I installed a simple 2 wire mechanical room thermostat to my combi and
to tall you the truth I am not happy with it so much. When I set it to
21 degrees it fells like the flat get wormer than that, around 24 and
after it click off it the flat cool's down for a long time (about 3
hours) the temperature in the flat drops to around 19.5 degree, and
than the thermostat click back on. I ask a friend of mine and he told
me that it is normal for this kind of thermostat to be inaccurate in
about 2-3 degree. He sad that in a month time when it's going to get
colder I will not feel big up and down in the temperatures. So I am
asking you what you think.

I did dome research about way my Worcester 24CDi fire every 3 min for
about 40 sec. at the beginning I was sure it is sort cycling and I try
to read here more about it. At the end I found on line the Worcester
installation and service instruction and I quote from the book
"Central Heating: With a demand for heating the burner will light at
its minimum setting and gradually increase to give the maximum output.
The output of the appliance is then automatically adjusted to maintain
the temperature of the system. The output can reduce down to its
minimum of setting. If the system no longer requires even the minimum
output to maintain the desired room temperature the burner will
extinguish. The fan and pump may continue to run to dissipate the
residual heat from the appliance. The appliance will remain off for a
fixed period of three minutes before re-lighting to automatically meet
the system requirements."

So to all the Worcester owners. 3 min timer is installed in to the
boiler. And this is not short cycling.