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.

John Rumm wrote:
* But the UHF rooms will have their own thermostat on the wall anyway
which controls a zone valve on the manifold

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?

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.

[...]
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?
[...]
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.

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

John Rumm wrote:
Jim wrote:
John Rumm wrote:
* But the UHF rooms will have their own thermostat on the wall anyway
which controls a zone valve on the manifold

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?

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.

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.

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/cac...art-1-of-2.pdf

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

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

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!

On 19/10/2009 13:14 Jim wrote:

FW100 Weather Compensation Controller 7716192067

Seriously expensive. Is there any advantage other than a comfortable
temperature a litle earlier?

It can't pay for itself.

Can it?

--
F

John Rumm wrote:
F wrote:
On 19/10/2009 13:14 Jim wrote:
FW100 Weather Compensation Controller 7716192067

Seriously expensive. Is there any advantage other than a comfortable
temperature a litle earlier?

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

It can't pay for itself.

Can it?

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.

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.

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

... 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.

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!