Why should a Digistat sometimes only call for heat for 40 seconds?

[old duffer]

As I say, my Honeywell allows such an adjustment so I'm not sure where you're going with that one.

Just read the data sheet. you can set a few pins high or low. I meant adjusting the logic modification.


The downside of that particular approach is that it is quite a crude controller and doesn't provide TPI control and so would be a step backwards in terms of stability of output. If @PaulUszak was looking for a replacement I'd recommend a Honeywell CM927.

If that example was too crude for you, then use this one

Or go full hogg and use opentherm adapter http://ihormelnyk.com/opentherm_adapter

Not sure why you think an ARM cortex IC running a full blown OS is a poor replacement for a simple RISC IC

 

[MJN]

@old duffer I think you're in the wrong thread.

 

[PaulUszak]

@MJN Yeh, I think so too...

 

[PaulUszak]

Partial solution:-

Set fuel type: heat pump (default: gas)
Set flow rate: slow (default: fast)

Fingers crossed...

 

[coopersim]

The obvious question and sorry if it's been answered is, are there external temperature influences that could be causing the setpoint to be reached, for instance, is the thermostat above / in close proximity to another source of heat (TV, monitor, plug in transformer, etc) ?

 

[PaulUszak]

@coopersim No there aren't, but worth re-considering. Background is that all I've done is replace the old control:-

with the new Digistat as it has more flexible heating programs. The old one worked perfectly thermally wise with a ~1.5 degree hysteresis.

I'm coming around to the notion that the new Digistat's quasi-PID algorithm just isn't tuned accurately enough for a 26 year old vented boiler. PID tuning is quite a science and entirely dependant on the environment, which in turn controls the process variable. Everyone now has combis and they're rated at >2x mine, and expected to run many many times a day as water is drawn. I hate combis.

If my tweaks don't work, I'll revert to the above

 

[magicmushroom666]

Watch out, I've got a similar Drayton units and even that isn't a simple mechanical on off switch, there's electronics inside and even when the temp is below setpoint, it still cycles the boiler sometimes!