Worcester Bosch 37 CDi Programmable Thermostat Wiring Advice

9 May 2010
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United Kingdom

I’m having a Worcester Bosch 37 CDi fitted next week by a Gas Safe plumber. If I have a Worcester Bosch DT10RF Digistat Optimiser (MkII) programmable wireless thermostat fitted, then I can (obviously) set the heating times and temperatures via the wireless thermostat. However, I see it also controls the hot water preheat to periods set on the plug-in receiver (as opposed to being preheated all the time). This sounds desirable to me.

I keep stumbling across poor reviews of the DT10RF but see that the Honeywell CMT927 rates very highly and, I like the idea of the functions on the Honeywell too. The down side of fitting the Honeywell would be that I lose the preheat time control offered by the DT10RF.

So, my question is (if you haven’t guessed), can the 37 CDi be wired to a CMT927 receiver and another timer to control the hot water preheat? If so, how?

I was thinking maybe a Worcester Bosch DT20 and a CMT927 however, maybe there is a simpler way using a single channel timer.

Many Thanks,
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I don't believe you can combine the DT timer with any room stat other than the FR10.

However, in the absence of any specific detail in the manufacturer documentation I suggest you call WB, you will find their technical people very helpful.

When we fit Worcesters (which is most of the time) we don't use any of their controls, preferring the Honeywell range for reliability and ease of use. In real terms you will find there is little advantage in timing HW preheat.
In real terms you will find there is little advantage in timing HW preheat.
Another sweeping statement based on no information about the needs of the customer. For someone on a schedule where water is required regularly at some times and little or not at all at other periods, this is a useful feature. Preheat doesn't quite suck up the gas the way it used to on older combis, but it is still a complete waste if you don't need the water and if the lost heat doesn't go somewhere useful. There is far greater scope for savings here than with some other features that are regularly recommended, in the right situation.

Or for that matter, the preheat cycle disturbs the sleep of some people and so gets turned off completely when it could be useful. Preheat also cycles the boiler every 10-20 minutes 24/7 which some fitters have claimed on this board will lead to failures within a short period. I'm a little sceptical about that, but you certainly can't claim that cycling on the heating every 10-20 minutes for a few hours a day is a bad thing but cycling on the hot water every 10-20 minutes every hour of every single day is OK.
What are your credentials exactly? Are you an avid forum reader or a central heating 'fitter'?

Your views are worth as much as mine on this forum, but can you qualify your comments on energy wastage and noise?

I don't go around telling my customers not to have preheat timing; but I do point out that the manufacturer proprietary controls for enabling that functionality are normally counter intuitive and needlessly complex. Vaillant and Worcester both fit this bill. I know that because I fit hundreds and take the calls from customers who have trouble with the controls.

I don't give out advice gleaned from surfing internet forums because
a) I don't how accurate the information is
b) my customers can look it up themselves and make their own mind up

Are you an internet regurgitator or do you know what you are talking about? From reading your post regarding cycling I know the answer.
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Insult me all you like. The statement remains. Fit what the customer needs instead of telling them they shouldn't be living that way and actually need "Gadget X".

I have no credentials. I have no NVQ Level 2 piece of paper and I have no Gas Safe badge. I am an ordinary person who respects the way that different people live their lives instead of ignoring them and selling them what the fitter thinks is great this week.

Thanks for your answers, I do feel you’ve gone “off topic” on my behalf.

As it goes, my circumstance is single and working so I only need preheat first thing in the morning and, maybe two or three hours in the evening. As I see it, I would save a fair amount of dosh if it was switched off twenty hours a day, maybe this is not the case?

Simond, you say you consider the preheat controls “counter intuitive and needlessly complex”, are you suggesting preheat doesn’t cost much to run 24 / 7 or are you saying don’t use preheat?

I'm saying that keeping 1 or 2 litres of water hot in the boiler is not going to be using a significant amount of energy.

I can see the advantages of the preheat function and in the past we would use Vaillant VRT340s and more recently VRT360s to give preheat timing, plus the Worcester in built digital clocks.

But most of our customers forget how to programme them and ring me up. Having called me on a Saturday they would then ask why I fitted such a complex controller 'when all I wanted was a simple controller'.

So by all means have the preheat programmable but this is not usually possible from 3rd party controls.
I rang Worcester Bosch, they basically said you can fit any twin channel programmer to achive this.
The preheat function alone uses 3-4kWH of gas daily. The exact amount depends on the boiler, the programmed water temperature, and the ambient temperature, but seems to be surprisingly consistent across a range of boilers. A fair proportion of the heat goes straight out of the flue, but the amount that goes into the water then leaks away over a couple of hours into the room housing the boiler. When there is central heating demand, the preheat will not fire and you should factor this into any cost calculation based on the average daily time when your controller will be demanding heat.

Each time that you demand hot water, you can consider that you are "saving" 0.25-0.5 kWH which would otherwise have gone to heating the primary circuit but has already been provided by the preheat (or you can visualise it as heat left behind after the hot water demand which doesn't then need to be provided by the preheat). With frequent water use, you obviously wouldn't "save" the full amount each time because the water would have still been warm even without the preheat and so you will never use less gas with the preheat on, but you may use more or less the same amount as if it was off. If you usually use hot water every few hours throughout the day and night then it costs you little or nothing to run preheat.

The other possible cost is wasted water. Without preheat you may run an extra few litres of water down the drain waiting for your shower to come warm. Or you may not, for example filling a bath. If you do, and if you pay for that water, then you could be saving about 2p each time by having the preheat function on. Sounds trivial, but add it up a thousand times a year!

Maximum cost of preheat (with natural gas fuel at UK 2010 prices) 30-40 quid a year, assuming you rarely use hot water. Only you can work out how much of this is wasted (boiler in loft, for example, or kitchen hot enough anyway), how much is usefully contributing to hot water you would use anyway, and the cost of water you might be saving.
What a fantastic and comprehensive answer on preheat costings.

As a single, full-time worker, I can see that if I want the luxury of preheat then installing a timed function is a no brainer. I’m surprised at the lack of competition for two channel wireless programmable thermostats, looks like I’m stuck with Worcester Bosch’s own unit.

Thanks for your answer ianniann, I was going down the wrong route; I was thinking timed preheat was desirable when in fact, for my circumstances, it’s as good as essential.

A Million Thanks,
I know this seems to be a dead thread but this will correct some mis-information contained in it. This is just for completeness to anyone discovering the thread in the future.

You can connect two controllers to a 37 CDi boiler (please check with Worcester Bosch yourself before making any commitment).

Today I emailed Worcester Bosch asking the question, fit a DT20 to the boiler and a third party controller to ST10. Here’s their answer:

Dear Mr SOUP

Thank you for your enquiry.

To do what you want you would need to set the heating on the DT20 permanently on then use the Honeywell programmable room thermostat to control the heating. The DT20 can then be used to control just the hot water function. Alternatively you could use our DT10 RF optimiser.

Kind regards
Technical Support
Worcester, Bosch Group
To clarify what WB are describing in their email.

The WB controller fits directly into the boiler fascia and has a ribbon connector direct to the PCB giving full control over anything they want. In this case, it is used to time the pre-heat function and to leave the heating permanently "on".

The second controller is wired to the normal rather simple "live return" connections and will over-ride the central heating "on" setting of the WB controller, but won't provide any more advanced functions.

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