2. On a relay input: set it to close at 750 ppm and open at 650, if its relay allows.

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

Outputs
0–10 V; optional normally open relay (adjustable)
Power
24 V DC ±20% or 24 V AC ±10%
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