Central Heating - timeswitch / thermostat

[Tim Smith]

Hi

I realise that this is a simialr question to one asked recently but I getting confused so I thought Id ask my specific questions.

I have a combi boiler heating system which obviously provides hot water on demand. This boiler has a timeswithc next to to it - old fashioned sem-mechanical type. There is also a dial type thermostat elsewhere in the house.

What I would like to do would be to repalce both these with something giving a bit more control.

What I am trying to understand then is can I use a programmable theremostat and simply replace the thermostat with this. If this is correct then I assume that the rahter than simply using the temperature the decide whether to turn the heating on, the thermostat factors in the time of day as well - is this correct ?

Furthermore, making the assumption that this is correct, I assume that I would have to leave the origianl timeswitch to permanently on and let the thermostat control the heating. Is this correct?

Can anyone recommend a particular model which is any good ?

Thanks for all your help.

Tim

[Christian McArdle]

Yes.

The facilities vary considerably and are down to personal preference. Some features that you might want to consider are listed:

1. Boost setting (1/2 hours, etc.)
2. Advance setting.
3. Manual Temp override.
4. 7 day, 5/2 day, 1 day operation.
5. Number of changes allowed per day.
6. Number of temperature settings allowed. (some are limited to 2 or 3 temperatures)
7. Optimum Start (Preempts change according to how cold it is)
8. Holiday mode (turns to frost setting for 'x' days)
9. RF wireless control.
10. Sophisticated display.
11. Appearance.
12. Granularity of timing.
13. Backlight.

I have a Sunvic model.

works slightly differently to many thermostats in that rather than having

2 or 3 periods to set during the day, it has 24 "dots" to represent each hour. Each hour can then be set to "day" or "night" temperature. This wouldn't suit everyone, but I quite like it. It also has a very good backlit display, which shows temperature, day/night mode, time, "call for heat" and the entire day's programme (as the 24 dots) simultaneously, so you can tell at a glance when the heating is due to turn on and off.

Other features is has are (almost) 7 day operation, advance (but not boost), manual temp override, optimum start and the best appearance I've seen on a thermostat. It is relatively shallow and looks very elegant. A fold down cover hides all the programming buttons.

Things it is missing are holiday mode, boost, rf, less than 1 hour granularity and having more than 2 set temperatures.

Christian.

[Dave Plowman (News)]

Yes. Mine - a Horsmann - has four periods per 24 hours. You can set these to what you want, time wise, and temperature wise. For example:-

Morning 0700-0900 20C Daytime 0900-1800 18C Evening 1800-0030 21C Night 0030-0700 15C

It has separate settings for nominated 'weekend' days when everyone is relaxing :-)

You can also over-ride the temperature at any time, and it will revert to the preset one at the next 'zone'

It's a bit fiddly to set, though.

I still use the normal two time periods controller, though, ie set to 'on twice'. If I'm out all day and the house is unoccupied. And only have it on overnight in the very coldest weather, where my system might struggle to reach the required temperature by getting up time. So when I'm in, use the 'on once' setting. And continuous for a cold snap.

It's easily recovered its cost in short order.

[Tim Smith]

I was just doing some testing on my existing thermostat as I dont believe it is working quite right anyway. It is an old(ish) Honeywell T40 electro-mechanical type.

It has 3 connections.

Terminal 1. Red Terminal 2. Blue Terminal 3. Yellow

The Red appears to be live (approximately 196V measured against terminal 3 on alternating voltage scale on the voltmeter).

The Blue appears to be either 1.5V or 3V (also measured against Termianl 2) depending whether the thermostat is turned right down or right up

Does this sound right ? I was expecting that the switched live would either be 0V or live. I have had problems with the heating being on but never really reaching the desired level (although I assume that these old mechanical types are reasonably inaccurate).

Thanks

Tim

[Dave Plowman (News)]

Yup. That will be the line in.

That's likely to be a neutral - needed for the accelerator coil in a mechanical stat.

The yellow will be the switch return to the pump. It should vary from near 0 to mains depending on whether the thermostat is satisfied or not.

They're not terribly accurate in terms of operating at the temp on their scale, but should be reasonably consistent. And of course any thermostat can only measure the local temperature. Where you sit in the room may well be colder.

Modern programmable electronic types are battery operated so don't need the neutral. The reason for this is they require continuous volts for their clock and electronics, and of course the live feed to a thermostat gets switched off if you switch off the system. Suppose they could have used a re-chargeable, but in practice the battery is cheap and has a long life.