Mixing Valve Calculator — Hot and Cold Blend, Stored Water Multiplier, and the Flow Range a Valve Must Cover

Hot and Cold Water Blend Ratio

The blend ratio is the one figure everyone comes for and the one nobody guesses correctly, because it is not a proportion.

Mixing is an energy balance with no loss term. The hot flow times the hot temperature plus the cold flow times the cold temperature equals the mixed flow times the mixed temperature, and the two input flows sum to the output. Solve that for the hot flow and the answer is a ratio of temperature differences: the delivery temperature minus the cold temperature, divided by the hot supply temperature minus the cold temperature.

Delivering 120 F from 140 F storage with 50 F cold water gives 70 over 90, which is 77.8 percent hot and 22.2 percent cold. At a 4 gallon per minute draw that is 3.11 gallons per minute of hot water and 0.89 of cold.

The reason it is so hot heavy is geometric rather than mysterious. The delivery sits 70 degrees above the cold supply and only 20 degrees below the hot one, so the blend has to lean toward the hot side in that proportion.

Change the delivery temperature and the ratio moves sharply. A 105 F shower from the same supplies is 55 over 90, which is 61.1 percent hot. That is a large change for a 15 degree difference in delivery.

Change the storage temperature and it moves the other way. That same 105 F shower from 120 F storage is 55 over 70, which is 78.6 percent hot. The same shower draws 61 percent hot from a 140 F tank and 79 percent from a 120 F one, which is the arithmetic behind a complaint usually blamed on tank size.

And the cold supply is in the equation too, which is the part most designs treat as fixed. That is covered in its own section below.

Stored Water Multiplier

Read the blend relationship backwards and it answers a different question: how much tempered water does a tank of hot water actually produce?

The multiplier is the hot supply temperature minus the cold temperature, divided by the delivery temperature minus the cold temperature. It is the reciprocal of the hot fraction, and it is greater than one whenever the storage is hotter than the delivery.

At 140 F storage, 50 F cold and 120 F delivery the multiplier is 1.286. A 40 gallon heater produces 51.4 gallons of 120 F water, or 194.7 litres from 151.4.

At a 105 F shower delivery the multiplier is 1.636 and the same 40 gallon heater produces 65.5 gallons of shower water.

That is a capacity argument rather than a safety one, and ASSE names it as such: a side benefit of an ASSE 1017 valve is that it allows the water to be stored at a higher temperature, extending the amount of hot water available while reducing the chance of Legionella. Manufacturer literature puts the same point in design terms, that a mixing valve can reduce the size or BTU requirement needed for peak demand.

The consequence reaches back into water heater selection. A tank sized for a 120 F delivery without accounting for 140 F storage is about 29 percent larger than the system needs at a 50 F cold inlet.

Three things the multiplier does not include, and all three matter on a real draw. It does not include recovery, so a heater firing during the draw delivers more than the figure suggests. It does not account for stratification, which reduces the usable hot volume below the nominal tank size. And it does not include heat lost between the valve and the fixture, so the temperature arriving at a distant tap is lower than the setpoint. The figure describes tempered water at the valve outlet from the stored volume, and nothing further downstream.

Winter Versus Summer Cold Water Temperature

The cold supply temperature is an input to the blend, and it is the one variable in the calculation that changes without anyone touching the system.

ASSE defines cold water as 40 F to 70 F, which is the seasonal span of a mains inlet in most of the country. That is not a tolerance; it is the range the same tap actually delivers across a year.

Work the blend at both ends, for 140 F storage delivering 120 F. At a 40 F winter inlet the hot fraction is 80 over 100, which is 80.0 percent. At a 70 F summer inlet it is 50 over 70, which is 71.4 percent.

So the same delivered gallon costs about 12 percent more hot water in winter. The multiplier moves with it in the same direction: 1.250 in winter against 1.400 in summer, so a 40 gallon tank produces 50 gallons of 120 F water in January and 56 in July.

That is why a system runs short in a season rather than failing outright. Nothing about the valve, the heater or the fixtures changed. The tank is simply emptying faster because each gallon out of the tap is taking more out of it.

The design consequence is which figure to use. For capacity, use the coldest expected inlet, because that is the condition the system has to survive. For the blend fractions, both ends are worth seeing, because a valve sized on summer figures will be drawing a larger hot fraction all winter.

The same seasonality reaches the valve duty. A higher hot fraction means the hot port is passing more of the total flow, which matters where a valve is near the ends of its rated range.

Mixing Valve Flow Range

A thermostatic mixing valve is specified by a range rather than by a capacity, and the range is the part that decides whether it works.

Every published valve carries a minimum and a maximum flow, and between them the valve holds its stated accuracy. Outside them it does not. One published series is rated from 0.5 to 12 gallons per minute, which is 1.9 to 45.4 litres per minute. Group control valves are described at 14 to 51 gallons per minute at 45 psi, which is 53 to 193 litres per minute.

