Best Thermostatic Mixing Valves for Solar Hot Water: 5 Compared

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As our scalding guide explains, a solar storage tank can hold water hot enough to scald, and a tempering (thermostatic mixing) valve blends cold water with the hottest tank water so the taps get a set temperature. For a solar system, the key figures are the highest hot inlet temperature the valve accepts, its outlet setting range, and which ASSE standard it is listed to.

Every specification on this page comes from the manufacturer’s own product page, checked on 26 October 2026. We have not used these products ourselves, so the picks are based on published specifications only.

Read preventing scalding from solar hot water and pressure and temperature relief before choosing a valve, and have a licensed plumber install it where your code requires.

How we chose these

• Published maximum hot inlet temperature, important with solar storage tanks.
• Outlet setting range and pressure rating stated.
• ASSE listing named: 1017 for valves at the water heater (point of distribution), 1070 for valves serving fixtures.
• Low-lead construction for drinking water.

ASSE 1017 valves temper water at the heater or distribution point; ASSE 1070 valves are for fixture-level scald protection. Codes differ on which is required where, so check yours.

Comparison table

Figures are from each maker’s sheet or product page for the size shown; other sizes and connection types can differ.

1. Caleffi MixCal 521 Series: for most residential solar tanks

Caleffi’s MixCal 521 is a three-way thermostatic and pressure-balanced valve for residential domestic hot water and radiant systems. It accepts hot water up to 200°F and sets the outlet from 85 to 150°F, and Caleffi lists it to ASSE 1017 and CSA B125.3, certified by ICC-ES. Anti-scale materials on sliding parts reduce jamming from lime deposits, according to Caleffi.

• Sizes 1/2 to 1 in, sweat or NPT union, plus 3/4 in press (Caleffi).
• Optional outlet temperature gauge; inlet check valves optional.

Pros: high hot-inlet limit; wide setting range; low-lead brass.
Cons: a point-of-distribution valve, not a fixture-level ASSE 1070 valve.

Check the Caleffi MixCal 521 Series on Amazon

2. Caleffi 5213 Series: for point-of-use scald protection

The 5213 is Caleffi’s point-of-use scald-protection valve with a locking setpoint and check valves in the hot and cold inlets as standard. Its setting range tops out at 120°F, and it keeps working at flows as low as 0.5 gpm, which suits a single sink or shower.

• Sizes 1/2 to 1 in, sweat and NPT union; low-lead brass (Caleffi).

Pros: locking setpoint; built-in check valves; low minimum flow.
Cons: lower pressure rating; serves fixtures rather than the whole house.

Check the Caleffi 5213 Series on Amazon

3. Watts LFMMV Series (LFMMVM1-UT press, 3/4 in): for baths and lavatories

Watts’ LFMMV is a lead-free thermostatic valve for supplying baths and lavatories with tempered water. A double-throttling design controls both hot and cold supply, with a wax thermostat, integral filter washers and check valves, and a locking temperature setting. This press-connection version is factory set to 120°F.

• Outlet accuracy listed at +/- 7°F for this model; minimum static pressure 30 psi (Watts).
• Also sold in threaded, sweat, PEX and CPVC versions.

Pros: listed to ASSE 1070 for fixtures; check valves built in; many connection types.
Cons: hot inlet range ends at 180°F, lower than the Caleffi 521 and Watts LF1170.

Check the Watts LFMMV Series (LFMMVM1-UT press, 3/4 in) on Amazon

4. Watts LF1170 Series (LF1170M2-US, 3/4 in): for whole-house tempering at the heater

Watts designed the LF1170 to install on water heating equipment and send tempered water to fixtures, and says it suits radiant heat too. It accepts hot water up to 200°F, adjusts the outlet from 90 to 160°F and flows up to 23 gpm, the highest flow of the residential valves here.

• Integral filter washers and check valves; outlet accuracy +/- 7°F for this model; minimum static pressure 30 psi (Watts).

Pros: high flow for a whole house; 200°F inlet; wide range.
Cons: a heater-level valve; fixture valves may still be needed by code.

Check the Watts LF1170 Series (LF1170M2-US, 3/4 in) on Amazon

5. Caleffi 5231 Series: for large systems and high flow

The 5231 is Caleffi’s high-flow valve for commercial point-of-distribution domestic water and radiant systems, in 1 to 2 inch sizes. It shares the 521’s anti-scale design and optional outlet gauge but needs a higher minimum flow to stay stable.

• DZR certified low-lead brass; sweat and NPT union (Caleffi).

Pros: highest flow capacity here.
Cons: oversized for most homes; high minimum flow.

Check the Caleffi 5231 Series on Amazon

Which one fits you

Practical tips

• Check the hottest temperature your solar tank can reach and choose a valve rated above it.
• Set and verify the outlet temperature with a thermometer after installation; see preventing scalding for why outlet temperature matters.

Sources

Specifications from each manufacturer’s product page, checked 26 October 2026: Caleffi MixCal 521 Series, Caleffi 5213 Series, Watts LFMMV Series (LFMMVM1-UT press, 3/4 in), Watts LF1170 Series (LF1170M2-US, 3/4 in), Caleffi 5231 Series. Makers can change specifications; check the product page before you buy.