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

Kostas Kavoussanakis wrote:
On Wed, 28 Oct 2009, John Rumm wrote:

Does modulation happen anyway as a result of demand variation, or
does one need suitable room or external sensors?

No, modulation will happen anyway. Without a weather compensator, the
boiler will maintain a set flow temperature, and also possibly attempt
to maximise condensing efficiency by keeping the return temperature
lower if it can. So as the return temp creeps up it modulates the
power down.

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.

POV) by the wiring diagrams of the VR61, it could be quite a task to set
this all up.

To an extent its a simple or as complex as you want to make it. You
could go for complexity not much dissimilar from a normal S or Y plan
system without any need for blended temperatures. Let the weather
compensator set a flow temperature for the whole house based on
conditions and then have conventional zones that all run at that
temperature.

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. This would lead to a certain amount of manual
intervention to get best performance from the system. The weather
compensator automates this, by dropping the temperature to the minimum
necessary to heat the place, it also feeds control to the boiler based
on actual temperature data rather than just "on" or "off" requests.

perhaps desire to not heat a zone too much in the mornings, but heat it
more in the afternoon; is that out of scope?

No, that falls into the scope of what a prog stat (and by extension the
weather controller does anyway).

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.

So many options, such high costs...

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)

--
Cheers,

John.

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