Have an Amtrol Well-X-Trol WX-250, Amtrol WH7L hot water tank, and Weil-McLain furnace. I need to take the pressure off the system to replace some other plumbing. How do I do this?

pump is producing pressure so it is ok
loosing pressure indicate pipes leaking or non return valve leaking

Water that goes in to this tank comes straight from pump, check the foot valve on your well might be bleed water back into well. Also check the air bladder on the tank. Make sure it has proper inflation

Is the pump running when power is ON?
Put ear against pipe and listen for pump.
Have you installed a check valve?
Check vlave lets water pass through the pipe and enter tank, but the valve closes after pump shuts off and keeps water from falling back down into well.

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Sand the points on the pressure switch
Replace pressure switch
http://www.amazon.com/dp/B000BQSERE/?tag=waterheaterti-20

Test you are getting 240Volt to pressure switch
Turn breaker fully off and then fully back on

Replace control box or replace start capacitor:
http://www.amazon.com/dp/B0060C7GFO/?tag=waterheaterti-20

Drain tank, and replace the air valve
http://www.amazon.com/dp/B000BQYKJK/?tag=waterheaterti-20

Gene

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either solenoid valve is closing or there is a large water hammer occurring

Tank is waterlogged.
Drain tank, install new air release valve

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Anode rod is reacting with well water
http://waterheatertimer.org/Replace-anode-rod.html#smell
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In a while the control unit will fail
On the Amtrol units the control is their weakness.Once it starts to buss it will last somewhere between a couple of weeks and a year, but it WILL eventually fail.
And while Amtrol advertise a "Limited lifetime warranty" the "Limited " part includes not putting any warranty on that control unit.