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Potterton/Baxi Promax Ultra 40 - Fluctuating temp

View the thread, titled "Potterton/Baxi Promax Ultra 40 - Fluctuating temp" which is posted in Central Heating Forum on UK Plumbers Forums.

R

Renato

Hi,

I have the combi boiler Potterton/Baxi Promax Ultra 40.

When I set the temperature to (e.g.) 45°C, the boiler fire up and stay at max power until the hot water temp increases to 57°C and them it turn off the flame.
When the temperature reduces to 42°C, it fire up again.

Doesn't matter the setpoint of the temperature. If I set it to 55°C, it keeps on until the temperature reaches 67°C and them turn off. And turn on when the temperature drops to 52°C.

Instead of reducing the flame to keep the temperature stable around the set temperature, it keeps turning on & off.
It looks like it's at 100% or 0%. No middle term.

If I increase the water flow opening 2 taps on the same time, let's say the shower + sink tap, the boiler modulate normally and keep the temperature stable. When the temp is stable, I close the sink tap and the shower keeps stable for the whole time.

Anyone have an idea what I need to do to fix this annoying problem?

Thanks in advance!
 

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