Heatmiser neoStat wifi boiler always firing

[GeoffreyGodwin]

My Heatmiser Slimline was recently replaced by a Heatmiser Neostat Wifi. The wiring was kept the same, i.e. permanent live to L, permanent neutral to N, the live linked to A1 and the boiler to A2.

However, when this is done the boiler fires irrespective of whether the thermostat is calling for heat.

The boiler still fires even when the front panel of the Neostat is removed, whilst the wires remain connected at the back. Everything else on the device looks to be working fine, including the wifi connection.

When the Neostat is removed and replaced with the original Slimline with the same wiring, the problem is resolved and the boiler works normally.

Is the Neostat likely to be faulty?

Attached is what the Slimline wiring looked like when I removed it and the wiring diagram for the neoStat.

 

[ericmark]

It may be a fault, but more likely the set up. Had the same with my Nest Gen 3 where legionnaires protection, pre-warm up, and a host of other automated features caused the boiler to fire up when I felt no need, I disabled most of them.

 

[CBW]

If it’s doing it with the cover removed, I’d say it’s a faulty unit.

 

[GeoffreyGodwin]

Not sure if this helps but I bought a second neoStat Wifi and replaced it - same issue.

Does it matter that this is for UFH? The pump is constantly running which it wasn't with the previous Heatmiser slimline.

 

[DetlefSchmitz]

The only reason that the boiler could be running is if the switch between A1 and A2 in your diagram is closed. It might be helpful to see the display of the Neostat at the same time as it is not calling for heat, but the boiler is running.

 

[GeoffreyGodwin]

Thanks. It still calls for heat even though both downstairs and upstairs thermostats are in standby mode. Also attached is a picture of the UFH setup.

 

[DetlefSchmitz]

Do you actually have a neohub - I notice the mesh symbol is displayed.

 

[GeoffreyGodwin]

No, they are "meshed" to my wifi router.

According to the product page, no neohub is required. In addition, after a factory reset (unpairing the wifi router) the same behaviour happens.

 

[RandomGrinch]

I think you can change the sensor selection to inbuilt - I may be mistaken, but frost protection seems to be active, which means it may be taking the temperature from a nonexistent sensor?

 

[DetlefSchmitz]

I've had the Slimline (12V) versions with UFH for many years now but some strange behaviour is causing me to look at the neostat 12V versions to try to rescue the system.
As I understand it the relay between A1 and A2 should never be closed unless the flame symbol is present. This would imply that even if there is some strange setting going on like frost mode or whatever, the flame should match the relay contacts.
There are two things I would try; if you have a spare unconnected one, I would check the resistance between A1 and A2 with nothing else connected to it. It should be open circuit. The other thing I would try is disconnecting the wire from A2 (and making it safe in a connector block) to see if the boiler still fires. Both of these are to find out if the neostat is closing the switch A1/A2 when it shouldn't. If it does then either the neostat is faulty, or it has some strange behaviour that doesn't seem to match its instruction manual.
Given that you said you have two that behave the same then most likely (as Eric said) it is some weird feature that makes no immediate sense.

 

[GeoffreyGodwin]

I think you can change the sensor selection to inbuilt - I may be mistaken, but frost protection seems to be active, which means it may be taking the temperature from a nonexistent sensor?

The default sensor is "built in" which is what it should be using. If I try to change it to the floor or air probes I get error codes (and incidentally it still calls for heat).

 

[DetlefSchmitz]

but frost protection seems to be active,

In all the Heatmiser stats I've looked at the frost setting uses the room setting at a settable temperature (like 16C). We set our conservatory to frost when we aren't expecting to use it for while.

 

[GeoffreyGodwin]

I've had the Slimline (12V) versions with UFH for many years now but some strange behaviour is causing me to look at the neostat 12V versions to try to rescue the system.
As I understand it the relay between A1 and A2 should never be closed unless the flame symbol is present. This would imply that even if there is some strange setting going on like frost mode or whatever, the flame should match the relay contacts.
There are two things I would try; if you have a spare unconnected one, I would check the resistance between A1 and A2 with nothing else connected to it. It should be open circuit. The other thing I would try is disconnecting the wire from A2 (and making it safe in a connector block) to see if the boiler still fires. Both of these are to find out if the neostat is closing the switch A1/A2 when it shouldn't. If it does then either the neostat is faulty, or it has some strange behaviour that doesn't seem to match its instruction manual.
Given that you said you have two that behave the same then most likely (as Eric said) it is some weird feature that makes no immediate sense.

I don't have a multi-meter at the moment but disconnecting A2 stops the firing.

 

[DetlefSchmitz]

I don't have a multi-meter at the moment but disconnecting A2 stops the firing.

So at least we don't have to look further than the neostat. What happens if you power the neostat down?

 

[GeoffreyGodwin]

I think the only way I can power the neostat down is by turning off the fused spur shown in the picture above. This stops the firing (and also turns off the UFH pump).