Hi Dave,

Dave wrote:
On 30 Nov 2003 18:53:25 GMT, Sam Norris wrote:
I'm looking for a digital replacement so I can time things more
accurately. Do these things typically fit to a standard backplate
so the existing hole in the wall can be re-used?

Most now fit onto a single gang box but also overlap varying amounts
left and/or right, top and/or bottom. If you have 6" clearance all
round from the edge of current plate that shouldn't be an issue.

OK, I haven't clearance on the left but at least I know now what to
watch out for.

My main reason for posting is I'd like more on-off periods a day but
3-4 seems to be the maximum. Anyone know of a 24-hour, or maybe
7-day, timer that has more, e.g. 6?

I can't see a reason for more than about 3 HW periods(*) morning,
lunch & evening. If you want better control on the CH then you'd be
better off looking at a programmable thermostat, these typically have
6 set temperatures through the day and each day can be different. The
more recent and cheaper programmable stats are a little more limited
in the temps/times etc than the older ones such as the Danfoss TP75 or
Honeywell CM67.

I really am limited to replacing the existing timer with another timer.

However you need to investigate the wiring of your time control and
existing thermostats (both HW cylinder and room) as it appears you
only have simple ON/OFF control for HW *and* CH from your current
controller.

That's correct; the timer has no selection over HW or CH as per my
wiring suggestion. And I don't have a room thermostat.

(*) I have reservations about having HW on a regular time (morning &
evening) control. It can waste significant amounts of fuel, depending
on your life style.

The HW cylinder is zoned off; the two-port valve is controlled by a
cylinder thermostat. So I use the simple timer to say `make sure
everything that wants heat gets it' and if the HW is satisfied then only
the rads get heat, and they're TRV'd.

The boiler only goes active if a flow sensor downstream of the pump
detects significant flow indicating either the HW circuit is open, or
sufficient TRVs are open. If all TRVs are closed, and the HW satisfied,
there's a by-pass before the flow sensor.

Hope this makes it clearer. Looks like there may not be a timer with
the number of on/offs I need. Could a room stat do the task if I set
the required temp. such that it was never met? That is, would it fit in
with the existing wiring?

Thanks,

Sam.