Knowledge/Fault finding/Most overheat faults are airflow, not gas
Most overheat faults are airflow, not gas
The overheat codes on both brands come back to restricted airflow more often than anything else. Closed outlets, a dirty return air filter, over-zoning, or a thermistor in the wrong place. Most "heater keeps cutting out" callouts are an airflow problem, not a gas problem.
- Brivis codes
- 40 to 43, 53overheat family
- Braemar codes
- FC10, FC19heat exchanger (HX) OT1, OT2 open
- Usual root cause
- Airflownot gas, not the valve
- Filter interval
- 2 weeksclean, per the manual
What do the overheat codes on both brands have in common?
Brivis and Braemar number their codes completely differently, but the overheat codes behave the same way on both. On Brivis it's 40 return air overheat, 41 supply air overheat, 42 supply air thermistor overheat, and 43 and 53 overheat lockout. On Braemar it's FC10 for HX OT1 open and FC19 for HX OT2 open.
In every one of those, the heat exchanger is making heat faster than the air can carry it off. So don't ask why the burner is too hot. Ask why the air isn't moving.
- Closed or blocked outlets
- A dirty return air filter
- Over-zoning, so too much of the system is shut at once
- A thermistor in the wrong spot, reading the wrong air
- Return air undersized for the unit in the first place
- Crushed or disconnected flexible duct
Which switch is actually tripping?
It's worth knowing which switch has opened. The set points differ by model, and the third one, the OT3 flame rollout switch, is a manual reset.
| Switch | Set point | Reset |
|---|---|---|
| HX OT1 | 110°C on most TQ3, TQ4, TQM5, BMQ, SSQ and TQS models | Automatic |
| HX OT1 | 80°C on TQ516 and TQ523 | Automatic |
| HX OT1 | 90°C on TQ532, TQM6 618, 623 and 632 | Automatic |
| HX OT1 | 100°C on TQD320 and TQD330, 60°C on TQD315 | Automatic |
| HX OT2 | 130°C across the entire range | Automatic |
| OT3 flame rollout | 130°C across the range | Manual reset |
That manual reset on OT3 matters when you're diagnosing. The service guide has a useful line. If 24v is present and there's no test point pressure, replace the valve. If 24v is not present, check OT3 is reset and there's continuity across the terminals. A tripped rollout switch looks just like a dead gas valve.
No
published
figure
There's no published temperature rise for these heaters
You'll hear people quote a temperature rise range in °C for indirect gas ducted heaters, the way you would for a furnace. Manufacturers don't publish one.
That's down to how the control works. The board reads the duct thermistor and speeds the fan up or down to hold the outlet temperature. The over temperature switches set the ceiling. So there's no target rise to hit. The fan just moves to keep the outlet where the board wants it.
So the numbers you can actually stand behind are the trip points above, plus two control settings. Brivis code 21, overtemp sensor trips at 90°C. Braemar FC09, thermistor above 60°C during start, which is the heater refusing to light into an already hot duct. If you need a rise figure for a commissioning sheet, it has to come from Australian Standard AS 4575 Appendix E or a real commissioning record, not from us.
Where do you start on an overheat callout?
- Pull the fault log firstEarliest recurring code, not the newest one. On an overheat chain the later codes are usually knock-on faults.
- Look at the filter and the returnIt's the check most likely to find it, and it takes a minute. The manual says clean the filter every two weeks. One nobody's touched in four years is your prime suspect.
- Count what's openOne outlet has to stay fully open at all times on a zoned system. If the family's learned to shut everything but one bedroom, you've found it.
- Then check where the thermistor isAbout 3 metres down the supply duct, in a duct that always has flow. Not in the return, not in a zone that can be closed off.
- Only then start on the gas sideIf the air path is genuinely clear and the codes still say overheat, then it's worth your time on pressures and the valve.
Had a fault that didn't make sense?
The ones worth talking about are the ones where the code pointed one way and the cause turned out to be somewhere else entirely.
Send it through. If enough people have hit the same thing it becomes a page.
Who this is for: small and medium HVAC contractors in Victoria. If you're a homeowner after a rebate or a repair, you need a licensed contractor, not us, and yours can point you the right way.
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Review date: 1 April 2027. Figures in this area change without much notice and this page carries some that will go stale. Every number above is dated and sourced so you can check it yourself. If you find something out of date here, tell us and we will fix it.