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Without regard to the failed block heater, forget about that for a second.
If the thermostat never opens, pressure is going to increase until something breaks. Could be wrong, often am.
Not to beat a dead horse, but the water pump is not a high pressure pump - I think you would be lucky to see 5 psi completely dead headed. Plus, think of it this way - if a closed thermostat caused enough of an increase in coolant pressure in the block to push out core plugs/block heaters, you would fail stuff every cold start when the thermostat is closed. If you are talking about pressure increase with rising water temperature, pressure will build until the radiator cap opens and vents the pressure to the overflow/expansion tank to maintain an even system pressure. The radiator cap is the only thing that regulates overall cooling system pressure. Take it off and the cooling system will operate equal to atmospheric pressure and zero gauge pressure.
I mean no disrespect. My point was to not create extra work and/or worry due to incorrect reasoning for the person whose truck spewed coolant everywhere. Let him replace the heater and head on down the road.
If you are talking about pressure increase with rising water temperature, pressure will build until the radiator cap opens and vents the pressure to the overflow/expansion tank to maintain an even system pressure.
I get it, if the thermostat is stuck closed, engine is still open through the water pump, bottom inlet hose to radiator.
It's Old Faithful at the radiator though, if the cap is taken off, right? Lol.
