HVACopediaNavien · Boiler
E736 Cascade communication fault
What the control is telling you
The main and sub boilers lost communication. Manual action: check the cascade cable.
Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers
Likely causes
- Ready-Link cable loose or damaged at CNK1
- Sub unit not registered / IDs lost
- Sub unit powered off
Test and repair steps
- Warning: Turn off power to all boilers before reconnecting cables.
- Check the Ready-Link communication cables at the CNK1 terminal on each boiler (up to 16 boilers).
- Power all boilers on, then on the main unit run Cascade Status and Settings > 1. Cascade System Setting to reassign IDs.
- Use 2. Cascade System Removal on a sub with a network problem to drop it from the cascade.
- Check 7. Cascade Info for the status of each unit.
Settings for these units
Installation clearances (indoor) · Navien NHB-H
| Side | Minimum clearance |
|---|---|
| Top | 9 in (229 mm) |
| Back | 0.5 in (13 mm) |
| Front | 4 in (100 mm) |
| Sides | 3 in (76 mm) |
| Bottom | 12 in (300 mm) |
| Vent to combustibles | 0 in |
Bracket gives 5/8 in (16 mm) from the wall; studs at 16 in on centre.
Common vent installs: 36 in above the top and 24 in in front are recommended for service.
Installation clearances (indoor) · Navien NFB-H
| Side | Minimum clearance |
|---|---|
| Top | 9 in (229 mm) |
| Back | 0.5 in (13 mm) |
| Front | 4 in (100 mm) |
| Sides | 3 in (76 mm) |
| Bottom | 12 in (300 mm) |
| Vent to combustibles | 0 in |
Do not install on carpet; indoors only.
Remove the wood pallet and refit the 6 screws to keep the cabinet airtight.
Models that show this code
6 models in Navien NHB-H, Navien NFB-H
Other codes on the same units
Save it for the next call
Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.
Source: Navien NHB-H Installation and Operation Manual, 11.14 Understanding Error Codes, p.119; Navien NFB-H Installation Manual, 11.7 Understanding Error Codes, p.116; NHB-H User's Information Manual (V1.0 Apr 2024), 5.2, p.24. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
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.