"usenet2012" <
usene...@invalid2012.org.uk> wrote in message
news:rhVbUSBZ...@stejonda.org.uk...
> In message <
aj2336...@mid.individual.net>, YAPH <
use...@yaph.co.uk>
> writes
>>On Fri, 14 Dec 2012 17:46:46 +0000, HRD wrote:
>>
>>> We have a Drayton Digistat RF1
> ...
>> And when the battery drawer on the sender
>>comes open because the stupid friggin clip that holds it shut has broken.
>>
>>Rinse and repeat until you get fed up enough to shell out for a Honeywell
>>CMT927 or suchlike :-|
I have a new Honywell CM 927 - which is why I asked if the 'call for heat'
sometimes worked and sometimes didn't.
The CM 927 is just too bl**dy smart at times.
It doesn't always respond to a 'call for heat' if it thinks it shouldn't.
From an online copy of the manual (at least one similar to the paper copy).
Symptom
"The thermostat's LCD display shows a symbol but the heating does not switch
on."
Posible Cause
"The heating is not required at this moment."
Remedy
"The heat demand is send to the HC60NG as a value between 0-100%.
The HC60NG will adjust the boiler on time to meet this requirement.
That is why the heating will be off sometimes even if the symbol is
displayed.
To check if RF system is communicating properly press the button to increase
the set temperature a few degrees above the current temperature. The heating
should come on after a few seconds."
Not quite the same wording as in my manual which says
"This is normal operation. The Relay Box cycles the relay on and off for
times proportional to the demand signal (0-100%) from the room thermostat.
The flame symbol only indicates that the demand value is greater than
0%.(!!)"
So the relay box is told by the thermostat that the room is colder than the
set value, and some heat is required.
The relay box then goes "Maybee I will, maybee I won't. It just depends.".
I think this is tied in with some Category 2 System Settings.
Ot is the minimum on time - set to 1 minute by default.
Cr is the Cycle Rate - set to 6 cycles per hour by default.
So if the thermostat calls for heat more than 6 times an hour you may not
get it.
Section 4.3 says that Cr 6 and Ot 1 is standard for gas boilers under 30kW.
It gives other settings for Oil boilers, thermal actuators and zone valves.
It doesn't however give values for gas boilers over 30kW.
Our boiler is rated at 38kW.
So at times it can be just too bl**dy clever by half.
If the thermostat is in an area with rapid fluctuation of temperature (e.g.
near an outside door - and a common location is the hall) then the
temperature can drop below the demand threshold each time the door opens.
You can see that if the house is warm and the hall radiator is warm then the
temperature could come up again quite quickly without firing up the boiler
so there does seem to be some sense in the setting.
However if you get a lot of in-and-out traffic at the front door this may
mean the house is without heating for the next hour.
I am currently puzzling as to what values to set for our 38kW boiler -
longer minimum run time, more cycles per hour, both.
So it would be useful if the OP came back with more information on the
problem.
Cheers
Dave R