Brivis MTS HE20E - Works correctly for 15 minutes then Busy 40 & 43 Error Messages

You are definitely correct - these heaters are often taken out by the failure of some part that can be replaced for a few dollars. The main problem, of course, is figuring out exactly what part it is, and yeah, sometimes it might turn out that the part really is unobtainium.

I guess you have ended up here because of the many threads about Brivis heaters. Have you looked at the other threads, in particular https://www.renovateforum.com/f193/a...sistor-110279/ and an older one pointed to by that one?

I'm not really familiar with these heaters, but maybe your one is the same as the ones mentioned in those old threads. Does it look like the ones in those threads? If so, even though you may have a different fault, the first of those threads in particular might help as somebody traced out the circuit. Some other Brivis heaters use similar circuits too.

That old thread also has a list of fault codes, but that is a bit cryptic and probably does not tell you anything new. The 40 code given there supposedly means "return air"overheat condition, so an obvious thing to check would be the sensor. The fault code list there mentions a "klixon", which is a thermostat (simple on/off switch), but in yet another old thread here somebody said that the "HE" model heaters use a thermistor (which can measure actual temperature).

I don't know how a problem with the thermostat/thermistor could be caused by the motor capacitor blowing. If anything that might take out the triac that controls the motor, but maybe that would cause a different fault code.

Also have a look at the PCB to see if there is any obvious sign of burning or overheated parts. Unfortunately, not all failed components will show signs.

Not familiar with the Brivis heaters, but a lot of other brands have a thermistor in the supply air duct (outlet from the heater). They should be installed around 3 meters from the heater itself in straight duct.

It is possible that the thermistor is faulty but you should also check the the fan is running correctly, there are sufficient ducts open, the supply ducts are not crushed, the return air filter is not blocked and the return air duct is not crushed.

If you have a thermometer handy you could place this in the duct at the same point at the thermistor and check what the temperature is, for Braemar heaters they switch to low mode at 60 degree and go overheat at 70 degree

I know very little about these heaters, but I always read threads about them, because we have an ancient Brivis heater, and I think that sooner or later it will be me with a problem, and I want to be prepared, by learning all I can.

Originally Posted by droog

Not familiar with the Brivis heaters, but a lot of other brands have a thermistor in the supply air duct (outlet from the heater). They should be installed around 3 meters from the heater itself in straight duct.

It is possible that the thermistor is faulty ...

From what I have been able to gather in the past, in heater lingo, the "supply air" is the hot air supplied to the house, while the "return air" is the air returning from the house to the heater. That is consistent with what you say there when mentioning the thermistor, and yes, our Brivis heater has a temperature sensor mounted in the duct where the hot air leaves the heater (it's a heat operated switch/thermostat rather than a thermistor, but that might be due to the great age of our heater).

The Brivis documentation does list error codes related to "supply air", and in fact hints that there might be more than one sensor for the "supply air" (there is one code for "S/A overheat switch open" and another for "S/A Thermistor exceeded 90 deg"). However, the error code received by Mudbirdiemont appears to relate to "return air" rather than "supply air", so maybe there are sensors in both ducts?

I'm pretty sure our heater does not have any sensor in the "return air" duct, but maybe more recent ones are different?

However, the error code received by Mudbirdiemont appears to relate to "return air" rather than "supply air"

You are correct "return air" is the air being returned from the house for the heater to heat and aligns with the Brivis install manuals, you also appear to be correct with the point above so I did a little more google digging.
The error appears to be a problem with the combustion fan rather than the supply air thermistor:

"The combustion fan overheat switch has been detected as opened forcing the overheat cool down process to be initiated. So long as this switch remains open this fault condition is reported"
Sourced from the manual here : https://docplayer.net/62644296-Servi...arpro-max.html
I realise this manual is not for the same model but Brivis do appear to retain the error codes across models.

Good pickup