Do I need a thermostatic mixing valve?
by Keith Tripp, February 2020
A client recently contacted me with a question about mixing valves. The mixing valve installed at his house was acting up,and he wanted to know if it could just be removed or if it must be replaced. Mixing valves are part of a temperature control system for hot water. The intention is to reduce risk of scalding by keeping water that will contact humans at temps less than 49C (120F). This is especially important for children and the elderly.
Here is my response: Your question about the mixing valve has a few angles to consider. Bottom line, if you want to skip all the details below, is that I recommend installing a new thermostatic mixing valve.
There are 4 levels to look at . One is what year code applies to the house, the second is the details within the code, third is whether an issue would arise when selling or having a home inspection done, the fourth is the practical safety issue.
Starting with #4, as stated above, I recommend installing a new valve. Especially if the house is a rental property, or elderly people are using the house, there could be liability issues by removing a valve that was originally installed for the purpose of temperature control. The valve also allows the tank temperature setting to be higher than 49C (120F). Some argue that this is necessary to reduce risk of Legionnaires disease, but that point is debatable. It does however allow for hotter than 49C water to supply a dishwasher or clothes washer if piping allows for that.
The code requirement: The key point is that the code ( even the current code) does not specify a thermostatic mixing valve. However it does specify the maximum temperature of hot water as 49C in 184.108.40.206. (See below) The thermostatic mixing valve has become the accepted method of controlling the temperature. You could argue that the water heater setting is controlling the temp, but in actual practice there are significant temperature swings in the supply temp. If you have a tankless heater with an electronic temp control, that may also meet the requirement without the mixing valve. Final decision on what code applies in each situation, and interpretation and enforcement of the code is the responsibility of the Building Official for your area.
What code applies? The code as is it is now with the temperature control requirement I believe came into practice around 2004 , but not certain on that. The codes are not retroactive. Many houses do not have the mixing valve, and when water heaters are replaced in older houses, from what I see, in most cases a thermostatic mixing valve is not installed. This house originally had a mixing valve so safe to assume it was code-driven at the time of construction.
Will it be an issue when selling? Home inspections are based somewhat on code, but codes can only be interpreted and enforced by Building Officials. A missing mixing valve may or may not make it to a home inspection report. In my reports, I include presence or absence of the valve as part of the plumbing description, but not necessarily as an issue that requires attention.
See below the section of code ( current) that applies .You can find the code on line at :. https://www.ontario.ca/laws/regulation/120332
7.6.5. Water Temperature Control
220.127.116.11. Maximum Temperature of Hot Water
(1) Except as provided in Sentences (2) and 18.104.22.168.(1), the maximum temperature of hot water supplied by fittings to fixtures in a residential occupancy shall not exceed 49°C.
(2) Sentence (1) does not apply to hot water supplied to installed dishwashers or clothes washers.
(1) Except as provided for in Sentences (2) and (3), all valves supplying fixed location shower heads, shall be individually pressure-balanced or thermostatic-mixing valves, conforming to ASME A112.18.1 / CSA B125.1, “Plumbing Supply Fittings”.
(2) An individually pressure-balanced or thermostatic-mixing valve shall not be required for showers if a single temperature water supply for such showers is controlled by a master thermostatic-mixing valve conforming to CSA B125.3, “Plumbing Fittings”.
Note: On January 1, 2020, Sentence 22.214.171.124.(2) of Division B of the Regulation is revoked and the following substituted: (See: O. Reg. 88/19, s. 181 (1))
(2) An individually pressure-balanced or thermostatic-mixing valve is not required for shower heads having a single tempered water supply that is controlled by an automatic compensating valve conforming to CSA B125.3, “Plumbing Fittings”.
(3) Deck-mounted, hand-held, flexible-hose spray attachments are exempt from the thermal shock requirements of Sentence (1).
Note: On January 1, 2020, Sentence 126.96.36.199.(3) of Division B of the Regulation is amended by striking out “Sentence (1)” at the end and substituting “Sentences (1) and (4)”. (See: O. Reg. 88/19, s. 181 (2))
(4) Pressure-balanced or thermostatic-mixing valves shall be,
(a) designed so that the outlet temperature does not exceed 49°C, or
(b) equipped with high-limit stops which shall be adjusted to a maximum hot water setting of 49°C.
Note: On January 1, 2020, Sentence 188.8.131.52.(4) of Division B of the Regulation is revoked and the following substituted: (See: O. Reg. 88/19, s. 181 (3))
(4) Pressure-balanced, thermostatic-mixing or combination pressure-balanced and thermostatic-mixing type valves shall be,
(a) capable of limiting thermal shock, and
(b) designed so that the outlet temperature does not exceed 49°C or equipped with high-limit stops which shall be adjusted to a maximum hot water setting of 49°C.