On Fri, 30 Oct 2009, John Rumm wrote:

Kostas Kavoussanakis wrote:
On Thu, 29 Oct 2009, John Rumm wrote:

Kostas Kavoussanakis wrote:

Thanks. So (having read the other thread, where it was claimed that, for
the W-B at least, there is no real gain for domestic applications) is there
no particular gain or timely return of investment from using the modulating
thermostats and weather sensors? Judging (from an ignorant

You mean for having weather compensation? My guestimate is it might reduce
the gas bill by about 10% - some gain from extracting a few % more condensing
efficiency from the boiler, and the remainder from maintaining better
temperature control with fewer overshoots in temperature. So you can
translate that into money and payback time based on your bills. In my case
its worth having, since it will payback fairly quickly.

How much of this can one get with a (programmable), decent, digital
room-stat, like the CM907? I mean, do you get better temp control and
fewer overshoots with the modulating room stats?

Re Vaillant, is the only option for 2 zones the 358-quid Vrc 430 + Vr 61
2 Heating Zone Kit, or can I just add another zone using any stat and a
valve?

The VR430 is usually about £100, and the VR61 about £70. There are quite
number of ways the above beasties can be configured, for multiple zones -
some offering temperature mixing for things like UFH. You could (I expect
- not read through in detail) do a bog standard S+ plan system with
ordinary stats and zone valves for a fixed flow temperature all over.

What does "fixed flow temperature" mean in this instance? I could

With a "normal" install you set the boiler flow temperature with a knob on
the front. It then generates water with that as a target flow temperature.
(apart from when recharging a hot water cylinder where it may run hotter). If
you set the temperature high then you reduce the boiler efficiency, if you
set it low then you may not get the house warm enough on cold days.

Is this correct even in modulating boilers (w/o external or modulating
temp sensors)?

Where is gets more complicated is when you want say a flow of water at 60 ish
for a rad zone, plus say 50 for an UFH one at the same time. The boiler can
only produce water at one temperature (even if the weather comp has final say
as to what temperature that actually is). Here you need a blended circuit,
with a variable mixer and sensors that can run the cooler zone using some
primary flow water mixed with some cooler return water to keep that zone
working at a lower temp. The VR61 will also let you do that (in combination
with the right valves) if you want.

Two things to say he

- Thanks!
- Wow!!!

(probably out of my scope this, but way cool).

Costs are not so bad if you shop about a bit.

http://www.plumbnation.co.uk/site/va...or-0020028520/
http://www.vhsdirect.co.uk/product/vr_61_mixer_module/

Thanks, I had seen that, but in browsing the literature it seemed to me
that you need more that a 430, a 61 and a 81 for two zones (and was thus
attracted to the kit). I guess I was wrong. Meanwhile, the CM907 is 50-55
quid.

You need "more" in that you also need the zone valves. I don't think you need
any extra electronic kit (the external sensor for the 430 comes with it, and
a cylinder sensor comes with the VR61)

Ah, thanks for this. Yes, the extra valve is unavoidable for the zone
(and I guess one can use any valve, rather than a Vaillant own
thing?).

This has all been very enlightening, many thanks.

--
Kostas