This is normal
Probably normal: a small shift that settles
A brief, small change when a toilet flushes is within what a working pressure-balancing valve does.
Why this happens
Delta designs its Monitor valves to hold temperature within about plus or minus 3.6F when pressure changes, such as a toilet flush. A small, short shift that settles on its own means the valve is reacting the way it should. Delta also notes that single-function Monitor valves may need manual adjustment for seasonal ground water temperature changes, so a shower that runs a little cooler in winter is not a broken cartridge.
What to do
- Leave it alone if the change is small and settles within a few seconds.
- If the whole shower runs cooler in winter, adjust the temperature limit stop by following your valve's guide.
- If the swing gets bigger over time, come back and choose the big-swing answer.
If it still worries you, tell the plumber
- If it gets worse: how big the swing is and how long it lasts.
- The valve brand and whether it is pressure-balancing.
Build the full list: what to tell the plumber.
Start this troubleshooter over
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