Wiring question series/parallel

[Robooo]

Hay,



I'm learning fault finding in CH systems which I'd need help with, please.

I've been given a set example: have got hot water but heating. Measuring the resistance around the room thermostat and the motorised valve together gives a lower total resistance than measuring the individual components. The example is not going into details just shows the expected values (total and individual programmer and 3 port valve) so I worked backwards that it is actually calculating as a parallel wiring not series one. (See picture)
I thought the controls are wired in series normally.
Is parallel wiring an option with these two controls?

Thanks!

 

[fowlerboi]

It would have to be in series for the switches to work otherwise they would just be bypassed?

I am no electrician

Maybe you can enlighten me but I'm not sure resistance is the way forward for finding broken components? It would be voltage at each component to ensure they are switching properly. You can't measure resistance over a programmer for example because the circuit is normally open and you want to check the switch in the normally closed position which it can't do without power, if it has power you can't check resistance (I think.) The same applies to the motorised valve which unless energised on the brown will be normally open across the grey to orange

 

[Robooo]

It would have to be in series for the switches to work otherwise they would just be bypassed?

I am no electrician

Maybe you can enlighten me but I'm not sure resistance is the way forward for finding broken components? It would be voltage at each component to ensure they are switching properly. You can't measure resistance over a programmer for example because the circuit is normally open and you want to check the switch in the normally closed position which it can't do without power, if it has power you can't check resistance (I think.) The same applies to the motorised valve which unless energised on the brown will be normally open across the grey to orange

Unfortunately, I'm not able to enlighten anyone yet
What the book says is that the room thermostat is set to high temperature so it is calling for heat but the system is isolated. So it would be a closed circuit between line and neutral from the wiring centre, room thermostat through the valve back to the wiring centre.
I know the two controls' resistance and the total resistance at the wiring centre. I'm supposed to figure out which component is faulty.
The calculation they gave it only works out if controls are wired in parallel because the total resistance is lower than the individual resistances.
It also states that it is a general fault finding which can be applied to for example to pumps etc.

 

[Robooo]

I'm supposed to be able to guess which component is faulty measuring resistance only by the wiring centre so I don't need to wonder around in the property.

 

[fowlerboi]

What terminal numbers are you using for each test?

 

[Robooo]

What colour and terminal number are you measuring for each test?

The book is not in reach at the moment but it measured the two components individually for resistance and the final resistance at the wiring terminals 2 and 4.
If I remember correctly one component gave 0.28 ohm and the other 0.36 ohm. The final resistance was 0.179 ohm. (The ohm meter was giving values the friction of 20M ohm so 0.28x20.000.000 ohm and so on).
I get the final 0.179 ohm only if the equation is 1/0.28 + 1/0.36 = 1/0.179 which would be wired parallel.
Calculating with normal straightforward addition the equation doesn't work out.

 

[Chuck]

My guess is that "I thought the controls are wired in series normally." may be the assumption that is stopping you solve the problem. You have a three-wire thermostat, not a two-terminal one. Maybe ask yourself, what are the three terminals connected to inside the thermostat? Do the same for the mid-position valve.

 

[Robooo]

My guess is that "I thought the controls are wired in series normally." may be the assumption that is stopping you solve the problem. You have a three-wire thermostat, not a two-terminal one. Maybe ask yourself, what are the three terminals connected to inside the thermostat? Do the same for the mid-position valve.

Maybe it is. I accept that. Probably the given example is wrong.
On the picture however in the first post it shows that:
The programmer connected to the wiring centre (terminal 4), this gives LIVE to the room thermostat which is calling for heat, this is coming back to the wiring centre and goes to the motor in the valve. From the valve motor the NEUTRAL is going back to the wiring centre (terminal 2). This makes me believe that there is a closed circuit present hence I can measure the total resistance at the terminal 2 & 4.
The math I mentioned above only makes sense if I do the parallel calculation.
I'll contact the study centre what they say.
Strange

 

[Chuck]

The math I mentioned above only makes sense if I do the parallel calculation.

If I've understood you correctly... The valve-motor is connected to terminals 5 and 2, the thermostat to terminals 5 and 4. So, for the resistance between 2 & 4 to equal the motor||thermostat you need terminals 2 and 4 to be linked. This would be the case if the programmer HTG terminal were neutral but you are assuming it is live.

 

[fowlerboi]

You don't have permission to view attachments. Attachments are hidden.

If you trace from 4 through the room stat through the nuetral through 3 port valve back to the room stat you can see you are in parallel while testing the nuetral

You don't have permission to view attachments. Attachments are hidden.

That may be a better way of showing it in parallel

All of the above is a giant educated guess

I still have no clue how resistance testing fits into all this. I understand how it works and have done it for things like underfloor heating mat but for this sort of thing checking voltages in the right places has been tried and tested for me.