How to set temperature and mixing valve for heat pump water heater?

Question

I have a Rheem heat pump water heater and a mixing valve. I have heard many (contradicting) advice for how to set water temperature. The mixing valve has simple settings but no thermometer and I haven't touched it since it was installed (by a totally incompetent contractor who I do not trust whatsoever):

What is the best approach to set both for highest efficiency and why?

To my understanding, the best would be to set water temperature the highest (150F) and adjust mixing valve to mix a bit of very hot water with lots of cold water. This I am thinking should maximize the amount of hot water. The tank itself is fairly efficient in storing hot water.

If this is correct so far, how do I adjust the mixing valve when there is no thermometer? Can I go to the closest sink, turn on the water and adjust mixing valve until the water has a certain temperature? If yes, what should be the target temperature?

(Note: While I enjoy as hot water as possible since I can mix at the sink anyway, I value efficiency much higher. So it should be the lowest acceptable temperature for lowest power consumption)

Answer 1

For a heat pump water heater, lower temperature settings are more efficient because the temperature differential on the heat pump affects its operating efficiency.

For a resistance heater, that doesn't matter. For potential leigonella prevention, minimum tank setting of 140°F/60°C is widely suggested.

Thus, unless you need more capacity than the tank offers, a higher setting is less efficient. If you regularly use all the hot water stored, and especially if you have the backup resistance element enabled, a higher setting might be more efficient, particularly if it allows turning off the backup resistance element while still having adequate hot water supply. If the hot water supply is adequate without engaging backup resistance heating, lower is more efficient, but under 140°F/60°C may increase odds of leigonella issues.

You set the mixing valve using a thermometer, (with hot water running from a faucet somewhere) preferably on the pipe coming out of it. I think mine included one that sticks right to the pipe. I find with minimal searching a clamp-on dial thermometer for 1/2"-4" pipes (for under $15 in 2024.) Alternatively, some multimeters can read an inexpensive type K thermocouple which could be tucked under the pipe insulation.

Measuring at a faucet might mean you get false readings if the faucet has anti-scald mixing built in, as many do these days.

Time it takes to get a severe burn from a scald:

• 155°F = 1 second
• 148°F = 2 seconds
• 140°F = 5 seconds
• 133°F = 15 seconds
• 127°F = 1 minute
• 124°F = 3-4 minutes

Source for scald time.

Stock advice for maximum temperature at point of use is 120°F/49°C

Answer 2

The mixing valve is what sets the temperature of hot water delivered to the house. You can set that by experimentation; there are standards but usually the essential thing is that it be below scalding temperatures so nobody burns themselves on your hot water.

The tank is set to a higher temperature so the mixing valve can bring it down to the working temperature. Exactly how much higher depend on the tank size and how you use hot water. Setting it higher will let you take more/longer showers before the hot water runs out. Setting it lower will save some energy; how much depends on how well your tank is insulated and how efficiently it is heated. There is a minimum temperature for this; I don't remember what it is offhand. (130F?)