Hello,
After the first few days in my new home, I’m having some issues with the heating system. Somehow, it feels like something isn’t working 100%... it heats very irregularly. In my experience, it only heats properly when as many rooms as possible are set to flow. Once most rooms have reached their temperature and only 1 or 2 are open, the system quickly cycles on and off...
- We have about 140 sqm (1507 sq ft) of floor area
- Underfloor heating throughout the entire house
- A Junkers Cerapur ZBS 22/100 S-3 MA gas boiler with hot water generation (22 kW)
- 17.5 cm (7 inches) calcium silicate stone + 14 cm (6 inches) external thermal insulation composite system (ETICS) (no special energy-saving regulations)
As mentioned, when only a few heating circuits are open, the boiler turns on (starting at about 25°C (77°F) flow temperature), heats up quickly (up to about 45°C (113°F) flow temperature), then drops off rapidly and starts again. So the heating circuits are only heated sporadically...? If I turn on more heating circuits, the warm-up time is longer and it gets warm... is the boiler oversized, since it heats single rooms poorly?
After the first few days in my new home, I’m having some issues with the heating system. Somehow, it feels like something isn’t working 100%... it heats very irregularly. In my experience, it only heats properly when as many rooms as possible are set to flow. Once most rooms have reached their temperature and only 1 or 2 are open, the system quickly cycles on and off...
- We have about 140 sqm (1507 sq ft) of floor area
- Underfloor heating throughout the entire house
- A Junkers Cerapur ZBS 22/100 S-3 MA gas boiler with hot water generation (22 kW)
- 17.5 cm (7 inches) calcium silicate stone + 14 cm (6 inches) external thermal insulation composite system (ETICS) (no special energy-saving regulations)
As mentioned, when only a few heating circuits are open, the boiler turns on (starting at about 25°C (77°F) flow temperature), heats up quickly (up to about 45°C (113°F) flow temperature), then drops off rapidly and starts again. So the heating circuits are only heated sporadically...? If I turn on more heating circuits, the warm-up time is longer and it gets warm... is the boiler oversized, since it heats single rooms poorly?
Jochen104 schrieb:
My approach:
The heating runs around the clock and circulates the warm water into the rooms according to the supply temperature based on the heating curve (currently around -4 degrees Celsius (25°F) outside temperature, about 30 degrees Celsius (86°F) supply temperature). The water returns at about 27 degrees Celsius (81°F); the heating raises it back up by three degrees and keeps it circulating.
You can calculate for yourself with which setup the heating cycles more often, consumes more electricity, or provides more even warmth in the rooms. But wouldn’t that mean all heating circuits are always open? Do you have a controller in each room where you can set the temperature? In my setup, these close off when the desired temperature is reached, so there is little or no circulation. The water in the boiler then flows very slowly and heats up too quickly (due to the output), which means the design temperature according to the heating curve is met.
In your case, the water enters the boiler at 27 degrees Celsius (81°F); it fires up because 30 degrees Celsius (86°F) is demanded. But before the water is fed back into the heating circuit, it already exceeds 30 degrees Celsius (86°F) inside the boiler (because of overheating). The boiler senses that all the water inside has reached 30 degrees Celsius (86°F) — so everything is heated — and switches off. Then the 30-degree Celsius (86°F) water is pushed through, and the 27-degree Celsius (81°F) water comes back in, starting the cycle over.
@everyone
I’ve read that the boiler only starts modulating after a certain period. Does anyone know when this happens with Junkers boilers? Since burner runtimes are only 20-30 seconds, it might be that the boiler never actually gets to modulate?
Basti2709 schrieb:
In your case, the water entering the boiler would be at 27°C (81°F)... the boiler turns on because 30°C (86°F) is set as the target... but before the water from the boiler is returned to the heating circuit, its temperature inside the boiler has already exceeded 30°C (86°F) (because it is being heated too much)... the boiler then assumes that all the water inside has reached 30°C (86°F)... so it considers everything heated and switches off... then the 30°C (86°F) water is pushed forward while the 27°C (81°F) cold water flows back in... and the cycle starts over...This is what your heating system is doing. Since the valves are not open, it cannot transfer heat, but it still has to heat up to 45°C (113°F), which is not ideal for underfloor heating. A more even flow and lower temperatures are better.
