Ideal Combi 35 + Honeywell CM707 Problem

25 Jan 2015
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United Kingdom
Good morning all.

I have an issue with my heating and wondered if anyone could help/know of anything to try before i call someone out.

I have an Ideal Combi 35 boiler and 2 Honeywell CM707 Thermostats, one upstairs, one downstairs, each controlling their own set of radiators. All around a year old.

The problem i have is that the radiators downstairs are constantly on regardless of whether or not the thermostat is on manual, auto, or off. This started happening out of the blue yesterday.

On the boiler it is set to the Hot Water and Radiator mode, which it has always been. I have removed the batteries from both CM707's but it still carries on heating. It is like it is ignoring the downstairs Thermostat and just constantly heating the radiators.

The weird thing is that when i turn the dial on the boiler to 'Reset" but then leave it there, everything operates at normal. If i turn the Thermostat downstairs to manual, it will fire up, if i turn it off it will turn off. Yet if i switch it back to it's original setting it will constantly be on 'C' mode with the blue operating light on until it reaches around 22 Degrees.

The heat setting on the radiator dial is set to 'E' about 2 o'clock on the dial.

It is worth noting that the thermostat upstairs works as normal and will heat the radiators upstairs when it should do.

Am i causing any damage / danger by leaving the dial set to reset whilst this problem is resolved?

Thank you very much!

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Look for two valves similar to this:

View media item 11762
One controls the downstairs heating; the other the upstairs.

Make sure both programmers are set to heating OFF.

At the end of each valve you will find a lever in a slot. labelled Auto at one end and Man at the other. The lever should be at the Auto end.

Try moving the lever to the Man end. You should feel resistance and the lever should return to the Auto end, with a whirring sound, when released.

If the lever is loose in the slot and stays still when released, the valve is stuck open. This will be the downstairs valve. Because the valve is stuck open, the microswitch in the valve will be closed and power is applied to the boiler, turning it on.

D_Hailsham, i found those 2 valves up above the boiler. The downstairs one is a Siemens branded one (czv222), and the upstairs is Honeywell. As expected from what you said, the upstairs one moves back to auto. However the Siemens one for downstairs does nothing when moved to manual mode.

But when i flicked it back to Auto, and turned the heating on downstairs with the Thermostat, it came on, and then when i flicked it off it went off, so seems to be behaving normally now.

I'm guessing this will happen again so the Siemens bit needs replacing?

Thanks again :)
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When it get's replaced i'll avoid them. I had my suspicions when i found it on the screwfix website and in the similar items were some Honeywell ones at double the price.
The downstairs one is a Siemens branded one (czv222), and the upstairs is Honeywell.
Makes you wonder why the installer didn't use the same brand for both when it's a new installation. Maybe he had them lying around in his van/workshop and couldn't be bothered to buy new.

Remove the actuator (metal box) from the top of the Siemens valve and check how easily the valve spindle turns. It should turn easily with the fingers, but not a full circle, may be 20 degrees only. If it's stiff. lubricate the spindle with silicon lubricant.

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