The Caleffi 280966A is a ThermoProtec boiler protection high-flow thermostatic mixing valve with 1 in. sweat union connections and a 140 F set point. It blends boiler bypass flow with system return flow to hold the water entering a non-condensing boiler at a fixed temperature, preventing flue gas condensation and the corrosion and tar deposits that follow.
Specifications sourced from Caleffi technical brochure 01223/23 NA and installation instructions NA10292.04. Last reviewed August 2026.
Key Features
Holds Boiler Return at 140 F
A wax thermostatic sensor immersed in the medium modulates a shutter, recirculating boiler bypass flow until the mixed temperature reaches 140 F. The bypass closes fully at set point plus 18 deg F (10 deg C).
Prevents Flue Gas Condensation
Keeping boiler surfaces above the flue gas dew point limits condensate, soot and tar deposits, which are a fire hazard and reduce heat exchanger efficiency and boiler life.
Thermostatic - No Power or Controller
The wax thermostatic sensor is fully immersed in the medium and acts mechanically. There is no wiring, controller or power supply to specify, commission or maintain.
Field-Replaceable Cartridge
The spring, inner valve and thermostatic sensor come out as one cartridge through the lower body plug. Cartridges are available at 115, 130, 140 and 160 deg F, so the set point can be changed without removing the valve body from the piping.
10 Cv High-Flow Body
Published flow coefficient of 10 Cv on the 1" size. Caleffi's sizing method is to calculate boiler flow from BTU output and thermal head, read the pressure drop off the hydraulic chart in brochure 01223/23 NA, then select against the available pump head.
Integral Gauge Pocket Wells
Pocket wells on the front and rear of the body accept Caleffi F29571 dual-scale temperature gauges for reading bypass from boiler, return from system and mixed to boiler.
Technical Specifications
| Product Type | Boiler protection high-flow thermostatic mixing valve |
| Application | Non-condensing boilers - solid fuel, biomass, gas, LP, oil |
| Series | ThermoProtec 280 |
| Connections | 1 in. sweat union |
| Connection type | Sweat union |
| Thermostatic set point (supplied) | 140 F (60 C) |
| Optional cartridges (field replaceable) | 115 F (45 C), 130 F (55 C), 140 F (60 C), 160 F (70 C) |
| Sensor cartridge accuracy | +/- 4 deg F (+/- 2 deg C) |
| Bypass from boiler complete closing temperature | set point + 18 deg F (+ 10 deg C) |
| Flow coefficient | 10 Cv |
| Suitable fluids | water, glycol solutions |
| Max. percentage of glycol | 50 % |
| Max. working pressure | 150 psi (10 bar) |
| Working temperature range | 40 to 212 deg F (5 to 100 deg C) |
| Dimensions | B 3-1/2 in.; C 7 in.; D 6 in.; E 1-5/8 in.; F 4-3/8 in. (letters per dimension drawing in brochure 01223/23 NA) |
| Body material | brass |
| Lower body plug | brass |
| Shutter | PSU (polysulfone) |
| Spring | stainless steel |
| Seal | peroxide-cured EPDM |
| Union seals | non-asbestos fiber |
| Thermostatic sensor | wax |
Documentation
Design Resources
Idronics_10_NA_Hydronics_for_wood-fired_heat_sources
Related Products
- Caleffi 280165A : ThermoProtec Boiler Protection Mixing Valve 130 F 1" NPT
- Caleffi 280166A : ThermoProtec Boiler Protection Mixing Valve 140 F 1" NPT
- Caleffi 280175A : ThermoProtec Boiler Protection Mixing Valve 130 F 1-1/4" NPT
- Caleffi F29571 : ThermoProtec Dual-Scale Temperature Gauge 32-250 F
Frequently Asked Questions
What does Caleffi 280966A do?
It is a ThermoProtec boiler protection thermostatic mixing valve. It mixes hot bypass flow from the boiler with cooler return flow from the system so water enters the boiler at 140 F, keeping the heat exchanger above the flue gas dew point where corrosive condensate and tar form.
Does Caleffi 280966A need power or a controller?
No. A wax thermostatic sensor modulates the shutter mechanically as temperature changes. There is no wiring, controller or power supply to specify, commission or maintain.
Can the set point on Caleffi 280966A be changed?
Yes. The thermostatic element is a field-replaceable cartridge accessed through the lower body plug. Caleffi supplies F29633 at 115 F, F29634 at 130 F, F29635 at 140 F and F29636 at 160 F.
Where is Caleffi 280966A installed?
Caleffi recommends installation on the return to the boiler in mixing mode, on either side of the boiler, vertical or horizontal. Hot from boiler enters the red dot port, cool system return the blue dot port, and mixed water leaves the port marked with the flame symbol. It can also be installed in diverting mode on the flow from the boiler.
Can Caleffi 280966A be used on domestic hot water?
No. Caleffi states this product is designed for closed hydronic systems only, must not be used in plumbing applications, and does not meet the low-lead plumbing standards of the U.S. and Canada.
What is the warranty on Caleffi 280966A?
Caleffi North America covers this product under its two-year limited warranty against defects in material and workmanship from the date of shipment. ZOT Supply is an authorized Caleffi distributor.
- Product Type:
- Boiler Protection Mixing Valve
- Application:
- Boiler Protection
- Series:
- ThermoProtec
- Set Point Temperature:
- 140 F (60 C)
- Connection Type:
- Sweat union
- Connection Size:
- 1 in.
- Flow Coefficient:
- 10 Cv
- Max Operating Pressure:
- 150 psi (10 bar)
- Medium Temperature Range:
- 40 to 212 deg F (5 to 100 deg C)
- Max Glycol Concentration:
- 50 %
- Body Material:
- Brass