This is the approach I used with our heat pump:
First, set all controllers to maximum. Then, lower the temperature curve of the flow temperature (currently 27°C (81°F) for us) in parallel until the desired room temperature is reached in the warmest rooms (in our case the bathroom and living room). For rooms that are still too warm (bedrooms), gradually reduce the flow directly at the heating manifold. You may need to make some additional fine adjustments here and there. This process does not work instantly but requires some time. If you later notice that it’s not working properly under different outdoor temperatures, you may need to slightly adjust the slope of the heating curve.
However, I’m not sure if a gas boiler can be handled similarly to a heat pump and if the same settings apply.
Maybe our expert @wrobel can help you further.
You can also review the following discussion:
https://www.hausbau-forum.de/threads/Nest-Thermostat.12769/page-7#post-104764
Musketier schrieb:
This is what your heating system is doing. Because the valves are not open, it cannot release any heat, but it still has to heat up to 45°C (113°F), which is not efficient for underfloor heating. A more consistent flow and lower temperatures are better.
Thanks in advance...
I just checked again... currently, downstairs the hallway is open, and upstairs the bathroom, corridor, and a children’s room... that’s almost 40 m² (430 sq ft)... yet the heating-up times are short and the boiler cycles on and off...?
Basti2709 schrieb:
Wouldn't that mean all heating circuits are always open? Yes.
Basti2709 schrieb:
Do you have a controller in every room where you can set the temperature? Yes, but as mentioned, they are disconnected.
Basti2709 schrieb:
In my case, these close off when the desired temperature is reached, so there is little to no circulation... Yes, they close completely.
Basti2709 schrieb:
The water in the boiler then flows slowly and is heated too quickly (due to the output), so that the design temperature according to the heating curve is reached... I don’t understand that sentence.
Basti2709 schrieb:
In your case, it would be like this: the water enters the boiler at 27°C (81°F)... the boiler starts because 30°C (86°F) is required... but before the water from the boiler is fed back into the heating circuit, it has already exceeded 30°C (86°F) inside the boiler (because it’s heating too strongly)... then the boiler thinks all the water inside has reached 30°C (86°F), so everything is heated and it switches off... then the 30°C (86°F) warm water is pushed further and the 27°C (81°F) cold water comes in again... so it starts all over... No, it only heats enough to raise the 27°C (81°F) return flow to 30°C (86°F) supply temperature. That’s why the heating system usually has sensors on both the supply and return lines.
How you reach the desired room temperature was already nicely explained by @Musketier.
If too many rooms are closed off due to an ERR, only small amounts of water flow through the heating circuit (and often too quickly, because most heating circuit pumps only switch on/off and do not respond to differential pressure). This small volume of water is then quickly heated to the desired temperature because of the high output of the boiler, since the amount is small and the distances are short. Warm water then returns quickly to the boiler, causing it to switch off again. However, in this short time, not nearly enough energy has been transferred to the screed (it only gets relatively warm near the heating pipes, but not in the wider area), so the water cools down quickly and the boiler turns on again. That is how I understand the problem.
Yes and no. If the boiler has too much output, it will heat somewhat above 30°C (86°F) at times, and sometimes a small hysteresis is "built in" (there are also minor temperature losses along the way). Ideally, though, it would work as described.
No, it only heats enough to raise the 27°C (81°F) return flow to the 30°C (86°F) supply flow. That is why heating systems usually have sensors on both the supply and return flow.
How you reach the desired room temperature has already been nicely explained by @@Musketier.
Yes and no. If the boiler has too much output, it will heat somewhat above 30°C (86°F) at times, and sometimes a small hysteresis is "built in" (there are also minor temperature losses along the way). Ideally, though, it would work as described.
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