That last phrase carries a condition that is easy to drop and it changes the number. A published range is quoted at a stated pressure differential across the valve. Change the differential and the range changes with it, so a range is a point on a flow chart rather than a fixed property of the device. A range used without knowing its pressure basis is preliminary, and this calculator says so when the differential is not entered.

ASSE approaches the same question from the accuracy side and gives a table rather than a single figure. For ASSE 1017 valves the allowable temperature fluctuation is plus or minus 3 F from 0 to 5 gallons per minute, plus or minus 5 F from 5 to 40, and plus or minus 7 F above 40. Those bands are 0 to 18.9, 18.9 to 151.4 and above 151.4 litres per minute, and the tolerances are 1.7, 2.8 and 3.9 Celsius degrees.

The table is introduced with the instruction that valves must be sized to match the flow requirements of the system, which makes the accuracy part of the sizing rather than a footnote on a datasheet. A larger valve is not just a larger valve; it is a less accurate one, and the setpoint has to move down to compensate.

Two figures are therefore needed to size a valve rather than one. The design flow sets the upper end. The minimum expected flow sets the lower end, and it is the harder of the two, which the next section covers.

Why Not to Oversize a Mixing Valve

On most components a size up is a margin. On a thermostatic mixing valve it is a failure, and manufacturer literature states the instruction in capitals: all thermostatic water mixing valves have limitations, they will not provide the desired accuracy outside of their flow capacity, consult the flow capacity chart and do not oversize.

The reason is the bottom of the range rather than the top.

Take a master valve chosen to pass a building morning peak of 45 gallons per minute, with a rated range of 5 to 50. At the peak it is comfortably inside its range. At two in the morning, when one lavatory runs at half a gallon per minute, it is an order of magnitude below its rated minimum.

At that flow the valve is not controlling. What leaves it is whatever the supplies give it, and on a system storing at 140 F that is a scald exposure rather than a comfort complaint. The building spends far more hours at the low end than at the peak, so the condition is not an edge case; it is most of the year.

The remedy is not a larger valve, and this is where the instinct does the most damage. A larger valve raises the rated minimum, which moves the low end further out of reach and makes the problem worse at exactly the hours it already fails. Where the required range is wider than a single valve covers, the answer is parallel or staged valves, or separate point of use protection so that the master valve is not the only thing standing between the storage temperature and a fixture.

The same argument applies at the fixture. Published guidance for shower valves notes that the showerhead flow rate must not be less than the manufacturer published minimum flow rate for the valve, so a low flow showerhead fitted to a valve with a higher minimum has the same problem at a smaller scale.

Sizing a mixing valve therefore starts with the smallest real draw rather than the largest. If only one figure is known, the minimum is the more useful one.

ASSE 1017 Versus ASSE 1070 Versus ASSE 1016

ASSE publishes a separate performance standard for each place a temperature control device can sit, and they are not interchangeable. The differences are set out in a matrix in the ASSE white paper on temperature control devices, and the columns that matter are what each device protects against and where it is installed.

ASSE 1017 covers temperature actuated mixing valves for hot water distribution systems. Scald protection: no. Thermal shock protection: no. Installed at, or near, the source. Downstream mixing allowed. Application recorded as temperature control for hot water distribution systems only, with additional downstream safety devices required.

ASSE 1016 covers automatic compensating valves for individual showers and tub shower combinations. Scald protection: yes. Thermal shock protection: yes. Installed at the point of use, with no further mixing downstream, and the final temperature adjusted by the user.

ASSE 1070 covers water temperature limiting devices for individual or multiple fixtures. Scald protection: yes. Thermal shock protection: no. Installed near, or integral to, plumbing fixture fittings, with the outlet adjustable up to a maximum of 120 F.

ASSE 1069 covers automatic temperature control mixing valves for single pipe gang showers and sitz baths. Scald protection: yes. Thermal shock protection: yes. Installed where the bather has no access to the temperature adjustment means, and designed to be the final temperature control.

Three more appear in the same matrix and are worth knowing. ASSE 1062 covers temperature actuated flow reduction valves, which reduce flow to a trickle within five seconds of the outlet exceeding a preset actuation temperature not to exceed 120 F. ASSE 1066 covers in-line pressure balancing valves, which give thermal shock protection for pressure disturbances only and no scald protection if an incoming temperature changes. ASSE 1071 covers valves supplying tepid water to plumbed emergency equipment, which by themselves do not meet ANSI Z358.1.

Read down the protection columns and one row stands out. ASSE 1017 is the only listing in the matrix with no in both. That is the subject of the next section.

Master Mixing Valve Is Not Scald Protection

This is the single most consequential thing on the page, and it is the one most often assumed the other way round.

A master mixing valve at the water heater does not protect anyone from scalding. ASSE states it in three separate ways in the same document.

In the matrix, ASSE 1017 devices are recorded as providing no scald protection and no thermal shock protection, with the application described as temperature control for hot water distribution systems only and additional downstream safety devices required.

