When hot water is on & toilet flushed the temp drops

First thing to try is remove the screen holder from the faucet to see if sediment has plugged it. Unscrews from the faucet where the water comes out. Turn on faucet with it removed to flush out any other minerals that may be in the faucet. Clean screen then replace.

Do You know if there is an anti scald valve in the shower valve? If so it may be scald up with corrosion and need replaced! To see if this could be the problem, flush the toilet next to the shower while the shower is running. If temperature changes in shower I would say that is your problem! If You need to replace it, it would be cheaper to do it yourself! They should be videos on Youtube on how to do it! Good Luck! Regards!

Water heater 14 years old, probably has never been flushed, Flush recommended at least every 6 months, Build up inside tank may be to late to flush, but you can try or replace heater.
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The water heater has nothing to do with it. You need a shower control valve that won't let the water pressure change when the toilet is flushed. Moen makes just such a valve called a pressure balancing valve. Moen L82683 Positemp Tub/Shower Valve. http://www.moen.com

Increase your flow, sounds like you are right at your activation rate.
1 way is to turn down the temp on the machine, so you dont have to mix in so much cold at your shower. Check to see if your mixing valve is bad, or your showerhead has restriction/scale built up in it.The other way is to turn on your lavatory fixture full hot before showering(which will keep hot water flowing through the machine as you adjust your temp to be comfortable.

Ok that sounds more life a faucet that is bad. In today world, the newer (20 years and less) they have pressure balance faucets. Have the faucet serviced. NOW lets look at an other area check out your home to see if you have a cross connection. A cross connetion is a place where hot and clod water meet. I know that everywhere but the toilet!

Yes thats true. But we buy all kinds of things that we attach to a fauects or tub spouts. Lets say you attached a hose to the sink in the garage to wash the dog with warm water. if you turned the water off at the handle of the hose and you left the water on at the faucet. BINGO

the hot and cold water mix and then up and down water temp while taking a shower.

So start looking first before spending any money. if ANYTHING is attached to any faucet take it OFF and then take a shower.

Be safe

Thanks

Tom ( how did we do ?) :)

well it is time to get the the two way valves to both side, I would use only a product the noritz says to use.

Now if you cant get it you can get other products that break up lime and scale from you local store that sells plumbing product. I you can see I dont list a store or a product I am here to help you repair your heater, Not have you buy from one store or an other.

With that being said you can vinegar but dont use acid. If yuo "burn" the inside of the heat exhanger you jusy might be buying a new unit. also Please flush out the system with cold water for at least 10 mins. with the 3 way valves that should be very easy to do.

You also might have a lime build uo in the faucet also which will affect how they work. You may want to replaced and cleanout the cartridge and see if you have a lime build up. If that is the case you might want to think about a low priced water filter or softener.

Hope this helps you out and please dont for get to rate me, and Be safe

Tom

when was the last time that you have "opened the flush valve (faucet) at the bottom of the tank to clear out the sediment? if it has been more then 4 years then you will need to do this depending where you are located this should be done once a year just to keep the tank clean. attach your garden hose to the valve and run it over to the sink open the valve for a few moments to clear out the tanks bottom (HOT WATER ALERT!) after doing this and if you have seen dirt come out then your tank should be clear. check to see what the tempature is every 1/4 inch is 10-30* increase hope this helps

This is typical for an installation without a balanced valve/anti-scald valve on the shower.

If you are using manual on/off valves on the shower, about the only way to fix this is to install the device mentioned in the previous paragraph.

I had a customer a few weeks ago that had the same problem. When the water heater was installed they crossed the supply lines. They had the inlet as the outlet. Esentially, the water heater was plumbed in backwards. Also there could be a miss piping somewhere in the house but it would probably be near impossible to find it right now without tearing up foundations. Also check that the faucet is not plumbed backwards as well. Especially if this is a single handle faucet that you are referring to.