HVACopediaFlowMax · Combi boiler
H2O Low water pressure in heating circuit
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
- Read the pressure gauge on the bottom of the unit; it should not be below 1 bar (14.5 psi).
- 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.
- If pressure keeps dropping, look for leaks in the system, the relief valve discharging, or a failed expansion tank.
- If pressure is correct but H2O stays on, check the pressure switch wiring.
- If wiring is good, replace the water pressure switch.
Settings for these units
Installation clearances · FlowMax condensing combi (FLOWMAX-90/120)
| Location | Service clearance (in) | To combustibles |
|---|---|---|
| X (side) | 0 | 0 in |
| Y (side) | 0 | 0 in |
| L | 16 | - |
| H | 24 | - |
| A (above) | 12 | 0 in from jacket top |
| B (below) | 12 | - |
| Front | - | 0 in |
| Rear | - | 0 in |
Wall-mount on a flat, solid vertical wall that can carry the weight.
Letters follow fig. 1 of the manual; L and H are the clearance zone in front/around the unit as drawn.
Hot water scald risk by temperature · FlowMax condensing combi (FLOWMAX-90/120)
| Temperature setting | Time to 2nd/3rd degree burn (adult skin) |
|---|---|
| Over 170°F | Nearly instantaneous |
| 160°F | About 1/2 second |
| 150°F | About 1-1/2 seconds |
| 140°F | Less than 5 seconds |
| 130°F | About 30 seconds |
| 120°F or less | More than 5 minutes |
120°F lowers scald risk; children, elderly and disabled people burn faster.
Fit a tempering (mixing) valve if water is stored or set hotter, e.g. with parameter 1 = 03 (up to 160°F).
Models that show this code
2 models in FlowMax condensing combi (FLOWMAX-90/120)
Other codes on the same units
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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.