Boiler not modulating?

[Bart B]

Hi,

Not sure if this is the place to post this, but hope some one can help.

My Central Heating water temperatures seem to fluctuate a lot, while I’m using a modulated system. Within an hour is goes from 50 degrees to 20 degrees many times, while the outside temp as wel as the room temp remain stable during this time.
Is a modulating system supposed to work like this?
In other words: Should it not level out and become stable after a while if the room/outside temperatures are not changing?

Details:
Using Open Therm I collect various info among which is the Boiler setpoint temperatures and CH water temperatures.
I'm using a weather compensation control setup and I can see the Boiler CH Setpoint temperatures nicely being set and following (opposite temp curve) the outside temperatures. I.e. the lower the outside temp's get, the higher the CH setpoint on the boiler is. So far so good.
When I then look at the actual measured CH water temperatures (using remote temperature sensors), I see that the temperature constantly fluctuates strongly and totally unlike the Setpoint. 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 and the power is maximized.

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

System used:
Boiler: Remeha Tzerra Ace 36 (year: 2022)
Nefit Easy room thermostat (using Opentherm gateway)

 

[John.g]

If its keeping the room temperature at its setpoint and by this I mean within 0.1 to 0.2C on either side of its setpoint then it might just be firing the boiler every 10 minutes or so for a few minutes if it has TPI control which operates in the on/off manner you describe.

 

[Chuck]

To expand slightly on @John.g's answer, many (most?) boilers have a minimum output of around 7kW, limited by the burner design. If your house requires more than this, e.g. in the winter, then the CH water temperature can stay stable but otherwise, e.g. in spring and autumn, the only way the boiler can match the demand is by switching on and off.

Modern boilers don't seem to mind this cycling too much; zone valves seem to suffer a bit from the abuse but you should still get at least five years out of them before the head needs replacing due to wear and tear.