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Chrono Proportional Control, Condensing Boiler and UFH


AliMcLeod

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A question/scenario for our resident plumbing experts. Doing some reading, I found out that our room stats (Danfoss TP5000si-RF) have a "feauture" called Chrono Proportional Control that  cycles the boiler on and off as the room temperature approaches the target set point. Based on the difference, it determines how long to keep the boiler on within each cycle period.

 

Danfoss sell its benefits here:

 

http://danfossheating.ourpressoffice.com/2016/why-fit-chrono-proportional-controls/


Post 22 here is a response from Danfoss with more details on the algorithm they use:

 

https://www.diynot.com/diy/threads/what-exactly-is-chrono-proportional.35622/page-2

 

I've just checked and our stats have the default set up of 6 cycles per hour. The question is, in my setup with a condensing boiler and no thermal store (UFH downstairs, rads upstairs, not particularly airtight house), would this not result in excessive short-cycling on the boiler? 

 

I'm wondering whether to turn this feature off, or reduce the cycles per hour, to reduce boiler cycling. The stat manual is here. Installation Option 34 controls the setting:

 

http://heating.danfoss.com/PCMPDF/40804v02s3-00.pdf

 

Any thoughts?

 

Edited by AliMcLeod
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In all probability the boiler already has anti-short cycling built in, in the form of a time delay that stops it firing up within a given time since it was shut down.  I know that our combi has this feature when it's in heating mode, and it's a pretty old model, so I'd be surprised if newer models weren't similar.

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Its a WB Greenstar 42CDi Classic and does modulate - from the manual

 

HEATING ECONOMICALLY

The boiler is designed to provide a level of comfort while keeping gas consumption and the resulting environmental effect as low as possible. The gas supply to the burner is controlled according to the level of demand for heat. The boiler continues to operate with a low flame if the demand of heat reduces. The technical term for this is modulating control. Modulating control reduces temperature fluctuations and provides even distribution of heat throughout the home. This means that the boiler may stay on for relatively long periods but will use less gas than a boiler that continually switches on and off.

 

I can't find anything that might imply some other level  of anti-short cycling though.

 

What is not clear is whether the additional on/off commands from the stat will increase the short-cycling. Just wondering if any of the installers here typically turn such features off with UFH. There's a few threads on the web where people debate whether Chrono Proportional Control is a good or a bad thing with condensing boilers, with no real consensus. 

Edited by AliMcLeod
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That model has a fairly short (3 minutes) anti-short cycle timer, plus the normal fan over-run time of 30 seconds after shut down, according to the manual available to download here: https://www.worcester-bosch.co.uk/professional/support/literature/greenstar-cdi-classic-combi-installation-instructions

 

So, the boiler is presumably OK if it fires up around ten times an hour (assuming that it would stay on for a couple of minutes or so whenever it fires up).  I'm not convinced that deliberately making the boiler short cycle as a way to modulate output below the lowest level is a smart thing to do, for a couple of reasons.  The obvious one is wear and tear on the boiler; the less often it fires up the longer it's likely to last.  Another point is efficiency.  When firing up initially most boilers are pretty inefficient for the first 20 to 30 seconds or so.  If you are near the flue when a boiler fires up you can detect this, as there will often be a slight smell from incomplete combustion on start up; it takes a short time for the flame to start burning cleanly.

 

 

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I have said this before, but there is nothing "new" in what this thermostat is doing.  All it is doing is mimicking how a mechanical thermostat with an accelerator heater behaves.

 

People shun them now as "old" and "inaccurate" but I am finding our new mechanical thermostat is working very well indeed.  To overcome the inherent hysteresis with a mechanical thermostat, they have a small very low power accelerator heater that warms the local environment very slightly when the heating is on with the effect of cancelling out the hysteresis.  A by product of this, whether by accident or design, is the modulate.  As you approach the set temperature the thermostat turns off, then a short while later back on. As it gets closer to the set point the on duration reduces and the off duration increases.

 

I think what I am saying is unless you actually want a programable capability (different temperatures at different times of day) then it is hard to beat the simple humble mechanical thermostat.  At least everyone knows how to work one of those!!!!

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2 hours ago, JSHarris said:

I'm not convinced that deliberately making the boiler short cycle as a way to modulate output below the lowest level is a smart thing to do, for a couple of reasons.  The obvious one is wear and tear on the boiler; the less often it fires up the longer it's likely to last.  Another point is efficiency.  When firing up initially most boilers are pretty inefficient for the first 20 to 30 seconds or so.  If you are near the flue when a boiler fires up you can detect this, as there will often be a slight smell from incomplete combustion on start up; it takes a short time for the flame to start burning cleanly.

 

Yes, that's why I'm wondering whether to turn the feature off or reduce the cycles.

 

With the default 6 cycles per hour, each 10 minute cycle is further split into an on and off period.

 

If the current temperature is more than 2C from the target set point, the call for heat will remain in place for the entire 10 minutes.  At 1C difference, the call for heat will be on for 5 minutes, and then off for 5 minutes. At 0.5C, the call for heat will be on for 2.5 minutes, then off for 7.5 minutes. At the next cycle, the delta is recalculated and the next on/off periods are determined.

 

If I changed it to 3 cycles per hour, each cycle would be 20 minutes, meaning potentially less on/off boiler cycles and fewer short cycles. I'm not sure that turning it off will be right though - the post I linked to above with the Danfoss response states that when in in/off mode, the  "thermostat controls using on/off control with a 1 DEG C". I suspect this would lead to overshooting, unless I had my set point at 1C less than what I really wanted (and there's no way my family would remember to to that). I could always use the advanced setting to changed the calibration to be -1 ?

 

Thanks for the link to the doc with the anti-cycle details - I'd not found that one.

Edited by AliMcLeod
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1 hour ago, ProDave said:

I have said this before, but there is nothing "new" in what this thermostat is doing.  All it is doing is mimicking how a mechanical thermostat with an accelerator heater behaves.

 

 

Definitely nothing new here - this system was installed in 2009.

 

1 hour ago, ProDave said:

I think what I am saying is unless you actually want a programable capability (different temperatures at different times of day) then it is hard to beat the simple humble mechanical thermostat.  At least everyone knows how to work one of those!!!!

 

This is exactly what we want (my bolding above). For example, in our bedroom, we typically want a lower heat overnight but with the room heating up in the morning ready for us getting up.

 

But I'd say these thermostat is very simple to use - a simple set of 4 set points per day (so no on/of settings, just "I want it to get to this temperature at this time") and ability to override up or down at any time.

Edited by AliMcLeod
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