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Hello, if there is no C terminal on the thermostat wire terminal strip in the furnace or air handler, then you can access C from the 24v side of the control transformer directly, that is *opposite* of the terminal that R on the thermostat originates from.
IF you need help locating that terminal use the paper clip icon at the right end of your tool bar to send a close up photo of the wiring diagram glued inside the blower service door. Photo must be taken from straight on, not at an angle, and be very will focused and well lit.
I can mark that up with instructions for you.
Hello, you do not take power from the red terminal.
Some photo's will work best.
If you do this correctly there are no compromise issues. If you do it incorrectly and attach to R for instance you will short the 24v circuit and blow out the transformer. This is why photo's work best I can mark them up for you. (I am 75 years old, retired last year after 55 years of working on these things... its quite do-able, there may even be a C terminal on the printed circuit board... I have to see it though, there are literally tens of thousands of variations on the market.)
Photo the diagram and the printed circuit card.
Hello again, you have a boiler there, not a furnace. With boilers there are most often more than one zone, that involves a number of pumps and a zone control panel. If the system is not set up for a thermostat with a C terminal it will be quite a bit of work sorting out how to use that thermostat on your system.
If you can tell me the brand and model number of the old thermostat that you are replacing, and tell me which color wires went to each terminal on that thermostat..I can most likely find a drop in replacement for the old thermostat that is programmable if you wish, but does not have a need for the C wire.
A photo of the thermostat wires where they exit the wall and attach to the old thermostat will be helpful, that will save some testing on the voltages applied.
We can go from there