Meross MTS200 thermostat wiring (again)

[NotherDIYer]

Hi all! I am trying to replace an old thermostat (Germany, underfloor heating, no boiler) where the casing chipped a bit with something a tad smarter and settled on the Meross MTS200. I'm stumped on the wiring though and reading all the other similar MTS200 threads didn't help me solve the problem.

I wired the thermostat as per the instructions on the Meross site, for what seemed like a texbook example. And for a brief moment it all looked fine except the actuator stays open no matter what - whether I set the temp above or below the room temp, or just set it to off. The thermostat looks like it's operating normally, I hear the relay as I play with the temp setting but the valve never closes. As far as I can tell the 2-wire actuator is NC, at least it's closed when I completely disconnect it from the power (and the electronics at the tip of the actuator are warm when the valve is open suggesting NC). I tried a spare actuator I found around the house and it has the same behavior. Before I play with the wiring more I'd love an opinion on what could be wrong.

On a different type of thermostat in the next room when some workers doing wall renovations dislodged it, the heating turned on (valve opened) and the previous owner just came, reseated the cables and it was fine again. This strange episode made me wonder why would a NC actuator open when it loses power, unless it didn't lose power, only COM? No idea, anyway, I attached a pic of the old wiring, maybe someone does have an idea what I could be doing wrong.

Best regards!

 

[RandomGrinch]

No idea, anyway, I attached a pic of the old wiring, maybe someone does have an idea what I could be doing wrong.

Can we have a picture of the rear of the Meross and how you have attempted to wire it? - there are multiple versions available.

Along with your actuators, the old thermostat is operating an N/C contact - try moving your switched live wire from N/O to N/C on the Meross, and see what happens.

 

[NotherDIYer]

It's mounted on the wall right now so until I open it I can use the official picture from the Meross page to show the wiring:

First wiring attempt was exactly as above. Blue in N, Brown in L, Black in NO, and a bridge between L and COM.

I also had the idea that I got it wrong and Black is in L and Brown is in NO as per the old wiring, everything else the same. Both scenarios resulted in exactly the same behavior. This is how it's wired now if I remember correctly.

I was thinking of switching between NO/NC just wasn't sure if something can go dangerously wrong. Before doing this, is your understanding that Black is L, Blue is N, and Brown is the wire to switch between NC/NO?

 

[RandomGrinch]

I was thinking of switching between NO/NC just wasn't sure if something can go dangerously wrong. Before doing this,

Nothing would go dangerously wrong; the switched Live wire is either going to be Live or not. When troubleshooting, it doesn't really matter (In Terms of the electrics) if it is Live when it should be dead, or the other way around!

Before doing this, is your understanding that Black is L, Blue is N, and Brown is the wire to switch between NC/NO?

Blue is N.
If the wiring diagram of the original stat is correct, then the black wire should be L.
It doesn't mean that it is though.
The simplest way to test - does the Meross power up with the Black wire in L, or the Brown one?

 

[NotherDIYer]

The Meross booted up just fine when I wired Brown-L and Black-NO, as well as when I swapped and put Black-L and Brown-NO (as per the old wiring as opposed to the example Meross wiring). Tonight when I get home I'll move Brown from NO to NC and see if that solves it. Any measurement I could take (as soon as I find my multimeter in the pile of moving boxes)?

 

[CountryFan]

I wonder if the neutral needs to loop as per original stat?

 

[NotherDIYer]

Still stumped. Switching between NO/NC made no difference. All the swapping I tried so far changed nothing, the thermostat comes up just fine, the actuator stay powered and open all the time. Any idea what I could measure to see if the wires are what they are expected to be for this thermostat model?

 

[CountryFan]

I wonder if the neutral needs to loop as per original stat?

Did you do this at all?

 

[CountryFan]

. Any idea what I could measure to see if the wires are what they are expected to be for this thermostat model?

You could test voltage between Neutral and Live to make sure it's 230/240vac. Then test between Neutral and NO in it's off state, if you have voltage I'd say it's a duff thermostat

 

[ericmark]

You could test voltage between Neutral and Live to make sure it's 230/240vac.

That was also my thoughts, wants a small load, so a lamp would be a good idea to test with. The old thermostat did not "need" a neutral, the compensation resistor will reduce the difference between on/off if there is a neutral, but it will work without one.

The new thermostat needs the neutral to work.

 

[NotherDIYer]

I will dig for my multimeter through my moving boxes, I'll try to test the proper way and come back with results.

I didn't try to create any different kind of loop to replicate the original wiring.

 

[NotherDIYer]

Ok, took forever and just as I was about to say "screw it, I buy a new multimeter", I found mine.

So the measurements are as follows: Blue(N)-Black(L) 230V, Blue(N)-Brown(NC) 230V, Black(L)-Brown(NC) 0V. Between Blue(N) and NO (which is now not connected to any wire) I get ~4V. The way the wires are connected is mainly because this is how they were connected to the old thermostat, as opposed to Meross' example photo where brown is L.

What exactly should I measure to see if I have the "proper" neutral type?

 

[ericmark]

So you have same reading Blue - Black as Blue - Brown, but no reference point. So we have no idea as to if blue is line or neutral, all we know is it is not same as brown and black.

We know if brown and black are connected together the heating runs.

So it would seem blue is neutral. But we have no idea if permeant line is black or brown, and it seems strange with all wires safely in a connection block to test, that both black and brown show as 230 to blue.

I would have assumed blue was neutral and if it did not work with black as line in and brown line out, I would have tried swapping them over, that is brown and black.

I would use a non contact voltage tester or a neon screwdriver to work out which is line and which is neutral, but there are only two options, if neither works, some thing has been damaged.

 

[NotherDIYer]

I actually have a non contact tester but the distance between the terminals is small enough that I'm not sure I get proper readings and no cross talk. And I'd rather not pull on the wires to distance them while energized. But I'll give it another try somehow.

I have multiple Meross thermostats, I swapped the front plate with the same result. I didn't swap the terminal block part, maybe this particular one is broken...

 

[NotherDIYer]

As I suspected, the cables in the wall box are too close to tell apart with my non-contact tester. But looking at a ceiling light that's maybe 2m away, closest to the thermostat, brown has current and blue does not. I'm assuming here the electricians kept the color standard all over...