Hi,

Andy Hall ) wrote:
: On Fri, 29 Aug 2003 23:26:54 GMT, (Dale
: Shuttleworth) wrote:

[...]

: 1) I want hot water at 60 degrees, what flow temperature does the
: boiler need to be set to to achieve this in a reasonable time?
: The Typhoon heat exchanger claims to be rated at 25kW but neglects
: little details like the temperature differential necessary to
: achieve this.
:
: You could contact the manufacturer for this but it would be 10 degrees
: approx for a conventional boiler, normally 20 for a condensing model.

Sorry, I was talking about the differential between the HW and the
boiler flow. Obviously, a given flow/return temp differential at a
given flow rate will deliver a given amount of power to the water,
I was hoping that I could deliver a reasonable amount of power with
a small flow / HW differential.

[...]

: 2) I want to run the radiators at as low a temperature as possible
: - for most of the year I suspect they will be sufficiently over-rated
: to operate with a 60 degree or even 50 degree flow.
:
: Almost certainly yes. I assume from what you've said that you have
: done the radiator derating calculations. Don't forget to derate
: further if you plan to do anything that obscures radiators like
: covers.

Yep, I've done the necessary derating calculations, looked at
radiators, got confused when there was a significant difference
in radiator size between Myson and Barlo for the same output,
scratched my head, read the data sheets again, realised that they
each quote based on different temp differentials, did my calcs
again and (I hope!) got it right. (Too late now - I got the
rads today!).

: 3) If I go for a simple boiler like the Keston Celsius then I suspect
: that the two requirements above may be somewhat incompatible.
:
: The Celsius will still do a respectable job, because it will attempt
: to supply heat as required by the load. For the radiators, when the
: weather is warmer, the TRVs will tend to close down and reduce the
: heat demand. The pump speed will drop, as will the boiler firing
: rate. Since the boiler is monitoring all the temperatures, it will
: try to get the flow and return to as low values as possible, while
: still delivering the required load.

This is the bit I don't understand. What is the boiler control
attempting to control? In the case of the Celsius, I assume that
it is attempting to control the flow to a temperature set by that
little knob on the front of the boiler. In addition, I assume that
once the temperature set point is reached, the boiler reduces its
output to maintain the minimum possible return temperature whilst
still maintaining the set flow temp.

The upshot of this is that I would think that the flow temperature
will remain constant but the return temp will fluctuate with demand.

In the absence of other inputs, the boiler cannot possibly know the
real heating load since the load is proportional to the flow temp.
If the boiler backed off the flow temperature when the return
temperature was high for example then it would effectively back off
the load leading to a very economical but ineffective system.

Of course, in the case of a clever boiler like the Micromat then
there are other inputs and the boiler can play games with the flow
temperature to reach maximum efficiency.

: When there is demand to heat the DHW, the boiler will "see" a fairly
: heavy heat load. This is because by the time the cylinder thermostat
: operates, there is already going to be a fair amount of cold water in
: the bottom of the cylinder. Therefore, the return water temperature
: will be fairly cool as well and the boiler will view it as a hefty
: heat load and fire up accordingly.
:
: As the DHW temperature approaches the set point, the boiler may start
: to back off a little as it sees the return temperature rising.

Yes, but I assume it won't back off the flow temperature, only the
power input, maintaining the flow but dropping the return temp.

The real question I have is that assuming I want hot water at 60
degrees then how low can I set the flow temperature on the boiler?
I'm hoping for something like 65 degrees.

[...]

: 4) I'm planning on a system with two zone valves, one for CH and one
: for HW ("S-plan" in Honeywell speak). I'd probably be looking at
: a programmable room stat (e.g. the Honywell CM-67 or similar) for
: controlling the main room temp downstairs and thermostatic valves
: everywhere else. Any comments on the best place to site the room
: stat (i.e. would the landing be better - the hall is not really a
: sensible option since it is very small and hence prone to drafts
: when the front door is open)? Any thoughts on the best choice of
: stat?
:
: Remember that if you use the living room as your controlled room, you
: can't really have any other form of heating in there like a gas fire
: because it will have the effect of backing the heating off to the rest
: of the house.

My concern at the moment is that I know the radiators upstairs will
be grossly overrated so I'm reluctant to put the thermostat on the
landing, even though all the rads will have thermostatic valves. The
radiator for the hall (and hence landing) is slightly underrated due
to space constraints. Last year with no radiator in the hall, there
was a very discernable temperature gradient as you went down the
stairs - there is a risk that I will have the same thing again but
to a lesser extent this year.

The downstairs radiators are fairly close to the mark for the rooms
they are in but at the moment downstairs as a whole is 1500W short
since I haven't planned anything for the kitchen - nowhere to put
anything (see question below). I have no other heating downstairs
at present (but I do have a hearth for a gas fire - if I do install
something it will be decorative rather than for heating though).

The hall or main room downstairs seem like the best bets under these
circumstances. (Someone suggested the kitchen but I think thats
cheating!).

[...]

: 6) I have noticed things like the Honeywell "Smartfit" system. This
: looks like it has everything conveniently integrated with control of
: both the CH and HW readily to hand. Does anyone have any views on
: things like this? I can see the convenience, I can also see the
: nightmare getting spares in ten years time...
:
: It is potentially quite a nice solution and can also be integrated
: with some home automation systems like www.comfort.org.uk
:
: Since it is Honeywell,who enjoy a good reputation, I would not be
: overly bothered, but the valves are proprietary.

I've decided to give it a try.

[...]

The only remaining issue now is the kitchen. At the moment I'm
probably going to survive this winter with it being heated
incidentally from elsewhere downstairs. There isn't space for a
radiator and I'm wary of the reliability of these fanned under
unit things - anyone have any experience of them?

Dale.