Honeywell CM 927 - Relay On/Off % Setting

eltel · 3 Jun 2011

Please could someone clarify the "Loss of Communication Setting" option (parameter 7LC) on the CM927. I understand the principle in that if communication is lost between the two units then fail-safe keeps the relay and hence boiler off as one option (0) but what exactly are the time periods used when set as 20% on 80% off (1)?

Is this 20% "on" in an hour/day/week etc? I know the total "on" figure will be the same i.e. 20% "on" in an hour will be the same as 20% "on" in any multiple of one hour but does anyone know what the base-line time is against which the 20% is calculated?

Hope this makes sense!

Thanks

D_Hailsham · 4 Jun 2011

I would assume it is the cycle time (parameter 2:Cr), which is normally 10 minutes for a gas boiler (6 cycles per hour).

So the boiler would be on for 2 minutes and off for 8.

I haven't bothered to turn this feature on as, if it is on, you may not realize there is something wrong!

eltel · 5 Jun 2011

@ D_Hailsham - thanks for your reply - I hadn't thought of the P2 setting. I'd assumed if it was in the failsafe state the red LED would be on on the relay box to let you know? I need to read the instructions again. My concern was that when leaving my property for a long period in the winter using this setting would at least gaurantee some heat if there was a thermostat failure - but then there are plenty of other things that might fail!

Cheers

D_Hailsham · 5 Jun 2011

eltel said:
I'd assumed if it was in the failsafe state the red LED would be on on the relay box to let you know?

You are right about that - see Trouble Shooting Guide on back page of Installation Instructions. But it assumes the relay box is in open view.

My concern was that when leaving my property for a long period in the winter using this setting would at least guarantee some heat if there was a thermostat failure

In that case it probably is a good idea to turn the feature on. You should also set the room temperature to a lower temperature during the day time, 5-10C to prevent freezing up. You could use the Holiday feature, which allows you to set a fixed temperature of 10C for up to 99 days. After which the regular time/temperature schedule returns.

eltel · 5 Jun 2011

Thanks for your advice - appreciated. One quick question - with my old Horstmann wireless thermostat under normal operation if there was a power cut when it was calling for heat then that was it until the next programme change which called for an increase in temperature - unless you manually lowered and raised the temperature to "trigger" a new heating cycle. This was simply because it only ever sent one start signal. I assume then that the Honeywell doesn't do this and under Auto mode would cycle on again once power was restored and not need a programme change or manual temperature change to restart the heating?

Or have I got that completely wrong (did I say a quick question!)

Thanks again.

D_Hailsham · 5 Jun 2011

That's correct. The thermostat is sending a signal to the boiler every ten minutes, telling it how long it should be on for over the next ten minutes. So, when the power is restored there might be a short delay (9'59" max) until the relay receives a signal from the thermostat. You could shorten this by manually altering the setting of the thermostat to fool the boiler into coming on.

Of course, if the room temperature has dropped by more than 1.5 degrees while the power has been off, the boiler will light as soon as the power is restored.

eltel · 5 Jun 2011

Excellent - thanks very much.

D_Hailsham · 5 Jun 2011

May I draw your attention to the little icon which appears in the top right corner of all posts, except your own?

That's how you earn brownie points!

I've given you one as a starter.

simond · 5 Jun 2011

I've given you one, D_H.

Seems like you were desperate, so I thought I would wade in. Well done, sleep tight.

Don't let the bugs bite.

D_Hailsham · 6 Jun 2011

simond said:
I've given you one, D_H.

Seems like you were desperate, so I thought I would wade in. Well done, sleep tight.

I think you need them more than me! So here's my contribution! But let's not start a mutual admiration society.

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Honeywell CM 927 - Relay On/Off % Setting

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