Vaillant ecotec plus 824 + Honeywell cm927

[HipHopAst]

I had new boiler fitted. Unfortunately it come with ST9100A as a programmer but it have only 2 daily programs and Honeywell Wireless Room Thermostat Y6630D1007. So you can only set one temperature for all day. As thermostat is wireless i replace Y6630D1007 for cm927. Simply turn ST9100A for continuous on and connect CM927 to existing HC60NG.
Room temperature is 20C and when i rise to 20,5C thermostat CM927 fire up boiler so it look all connected well. Boiler is start working but only on lowest possible burner modulation rate. So after while flow temperature raise to max 40C even if system is set to max as operating manual says (5.2.2).
Sometimes when i change target temperature to 25C boiler start working "normally".
Sorry for my bad English. Hope you understand what i mean.

 

[D_Hailsham]

Room temperature is 20C and when i rise to 20,5C thermostat CM927 fire up boiler so it look all connected well.

The CM927 is only accurate to within ±0.5C and only displays in 0.5C steps. So a change from 20C to 20.5C is not very significant.

Boiler is start working but only on lowest possible burner modulation rate. So after while flow temperature raise to max 40C even if system is set to max as operating manual says (5.2.2).
Sometimes when i change target temperature to 25C boiler start working "normally".

The boiler always starts at minimum modulation and only raises the temperature if needed.

If you read 5.2.2 carefully it says:

If your boiler has a weather compensator or a room thermostat control system, you must make the following settings:

> Set the maximum heating flow temperature on the boiler

The actual heating flow temperature is automatically adjusted by the controller (for information about this, see the controller operating instructions).

That means the boiler only heats the water up to the temperature necessary, not to the maximum temperature. In your case this is controlled by the CM927.

When you turned the temperature up from 20C to 25C, the boiler will run at full output until the temperature reaches 23.5C. The CM927 then starts to cycle the boiler with reducing on time as the room temperature gets closer to 25C. This will mean the water does not get so hot, so the boiler will only need to run at a lower modulation.

The CM927 appears to be working correctly.

 

[HipHopAst]

In that case can i manually set flow temperature to 50C as i don't like that way.
My last boiler (over 10 year old) and CM907 heat up to set flow temperature. This one not.

 

[D_Hailsham]

In that case can i manually set flow temperature to 50C as i don't like that way.
My last boiler (over 10 year old) and CM907 heat up to set flow temperature. This one not.

Your old boiler could not modulate so, whenever it turned on, it always ran at maximum output and heated the water to maximum temperature, even if it was not necessary.

I don't understand why you "don't like it that way". As the boiler's User Instructions make clear, that is how the boiler is supposed to work.

If you set the flow temperature to 50C, all you are doing is setting the maximum temperature the boiler is allowed to reach. It will still modulate to lower temperatures, where necessary. In any case, a temperature of 50C may not be sufficient when the outside temperature is very low.

The radiators do not have to be at maximum temperature all the time for the room to reach and maintain the desired temperature.