Programmable Thermostat vs Programmer

[rudeboy]

I've read through some posts on PT's and there is no doubt on their usefulness. I intend to replace a mech. stat with a Honeywell CM67/907.

What is usually done with the standard programmer for the CH? To make full use of the PT, it would appear the heating should be on 24/7 to allow for frost protection as well as Optimum start, and leave the thermostat to call for heat as and when it wants?

If this is the case, other than providing switching for HW, what is the point of the programmer? Also if timer IS set for CH, say, to start at 06:00 and on a cold day the CM67 wants to kick off at 05:30, does it just start as soon as it can?

 

[Softus]

What is usually done with the standard programmer for the CH?

Depends on the system, but very often it's left in place and uncoupled with the CH channel disconnected. It helps if you have a wiring centre in place.

To make full use of the PT, it would appear the heating should be on 24/7 to allow for frost protection as well as Optimum start, and leave the thermostat to call for heat as and when it wants?

If it's disconnected on the CH side then it doesn't matter what CH periods remain set or not set.

If this is the case, other than providing switching for HW, what is the point of the programmer?

Nothing.

Also if timer IS set for CH, say, to start at 06:00 and on a cold day the CM67 wants to kick off at 05:30, does it just start as soon as it can?

See above - the programmer shouldn't needn't be left to control anything to do with the CH.

Bear in mind that you should wire your bits such that all components can be isolated electrically using a single switch, to avoid confusion.

 

[simond]

We just advise customers to set the CH side of their twin channel prog to 'on 24hrs' or similar.

Nearly all HW/CH programmers have a permanent running mode selectable in each channel.

The CM67 is now obsolete, by the way.

 

[Softus]

There's still stock around though.

 

[rudeboy]

Yeah, just picked one up on eBay. The 907's very pretty, but twice the price.

Thanks guys, you've confirmed what I suspected. New wiring centre's in the post too

 

[simond]

Earlier versions of CM67 had a dubious implementation of optimum start.

Heating would come on at 3am to be ready for 7am etc. If in any doubt of the age of the unit, turn off optimum start in the second level installer menu.

A few years back we used to turn off OS on every install, as we had a few callbacks. Newer models seem OK in this respect.