On 12/01/2019 10:25, RJH wrote:
On 12/01/2019 07:30, John Rumm wrote:
On 12/01/2019 00:55, RJH wrote:
On 11/01/2019 21:30, John Rumm wrote:
On 11/01/2019 18:52, tim... wrote:
I had a new Combi fitted today (in the to-be-moved-to house).

Fitters told me that I should run this at 74 degrees.

snip

Anyhow, at my current house it is 55 and works perfectly well

what do you guys/galls do

I have weather compensation on mine, and so it chooses its own
temperature based on the outside temp. Basically that means its runs
as cool as it thinks it can get away with and still be able to reach
the target set point temperature in a reasonable amount of time.
(the relationship is set by choosing a mapping curve that reflects
the rate of heat loss of the building).


I don't follow how that can work properly, as for most homes
different rooms will have a different 'curve'. Or does tweaking the
TRV compensate?

It seems to work well enough in practice. Each room also has a TRV,
and I have the place split into two zones; upstairs and downstairs, so
that will account for some variation.

It needs a little bit of experimentation to set the profile the first
time - basically waiting for colder days and seeing if the system
still heats the place comfortably and quickly enough. If it doesn't,
then you just tweak it up to a steeper curve. It also helps if you
have appropriate rad sizes for the rooms, and the system is balanced.

The response curves look like:

http://wiki.diyfaq.org.uk/index.php/...tingCurve1.png

IIRC the system defaults to the 1.2 line. If you live in a super
insulted place / particularly sheltered location then you would tweak
down. In my case (exposed location - solid wall construction), I
needed to go up. I found the 1.8 curve worked well.

The system is also smart enough to automatically shift the response
curve vertically based on the currently demanded internal target
temperature[1]:

http://wiki.diyfaq.org.uk/index.php/...tingCurve2.png

So if you tweak the room temp up or down during the day (or have
different times programmed with different set point temps), then it
can vary the flow temperature to match the requirement.

[1] Note that all the system temperatures sensors (internal
downstairs, internal upstairs, DHW cylinder, and external) are
digitized and processed as actual temperatures, not just as on/off
"call for heat" style demands.


Thanks for that, interesting. And I hadn't realised such systems had a
collection of sensors.

Probably no more sensors, than any other zoned system (i.e. one for each
stat, plus the external one), but the fact that it can read actual
temperatures from each does open up some more possibilities.

OT but loosely relevant, I use my CH as 2 zones - upstairs and
downstairs. I'm really not going to replumb to add a properly zones
system, and I was wondering if it's possible to isolate the upstairs
zone with a three way motorised valve - simply cutting out the upstairs
'circuit'.

You could do, although a pair of two port valves is the more common way
of doing it. It does depends a bit on how the pipework is done. Some
systems are easy (say where pipework for each floor of rads is run under
the respective floor), and some very difficult (say where all the
pipework is under the upstairs floor, and then branches both up and down
to feed rads).
A single switch would be easier than faffing about with 4 TRVs each
evening.

Indeed - or even a timer / programmable stat for each floor.

--
Cheers,

John.

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