HVACopediaNTI · Boiler · Freeze protection
Freeze protection stages
What happens, in order
- Stage 1 when NTC1 or NTC2 water temperature is 39°F to 46°F, the internal pump runs at high speed.
- Stage 1 the 3-way valve alternates 1 minute heating, 1 minute DHW until temperature reaches 46°F or more.
- Stage 1 if conditions last 20 minutes, go to Stage 2.
- Stage 2 when temperature is below 39°F, the burner fires at minimum power.
- Stage 2 the 3-way valve sits on DHW and switches every 30 seconds between CH and DHW.
- Stage 2 burner turns off at 104°F and the boiler holds 95°F to 104°F for 45 minutes.
- Stage 2 after 45 minutes, 2 minutes of post-heating circulation.
- Stage 2 if temperature falls below 46°F within 150 minutes, the burner restarts immediately.
Timings and thresholds
| Item | Value |
|---|---|
| Stage 1 duration before Stage 2 | 20 minutes |
| Stage 1 valve switching | 1 minute heating / 1 minute DHW |
| Stage 2 valve switching | 30 seconds CH / DHW |
| Stage 2 hold time 95°F to 104°F | 45 minutes |
| Post-heating circulation | 2 minutes |
| Restart window | 150 minutes |
Applies to: TRX boilers
More NTI boiler sequences
- Combustion checking procedure · Startup
- Power-on and mode sequence · Startup
- Power-up and what lets the boiler start a call · Startup
- Failed ignition and lockout · Ignition
- First ignition start-up · Ignition
- Light-off sequence and retries · Ignition
- Central heat call (CH1) from call to satisfied · Heating
- Indirect tank (DHW) call · Domestic hot water
Related
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Source: NTI TRX150 manual, p. 14. Written in our own words; timings can differ by model and software version, so check the manual for your exact unit.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.