Flow temp vs power rating

[Johnytuono]

So I've monitored my boiler this afternoon. It doesn't make sense...

So flow temp is 60C
Anti fast cycle flow temp is -14
Anti fast cycle time is 15 mins

12:00 - boiler comes on. Flow temp initially goes to 43C, then settles down to 30C and builds from there.

12:29 - reaches the 60C flow temp and continues at that for a while.

13:09 - hits 61C as one or more of the TRVS presumably reaches setting temp and starts to close radiator. The boiler has been running continously since it started and burning gas. House temp has increased by 2.5 - 3C.

13:19 - hits 64C. Boiler stops burning gas after about 30 seconds. Fine. It is from here that I am interested to see what the anti fast settings do. From the point it stopped burning gas it continues cycling for about 5 mins. Flow temp falls quite quickly to 59C.

13:23 - cycling stops. 57C. Boiler now idle.

13:29 - cycling starts again, no gas. Temp goes from 59C - 51C.

13:33 - cycling stops. 50C. Idle.

That's two 5 min cycles without gas, 5 mins apart.

13:38 - 50C, boiler fires up, burns gas. Temp goes to 58C quickly, but then settles down to 53C.

13:40 - gas stops, but cycling continues.

13:44 - 46C, cycling stops. Boiler becomes idle.

So this is the end of a third cycle without gas although the third one began with a brief period of burning gas. So about 15 mins of cycling. And the -14C point has also been reached...

13:46 - 47C. Idle.

13:50 - another cycle with no gas. Temp goes from 48C to 41C.

13:53 - the gas comes back on and now the boiler seems to begin again a proper burn back towards set temperature. Gets up to 56C by 14:00 before turning off with timer.


So I'd appreciate thoughts. It didn't seem to fire up at -14 below set temperature. Or fire up properly at -14 below SP+4. And it isn't clear how the 15 minute setting came into effect either. What was that mini burn all about at 13:38?

The anti cycle time, on this burn, will have been used up because of the length of the burn (about 1 hr 20 mins). so therefore it will be on the set anti hysteresis only.

The anti hysteresis drop is from the original flow temp (60-14=46)

Somebody may have just run some hot water just prior to the 12:00 start time therefore showing the water temp on the display (I've noticed this happens with mine)

13:19 is probably your pump still running for around 5 minutes
13:29 also sounds like the pump running
13:40 also sounds like the pump as no gas is being used

13:38 Sounds like you have hot water pre heat on OR did someone use a hot tap? If this is the case, there is a setting where the boiler lies idle for a certain amount of time after hot water is demanded (mine is set at 1 minute default)

Sometimes, when it is very near the temp drop of the anti hysteresis it doesn't show the exact temp. number on the display.

I'm assuming this is a combi and not a heat only boiler i.e. have you a separate water cistern that it is heating?

 

[Johntheo5]

Its a 25I Combi (post #1), does the DHW usage influence the CH side?, it shouldn't as the flow temperature can regularly exceed the CH SP+4C and the burner won't trip.

 

[Johnytuono]

Its a 25I Combi (post #1), does the DHW usage influence the CH side?, it shouldn't as the flow temperature can regularly exceed the CH SP+4C and the burner won't trip.

yeah it can throw it John, it does on mine anyway, ours was in the middle of a burn, someone ran the hot tap for a while which put the temp on the display up, I then noticed that the boiler had gone into the anti hysteresis mode (204 as it had run past my anti cycle time of 5 mins), after they had finished (and the 1 minute wait time had elapsed), I definitely know the boiler on the CH side wasn't at my SP+4 (50+4) as I was stood there timing it...

 

[Johntheo5]

OK, not very clever, but if it then restarts on CH and reaches SP+4C and burner trip, will the burner stay off until the flowtemp falls to SP- hysteresis temp, assuming no DHW demand?.

 

[Johnytuono]

OK, not very clever, but if it then restarts on CH and reaches SP+4C and burner trip, will the burner stay off until the flowtemp falls to SP- hysteresis temp, assuming no DHW demand?.

Yes it will.

If DHW has been used in the middle of a burn and then the tap is switched off, the display shows the DHW temp, after 1 minute ( the default time) the boiler will then fire up and show a lower temp on the display ( I assume the CH temp) and then burn as normal.
After reaching SP+4 it will then wait to drop to the anti hysteresis before re firing again (if needed).

What I am wondering though in the other Johns' case is, has his anticycle time of 15 minutes (burn/run time ) been interrupted (especially if DHW has been used after a fire up and is therefore using THAT time up (assuming it's longer than his anti hysteresis setting)?

EDIT:
Ok I just tried it
CH was last used an hour ago
Flow showed 36c
Ran DHW tap for a few minutes and display showed 44c
Turned tap off, display showing 44c and then waits a minute.
I turned the CH on, display went up a couple of degrees to 46c and then went down to 38c wherein it then started to climb again and I know it would eventually have reached SP

 

[John Smith Esq.]

No hot water was used throughout the monitoring. Only me in the house.

Why would the anti cycle time be used up whilst heater is burning gas over a long cycle?

