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Replacing the thermostat in the Watts 70A tempering valve

I have a 70A Watts tempering valve hot water system. I have my oil burner man replace the thermostat every year when he does the maintenance on the heating system. He thinks I am foolish to do this but I have seen the thermostat and spring turn to dust after a couple of years from the minerals in the water I guess. Rather than waiting for the thing to go and have to call and have him come over just for that I just include it in my yearly maintenance, as the repair kit used to cost about $16.00 and is now $30.00 and should take him about five minutes to change. I would like to be able to do this myself (using the 70A-RK repair kit) but am unsure what valves to turn off. I have read that you should turn off both the hot and cold water valves before the tempering valve and to turn on the cold water before the hot so you don't ruin the valve. Plus turn off the oil burner? Water where it comes into the house? Open the faucets to bleed the pressure before opening the valve to get at the thermostat?
Any help would be appreciated.