Tankless or tank water heater

[Ladiesman271]

That's why I told the OP to measure the water temperature with a thermometer. Just because the thermostat is set to 170 does not mean he has 170 degree water at the tub.

He may not even have 170 at the water heater. If he has a tempering valve, it could be set low or it could even be bad.

Measure the water temperature, don't guess!

 

[Jadnashua]

A bad tempering valve can always be mixing in more cold than it should. If you have one, it's hard to actually test the tanks output. You could do it with a contact thermometer after running the water a bit to heat up the pipe, but normally you wouldn't have a tap before the tempering valve.

If you feel the in, cold, out branches of the tempering valve, they should be at radically different temperatures if you have it set to 170, and it really is there. I'd never run a WH that high without a tempering valve...too dangerous. Raising the tank's temp does give you more capacity at normal temperatures, but the recovery time is extended quite a bit.