Hopefully someone out there has this problem too. One of my showers has this insane problem of changing water pressure and water temperature when someone else is in the other shower in my house. The pressure starts out OK, but then once someone else gets in the other one, the water either gets insanely hot for a few seconds or turns cold/luke warm for a longer time and the water pressure drops. My dad has tried to figure it out and can't at all. He's asked around at Home Depot and Lowes and no one seems to know. So if anyone out there has any ideas, I'll be glad to pass them along to him.
This is very common. You have one pipe feeding the house. It is split into two in the house, on for hot, the other for cold. A given size pipe can supply a certain amount of water. When two faucets are opened, the pressure will drop some in the system and neither gets full pressure. Depending on whether the hot or cold is opened on the second faucet, that will increase or decrease the temperature coming out of the mix on the other faucet.
To (partially) solve the temperature problem, install a temperature sensing shower control. There are two types, one is an anti scald, the other is a temperature control. The anti-scald will prevent the water from exceeding the temperature the user has set while showering. If another faucet is opened, the flow may drop a bit, but you won't get burned. The down side is that if the other person turns on the hot water, you will get less and you can get a chill. (I have this in my house)
More expensive temperature control units will compensate for either hot or cold changes.
Another part of the problem is the amount of water and pressure available to you from the main. If it is an older house, chances are the water fed is
3/4" rather than the larger 1" commonly used today. No matter what fixes you do in the house, only so much water can pass though the pipes. If you don't already have them, low flow shower heads will help also. One of the best for the money is Saver Shower (do a Google search) as they still feel good with reduced flow.
Other factors are the incoming water temperature, the setting of the hot water, size of tubing at the showers. They won't fix the problem, just change the degree of it. You can't change the temperature the water comes in from the street or well.
As you stated, setting of the hot water heater too high can cause some of this, especially when flushing a toilet while someone is taking a shower. If the temperature is set too high, it takes a lot more cold water to make it comfortable for a shower. So when another cold water faucet is turned on, the hot water immediately becomes too hot. Keep the water heater down around 120F like they recommend.
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