Worcester Bosch Weather compensation and UFH

I've read through a lot of the chat about boilers and heating controls that's been posted in the last few months, but haven't managed to find an answer to my specific question, so here goes:
We are in the later stages of building an extension, which has involved removing our tiny old Ideal combi boiler. It's going to be replaced with a shiny Worcester Bosch Greenstar 42CDi combi (not negotiable, there is a sizeable discount available to us on Bosch kit).
The heating system is a bunch of radiators, and wet UFH in the new lounge, stairwell and kitchen.
This doesn't sound like it should be particularly complex or difficult to control: but we need to make a choice between:
FR110 Programmable Room Thermostat 7716192066 FW100 Weather Compensation Controller 7716192067 DT10RF Digistat Optimiser 7716192053
Now these all have slightly different features:
* The RF one allows us to position the thermostat anywhere we like in the house * But the UHF rooms will have their own thermostat on the wall anyway which controls a zone valve on the manifold * The weather compensation controller claims to save energy by monitoring the external temperature and measuring how quickly the house normally heats up * The FR and the FW claim to modulate the boiler to stop temperature cycling.
What I'm inclined to do is to buy the FW100 and put the thermostat in the master bedroom upstairs (which is the only room we always want to heat which isn't on UFH), the rest of the radiators on TRVs.
But the question is, how weather compensation and intelligent boiler controls can be connected to a UFH system which just calls for heat in an on-off fashion? And does this destroy the efficiency?
We are still trying to choose between UFH systems from Polypipe and Robbens Systems, to add a bit more confusion.
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Jim wrote:

I commend you on your perseverance! ;-)

Does this zone valve do any blending to achieve lower flow temperatures typically used for UFH, or does it just modulate the on/off demand to control them?

If that is the room that typically heats last, then its an "ok" place for the stat...

To answer the second question first, no it will not destroy the efficiency. The weather compensation achieves the results it does by adjusting the temperature to suit the outside temperature and the characteristics of the building. In the past that gave greater comfort, and these days it also maintains optimum condensing efficiency for more of the heating season. Where you sink that heat is not directly of concern to the boiler.
The first bit, about how you interface an external demand I can't answer definitively with that stat / boiler combination - since its not covered in the compensator docs (it may be in the boiler manual - which I have not read in detail). I would assume that any additional demand would be met with a flow temperature as dictated by the compensator. The only time that might be a limitation the UFH is if you are using some fixed blending arrangement rather than a thermostatically controlled one; since this might give a blended temperature that is too low to be useful.

--
Cheers,

John.

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John Rumm wrote:

Yes, there is a separate UFH pump and blending valve so the temperature of the UFH circuit may be set correctly at a lower value than the primary boiler/heating circuit.
[...]

[...]
This is how we'd imagined it could work: the trouble is that Worcester Bosch are being singularly unhelpful!
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Jim wrote:

So long as the arrangement can take account of the fact that the flow temperature on the primary circuit will vary (under weather compensation), then it ought to be ok.

Perhaps John S or someone who have played with some of these in the field would be able to comment?
--
Cheers,

John.

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John Rumm wrote:

That's a good point, yes it should keep the UFH constant even if the primary circuit varies.
The "intelligent" controls Bosch supplies (which include the weather compensation one, an RF thermostat, and a combined thermostat/timeclock) communicate over a proprietary bus.
There is also a connection on the boiler (p30 of the installation and servicing PDF) for "230V room thermostat and/or external timer".
It's unclear whether you could utilise both connections at the same time, connecting the UFH to this external connection, IYSWIM.
http://www.worcester-bosch.co.uk/cache/file/3/installation-and-servicing-instructions-for-greenstar-cdi--part-1-of-2.pdf
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Jim wrote:

Good, tath solves one potential problem then...

http://www.worcester-bosch.co.uk/cache/file/3/installation-and-servicing-instructions-for-greenstar-cdi--part-1-of-2.pdf
Perhaps that is an easier question for their tech support people - "can I use both at once?". If you can then it seems like it would do what you need.
--
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John.

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On Tue, 20 Oct 2009 16:03:11 +0100, John Rumm wrote:

Sorry, I only ever fit the toy^H^H^HJunior models whose most sophisticated controls are ON and OFF :-)
Can vouch for W-B's helpfulness on anything slightly out-of-the-box, though.
--
John Stumbles

DEATH TO FANATICS!
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On 19/10/2009 13:14 Jim wrote:

Seriously expensive. Is there any advantage other than a comfortable temperature a litle earlier?
It can't pay for itself.
Can it?
--
F

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F wrote:

It won't get there any sooner, it will just do it more consistently and with less overshoot.

Eventually - it will reduce the gas usage a bit and improve the condensing efficiency of the boiler a bit. I calculate on gas bills like mine it could save about 100/year. So on a DIY install, payback in a couple of years perhaps.
--
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John.

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John Rumm wrote:

You haven't accounted for the saving of not having to buy separate timeclocks and thermostats: a decent digital thermostat will set you back about this much anyway.
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Jim wrote:

Well, all you would need normally would be a prog stat / programmer - something in the range 30 - 70 typically. The weather compensator replaces both at about 100 - so not much extra. However in the case of the WB CDi, you also need the 80 diversion valve kit, and then separate flow and return plumbing between the boiler and the hot tank.
So comparisons are not straight forward and real costs can vary quite a bit depending on layout etc.
The thrust of what I was saying is that it will in general save you some money, so long as you don't have to spend an extra grand having it installed!
--
Cheers,

John.

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On Wed, 21 Oct 2009 12:21:40 +0100, John Rumm wrote:

I suspect you can probably dispense with this and fit external valve(s) and energise the call-for-DHW terminals on the PCB via suitable smarts, if that suits you better (e.g. replacement boiler fitted miles from existing valves so separate return for DHW impracticable). After all, if you look at W-B's diverter kit instructions, the kit doesn't actually connect electrically to the boiler in any way other than the stepper motor for the diverter valve itself (as fitted to the combi).
Must actually try it sometime, otherwise you'll all think I'm an alter ego of you-know-who ;-)
--
John Stumbles -- http://yaph.co.uk

Procrastinate now!
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YAPH wrote:

That would take some doing!
--
Cheers,

John.

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