For years our hot water would be a certain temperature after sitting (no hot water use for X hours), and 10° hotter than that if a fair amount was used followed by a brief recovery time (approx. 30 minutes). Today after returning from a weekend trip I found both of those temperatures were 10° less than usual (checked with two thermometers). I turned up the adjustment knob on the heater and was able to get the same high temp as before. Why would the system suddenly be performing ten degrees cooler than it had been?
Maybe scale or sediment reducing heat transfer effectiveness?
My guess is that either your thermostat is starting to go wonky, or if this is an electric heater, one of the elements has failed or is failing.
Update: now this morning, it’s hotter than it had been, so I’ve just turned the adjustment back to where it was before.
Thanks for the replies. I’ll keep those in mind if it continues to be inconsistent.
The ambient temp can have a huge effect on the temperature of the water coming out of the faucet. Some heat is lost traveling through the pipes, which can vary depending on how far the water has to travel and how cool or warm the space the pipes pass through is.