Chrono proportional control - what does it do?

Looking at replacing the old programmable CH stat for the new combi and found mention of 'chrono proportional control' on the Danfoss site. As it also mentions 'cycling rates of 3, 6, 9 or 12 per hour can be selected', I take it that the it could be switching the boiler on for a limited period, but I could be completely wrong. If this is so how would this improve the efficiency?

-- rbel

Reply to
rbel
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It happens that rbel formulated :

I would guess it aims to stop the boiler constantly cycling on and off, by limiting the number of times per hour it is allowed to fire.

Reply to
Harry Bloomfield

In article , Harry Bloomfield writes

IME, quite the reverse, it forces the boiler to cycle n times per hour instead of giving a long slow burn. I've experienced this on Honeywell programmable stats where it permanently on w/o option and I will be scrapping my current one for an accurate but non propo stat before winter, probably an ugly but effective Siemens

Reply to
fred

A purely analog(ue) proportional controller would have a continuously variable output which might control (say) the position of a valve or the speed of a motor.

A CH stat doesn't have anything continuously variable to control - its output is simply ON or OFF. It does a similar job to an analog controller by controlling the mark/space ratio[1] of its ON/OFF function. This gives more accurate control than waiting for the set temp to be achieved before switching off, and reduces over-shoot. It would be most accurate if it had very short on/off periods, but that would result in excessive boiler cycling. So, the compromise is to limit the number of switching operations per hour.

[1] Hence 'chrono' refers to the on and off time periods
Reply to
Roger Mills

It cycles the boiler on and off once it gets into the proportional band width. The proportional band width can be set but on my Honeywell CM927 it used to be at 1.5ºc before the target temperature; so if the desired temperature was 20ºc and the current temperature was 16ºc, the boiler would fire continually until the temperature reached 18.5ºc, at this point the boiler would start to cycle more and more until it reached

20ºc. Once 20ºc had been reached it would cycle a number of times per hour to maintain the temperature. The idea of this is supposedly to obtain a lower average flow temperature.

However, I think it is more suited to non modulating boilers that are either 100% on or 100% off. It didn't work well with my modern Vaillant condensing boiler; the control had a different logic to the boiler wherein the boiler preferred a long burn modulating down as it approached the flow temperature. I have since got rid of it and replaced it for a Vaillant weather compensator which is much better.

Reply to
gremlin_95

Aye, should avoid so much overshoot. But chronoproportional control is not recomended if there are motorised valves in the system as it cycles them as well.

Reply to
Dave Liquorice

Many thanks to all for the explanations. As the new combi is a modulating unit I think I will give the chrono proportional option a miss.

-- rbel

Reply to
rbel

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