Something has changed (plumbing question)

Hello, I have been away from home for about three months due to work etc.
Three months ago, (and for the past 8 years) while you were showering, anyone else could run a tap or flush a toilet without disturbing the temperature in the shower. Now when a toilet is flushed the water turns to total scalding heat in the shower. Any ideas on what could have possibly changed this? No plumbing modifications have been done. We are on a well system with a pressure system. All the water pressures seem normal like they were. Any ideas as this really has me confused....thanks...Jim
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Jim wrote:

Is the shower control a pressure balance or scald-gard type? If so, it may have failed (stuck) internally.
Do you have a free-standing water heater (gas or electric)? Or is water heated from a coil in the boiler? If so, there may be a tempering valve which has failed.
See what happens to the tap temperature in the lav basin when a toilet is flushed,. That may give a clue as to whether just the shower control is affected.
Jim
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Hello Jim and thanks for your help in this thread, I'll try to answer your questions as well as I can.

I don't know nor know the difference. How do I find this out?

I have a gas free standing water heater.

Okay, will do that Jim..... wife is in bed and will do this test tomorrow when she awakes.....will let you know

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Jim wrote:

This is typically a sign that older galvanized pipes are starting to clog up with mineral deposits and rust. Replace with copper. Just DO NOT connect the two together directly. Use a 6" brass nipple.
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Hello Inspector D....This home is 10 yrs old and has no galvanized pipes. it is all copper....Regards...Jim

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Jim wrote:

The anti-scald valve in the shower may be stuck. Ask TOH had a segment on it recently. When the shower is left unused for a while corrosion and sediment have a chance to work.
Brad
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