March, 2003
Inspection News and Views from the American Society of Home Inspectors

Keeping Out of Hot Water


All of us are familiar with two warning labels found on modern water heaters. One warns of the risk of explosion if the ignition source is not raised 18 inches above a garage floor. The other warns of scalding risks by stating, “Water temperatures over 125 degrees F can cause severe burns instantly or death from scalds. Children, disabled and elderly are at highest risk of being scalded.” The label usually includes the statement, “Temperature limiting valves are available.”

The basic method of controlling water temperature is to set the thermostat on the heater at a safe level.

As home inspectors, however, we occasionally find domestic water heating equipment that does not have this scald hazard label, and does not have any way to thermostatically control the temperature of the water produced. The most common of these devices is the “domestic coil” type of tankless water heater. This device is simply a heat exchanger that is installed in a gas or oil-fired boiler to heat household water. The temperature of the water heated by this method varies with the jacket temperature in the boiler, the amount of time the domestic water has been in the exchanger, and the rate of flow.

Two additional types of water heating appliances typically do not come with installed thermostat controls: water heating coils in wood-burning appliances and solar units.

The ASHI Standards of Practice require us to inspect the “normal operating controls” and the “automatic safety controls” on water heating equipment. In my area I see quite a few domestic coil installations, especially on oil-fired boilers. When inspecting, I always look for a thermostatically controlled tempering valve. Most often if the coil has any tempering valve installed, it will merely be a manual valve, which mixes some cold water with the flow of hot water from the coil. In my opinion, this set up does not provide an adequate level of protection. Varying flow rates can drastically affect the temperatures in this type of an installation. Thermostatically controlled tempering valves are relatively easy to install and well worth the effort.

Tempering valve with tankless coil

a temporary valve mixes some of the incoming
domestic cold water with the “too hot” water
coming out of the tankless coil to bring it down
to a temperature suitable for domestic use

Illustration courtesy of Carson Dunlop & Associates, Ltd.. Used with permission.

The following information is reprinted with permission from a Watts Regulator Co. Safety Mixing Valve Product Guide. Additional information is available at

How they work

Upon use of tempered water, a thermostat in the mixing chamber of the valve senses the out temperature The thermostat automatically positions a shuttle or valve which controls the flow of hot and cold water supplied to the mixing chamber. A mechanical adjustment permits selection of the desired outlet water temperature within range of the valve.

Why they are used

Supply Systems
Mixing valves provide the following benefits:

The water temperature that is discharged directly from a water heater can vary ± 10°F or more. This can be due to tolerance of control devices, inlet water temperature changes and stacking (higher water temperatures at the top of the water heater).


Illustration courtesy of Carson Dunlop & Associates, Ltd.. Used with permission.

Use of a thermostatic mixing valve assures constant outlet water temperature even with variations in hot or cold water supply temperature. If a water heater is operated at lower temperatures, high flow demand situations can result in a reduction of hot water supply temperatures if the recover time of the water heater is not sufficient for the flow demand. Using a hot water extender or mixing valve allows the water heater to be operated at higher temperatures, extending the effective system flow rate and preventing the growth of Legionella.

To be able to supply peak demands for domestic hot water it is often necessary to provide high capacity water heaters with high recovery rates. Use of a hot water extender can reduce the size/BTU requirements needed to provide capacity for peak system demands.

At the fixture
Heat loss in the system piping can result in varying temperatures of the hot and cold water supplied to fixtures. The use of thermostatic mixing valves assures constant safe hot water temperature at the point of delivery to the fixture.