I thought the point of these settings is to adjust what the boiler does after it hits SP+4. One is an anti fast cycle temperature (minus), one is an anti fast cycle time. That's what these settings are called on my boiler

In case I'm confused, could somebody tell me what the fundamental reason for these settings is? I thought it was so you can delay how long it takes for the boiler to refire after it hits SP+4.

 

[Johntheo5]

Yes, thats what should happen, in your case, after 15 secs or SP-14C, 46C, whichever is the longest, but WB have designed this AFCtime to reduce each time the boiler refires after reaching SP+4C, so If I am interpreting it properly, the remaining AFCtime will reduce zero after a few cycles but the boiler should then not refire until the flowtemp falls to 46C, SP-15, that you set it to, presumably, this willcontinue to happen until the boiler is programmed off or the roomstat shuts it down, the AFCtime should then reset to 15 seconds.

 

[Johnytuono]

This is the email I got back from WB Technical :

Dear John

The anti-cycle time is to protect against multiple burner start ups and so is calculated from the time in which the burner starts. If the burner is lit for a longer duration than the anti-cycle time, then this element has been satisfied when the burner shuts down and so the central heating hysteresis based on temperature will determine when the burner re-activates.

The recommended settings would be based on system requirements which in most cases would deem that the boiler should not cycle during a demand which would extend the demand period and not satisfy requirements. The boiler should modulate to maintain the required flow temperature.

Poor circulation or too greater heat output from the appliance could cause the boiler to cycle as the temperature is not being dispersed.

Best regards,

Technical Support
(HC/SGB-I2R-TC)

The anti cycle time is so the if the boiler fires for say, only 8 minutes, then it will be off for at least another 7 minutes (if you have set it at 15 mins) OR the set anti hysteresis, whichever is longer (therefore guaranteeing a temp flow drop) and then the next burn should then be longer and hopefully satisfy the room stat temp..

It is also what I observe happening with mine, do you have the DHW set to Eco or Preheat John?

Also, maybe your pump is set too low of a speed , I tried mine at speed 1 (7 rads + towel rail, totalling about 9kw) and it wasn't sufficient to heat the rads to given temp.

 

[Johntheo5]

Just wondering how do external anti rapid cycling controls do their sums, a lot of these like Evohome which makes a art out of cycling boilers to give pinpoint room temperature, can give up to 6 cycles/hour, how is it calculated/done?

 

[Johnytuono]

Just wondering how do external anti rapid cycling controls do their sums, a lot of these like Evohome which makes a art out of cycling boilers to give pinpoint room temperature, can give up to 6 cycles/hour, how is it calculated/done?

Maybe it’s once it’s reached room temp and then fires the boiler for so long to keep topping up to maintain room temp ? Honeywells also have always puzzled me

 

[Johntheo5]

Maybe it’s once it’s reached room temp and then fires the boiler for so long to keep topping up to maintain room temp ? Honeywells also have always puzzled me

WB Technical, above :
"The recommended settings would be based on system requirements which in most cases would deem that the boiler should not cycle during a demand which would extend the demand period and not satisfy requirements. The boiler should modulate to maintain the required flow temperature."
which is a bit rich really as the set/remaining time is very quicky swallowed up and relies on the programmer/roomstat to reset it. Because most boilers do reach SP+4/5C quite regularly then one would think there would be more logical method with the time factor allways incorporated.

For interest, this is from some notes I have on Evohome.
"Every HR92 sends a heat demand from 0 to 100% wirelessly to the Evotouch. This is proportional to how much "heat" it thinks it needs from the boiler to reach or maintain the set point. The controller combines the demands of all the different HR92's in some way and forms an aggregate demand also from 0 to 100% which it sends to the boiler relay.
The boiler relay then switches on for this percentage of time in each cycle. So with the default 6 cycles per hour and 1 minute minimum on time a 10% demand is the minimum required to get the boiler to fire at all. A 30% demand would mean 3 minutes on 7 minutes off."

 

[Johnytuono]

WB Technical, above :
"The recommended settings would be based on system requirements which in most cases would deem that the boiler should not cycle during a demand which would extend the demand period and not satisfy requirements. The boiler should modulate to maintain the required flow temperature."
which is a bit rich really as the set/remaining time is very quicky swallowed up and relies on the programmer/roomstat to reset it. Because most boilers do reach SP+4/5C quite regularly then one would think there would be more logical method with the time factor allways incorporated.

For interest, this is from some notes I have on Evohome.
"Every HR92 sends a heat demand from 0 to 100% wirelessly to the Evotouch. This is proportional to how much "heat" it thinks it needs from the boiler to reach or maintain the set point. The controller combines the demands of all the different HR92's in some way and forms an aggregate demand also from 0 to 100% which it sends to the boiler relay.
The boiler relay then switches on for this percentage of time in each cycle. So with the default 6 cycles per hour and 1 minute minimum on time a 10% demand is the minimum required to get the boiler to fire at all. A 30% demand would mean 3 minutes on 7 minutes off."

In bold : Thats why I'm trying to depend on the hysteresis temp drop, at least it's guaranteed and (hopefully) would satisfy the room stat on at least the second burn.

I've had Nest for quite some years now and been quite satisfied with it AFTER I had turned off all the learning 'software' etc.

Evohome does sound interesting though and it may be something I would look into in the future.