Hi, we have a thermostat in our entrance hall for the central heating (boiler upstairs with the main timer unit near the boiler).. We turn the temperature down overnight and I am continually forgetting to do so at night, meaning a trip downstairs to do it. I believe one can get a thermostat with a timer which, I understand, I can set with different temperatures for day and night. But does anyone know if the wiring is different for that compared to a simple thermostat. Can I just replace the thermostat with a timer one or does it need additional wiring connecting it to the boiler. Any advice, please, Lawrie
Yes, a programmable thermostat will give you exactly that. I would personally recommend the Honeywell CM907 which will give you full 7- day control and upto 6 different temperature settings per day (you can delete unused ones to keep things simple) all wrapped up with a easy- to-use intuitive control interface. It has lots of other nice features such as day-off/holiday/party modes, optimum start and time- proportional-integral operation to maintain comfort levels.
It will be a straight swap. It's a 2-wire connection - permanent live coming in and a switched live going out. If you have a neutral present (and/or earth) these can be left disconnected.
As the stat will now also control the timing elements you should leave any existing timer on 'permanent-on' (and/or leave it to handle the hot water if under separate control).
I had a Centaurstat for many years - absolutely brilliant. I now have a RF one from Drayton to go with my new boiler.
All as Matthew said - just forget the timer - leave it on 24/7 and let the programmable stat do everything. You will not want to go back to the old way. I have mine set for 16 degrees during the night - as good as being off - but if it did get really cold then it would prevent the house from getting too cold.
Some have a terminal for you to 'park' the neutral as it is not required - take care over this. Make sure you identify it and seal with it correctly as my neighbour got the wires mixed and got a dead short when he switched on.
If your existing room stat is the mechanical type, it will almost certainly have an accelerator heater (resistor) which gets warm when the stat switches the boiler on in order to reduce hysteresis (the difference in temperature between its ON and OFF point). In that case, the stat will have a live feed, a switched live AND a neutral return for the accelerator heater. It's not simply a case that any neutral and earth wires can be left disconnected - they *MUST* not be connected to your new digital stat - or else they will cause a short circuit and a potentially expensive bang! They must be taped off and made safe. The new stat will be battery powered for its timing and logic and is effectively an on/off switch to which the live and switched live wires must be connected. However, because it can also handle cooling (air conditioning) applications, its switch will be of the change-over type with 3 connections. The 3rd connection is not used but, because it's there, some people are tempted to connect the neutral wire to it. *Don't!*
All of the above assumes that you will use a wired programmable stat, installed as a direct replacement for the existing stat, and utilising (some of) its wires. You can also buy wireless programmable stats - which come as two units. The base unit is mains powered, and does the actual switching - and is usually installed close to the boiler. The remote unit is battery powered, and free-standing - so you can move it around to find the best location for it. It's this unit which you program, and which has a display and contains the temperature sensor. This unit uses radio signals to communicate with the base unit, telling it when to switch the boiler on and off. This is probably an overkill for your application, and would require some wiring changes in order to install the base unit - but I thought I would mention it for completeness.
If it doesn't use neutral, how do the batteries which run it get recharged?
Do you have to keep changing them, or does it have some clever way of using the voltage between the switched and unswitched live wires (when it has switched the heating off) to power an internal charger? If it did this, it would obviously need to ensure its current draw is low enough not to trigger any switching relay which might be at the other end.
When I had one of these, a set of 3 Duracells would last over 3 years. Note the Neutral "parking" terminal. It is electrically isolated. The switching is called "Volts Free" - I guess it is a battery operated reed switch type of relay.
I strongly recommend a programmable thermostat to all and sundry. Stop fiddling with the old room stat - get this set once and enjoy the lack of extremes.
Not all makes of stat have a 'parking' terminal. I'm pretty sure that Honeywell stats don't. So you just have to make sure that the end of the old neutral wire is insulated and tucked out of the way. Or do like I did - by terminating the old cable in a junction box inside a cupboard, and running a 2-core cable from that to the stat.
