Evohome continually bringing boiler on for one minute then off for 10 minutes

[colin_]

It's not hard to change it and try it, but so be it. Wait for your reply.

Good luck getting the system sorted.

Thank you for your help and assistance.

Regards
Colin

 

[John.g]

Thanks, I think I'll need it. (hoping the system will not be 3 minutes on/ 7 minutes off for the rest of the night as may be tempted to turn the boiler off.

I have set to multiroom on this zone to see if anything changes.

Your house must require some level of heat to keep it warm so even though you are not seeing any rise in room temp this just proves in a way that the evohome is working by just firing the boiler long enough to keep the house exactly at your required temperature by keeping the rads at the required temperatures.
Except that your rads are at the room temperature then they must be emitting some heat so even if they are only at 25C and the rooms temperature is 18C then they will still emit ~ 8% of their rated output say 1.6 to 2 kw of a 20kw rating which isn't going to break the bank.

 

[colin_]

Your house must require some level of heat to keep it warm so even though you are not seeing any rise in room temp this just proves in a way that the evohome is working by just firing the boiler long enough to keep the house exactly at your required temperature by keeping the rads at the required temperatures.
Except that your rads are at the room temperature then they must be emitting some heat so even if they are only at 25C and the rooms temperature is 18C then they will still emit ~ 8% of their rated output say 1.6 to 2 kw of a 20kw rating which isn't going to break the bank.

Hi John

I think the main issue is that the boiler starts up at its min modulation (10%) which means 3KW of heat to warm up the water. It remains at this level for several minutes before ramping to the next level. It keeps ramping up and it can take over 12 minutes to get to 70 degrees.

I heave set the Evohome for a minimum on time of 4 seconds and iduring this time it can barely raise the water temerpature to 45 degrees (from around 30). It thus keeps coming on for 4 minutes in every 10. It eventually gets to around 60 degrees but that takes arind 5-7 cycles so around an hour. It is worse if more radiators want heat.

I suspect that the system will work much better if the start modulation is set higher (20% -> 6KW) and I am waiting on the installer to get back to me and set it as a test.

Thanks
Colin

 

[John.g]

Yes, understood, but Evohome seems to be saying so that as long as the the room temp is within 1.5C of its setpoint that whatever minimum time you have set is in fact a fixed time and not just a minimum?. So if the temperature offset is 0.75C then the controller just looks for the TRVs to be open say 50% or something like that and during this x weeks learning curve if the room temperature isn't rising then it might look for 75% opening or whatever but has no effect on the actual run time?

Is this your understanding of how it works?

 

[colin_]

Yes, understood, but Evohome seems to be saying so that as long as the the room temp is within 1.5C of its setpoint that whatever minimum time you have set is in fact a fixed time and not just a minimum?. So if the temperature offset is 0.75C then the controller just looks for the TRVs to be open say 50% or something like that and during this x weeks learning curve if the room temperature isn't rising then it might look for 75% opening or whatever but has no effect on the actual run time?

Is this your understanding of how it works?

Hi John

I think that when near temperature the Evohome is supposed to know how long to bring the boiler on to get the desired temperature (it is supposed to learn this over several weeks).

In the settings you can set the minimum time that the Evohome will bring the boiler on for, the defualt is one minute every 10.

I have had to set this to 4 to even get the radiators warm (it gets the temperature from the boiler to ~50-55 degrees in an hour of cycling 4 minutes off, 1 minute on).

I am now on its max setting of 5 (so boiler spending 50% of its time on whilst doing this - but presumably ok as modulated down to say 10% for firt couple of minutes).

So far the Evohome has not worked out that it needs to adjust the on time.

I do believe from what others have said is that the Evohome tells the HR92 how far to set its actuator.

Thanks
Colin

 

[fronty]

Another evohome user here, been reading this thread with interest, I do believe when evohome is doing this TPI thing (eg firing for 1 minute in every 10), the flow temps are not supposed to get up to anywhere near what you'd expect, it's literally just warming the water slightly to dump a small amount of heat into that zone, the idea is to prevent an overrun.

My system seems to work quite well as it will maintain the temps in each room once the set point has been reached. However what I did find is that some rads did not get warm at all despite calling for heat. It seems than for low demands of 10-20% when the boiler is firing as per TPI schedule, the valve does not open enough inside the TRV to let any water pass, I found that on a couple of rads I had to set the HR92 stroke parameter (setting 6) to 1. That seemed to fix the problem and even on low demands I can feel some heat in the rad, it's not hot just warm..

