The CH and HW is driven by a Danfoss 24-hour `clock face' timer that has two on-off periods a day. It's six terminals seem to be wired
1 switched live output 2 unused 3 switched live input, connected from 6 4 earth, not connected - is this normal? 5 neutral 6 live
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?
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?
Hi Most now do 3 time zones and 7 day mine came from screwfix and combined with a digital stat that has 2 periods this combination should be enough for any one. and the mostly do use the standard plate.
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.
There has been some standardisation on back plates over recent years. One of the makers has a nice free booklet that details old controllers and their wiring and how to wire up to (their) new controllers. This is from Horstmann I think.
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.
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.
(*) 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. I had a simple mechanical timer on the HW/CH it broke but at the time I was living alone, the HW tank was well insulated and would last 2 days hot enough for washing/washing up. If I wanted a bath I'd manually switch on the boiler, reheat time was about 20mins. There was about =A310/qtr reduction in may gas bill between having the timer and manual system. Needless to say they timer didn't get replaced until I sold and moved out of that flat...
A have a programmable room thermostat (Sunvic) which can have up to 12 on-off periods a day. However, it can only be switched an hourly intervals. You'd need to replace your room thermostat with it and set your original programmer to always on.
Now if you'd posted properly, below the section of the post that you are refering to and trimmed out anything not relevant I wouldn't have to guess as to which part you are commenting.
I think that although the back plates are pretty standardised now, unfortunately they weren't when the Danfoss mechanical timer was designed. I remember years ago trying to do what you want to, and in the absence of any decent wiring instructions I found the whole jumble of wires behind the controller so confusing I ended up refitting the Danfoss and giving up on the idea!
Have a browse around
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though - they have some very useful reference material on CH timers amongst other things.
The system doesn't have a room thermostat and I don't wish to add one given the current task in hand. All I need is a more precise timer, e.g. on at 7am sharp for 90 minutes, and more on/offs a day. Three on/offs wouldn't really be much of an improvement over my current two. Any other ideas?
OK, I haven't clearance on the left but at least I know now what to watch out for.
I really am limited to replacing the existing timer with another timer.
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.
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?
One solution would be a plugin timer of the sort that you move lugs in/out or insert pins into to set on/off times. Not particulary elegant or easy to to wire into your existing cabling but not impossible.
Best bet is to dig about on the various manufactures websites, google is your friend. Remember if you get a CH HW controller with 3 independant periods/day you could parallel the contacts and get 6 periods. Horstman do a 3 channel timer...
In which case it would always be on... A programable room stat turns on/off in response to the rooms temperature, the time element controls the times that a given temperature is set. eg. 0700 > 0900 20C, 0900 >
1600 18.5C, 1600 > 2300 21C, 2300 > 0700 15C.
I think my TP75 has an "off" setting as well as various temperatures but to use it just as a time switch would be a little wasteful.
It is permissible to have all TRVs instead of a room thermostat and TRVs in other rooms. However, your system should have a boiler interlock. This turns off the boiler when all the TRVs are closed. This is done with a flow switch on the boiler automatic bypass, or on the central heating circuit. Your system may not have such a bypass or flow switch, in which case it may be inefficient. Also, without an automatic bypass, you may find that the pump overrun is ineffective, which can cause problems for your boiler.
Been there, and think I've figured out what the wiring is doing.
There's L, N, and E supplied through a fused switch. L and N supply 5 and 6 on the Danfoss timer.
1 switched live output 2 unused 3 switched live input, connected from 6 4 earth, not connected - is this normal? 5 neutral 6 live
The LNE also continue up from the kitchen to the control unit in the airing cupboard. From there another cable comes down which is used for L, N, and E for the boiler plus a pump-overrun live, and a switched live.
Thanks, I found this helpful. They have two `Wire-in Time Clocks, Fused and Switched' --
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the 7 day one, FST17 seems to have 6 on/offs a day.
They provide a PDF of the installation instructions that covers both the FST11 (24 hour) and FST17 (7 day) --
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-- which says "A normal switched output is available as well as the timeswitch output" and refers to Fig. 3. But where's the "normal switched output" as terminal 1 is the timeswitched live?
Apart from that, it would seem suitable, no? I don't need the fused switch aspect since there's already one sitting next to the Danfoss timer but I guess two in series won't do any harm. Alternatively, I could replace the existed fused switch with the FST17 and just leave the Danfoss sitting there unused, assuming the cables will reach.
Ah, you mean I've 3 on/offs for HW and 3 independent on/offs for CH, but in my case they'd both just feed a single switched live through to the rest of the system. I didn't appreciate that when something like the Horstmann C27 programmer, Screwfix 11225, says 3 on/offs per 24 hours that would be 3 for HW and 3 for CH and not 3, each of which can be HW, CH, or HW+CH.
Yes, you just have to ensure that the programmer has independant switching for HW and CH, most these days do.
A switched output.
Wonderful devices, the house is always the right temp, once you've got it programmed for the correct temps/times it's fit and forget. Noticable reduction if fuel consumption over a simple room stat/time switch combination and the house is far more comfortable.
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