Worcester Bosch TR2 room stat replacement

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Place holder - just rescuing this from the NG, needs expansion into an article:

John picked up your e-mail from a discussion about adding zone control to a Worcester Bosch boiler. I appreciate you are not the poster of the query but I am unable to post a reply and thought you may be able to post/forward/explain how I can post. I have similar problem with this boiler and have reverse engineered the TR2 interface (bit of an exageration for what it is). I came up with what now seems an overly complex solution which fooled the boiler into thinking the tr2 was a) in constant mode and b) demanding max heat. this is done by inserting an 8.95 kohm (a standard 9.1kohm should still work) resistor across pins 4 & 3 of the tr2. The tr2 presents a resistance (my actual measured) to the boiler based on the demand temperature on the rotary dial and the position of the 4 position switch as follows:

switch position 30Deg .............0Deg constant 8.95 kohm 11.27 timed 6.22k 8.54k night(econ) 13.96k frost 12.64

 From observation of the pcb the rotory dial is a 2.2k pot. in series

with a 6.2k (3.5k +2.7k) and a 2.7k which is switched in for constant (i.e un timed operation. it is worth noting that the pot is low for high temp demand and 2.2k for low temp (confused me for a minute did that). The night and frost are fixed 14k and 12.7k resistors.

looking back I think a simpler solution is to open/short circuit the temperature sensor, which looks like a thermistor, between pin 4 and pin F to fool the boiler into thinking it was cold. May need to insert a specific value in/across. If there is any interest will look into this option and report back.

Craig Nixon"

So there you go, it sounds rather like one could introduce a conventional external battery powered stat (programmable if required), by connecting a 8.95K and 2.2K resistors in series across the control pins on the boiler, and then connecting the normally open contacts of the stat across the 2.2K resister. That way as the external stat calls for heat it would bridge the 2.2K out of circuit and change the demand temperature sensed by the boiler from min to max.

My suggestion at the end would look something like:

The note I posted at the end of that post ought to do what you need:

                     / RL1
              |=====o o========|
              |                |
              |                |

4 ====\/\/\/\/========\/\/\/\/========= 3

        R1              R2

R1 = 9k1 (nearest preferred value to 8.95k measured). R2 = 2k2 RL1 = the two normally open contacts of a external programmable stat (something like a Horstman C27) 3 & 4 = pins on the boiler.

However the thing missing from the TR2 above is an actual temperature sensing element since the fixed resistors and the pot on the TR2 give the required mode and the set temperature, but they won't tell the boiler what the actual temperature is. I believe the TR2 used a third wire for this.

So I think you will may also need a link of some kind between terminals 4 and F on the boiler to look like the resistance of a thermistor.

However since the thermistor is likely the negative temperature coefficient type that seems common in heating systems, it will look like a high resistance when cold. So open circuit may be good enough to convince the boiler its 'kin freezing!

If that does not work, then some experimentation may be required, say using a 100K or a10K fixed resistor between 4 and F.