bar mixer behaviour - is this normal ?

[sm_jamieson]

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.

[Phil]

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).

[manatbandq]

Not neccessarily (better control system theory).

MBQ

[sm_jamieson]

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.

[Phil]

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

[sm_jamieson]

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.

[Ed Sirett]

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.

[sm_jamieson]

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.

[Andrew Gabriel]

Well, my Gainsborough doesn't do this.

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).

[Ed Sirett]

The overshoot is a misnomer here. The wax pellet mechanism takes as it's input the the outlet temperature of the mixer. There is no overshoot if the setting is changed (except at the start).

When the mixer is initially turned on the mechanism calls for 100% hot as the HW temp is less than the set temperature. When the HW comes through it does so at a temperature exceeding the set point. After a short while the wax pellet and gubbins react to the temperature and mix the output correctly. However there will be a couple of seconds where the wax has not heated up and the mixer lets through water that is too hot.