What would cause a variation in the water temperature from the bathroom sink to the shower? The sink water heats to the temp set on the thermostat but the shower water never gets to the proper temp. The diference is about 30 degress. This is a Rinnai system that is 6 years old. Model # R85 and Thermostat MC-91-1-US-S. Thanks

Reduce hot water do not turn full on then add cold to suit

This is going to be a water heater issue. Not the head.

A lot depends on the size of the tank. And when it heats some systems only heat at night , this is ok if the system is large , on most heaters there is a thermostat on the side that can be used to set to a higher temp. the thermostat on your heater may be faulty.

Check for crossover problem where cold water is getting into hot line and causing less flow on hot line that cuts off heater and reduces temp at faucet.
http://waterheatertimer.org/Crossover.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters .

Hello mkh....Changing the water temperature setting is not difficult. Go to the hot water tank and you will see the temperature setting knob. Sometimes it is down low and other times it is behind a small door which you can open or sometimes even have to remove a screw or two to get to. Make a small change in the upward direction and see if that solves it for you. Chances are the 2nd bathroom is further from the water heater and is loosing heat along the supply pipe. Consider insulating that pipe if it is out in the open. Joe

Thanks for choosing Fixya!

Auto/self adjusting anti-scald feature in shower body and /or flow meter defective/cloged.