I entirely agree with that. It will control the temperature much more closely than a manual stat, and save energy by not overshooting. It will also allow you to set different temperatures at different times of day - or even different days of the week - if you wish, and will also act as an automatic frost stat.
I'm also considering a programmable thermostat. But I wouldn't be happy with having two timers in two rooms, one for the central heating and one for the hot water. That rules out the Honeywell and most other programmable thermostats for me.
But I quite like the look of the Danfoss-Randall TP9000 with its remote temperature sensor. The remote sensor replaces the existing thermostat and needs only a two-wire connection. The TP9000 replaces the existing timer. It has lots of useful features such as different programmes for different days, intelligent switch-on times, simple overrides, and automatic summer/winter time adjustment. And, importantly for me, it does the hot water as well. And there are no batteries to worry about.
If you can arrange a cable with sufficient cores, you could use one of the PRTHW or PRTHW-N models. You might need a couple of relays in your wiring centre, depending on the inputs to your boiler
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about five years, the clock on mine now seems to drift at about half an hour a day, no idea if that that's a typical fault, somehow I doubt it.
Why not? It means - among other things - that the two are timed independently, and can be on at different times from each other so as not to compete for boiler output, if you so wish. You just have to make sure that anyone who may need to operate the controls knows what's what.
This arrangement works perfectly well for me, with the main timer (only effectively timing the HW) in the meter cupboard and a programmable stat in the hall, timing the CH - oh, and for good measure, a wireless programmable stat in the kitchen timing the under-floor heating in that room.
Thanks. Unfortunately it would be a lot of trouble to install those extra cores. I did take a good look at their wireless models PRTHW-W and PRT/HW-WTS but they were less than ideal in several respects.
I too thought that when I moved from a combi to a conventional boiler (as a result of moving house). I thought it would be a faff at best to have to manage two timers and was disappointed, and somewhat surprised, to find there are limited options when it comes to programmable stats with 2 channels (I think the only one I found was one of higher-end Heatmisers as already suggested).
However it then dawned on me that I rarely, if ever, change the HW timing - it just goes on for half-an-hour every day and that's enough for us. The only occasion when we need to deviate from this is for adhoc hot water requirements such as a bath in which case we'd need a boost button rather than a permanent modification in timings.
Hence I ended up with a nice simple timer in the airing cupboard (Sangamo PSW) for just the HW and a Honeywell CM927 for the CH. The setup works perfectly and I do not consider there to be any duplication/confusion of function.
Might it be worth you reconsidering? There really are significant benefits in having a programmable stat...
So when they do run out, the programmer will have forgotten the program, and you'll have forgotten not only how to program it, but also where you've put the instructions. :-(
No, all the settings on the Honeywell are held in non-volatile memory so will not be lost. There's a low battery warning so you'd receive plenty of warning.
Besides which, you wouldn't need the instructions as it is so easy to program anyway! ;-)
No, all the settings on the Honeywell are held in non-volatile memory so will not be lost. There's a low battery warning so you'd receive plenty of warning.
Besides which, you wouldn't need the instructions as it is so easy to program anyway! ;-)
Mathew
Instructions are on line for most stuff nowadays - I have a folder on my PC for instructions for most of the stuff I own.
On the Centaurstat the settings are not lost when the batteries go and you get a warning that they are going - as I have said - about 3 years life out of Duracells.
Thanks for your comments. I agree that there are considerable benefits, which is why I've been looking at my options. I think I've found a programmable stat (Danfoss-Randall TP9000) that does hot water, fits my existing wiring, has all the options I need, is a doddle to use, and doesn't use a battery. It's not particularly expensive either. I've pretty-much decided on that and only raised the subject here in the hope that it might prove informative for the OP.
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main thing I dislike about it (so far) is that according to the manual the "extra hour or two" function is disabled when the heating is "off". Why do they do that?
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