"Andy Hall" wrote in message
...

snip cut and pasted explanation

There are three basic modes of control:

1. Proportional - operates on off-set and never reaches setpoint
2. Integral - time.
4. Derivative - anticipation, measurting rate of change.

PID controllers, which incorporate all three, are generally better.
Certainly more flexible. In short, the PI of the CM67: proportional
gets
it
near the setpoint, but never reaches setpoint because it operates on
"offset". A proportional controler can sit there all day and not move
and
the setpoint will not be reached. They need a prod. The Integral
mode
of
control operates on time and pulses the offset (the differnce bewteen
the
setting the proportional mode has decided to stay on an the setpoint)
up
to
setpoint, giving high accuracy.

Be careful not to have the proportional band too tight as "hunting"
will
occur, known is short cycling to you.

Hunting is hunting. It is not short cycling but movement around the
set point.

It is a form of short cycling an easier for others to understand.

It's a totally different mechanism

It constantly moves up and down in an on-off manner.

In a CM67, the control aspect that times the delayed start is separet
from
the PI temperature control.

They could conceivably be related. the manufacturer doesn't say.

Controlwise they have to be separte. The user doesn't know this nor
care.

Not true. The piece controlling the optimised start could make very
good use of knowledge of the characteristics of the PI part. Whether
it does or not is a matter for the manufacturer.

It could supply data to other control processes, but it is a separate
control process.

The time proportions of the CM67, are a crude way of keeping an on-off
boiler within the proportional band.

That was already covered.

The CM67 can control an electric actuator. A better system would be a
heat
bank, heated by a basic on-off control boiler (these are cheaper and
have
simple more reliable basic controls. The CH flow and return pipes
would
have a modulated 3-way valve, moved up and down by and electric
actuator.
This can stop at any point in the travel. This way only PI control
would
be
used and guarantee much more accuracy of setpoint temperature control.
As
many heat bank and thermal stores are recommended to be on 24/7, the
boiler
can be controlled only by the heat bank and the CM67 moves the
modulated
valve and switches on the pump when it calculates the start time.

I think you are confusing the issue.

Not at all.

You are because the heatbank is only a source
of heat if designed to be oversized by a reasonable
amount so that the boiler behaviour goes
out of the equation.

And, the modulating 3-way valve has finer accuracy to maintain a setpoint
temperature than switching on-off a boiler.

From the control mathematics
perspective this is PI control exactly the same. All that this
achieves, although it will work is perhaps finer grain control of heat
delivery from the heatbank to the radiators. Of itself, the heatbank
is a red herring in this if the boiler is not part of the main control
loop.

Not so. The heat bank can sit there 24/7 keeping itself to its own
setpoint
providing a nice constant heat source. Just a store of heat.

Exactly, but that's all.

See above.

The CM67 will
then just switch on the CH pump and modulate the CH actuator. Modulating
a
3-way valve gives far more precise temperature control than switching in
and
out a boiler.

I don't disagree with that notion
but can you suggest a make and model
that can be modulated in a precise "analogue"
way (i.e. part opened) by a CM67 type of
controller with a long cycle time?

The cycle times are not a part of the Integral aspect of the controller
AFAICS.

There are certainly valves that can be
modulated with a DC voltage (I
have some) and I could imagine pulse
width modulated using a much
shorter cycle time - e.g. in the way that
model control servos work.

There are 230v electric modulating valves available. Look in the CM67 pdf,
it has a diagram of one.

If you can't suggest one that will work with the CM67 type of
controller, then the argument becomes a bit academic because either a
different type of controller is needed, which is not in the same price
bracket, or you would be talking about a standard motorised zone valve
and proportioned on/off control of it. That would be a bit better
than controlling a boiler on/off, although with a lightweight heat
exchange type, I don't think that the difference would be substantial.

What I am on about is a "modulted" 3-port mixing valve, with an electric
actuator. I have a few here.

Another way is that the thermal store can be switched on also by another
CM67. The desired time must be earlier than the time of the heating.
This
is if you can replace the room temperature sensor with a water strap-on
cylinder sensor of course.

Yes of course, but this is adding extra complication. As you said, a
thermal store ought to be on 24/7 anyway.

It depends on how far you want to take it. A CM67 on a themal store will
learn how long it takes the boiler to heat it up, and bring it on at the
right time.

I would also be surprised if a valve for this application would be
modulated with a ten minute or even 5 minute cycle time. but rather
that it would have some form of electronic control requiring a much
shorter cycle time -seconds or even sub second.

All those cycle times in minutes would be redundant, as only PI control
on
an actuator is required.

The proportional pulse width output from the controller would be used,
but can you suggest a valve that will take this and use it to operate
the valve to arbitrary positions.

AFAIK, the cm67 when controlling the setpoint temperature, only applies PI
control modes. The cycle times appear to be an underlying control layer
which only allows the boiler and PI control on it at certain time periods.

snip crap about snakes, which should all be banned.
If Brazil is short of snakes then you should not be having one.