We’ve been explaining to a lot clients what the mixing valves installed at the water heater and/or under the (typically) MB vanity sink are intended for, they are almost never correctly adjusted (in our area). Recently I have seen some Facebook home inspector group posts inquiring about mixing valves. Here is a few links for those wishing to research about water mixing valves:
This is the narrative we include when we don’t find a mixing valve installed at the water heater.
There did not appear to be a temperature mixing valve installed at the water heater. These mixing valves are typically installed to allow the water heater to be operated at or above 140 degrees F to kill bacteria such as Legionella. Legionella is an aquatic bacteria that thrives in warm water environments, and can cause Legionnaires Disease. It is commonly found in potable water supplies, hot tubs, cooling towers, fountains, swimming pools, etc. and multiplies in warm conditions especially between 68 – 122 °F. The Legionella bacteria can spread to humans when breathed in. We recommend installing a mixing valve and setting the temperature of the water heater to 140 degrees F and adjusting the mixing valve so that the water delivered to the home is in the generally acceptable range of 110 to 120 degrees F to prevent scalding hazards while protecting against Legionella. Also note that most electric water heaters have energized electrical components at the temperature adjustment area and should be adjusted by a qualified person in accordance with the water heater manual installation/operating instructions.
I have begun to see these on new construction/ 11 month warranty inspections. Homeowners are not happy because the tub that they are usually attached to doesn’t get hot enough to take a bath in.
I’ve seen hundreds of them the last few years…one was adjusted correctly. Today’s water heater was set above 140 but the mixing valve was not correctly adjusted. Some of the plumbers around here put a sticker right on the side that tells how to set the tank and the valve… but still don’t do it.
we use the 2009 irc in wv so i’ve never seen one on an inspection either yet. seems like 120*f is hot enough for a bath so why does it not allow the water to get hot enough for a bath if there is one installed? can you not just turn on the hot water and use that? i certainly can’t stand water at that temperature for too long.
If the mixing valve is installed on the hot water line, it is limiting the temp of the hot water that is coming out of the faucet. It does this by mixing in cold water prior to the faucet.
In my area, we have quite a few water heaters, including mine, on energy saving programs. The program involves only running your water heater overnight, when electricity is typically in low demand. In exchange, we get that electricity for half price on our bill.
To make this work for some larger households, the water must be warmed higher than the suggested 120 or so desired at the faucet. A mixing valve is installed at the water heater to bring the outgoing temp down. This also has the effect of increasing the capacity of the water heater to provide enough hot water throughout the day, as will not actually heat water again until the next night.
Of course having a very well-insulated (and sometimes larger) water heater is also beneficial on this program. Below is what most use…