Setpoint vs. Boiler temperature mismatch

[Bartb]

Hi,

Not sure where to post, but I thought I'd start here.

Using OTGW I collect various info among which the Boiler setpoint temperatures and CH water temperatures.

I'm using a weather compensation control setup (weersafhankelijk) and I can see the Setpoint temperatures nicely following (opposite temp curve) the outside temperatures. I.e. The lower the outside temp's get, the higher the setpoint on the boiler is. So far so good.

When I then look at the actual measured CH water temperatures (aanvoer temperatuur), I see that the temperature fluctuates strongly, much more then the Setpoint does. It seems to act much more like a on/off switched boiler, rather than a nicely modulated system.
To be sure I checked the CH return temperatures not rising to high (waterzijdig ingeregeld). So this should not have an impact.

Can anyone explain why I do not see a much more stable CH water temperatures. The systems does not look efficient this way.

I've added some graphs to show the effect.
(Side note: the short peaks in the graph going up to 75 Celcius are related to Domestic hot water (warm water) being used. Please ignore.
Looking forward to any idea's

Outside temps vs Control setpoint temps over 5 days:

Outside vs setpoint temps 5d.png (58.65 KiB) Viewed 2904 times

Temps over 5 days:

boilerwatertemp.png (68.55 KiB) Viewed 2904 times

setpoint 5d.png (56.07 KiB) Viewed 2904 times

Temps over 3 hrs:

boilerwatertemp-3hrs.png (58.12 KiB) Viewed 2904 times

setpoint 3hr.png (51.12 KiB) Viewed 2904 times

[wintersteiger]

I have no idea whether that's really the case, but I think this could be due to the fact that your boiler can't get the heat into the house. The minimal modulation of your boiler is 8.5kW, which is quite a bit of energy. What happens is that a control set point is set, but after a little time, the flow temperature rises higher than the setpoint, because the heat could not get out of the system and the boiler can't modulate any lower. At that point, the boiler cuts out and waits until the flow temperature is back below the setpoint. So, it's cutting in and out repeatedly.

My system behaves the same. Initially I thought my boiler is just too powerful, but last winter there were some very cold days and it couldn't keep up. The real reason is that my radiators are just too small and can't get the heat into the house quick enough, so I'm contemplating installing some additional ones now. During warmer times, e.g. when heating the house from 20 to 21 C, it does it quite frequently (the smaller the difference, the harder it is to transfer the heat).