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.

On Fri, 17 Nov 2006 14:20:17 -0800, Golan wrote:

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.

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).



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.

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



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html
Gas Fitting Standards Docs he http://www.makewrite.demon.co.uk/GasFittingStandards

In article .com,
Golan wrote:
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.

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.

--
*We are born naked, wet, and hungry. Then things get worse.

Dave Plowman London SW
To e-mail, change noise into sound.

In message .com,
Golan writes

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.


Or more precisely, it's a software function

--
geoff