CENTRAL HEATING TEMPERATURE CONTROLLER/THERMOSTAT
- Thread starter Peter GEE
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There was a thread about swapping the 425 (with some wiring work) to a Hive system.
https://www.diynot.com/diy/threads/changing-horstmann-425-diadem-to-hive-dual-channel.491978/
https://www.diynot.com/diy/threads/changing-horstmann-425-diadem-to-hive-dual-channel.491978/
I had similar controls but when I had the system upgraded a couple of years ago, they fitted a Honeywell DT90E room stat
And a Honeywell ST9400C Programmer.
I’m very pleased with both.
And a Honeywell ST9400C Programmer.
I’m very pleased with both.
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@Peter GEE You don't give any details of the heating system being controlled. Some old systems (ie those without motorised valves) will only allow the central heating to operate if the hot water is also 'on' at the same time. The existing Tiara programmer can be set to work with either older and newer systems. (there's a screwdriver slot at the top of one of the sliders) If it is an older system that doesn't presently allow independent control of the heating and hot water; when the CH slider is moved up to put the heating 'on' the hot water slider will automatically move up with it at the same time.
I mention this because some have wanted to fit modern controls to an old heating system, only to find out later that they don't work as they were expected to due to the restrictions of the plumbing. This won't be an issue if the existing system already allows independent control of the heating and hot water.
I mention this because some have wanted to fit modern controls to an old heating system, only to find out later that they don't work as they were expected to due to the restrictions of the plumbing. This won't be an issue if the existing system already allows independent control of the heating and hot water.
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It seems odd to have hot water controls like you show with a combi boiler, with a combi boiler normally there is only a wall thermostat and programmer for the central heating, which can be combined into a single unit like this 
which instead of switching on/off at set times switched up/down so for example 17°C over night and 20°C in the day.
To include domestic hot water the price tends to jump from £35 to £135 with units like Hive and Nest.
There is also a big difference between hard wired and wireless, the hard wired any cheap programmable thermostat will do, with wireless the unit needs to fail safe, the thermostat shown uses 2 AA batteries which last around 2 years, so swapped every year no problem, if they go flat no central heating, but with cheap wireless flat batteries can cause it to stick on.
Often all the wall thermostat does is turn off heating in the summer, it stops the boiler cycling on/off, using some thing like Nest Gen 3 to do that is like using a sledge hammer to crack a nut. So what do you have, some more details.
which instead of switching on/off at set times switched up/down so for example 17°C over night and 20°C in the day.To include domestic hot water the price tends to jump from £35 to £135 with units like Hive and Nest.
There is also a big difference between hard wired and wireless, the hard wired any cheap programmable thermostat will do, with wireless the unit needs to fail safe, the thermostat shown uses 2 AA batteries which last around 2 years, so swapped every year no problem, if they go flat no central heating, but with cheap wireless flat batteries can cause it to stick on.
Often all the wall thermostat does is turn off heating in the summer, it stops the boiler cycling on/off, using some thing like Nest Gen 3 to do that is like using a sledge hammer to crack a nut. So what do you have, some more details.
Are sure ? if so why do you have a programmer ?post a pic of the control pnel on your boiler, do you have a hot water cylinder ?
All I know is I inherited the boiler and it had the programmer already there but thinking it through I have never tried demanding hot water with the HW control set to OFF. I will try it. May be the previous installer only had the HW CH programmer and no other?
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I think that would be unlikely. With an installation having just the components you describe there are a maximum of 3 things that can be connected to the Tiara programmer. The mains 230V supply, the room thermostat and the boiler. And, if the 230V supply was connected to the boiler first instead; only two: the thermostat and the boiler. However, I can see at least 4 cables connected to it. [Although it's possible not all of the wires in them are used.] However, taking this, the unused hot water function, and the vintage of the controls into consideration, I suspect that the controls remain from an earlier installation that did have a hot water cylinder, and has been 'mashed up' to control a new combi boiler. If so, for me there would be no option other than to take the old controls out and start again from scratch. Unfortunately it will be difficult to give any further advice regarding the existing installation without knowing what each of the wires in the existing cables is connected to at their other end.
That new combi boiler is at least 30 years old, the installation instructions back then showed a Horstman time switck, looks like someone has used a programmer and the HW side wont be wired to anything, but who knows what the untrained do
Thanks interesting viewpoint. I know for a fact that there never was a hot water cylinder as the property was built in April/May 1991 and boiler fitted in July 1991.
Good guess at 30 years old then
Wow!! a combi that's lasted longer than 15 years, that's a new one on me. 
Still don't get all the cables though. Unless they only had 3 core cable 30 years ago; or combi's were wired differently back then. I would expect to see no more than 3 cables at the programmer.
Should be fairly easy to find out how it's wired from the wires at the Tiara, terminals 1 & 3 (hot water control) won't have any wires in.
Still don't get all the cables though. Unless they only had 3 core cable 30 years ago; or combi's were wired differently back then. I would expect to see no more than 3 cables at the programmer.
Should be fairly easy to find out how it's wired from the wires at the Tiara, terminals 1 & 3 (hot water control) won't have any wires in.
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