Is the heating oversized?

My version: The heating runs around the clock and pumps the warm water into the rooms according to the flow temperature based on the heating curve (currently about 30 degrees flow at -4 degrees outdoor temperature for me). The water returns at about 27 degrees, the heating raises the three degrees again and keeps it circulating.

You can figure out yourself which variant causes the heating to cycle more, has a higher power consumption, heats the rooms more evenly, etc.

But that would mean that all heating circuits are always open? Do you have a controller in every room where you can set the temperature? For me, these close when the desired temperature is reached... so there is no or very little circulation... the water in the boiler then flows only slowly and is heated too quickly (due to the power)... so that the design temperature according to the heating curve is reached...

In your case, it would be like this: water goes into the boiler at 27 degrees... it switches on because 30 degrees are desired... but before the water from the boiler is fed back into the heating circuit, it has already exceeded 30 degrees inside the boiler (because it is heated too strongly)... then it thinks, all water inside has reached 30 degrees... so everything is heated... switches off... then the 30-degree warm water is pushed forward and the 27-degree cold water comes back in... so again from the beginning...

@everyone I have read that the boiler only modulates after a certain time... does anyone know when that is the case with Junkers... since burner times are only 20-30 seconds, it may be that the boiler does not get to modulate at all?

In your case, it would be that the temperature goes into the boiler at 27 degrees... it turns on because 30 degrees are desired... but before the water from the boiler is fed back into the heating circuit, it has already exceeded 30 degrees in the boiler (because it is heated too strongly)... it then thinks, all the water inside me has reached 30 degrees... so everything is heated... switch off... then the 30-degree warm water is pushed on and the 27-degree cold water comes back in... so it starts all over again...

That is what your heating does. Due to the lack of opened valves, it cannot dissipate any heat, but it has to heat up to 45°, which is not sensible for underfloor heating at all. Better is even flow and low temperatures.

This is how I proceeded with our heat pump:

All controllers set to max first.
The temperature curve of the flow temperature (currently 27° for us) is shifted downward in parallel until the desired room temperature in the warmest room (for us bathroom and living room) is found. The rooms that are still a bit too warm (bedrooms) then gradually have their flow throttled directly at the heating circuit distributor. Possibly some fine-tuning here and there. This does not work instantly, but is a process. If at some point it is noticed that it does not fit at other outside temperatures anymore, then the slope of the heating curve needs to be adjusted if necessary.

However, I am not sure whether a gas boiler can be handled similarly to a heat pump and if the same settings can be made.

Maybe our expert can help you further.

You can also read more here:

That is what your heating does. Due to lack of opened valves, it cannot dissipate any heat, but has to heat up to 45°, which is not at all sensible for underfloor heating. A more even flow and lower temperatures are better.

Thanks in advance...

I just checked again... downstairs we currently have the hallway open and upstairs the bathroom, the corridor, and a child's room... almost 40 sqm... nevertheless, the heating times are short and the boiler cycles....???

40sqm is nothing, especially if the rooms are partly interior. You only need about ONE kW when it's really cold, your heater delivers at least 7kW. It's like trying to keep 3 drops of water on a gas stove at exactly 45 degrees.

But that would mean that all heating circuits are always open?

Yes.

Do you have a controller in every room where you can set the temperature?

Yes, but as described, they are disconnected.

In my case, these "close" when the desired temperature is reached... therefore, there's no or very little circulation...

Yes, they close completely.

The water in the boiler then flows only slowly and is heated too quickly (due to the power)... so that the design temperature according to the heating curve is reached...

I do not understand that sentence.

In your case, it would be such that 27 degrees goes into the boiler... it switches on because 30 degrees are desired... but before the water from the boiler is fed back into the heating circuit, it has already exceeded 30 degrees in the boiler (because it is heated too strongly)... then it thinks all water inside has reached 30 degrees... so all heated... switches off... then the 30-degree warm water is pushed further and the 27-degree cold water comes in again... so again from the beginning...

No, it only heats enough so that the 27-degree return flow is heated up to the 30-degree supply flow. That is why the heating usually has sensors at both supply and return. How you arrive at the desired room temperature has already been nicely explained by .

If too many rooms "shut down" because of an ERR, then only small amounts of water flow in the heating circuit (and often too quickly, because most heating circuit pumps only switch on/off, but do not respond to the differential pressure). This small amount of water is then quickly heated to the desired temperature due to the high output of the boiler, because the volume is small and the paths are short, so warm water quickly returns to the return flow, causing the boiler to switch off again. However, in this short time, by no means enough energy has been transferred to the screed (it is only relatively warm near the heating pipes, but not in the wider surroundings), so the water cools down quickly again and the boiler switches on again. This is how I understand the problem.

No, it only heats so much that the 27-degree return is heated up to the 30-degree flow. That’s why the heating system usually has sensors on the flow and return.
How you reach the desired room temperature @@Musketier has already nicely explained.

Yes and no, if the boiler has way too much output, it will also heat a bit above 30 degrees, sometimes a small hysteresis is "built in" (there are small temperature losses along the way). But that would be optimal.