Room Thermostats

[Alfie25]

Apoogies for another thread on thermostats but I'm still trying to get my head round them. I currently have an old Sunvic TLX 2259 which has 3 wires + earth. This is an old model from the 1980's - is this likely to have an anticpator, hence the 3 wires?

Some of the replacements I'm looking at require 2 wires only. Some are live and neutral, others are live and switched live.

I've read posts where people are talking about replacement stats requiring live and switched live (no neutral), but the mechanical sunvic tlx 4101 2 wire seems to need a live and neutral? Same goes for the honeywell DT90E electronic stat. Can anyone clarify please what the differences are please, and why some need live and switched live, and others need live and neutral.

 

[Dan Robinson]

DT90e does not need a neutral.

Switched live in.

Switched live out.


Can't get much simpler. If this confuses you then I would politely suggest getting a professional.

 

[Whitespirit66]

The TLX2259 does have an anticipator.

Also, the Sunvic is the same as the DT90e, in that it doesn't have a neutral. If you look at the wiring diagrams for both, the live goes in on one terminal (term 2 for Sunvic, term. A for the Honeywell), then through a switch and out of another terminal (term. 1 for Sunvic, and term B for the Honeywell). The neutrals are only connected to the load and go nowhere near the stats.

Your stat has a neutral, as it needs L and N to power it - mainly the anticipator. The others have batteries, so don't need the neutral.

 

[Alfie25]

Ok thanks, I'm not going to be touching it but I'm just looking at wired replacements as I don't fancy wireless, and thats all installers seem to be pushing these days.

Do you guys still fit a regular thermostat and separate timeswitch/programmer if the customer so desires? These programmable thermostats seem to be all the rage these days but I'm really not bothered about varying the temp at different times of the day, so I'm looking for something simple.

The honeywell cm907 looks pretty decent in that regard, with a good range of functions, but the chrono proportional control doesn't appeal to me, cycling the boiler on and off. I see the Danfos TP7000 allows you to turn it off, however its range of features is pretty limited, which is why i've arrived back at the separate thermostat and programmer idea.

 

[Dan Robinson]

CM907 is very easy to use and effective. You can turn off the optimisation if needs be - in fact it comes disabled. Personally I love the optimisation features and they work well in my house - complete with weather compensation.

No decent installer will fit a single product. Always best to suit the installation and customer.

Only last week I put in a traditional Y plan with a T6360 twiddly dial thermostat and dual channel timer; as it was a rental flat and tenants are inherently stupid and the Landlord wanted to keep things simple.

 

[Alfie25]

Haha I hope you're not kindly sayng I'm stupid

Regardless, the optimisation feature of the cm907 is one thing (and can indeed be switched off), but the TPI/chrono proportional control is another and cannot be switched off.

I do like the look of the T6360 dial stat, looks like a direct replacement for the old tlx2259 i have now. It says there's a lamp on it that illuminates when the stat calls for heat. Do you know if it's only when the timeswitch is set to On, or does it light whenever the room temp is below the set temp?

Oh, and as a rule do you need to get a volt free stat for a combi boiler or will it take a mains voltage one?

 

[Dan Robinson]

I rate most Sunvic stuff the same as I would Gonorrhoea so I'm not familiar with the various models.

TBH I hardly ever do any bimetalic room control stuff nowadays as the DT90e and CM907 are as basic as most things need to be.


If you saw some of the stupid questions we get asked by tenants you might understand the position of professionals with regards a tenants IQ. If we are being fair we put it down to them not owning stuff so not caring as much about how/why things work and how much stuff is in £'s.


Stick to Honeywell anyway.

 

[Alfie25]

If you saw some of the stupid questions we get asked by tenants you might understand the position of professionals with regards a tenants IQ. If we are being fair we put it down to them not owning stuff so not caring as much about how/why things work and how much stuff is in £'s.

Hit the nail on the head there Dan. Now I've bought a house I do care about how/why things work. The cost doesn't really bother me (within reason) I just want simple controls, with my general mantra being the less complicated it is, the less there is to go wrong.

 

[Dan Robinson]

DT90e all the way then.

Although don't discount the CM907 out of hand.

 

[Alfie25]

Well as I understand it, from Honeywell's own site, the DT90e has TPI control (time proportional integral) which switches the boiler on and off in cycles as the stat approaches the set temp, to prevent overshoot.

The cm907 has chrono proportional control but by its description, this is essentially the same feature as TPI, i.e. cycling the boiler as the stat gets within 1.5 degrees of the set temp.

Now I know a lot of people see this boiler cycling as a good thing, I'm not sold on the idea and would prefer a stat that shuts off when the set temp is reached, rather than one that beats about the bush (so to speak) to maintain the set temp.

 

[Dan Robinson]

The difference between the two is that the CM907 will work out what time it needs to start up in order to reach the target temperature and the time programmed in. That can bit can be turned off.

Boiler cycling vs. temperature overshoot - that is a personal decision.

I hate boiler cycling too but for different reasons. Temp overshoot is just wasting energy.


You can always reduce the proportional bandwidth.

 

[Alfie25]

I think we're at cross purposes here. I do know what you're talking about with regards the cm907 working out what time it needs to come on vs. the time programmed in, and that being the 'optimisation' feature which can be disabled.

What's more concerning for me is the chrono-proportional control that cycles the boiler as it approaches the set temp. As you know, this will cycle the boiler (default is 6 times an hour for a gas fired boiler) in order to maintain the set temp. Personally I don't want this feature and would rather the stat reach the set temp and then shut off the boiler until the temp drops. Yes this would switch the boiler off then on, but at a much slower rate than cycling it 6 times an hour to maintain temp.

Cheers for the discussion by the way, i'd love to get this bottomed out and get a programmer/stat bought.

 

[Dan Robinson]

Hence reducing proportional bandwidth.

Why the thing about boiler cycling?

 

[Alfie25]

Sorry I'm not following you on the proportional bandwidth thing. Can you explain please?

If you mean reducing the cycle rate, I believe the cm907 can be set to a minimum of 3 times per hour, but thats for oil. For gas, it recommends 6 times an hour. In my eyes, no good for the boilers components constantly cycling like that.

Oh, and is there any other benefit to a 'one unit' programmable stat cm907, vs. a separate stat (T6360) with a timeswitch (ST9100c)

 

[Dan Robinson]

Having a cm907 means the heating program can be accessed usually from some where more amendable than the airing cupboard.

Proportional bandwidth is the area in the temperature range where tpi kicks in. Lowering it to say 0.5 means it works much more closely to your target temp.

Search the forum for tpi, as a handy off is linked to that explains it nicely.