Krodal:
You can't measure an accurate temperature with an IR temperature meter. The material you are pointing at influences the temperature.
I know that copper is good at reflecting heat, so I put black electrical tape on the spot where I check with the IR. Didn't change the reading much, though, if at all.
dhenry:
That's assuming that the other measurements are more accurate than the DS. How do you know that?
I don't know for sure, but I admit that I assumed that the sensors on the boiler are more or less accurate. Even if they aren't, and the ds18b20 is, then could they be off by more than 10° at the temperatures they were designed/installed for (showing 55°C when the ds18b20 shows only 44°C)? I woundn't expect that on a not-too-cheap boiler.