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You can connect the volt free contacts of a programmable thermostat (such as a Honeywell CM907) to the thermostat terminals described in section 9.4 and shown in Fig 7 of the manual: http://www.heatline.co.uk/assets/boilers/products/manuals/Vizo%20Plus%2024%20manual.pdf. Set the timer to...

Not sure what you mean by 3 program settings. The CM921 can be programmed with 6 temperature changes through the day. Do you need more? If you want different programs every day or weekday/ weekend then you need a CM927.

If you have measured across the output terminals when the heating is on, there will be no 230V as you are measuring across a closed contact. If you measure from neutral to those terminals, you should see 230V. You are correct that the receiver should be connected as shown for the volt free...

It sounds like it has been wired incorrectly or the programmer contacts are permanently closed due to a fault.

At 6 cph, it means it will be on for as long as it needs in each 10 minute cycle to supply the required amount of heat. So it might be on for 2 minutes, then off for 8. If the weather is colder, so more heat is required, it may be on for 4 minutes, off for 6. In mild weather, the cycle will...

No it doesn't, because the RTS1 requires Live, neutral and switched live. See http://www.draytoncontrols.co.uk/WorkArea/DownloadAsset.aspx?id=10497

Is the problem that the thermostat does not work when connected into your heating system, or that it does not turn on the lamp in your test rig, or just that you can’t explain why you should measure 60V across the lamp when power is not applied to it?

If the thermostat is operating the heating system but not the led, it would suggest that you have no connection to neutral at the thermostat.

Installing a programmable thermostat is quite simple but does involve connecting to mains voltage wiring. It can simply replace the existing thermostat, so there is no need to touch the boiler. However, from your original description it is not entirely clear what the problem is you have now...

The requirements for room thermostats are contained in the Domestic Building Services Compliance Guide (section 2, table 2). This is a guidance document that supports Part L1 of the building regs.

It would be preferable to replace the thermostat with a programmable type so that the timer can be left permanently ON, or removed. The programmable thermostat will then control heating times and temperatures.

As an option, you can add a tank thermostat and achieve a better control than you have now without doing any plumbing - as shown below. With this interlock, the boiler will only fire if the tank thermostat is demanding heat or the room thermostat is demanding heat. You still need to have...

If you use a wired programmable thermostat, such as the Honeywell CM907, that can simply be fitted in place of your existing thermostat. You will need to keep the programmer to control your hot water heating times but simply set the programmer CH to permanently on and let the thermostat control...

A programmable stat is exactly what you want. As you say, keep a night set-back temperature of 14 or 15C, then set the optimum start function that will decide when it needs to bring the heating on in order to reach your set temperature. This is how I operate my heating system using a Honeywell...

The receiver probably won't pick up a signal if it's inside the boiler.

From doing a quick search, it seems that the BG PT2 is a 7 day programmable thermostat (a badged Potterton Myson?), so you should not have a separate programmer because it is controlling a combi. I can't find a manual for the PT2 on line, so can’t be much help. Do you understand the...

Terminals A and B of the thermostat receiver should be connected to terminals 3 and 4 of the boiler terminal strip (see Fig 4.15 and 4.18 of the Vaillant manual), with the link wire removed.

You mention ebus. The CM727 should not be connected to the boiler’s ebus (serial data bus). The A & B connections should be connected as per a conventional switched thermostat circuit to the boiler. Check with the boiler installation manual.

Yes, that's a good point. The same thermostat unit can then be used for either.

It does seem odd that in the wireless version they split the control function between thermostat and receiver. They must have had some good reason.