Thermostat question and a Worcester Fact about short cycle

[Golan]

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.

[Ed Sirett]

With a simple thermostat you should have also provided a neutral to run the "anticipator" inside the unit. 2-3 degrees is what you will get with only 2 wire. (See just how much you have to turn the dial back to click off after is has clicked on.).

Simplest fix is to fit an electronic unit (e.g. a programmable stat).

That's right - 20seconds on 20 seconds off is _short_ cycling. Some boiler let you set the off period.

[Dave Plowman (News)]

A mechanical thermostat of the two wire type *will* be inaccurate much of the time. Most have an accelerator coil to help this so need a neutral, so three wires. If you wish to stick with a two wire circuit, get a battery operated programmable type. Those use a modern solid state sensor which is far better.