Add-on cooling needs a second thermistor, and here's why

What is add-on cooling?

It's a cooling-only outdoor unit with an evaporator coil sitting in the supply duct, after the gas heater. The heater's own room fan and ducts move the air for both heating and cooling, and the heater's controller decides which one runs. Seeley sells it as Braemar AOCF, TCCF, SACC, SACV, SCHV, KACV and KCHV. Brivis and Bonaire call it Dual Cycle.

That shared fan and shared ductwork is exactly why it's cheap to retrofit, and exactly why it's a compromise.

What does the manual require?

The installation manual is clear about it. "All Braemar ducted gas heaters fitted with a V1.10 ICS board or later, where add-on refrigerated cooling is installed, require a second thermistor for correct heating and add-on cooling operation."

It gives the reason too. "The second thermistor is required to remove false FC16 conditions when a heater is started with a cold cooling coil and is unable to generate the temperature rise required to allow the heating cycle to continue."

In plain terms, a cold coil soaks up the heat, the heater can't see the temperature rise it's waiting for, and it throws FC16 even though nothing's wrong.

Where each one goes

• The HEAT thermistor must go before the add-on cooling unit
• The COOL thermistor must go after the add-on cooling unit
• Both connect side by side on the ICS board, labelled COOL and HEAT

Why is this a design conflict, not a fault?

It's worth a second, because it explains a whole category of callout. The heater proves it has lit by watching the supply air temperature climb. A cold coil sitting in that supply duct soaks up the heat before the thermistor sees it. So the control logic decides the thermistor isn't in the outlet duct and throws FC16, which is the location fault.

Nothing is broken and nothing is installed wrong. The two systems are just fighting over the same duct. One thermistor can't serve both, so the fix is a second probe, not a relocation, a new board or a gas valve.

If you take one thing from this page, take that. On these systems a fault code names a symptom, and the cause is often a design compromise somebody accepted years ago.

What else should you expect from add-on cooling?

The room fan was picked for gas heating, which moves less air at a big temperature difference. Cooling needs more air per kW, and now the fan has to push it through a coil it was never chosen for. Airflow drops, coil temperature drops, and you get less real cooling of the air (poor sensible capacity), coil icing and short cycling. And when it's heating, that coil is extra resistance in the duct all year round.

The distribution works against you too. The ducts, outlets and return were all sized for heating, so you've usually got floor registers blowing sideways and a low-level return. Cold air dumped at floor level with the return also low is close to the worst case for cooling. Manage that expectation at quoting stage.

A few practical notes

• Set continuous fan and fan run-on time in MagIQtouch. They help dry out the coil and clear leftover heat from the heat exchanger.
• On Bonaire, a 24 V manual digital controller gives you no zoning, no evaporative or add-on cooling operation and no room fan speed adjustment. So add-on cooling needs the full digital controller, not a two-wire thermostat.
• P6 on SCHV and KCHV is a communication error between the outdoor control PCB (printed circuit board) and the outdoor drive board.