Knowledge/Install and commissioning/The outlet thermistor goes 3 metres down the supply duct
The outlet thermistor goes 3 metres down the supply duct
It goes about 3 metres along the supply air duct, somewhere with constant airflow. Not in the return air duct. Not in a zone that can be closed off. The distance is there so the air has mixed enough for the thermistor to read an average. Get it wrong and the heater shuts down after 90 seconds.
- Position
- About 3 malong the supply air duct
- Never in
- Return airor a zone that can close
- Symptom if wrong
- 90 secondsthen it shuts down
- Fault codes
- FC15, FC16thermistor in the wrong spot or missing
Why 3 metres and not at the plenum?
The manual explains why, and it's worth knowing. Braemar says the distance is there so the outlet air can mix and the thermistor reads an average outlet temperature.
Right at the heater the air hasn't mixed. You've got hot streaks off the heat exchanger with cooler air around them, so the reading depends on which part of the stream the probe happens to sit in. Three metres down, it's one temperature.
On a multi-duct system it has to go in the duct that always has flow. Put it in a branch that a zone can shut and you've built an intermittent fault that'll take somebody three visits to find.
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It must not go in the return air duct
The manual says it outright. It must not be installed in the return-air duct, or in a zone that can be closed off. It gets done anyway, because the return is easier to reach and it's the first bit of ductwork you meet.
The symptom is specific, and it's the giveaway. The heater shuts down after 90 seconds. The control logic is waiting for a temperature rise that proves the burner is doing something, and a probe sitting in return air never sees it.
So if you're called to a heater that runs for a minute and a half and stops, check where the thermistor is before you go anywhere near the gas valve.
How do you check the thermistor itself?
It's an NTC (negative temperature coefficient) type, so resistance falls as temperature rises. Braemar publishes the curve, so you can check a suspect probe with a multimeter instead of guessing.
| Temperature | Resistance |
|---|---|
| 10 °C | 6.6 kΩ |
| 15 °C | 5.1 kΩ |
| 20 °C | 4.2 kΩ |
| 25 °C | 3.3 kΩ |
| 30 °C | 2.7 kΩ |
| 40 °C | 1.8 kΩ |
| 50 °C | 1.2 kΩ |
| 70 °C | 580 Ω |
Which fault codes point here?
- FC15, thermistor in a cool duct location
- FC16, thermistor not installed in the outlet duct
- FC09, thermistor reading above 60 °C during start, so the heater won't start into an already hot duct
- FC04 and FC05, thermistor and temperature sensing faults
FC16 has one more cause that isn't an install error at all. If the system has add-on cooling, a cold coil in the supply duct can cause a false FC16, because the heater can't produce the temperature rise it's looking for. That needs a second thermistor, not a relocation.
Chasing a figure the manual doesn't give you?
Some of them genuinely aren't published, and some of them are published somewhere nobody thinks to look. Either way you shouldn't be guessing.
Tell Kevin what you're after and he'll point you at it or tell you it isn't there.
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.
Related
- Duct sizing, bends and return air on gas ducted heatingAirflow per outlet, when to upsize the duct, minimum bend radii, and the grille table that shows exactly what a filter costs you in return air size.
- Relief air, and the two tests that prove you have enoughTwice the vent area per room, and two field tests you can run with no instruments at all. Plus the published ratio that doesn't exist, whatever you've heard.
- Commissioning the water side of an evaporative coolerSupply pressure, water level, prewet and the pad flush. Plus two plumbing traps, one that stops the water shutting off and one that locks up the solenoid.
- Add-on cooling needs a second thermistor, and here's whyA cold coil in the supply duct soaks up the heat rise the heater looks for to confirm it has lit. That's why V1.10 boards onwards want two thermistors.
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.