HVACopedia

FlowMax · Combi boiler

H2O Low water pressure in heating circuit

Medium severity FlowMax condensing combi (FLOWMAX-90/120)

What the control is telling you

The water pressure switch has opened because heating system pressure is too low; the unit stops at 0.3 bar (4.3 psi). Normal fill pressure is 1 bar (14.5 psi), around 1.2 bar mid-green on the gauge. The 120 troubleshooting table prints it as 'H20'.

Applies to: FLOWMAX-90, FLOWMAX-120

Likely causes

  • Low system pressure (leak, air vented off, never filled)
  • Water pressure switch wire disconnected
  • Water pressure switch faulty

Test and repair steps

  1. Read the pressure gauge on the bottom of the unit; it should not be below 1 bar (14.5 psi).
  2. If low, open the fill tap and bring the system to about 1 to 1.2 bar, watching that H2O clears, then close the fill tap. The unit restarts on its own.
  3. If pressure keeps dropping, look for leaks in the system, the relief valve discharging, or a failed expansion tank.
  4. If pressure is correct but H2O stays on, check the pressure switch wiring.
  5. If wiring is good, replace the water pressure switch.

Settings for these 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.

Open the app

Source: FlowMax FLOWMAX-120 Instruction Manual, 6.10 Troubleshooting table, p.66; FlowMax FLOWMAX-120 Instruction Manual, 6.11 Diagnostics error code list, p.67; FlowMax FLOWMAX-90 User Manual, Diagnostics - Error codes, p.8-9. 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.