Replacing Potterton PRT2 with Computherm Q7RF(RX)

[Argual]

Hi,

I am planning to replace my old Potterton PRT2 thermostat with a Computherm Q7RF (RX) receiver.

The potterton has 4 connection points:
2 - COM - Brown wire (jumped from 4)
3 - H - Yellow/Green wire
4 - TL - Brown wire
5 - N - Blue wire

The computherm also has 4:
N
L
1 - NO
2 - COM

I *suspect* that the brown wire is live, blue is neutral and the yellow/green is ground. I am saying "suspect" because it was installed by my grandpa, who has passed a long time ago and he did not really stay consistent and sometimes used whatever wire he grabbed first.

My problem is that there is no outlet nearby, so I would like to reuse the existing 3 wires if possible. I think I should connect the brown wire to L, jump it to COM, connect the blue wire to N and connect the yellow/green wire to NO:

I am also considering that maybe the ground wire should be unused and I should jump N to NO instead:

I could not find a manual for the potterton, but the computherm manual is available here.

Which of the two (if any) is the correct solution? Please assume I have none to some very basic understanding of how these systems work. I would appreciate any support

 

[stem]

The terminals of both are marked the same, so simply swap the wires over 'like for like' including the link between L and COM. So:

Live = Potterton TL (Thermostat Live) = Computertherm L

Neutral = Potterton N = Computertherm N

Normally Open Contact = Potterton (3) H (Heat) correctly marked by you as NO = Computertherm NO

Common Contact = Potterton (2) COM = Computertherm COM

Based on this the first diagram appears to be correct.

Computertherm terminal NO (Normally Open) becomes live when the thermostat switches on. In your second diagram below you have connected NO to the N. Connecting an L and N together is never a good idea.

Historically 4 core cables were hard to come by so often the green/yellow wire was used for a function other than earth. It's considered bad practice today, but is still often seen, usually a brown sleeve is put over it to indicate it is being used as a live wire.

The Potterton live is marked TL (Thermostat Live) so this means it is not a permanent live. Normally it comes via a timeswitch or programmer. If this is the case with yours, and the Computertherm requires a permanent live, the timeswitch should be left permanently 'on' 24/7

 

[Argual]

The (NO) mark under the Potterton 3 (H) is my assumption, I mistakenly left it in the picture. Was my assumption correct or does this change the situation?

Thank you for your help, I will mark the answer when I replace the thermostat.

 

[stem]

Your markings are correct according to a translation of the manual, which says.

The receiver is a change-over contact, potential-free it controls the boiler (or air conditioner) via a relay, the connection points of which are: 1 (NO), 2 (COM) and 3 (NC)

Although being in Hungary I assume you can read it in its original language.

 

[Argual]

I meant the marking on the Potterton label.

The (NO) under the 'H'. That is only just my assumption, as I could not find the manual for the Potterton. I found that it may be the 'termostat switched live', but could not find official doc. I think it is the normally open, since if it is the switched live, I think it is closed only when the heating should be on, hence it is normally open, until heating is needed.

 

[Mister Banks]

I meant the marking on the Potterton label.
View attachment 327687
The (NO) under the 'H'. That is only just my assumption, as I could not find the manual for the Potterton. I found that it may be the 'termostat switched live', but could not find official doc. I think it is the normally open, since if it is the switched live, I think it is closed only when the heating should be on, hence it is normally open, until heating is needed.

It says NO below the H so it is the normally open.

 

[Argual]

It only says NO because I used Gimp while I was thinking how I should wire it, and when I decided it is probably the NO I wrote it there. Normally the NO is not there, and I forgot to hide the layers with my assumptions on them when I created the image I uploaded. That is the only assumption (aside from the wiring) I forgot to remove.

 

[RandomGrinch]

. I think it is the normally open, since if it is the switched live, I think it is closed only when the heating should be on, hence it is normally open, until heating is needed.

It is; even without NO written on the stat - the symbol for the switch shows a NO contact.

 

[stem]

H stands for 'Heat'. When the contact closes it starts the heating. So, as has already been pointed out it is the 'Normally Open' contact, when it closes it switches on the heating.

 

[Argual]

In the end I took the time to properly wire it in using a second cable to power the receiver. Thank you all for the help!