Right, I've at last fitted my Triton Tyne bar mixer. When you adjust
the temperature in a certain direction, you get an initial rush too far
in that direction before it settles down. The faster you make the
adjustment, the bigger the rush. When increasing temperature you thus
get hotter than expected, and I assume above the set temperature, so
this could be a safety issue. This is not the boiler, since the
temperature change is too quick.I was expecting more like a gradual
move towards the desired temperature.
Is this a usual behaviour of these types of mixers ?
This also makes me wonder exactly how the thermostatic cartridges work.
Anyone know what happens in there when you change the temperature ?
Simon.

You will get overshoot (control system theory) unless you make the
change slowly. An instant response would result in oscillation!
Consider adjusting the DHW temp from your boiler or restricting the
flow of the cold water so the valve uses more hot resulting in a better
balance (assuming a combi here where faster DHW flow = lower temp).

Phil wrote:
You will get overshoot (control system theory) unless you make the
change slowly.

Not neccessarily (better control system theory).

MBQ

You will get overshoot (control system theory) unless you make the
change slowly. An instant response would result in oscillation!
Consider adjusting the DHW temp from your boiler or restricting the
flow of the cold water so the valve uses more hot resulting in a better
balance (assuming a combi here where faster DHW flow = lower temp).

I can reduce the cold via the gate valves I put in the inputs. But how
do the internal workings of the unit account for this overshoot ? Are
we talking about a mechanical overshoot when you adjust the knob ? How
does the knob mechanically connect to the thermosensing unit ?
We need diagrams folks !
Simon.

You will always get some overshoot in practice as even critically
damped systems move too slowly to achieve SP IRL

You will always get some overshoot in practice as even critically
damped systems move too slowly to achieve SP IRL
I cannot see why overshoot should occur in this case. Surely the system
is damped
by the temperature control knob. This knob does not oscillate however
fast you
turn it ! Also, damped by the wax itself. Where is the undamped item
that would
result in oscillation. Control system theory must be related to a
real-life example.
Of course in really there are very small oscillations all over the
please, but the
amplitude is too small to be of any consequence.
*However*, we still have not explained the overshoot.
Simon.

On Tue, 16 Aug 2005 08:06:47 -0700, sm_jamieson wrote:

You will always get some overshoot in practice as even critically
damped systems move too slowly to achieve SP IRL
I cannot see why overshoot should occur in this case. Surely the system
is damped
by the temperature control knob. This knob does not oscillate however
fast you
turn it ! Also, damped by the wax itself. Where is the undamped item
that would
result in oscillation. Control system theory must be related to a
real-life example.
Of course in really there are very small oscillations all over the
please, but the
amplitude is too small to be of any consequence.
*However*, we still have not explained the overshoot.
Simon.

The delay and overshoot come from the time is takes to heat up the wax
pellet and for that to expand.

The really good mixers (eg Aqualisa) use a bi-metal mechanism which can
react faster.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

The delay and overshoot come from the time is takes to heat up the wax
pellet and for that to expand.
Hold on. The delay due to the wax expanding accounts for the time to
react to
incoming water temperature changes. But how does it account for
overshoot
in the adjusting of the temperature knob ? That is nothing to do with
the wax
expanding. Presumably a spring acting against the wax is adjusted,
making the
capsule smaller when you want the shower hotter. The wax expanding does
not
control the temperature setting, just the temperature stability. The
temp setting
must be calibrated against the spring.
I think overshoot must be to do with the spring oscillating a little
bit, but this would
assume the wax is fairly liquid - any know if this is the case ? Also,
maybe the wax
has a reaction to the spring that is built into the calibration. That
would be something
to explain the overshoot.
Simon.

In article .com,
"Phil" writes:
You will get overshoot (control system theory) unless you make the
change slowly. An instant response would result in oscillation!

Well, my Gainsborough doesn't do this.

Consider adjusting the DHW temp from your boiler or restricting the
flow of the cold water so the valve uses more hot resulting in a better
balance (assuming a combi here where faster DHW flow = lower temp).

You normally require a special mixer for a combi or multipoint,
to cope with wide pressure range on the hot, and it has to know
that just turning down the hot flow is no good for temperature
control (as the hot water simply gets hotter, exactly canceling
out the change).

--
Andrew Gabriel