I don't know anything about Arduino but I would like to. On my boiler, I have no t-stat. The boiler (most boilers) use a thermister (they range from 1k or 20k nominal at 20or 25C nominal) which gives the boiler a value to compare the boiler water temp to.
Here is a Viessmann outdoor control programming guide. On page 17 it gives you a sample thermister curve. I wouldn't use this value of thermister as it draws a bit more current than I would like but the programming tips are good.
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