Programmer still firing boiler despite set point achieved

[diyn24318]

Many thanks again D_H for your post.

I have "rebalance rads with a greater drop" on my TODO list...

The boiler is designed to have a 20C drop.

I did ask in another thread but got no response - my gut did tell me I need as near to 20C as I can get and will attempt this rebalancing soon.

The 400 series boilers has caused problems with Micro-firing. A modified PCB is supposed to fix this.

Yes I did read this blog before I attempted my rebalancing a few months ago, though my understanding from last time of reading was such PCBs were only made/altered in small, pre-production numbers and you had to jump through hoops to have any chance of getting one (and the blog is a few years old now too).

This could be the room stat turning the boiler off

I think this is the MOT in operation - were this scenario to occur all the time I'd suggest the "system" is working as designed

MOT?

I borrowed your acronym from your post on Mar 09 at 12:19pm! Minimum on Time.

Yes, the boiler could be turning itself back on after five minutes. It's all to do with the anti-cycling time. The manual says that this is 20 mins but, as the link above explains, this is not a fixed time. It varies with required flow temperature.

I guess this could be a good thing, though it's certainly annoying they don't mention in the manual of the potential for some of setting values to change themselves!

Have you made a note of the Status Codes when the various events happen? They would tell tell you what is causing the boiler to go on and off.
The Fault (F) codes may also provide a clue.

That's a very helpful reminder - I will do so.

My experience of Honeywell TPI thermostats (3 properties) is that they do what it says on the tin.

Likewise previously - I hope the status and fault codes revert my current thinking!

 

[D_Hailsham]

Yes I did read this blog before I attempted my rebalancing a few months ago, though my understanding from last time of reading was such PCBs were only made/altered in small, pre-production numbers and you had to jump through hoops to have any chance of getting one (and the blog is a few years old now too).

I re-read the blog (and comments). The last entry, in 2014, said that the new PCB is working OK with no sign of micro-firing. There was also an interesting reference to the standard PCB causing the water temperature to rise too quickly causing overheating, similar to your situation.

The current PCB is Vaillant Part No: 0020036861.

I borrowed your acronym from your post on Mar 09 at 12:19pm! Minimum on Time.

It's impossible to say if the cycling is caused by the MOT or by the boiler's anti-cycling system without checking the status information.

It might be worth experimenting with the "Proportional Band" parameter on the Honeywell as this effect the rate of temperature rise. Normally the boiler will run continuously (apart from when it is in anti-cycling mode) until room temperature is within 1.5C of target. The room stat then controls the on time as a reducing proportion of the cycle time as the target temperature is approached. The PB parameter can increase the size of the band up to a max of 3C, which means the room temperature will be approached more slowly thus reducing the possibility of overheating. If you decide to try this, I suggest you go the whole hog and set it to 3C.

Do you have Optimization turned on?