I don't think anyone has mentioned this so give it a try.

 

[colin_]

Another evohome user here, been reading this thread with interest, I do believe when evohome is doing this TPI thing (eg firing for 1 minute in every 10), the flow temps are not supposed to get up to anywhere near what you'd expect, it's literally just warming the water slightly to dump a small amount of heat into that zone, the idea is to prevent an overrun.

My system seems to work quite well as it will maintain the temps in each room once the set point has been reached. However what I did find is that some rads did not get warm at all despite calling for heat. It seems than for low demands of 10-20% when the boiler is firing as per TPI schedule, the valve does not open enough inside the TRV to let any water pass, I found that on a couple of rads I had to set the HR92 stroke parameter (setting 6) to 1. That seemed to fix the problem and even on low demands I can feel some heat in the rad, it's not hot just warm..

I don't think anyone has mentioned this so give it a try.

Hi

I am not sure what the system is supposed to be doing when in TPI mode but it seems that if I put the on time of the boiler to 4 minutes then the radiators will get hot (obviously as more time to heat) and that allows the system to get to/past the set temperature quicker and after a few cycles of that it will no longer need to come on every 10 minutes as no longer calling for heat.

My situation is made worse by the fact that the bathroom radiator is, at the moment, fully open and being used instead of an automatic bypass and if I do not have the boiler hotter then any heat that is output (even at 3 minutes, hence move to 4 minutes) is lost into this radiator and does nothing for the radiators that are wanting the top up heat.

I also have a Worcester boiler that seems to slow start. It comes on at a minimum modulation (default 10%, mine now 20%) and will stay there for several minutes before deciding it needs to up its modulation. This again means I have to set the minimum on time high. I hate to think what the gas usage is going to be like (Honeywell mention that the system will use less gas but in my case it looks like it will be much more).

So conflicting system needs and trying to get it sorted is a nightmare (Honeywell connected specialist installed it - they say they have escalated it with Honeywell). It would have been nice if Honeywell state what happens in TPI mode, especially with what the boiler is expected to do.

[Q1] When in TPI, do you happen to know what temperature your boiler is getting up to when it is turned on - when I had mine at 3 minutes it barely got to 40-45 degrees out of the boiler and so much less at the radiators?

[Q2] When in TPI mode you mention that the radiators are only warm, on yours is the most warm near the top of the radiator with the rest simply warm or is it evenly distributed around it?

[Q3] When in TPI mode, I presume that it takes many 10 minute cycles before the HR92 units are demanding 0% (hence why mine seemed to be coming on every 10 minutes for 4-5 hours)?

BTW: I had to put all my HR92 into full stroke mode to ensure that they opened far enough given what little heat was being given out.

Thanks
Colin

 

[John.g]

This means so that its opened more for any given HR92 deltaT?,

I never see any mention of the position of the HR92, ie on the rad flow or return as if installed on the return will give a more representative room temperature, obviously doesn't make any difference with a cold rad and a few bursts of 1 minute boiler run time.

 

[colin_]

Dear John

I think the Evohome may decide (as part of its learning process) to open it up more if the temperature has not moved in the last 10 minutes so that next time it gets more heat through it but that is just conjecture on my part as not looked closely at that yet.

Thanks
Colin

 

[John.g]

Talking generally,

"Normal" PI control integrates the (proportional) error over given time, for example if the error was looking for 10% valve opening then with a 1 minute integral time the valve opening will increase by another 10% in 1 minute (if the error remains constant) eventually and would be 100% open after say 9 minutes, normally the error is reducing as the valve opening increases so the integral add on progressively reduces until there is no error and the valve will stop opening, eventually the measured value may be > the SP value (integral overshoot) so the integral effect starts reducing the valve opening until eventually the SP value is achieved.

EH must clearly interrupt this integral effect when the boiler is not (EH) switched on otherwise all the HR92s would be 100% open irrespective of their room temperatures after a relatively short period and certainly after a overnight boiler shutdown.
If, as you think, the EH does not increase/decrease the valve opening with integral effect until there is a change in temperature after a certain time then that is not true PI control so maybe this is why its called timed PI control and doesn't increase/decrease the valve opening until each burst of run time unless the temperature has actually changed.
Or maybe the EH integral time is just the on run time/cycle.

Still a bit confusing to me IF it doesn't increase the valve opening to 100% say overnight whatever way I look at it.