In the device description, ASSE 1017 valves shall be installed at or near the outlet of the hot water source only, and are not intended for point of use applications. Used alone they do not provide thermal shock protection or adequate scald protection.

And in a worked question, asked because installers keep asking it: installing a device listed to ASSE 1017 at the water heater will not provide adequate scald protection.

The reason is what the valve is controlling. A master valve holds the distribution temperature steady against changes in the storage temperature and the incoming supplies. It does not know what is happening at a fixture, it does not respond to a toilet flushing on the same branch, and it does not limit what a user can select at the tap.

So the two devices are in series and neither replaces the other. The master valve solves the storage problem: it lets the tank sit at 140 F for Legionella control while the distribution runs cooler. The fixture device solves the user problem: it limits or compensates what arrives at the outlet. Removing either one leaves a real exposure.

Federal design guidance shows both requirements arriving together. It sets storage at not less than 140 F, requires a master mixing valve under ASSE 1017 to bring the distribution down to 131 F, and then notes that on existing systems steps must be taken to avoid scalding occupants at that distribution temperature. That last clause is the fixture level requirement, stated from the other direction.

Multi-Listed Mixing Valves

Many valves carry more than one ASSE listing. One published series is listed to ASSE 1017, ASSE 1069 and ASSE 1070 at the same time, and the natural reading is that such a valve can do all three jobs at once.

ASSE addresses that reading directly and the answer is no.

The rule is that the listing follows the installation position rather than the label. A device listed to multiple standards installed at the water heater to regulate the distribution temperature serves as an ASSE 1017 point of source application only. In that position it provides temperature control, and it does not provide the required scald protection.

The white paper works through the case that prompts the question. A plumber proposes installing a multi listed valve at the water heater with its limit stop set to 110 F, intending it to control the distribution temperature and prevent scalding at the fixtures in one device. The answer is that the device will not be able to provide the required scald protection in this application, and that further point of use temperature control devices must be installed as required.

The underlying principle is stated plainly: a single device should not be installed to control the entire temperature of the distribution system while simultaneously serving the needs of a fixture. Those are two jobs with different response requirements at two different places in the system.

What the multiple listing does buy is flexibility in specification. The same product can be stocked for either position, and it will perform to whichever standard applies where it is fitted. What it does not buy is a device count of one where the design needs two.

Setpoint Margin and Valve Accuracy

A limit is not a setpoint, and the gap between them is the valve own published tolerance.

Every thermostatic mixing valve has an allowable temperature fluctuation. A published point of use series states plus or minus 3 F across its rated range. ASSE 1017 publishes the figure by flow band for master valves: plus or minus 3 F from 0 to 5 gallons per minute, plus or minus 5 F from 5 to 40, and plus or minus 7 F above 40.

Set a valve to a limit and it is permitted to exceed it. A 120 F setpoint on a valve with a 3 F tolerance may deliver 123 F, and the valve is performing exactly to specification while the installation is outside the limit. Set it to 119 F and it may still deliver 122 F. Only 117 F or lower holds a 120 F limit on that valve.

The arithmetic is the limit minus the tolerance, and it moves with the flow band. Holding a 120 F limit needs a setpoint of 117 F on a small valve, 115 F in the middle band and 113 F above 40 gallons per minute. Holding a project limit of 115 F needs 112, 110 and 108 F respectively.

The manufacturer recommendation of a maximum of 110 F for shower and bathing fixtures works the same way: a 3 F valve needs a setpoint of 107 F to hold it.

Two practical points follow. A larger valve needs a lower setpoint for the same limit, which is a second reason not to oversize beyond the flow argument. And the tolerance to use is the one published for the actual product and its listing. The ASSE 1017 bands are published in ASSE 1017 for ASSE 1017 devices, so they do not transfer to an ASSE 1070 or ASSE 1016 device, which carries its own figure.

What Is a Mixing Valve

A mixing valve blends hot and cold water to produce water at a controlled temperature. In domestic hot water systems it exists to resolve a contradiction that has no other solution.

Water has to be stored hot. Federal design guidance sets storage at not less than 140 F to suppress Legionella, and published guidance citing ASSE puts the figure for killing 99 percent of the bacteria at 140 F over 32 minutes.

Water has to be delivered cool. The same published guidance notes that water at 140 F causes a third degree burn in five seconds. ASSE reports that hot water from the tap accounts for more than 25 percent of all scald burns in children, with the elderly and the physically impaired at increased risk.

The two requirements are the same temperature and they point in opposite directions. A mixing valve is the device that lets a system store at 140 F and deliver at 120 F or lower.

There is more than one kind, and they are not interchangeable. A master or system valve at the heater tempers the whole distribution system. A point of use device limits the temperature at a fixture. A shower valve compensates for pressure and temperature changes while someone is standing under it. ASSE publishes a separate performance standard for each, and the white paper setting out where each belongs was written because inspectors and installers kept asking whether one device could be used in place of another.

The answer, in most of the cases people ask about, is no.