Mikepier wrote:
I agree with Paul with something additional to add. You claim that
when
the T-stat is off the 2nd floor would be boiling, and when you turn
the
T-stat up, it's cold. All this started after you replaced the T-stat.
Could it be possible that you mis-wired the T-stat and that it is in
"cool" mode. Anotherwords you wired it up as an A/C thermostat so
when
you turn down the T-stat, it would actually turn on your boiler
instead
of turning it off, and vice-versa when you turned up the T-stat it
would turn off the boiler.
A good way to check is to remove the wires from the t-stat and short
them together. If you start to feel heat coming up to the second
floor,
that means that everything is fine, you just have to wire the T-stat
correctly.

Hey there, thanks for the advice, I will definitely try shorting the
cables together. Just to clarify, the thermostat that was behaving
badly was the old one, and I didn't want to imply that it always
behaved the opposite of what I set it to do. I was just trying to say
that sometimes the heat was as expected, other times it was very warm
or cold, irrespective of what we put the thermostat to.