Worcester Bosch R 28 HE system (ZB 7-28 R 28 HE)
System boiler, natural gas and LPG (by conversion kit)
Combustion figures
| Figure | Natural gas (G20) | LPG (G31) |
|---|---|---|
CO₂ at high rate More infoHide infoFrom Table 16 (MI 7.1, Setting the gas/air ratio), CO₂ reading at max. rated heat output. Measured at the flue gas testing point (234) with the probe about 135 mm in, outer case removed, boiler set to max. rated heat output (display 2.) in service function 2.0. The MI gives no tolerance and doesn’t say how long after firing to measure. The LPG figure is for LPG G31 (propane); the boiler is converted from natural gas to LPG with kit 7 710 149 049 (MI 7, Table 15). The technical data (MI 1.9, Table 2) gives the same figures. The MI says the CO₂ level is set at the factory and no adjustment is needed when installing a natural gas appliance (MI 5.1); the gas/air ratio only needs setting if the appliance has been stripped down and reassembled, the fan, burner or gas valve replaced, or it has been converted to another gas (MI 6.3). Combustion must be checked before and after any servicing work on the combustion and burner components (MI 8.2). After installation, a long period out of use or a mains interruption, the trap filling programme holds the boiler at minimum output for 15 minutes; the display alternates between -II- and the flow temperature while it runs (MI 5.2). | 9.2% | 10.8% |
CO₂ at low rate More infoHide infoFrom Table 16 (MI 7.1), CO₂ reading at min. rated heat output (display 1. in service function 2.0), same test point and conditions as at high rate. The MI gives no tolerance. The technical data (MI 1.9, Table 2) gives the same figures. | 8.8% | 10.5% |
Maximum CO More infoHide infoThe MI gives no CO limit. MI 7.2.2 (Testing CO and CO₂) says to measure CO and CO₂ at the flue gas testing point (234, probe about 135 mm in) in chimney sweep mode, but gives no figures to compare them with. | Not given in MI | Not given in MI |
CO/CO₂ ratio | Not given in MI | Not given in MI |
O₂ More infoHide infoThe MI gives no O₂ figure for the flue gas. It gives O₂ only for the flue integrity check in the combustion air (see Flue integrity). | Not given in MI | Not given in MI |
Working inlet pressure More infoHide infoMI 5.1 says to check the gas supply pressure before commissioning at the gas supply pressure test point (Fig. 2, item 7, MI 1.6), which Fig. 3 (MI 1.7) shows on the inlet side of the gas valve. Natural gas appliances must not be operated if the gas supply pressure is below 18 mbar or above 24 mbar (MI 5.1 and MI 7). For LPG, MI 5.1 says the appliance must not be operated if the supply pressure is not 37 mbar at the inlet to the appliance, and MI 7 says not if it is below or above 37 mbar. The MI doesn’t say whether the boiler should be running during the check. The technical data (MI 1.9, Table 2) gives a gas supply pressure of 20 mbar (natural gas G20) and 37 mbar (LPG), and MI 7 says natural gas G20 appliances are factory set to Wobbe index 15 kWh/m³ and 20 mbar supply pressure. |
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Gas rate at maximum More infoHide infoFrom the technical data (MI 1.9, Table 2), maximum gas flow rate after 10 minutes from lighting: natural gas G20 (CVnet 34.02 MJ/m³) and LPG (CVnet 88 MJ/m³). Max. rated heat input net is 27.7 kW on both gases. Table 20 (MI 9.3) gives a natural gas volume flow rate of 48.5 l/min at display code 100 (27.7 kW heat input, flow/return 80/60°C); MI 6.2.6 says to measure the gas flow rate and compare it with the figure for the displayed code when setting the heating output (service function 5.0). The LPG table (MI 9.4, Table 21) gives no gas flow rates. | 2.9 m³/h | 2.1 kg/h |
Flue integrity More infoHide infoFrom MI 7.2.1 (Testing the O₂ or CO₂ level in the combustion air), which checks the gas tightness of a type C13 or C33 flue system. Printed with decimal commas (20,6 % and 0,2 %). Measured at the combustion air testing point (234.1, Fig. 50) with the probe about 80 mm in, in chimney sweep mode, where the boiler runs at max. rated heat output or the set heating output for 15 minutes before returning to normal mode. The MI doesn’t say whether the case should be on or off. The MI words it as O₂ must not be less than 20.6% and CO₂ must not exceed 0.2%. | O₂ ≥ 20.6% CO₂ ≤ 0.2% | O₂ ≥ 20.6% CO₂ ≤ 0.2% |
From 6 720 611 443 GB (05.04)
Enter service mode for high and low rate
High rate
- Hold the service button (spanner symbol) until the display shows – –. The button lights up.
- Turn the central heating control (radiator symbol) until the display shows 2.0. After a short delay it shows the current mode (0. = normal).
- Turn the hot water control (tap symbol) until the display shows 2. for maximum. The display and service button flash.
- Or: hold the chimney sweep button until – – shows. Runs at maximum or the set heating output (Service Function 5.0) for 15 minutes.
Low rate
- Turn the hot water control (tap symbol) anticlockwise until the display shows 1. for minimum.
Ending the test
- Turn the hot water control anticlockwise to 0., hold service until [ ] shows, then set both temperature controls back where they were.
- Chimney sweep mode: hold the chimney sweep button until – – shows. It also ends by itself after 15 minutes.
Models and GC numbers
| Model | GC number | Gas |
|---|---|---|
| ZB 7-28 R 28 HE System | 41-311-62 | NG |
Source: Worcester R 28 HE system wall mounted condensing boiler for central heating installation and servicing instructions, 6 720 611 443 GB (05.04).
Sections: Cover (GC number); 1.6 Layout of appliance (Fig. 2, item 7); 1.7 Function (Fig. 3); 1.9 Technical data (Table 2); 5.1 Commissioning; 5.2 Switching the appliance on/off (trap filling programme); 6.2.6 Setting the heating output (Service Function 5.0); 6.3 Setting the gas/air ratio; 7 Converting the appliance to different gas types (Checking the gas supply pressure, Table 15); 7.1 Setting the gas/air ratio (Table 16); 7.2.1 Testing the O₂ or CO₂ level in the combustion air; 7.2.2 Testing CO and CO₂; 8.2 Description of servicing operations; 9.3 Heating/hot water output settings (N.G) (Table 20); 9.4 Heating/hot water output settings (L.P.G) (Table 21).
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Check these figures against the MI for the appliance in front of you, matched by GC number and manual date. Combustion work needs a calibrated analyser and a Gas Safe registered engineer.