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http://s3.pexsupply.com/product_files/T498B1553-Submittal%20Sheet.pdf

Scroll down to Fig. 3. In your case both L1 & L2 would be hot.

Yeah, the two red are one side of the thermostat, the two blacks are the other. So now when you turn up the thermostat it closes red to red and creates a 240V dead short.
Did you see the wiring...

Then you wired it wrong.

The term 120/240v is a service rating. You have 120V loads in the house, along with 240V loads (like an A/C, power tool, motors, electric heat, etc), as well as 120/240V...