Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
Boilers.ca is a brand of ROMA Heating & Cooling · Licensed gas contractor LGA0202903
Bosch Thermotechnology · Series
The Greenstar Wall line is Bosch's wall-hung condensing boiler family for Canada: heat-only ZBR 21-3A, 28-3A, 35-3A and 42-3A models plus the ZWB 35-3A combi, all in a 440 x 850 x 353 mm cabinet with an aluminum-silicon heat exchanger and a Heatronic control that shows two-character fault codes.
Series facts
Bosch's CA product pages market 95% AFUE efficiency for the Greenstar family; the 11.2025 I&O manual technical tables do not print an AFUE row (the AFUE figure is published in the AHRI directory).
The Greenstar Wall I&O manual (6721853955, 11.2025 US/CA) documents the current wall-hung generation of the family Bosch markets in Canada simply as 'Greenstar Boilers': the ZBR 21-3A, ZBR 28-3A, ZBR 35-3A and ZBR 42-3A heating-only models, whose engineering codes share the 'Z' wall-hung prefix and the '-3A' generation suffix. The same manual also carries technical data for the wall-hung combi models ZWB 28-3A, ZWB 35-3A and ZWB 42-3A, which Bosch's CA manual library splits across two titles; Boilers.ca documents the ZWB 28-3A and ZWB 42-3A under the Greenstar Combi (100p/151p) series and the ZWB 35-3A here, because the combi-100p/151p manual covers only the 28-3A and 42-3A sizes.
All wall models share one 440 x 850 x 353 mm cabinet. Heating-only ZBR models weigh 103.6 lb without packaging, while the combi ZWB models weigh 110.2 lb because of the built-in plate heat exchanger and three-way valve. Bosch's CA pages market 95% AFUE; the I&O technical tables print max input from 79,200 BTU/hr (ZBR 21-3A) to 151,600 BTU/hr (ZBR 42-3A) on natural gas, with altitude tables through 7,000 ft.
The Heatronic control shows text-style readouts plus two-character fault codes such as A7 (DHW temperature sensor), d3 (temperature high limit), E9 (safety high limit or flue gas temperature limiter), EA (no flame detected) through Fd, listed in the manual section 'Faults shown on the display'. Bosch resets some faults from the keypad and others only after the cause is corrected; Boilers.ca reads every code against this 11.2025 manual because code meaning and reset behaviour belong to the generation.
Highlights
Models
One canonical page per model with input, output and efficiency kept separate.
Bosch Thermotechnology
The Greenstar ZBR 21-3A is the smallest current wall-hung heating-only Greenstar boiler: 24,600 to 79,200 BTU/hr natural-gas input at 95% AFUE family rating (per Bosch CA marketing), 103.6 lb in the shared 440 x 850 x 353 mm wall cabinet.
Bosch Thermotechnology
The Greenstar ZBR 28-3A is the 100 MBH-class wall-hung heating-only Greenstar: 24,600 to 100,800 BTU/hr natural-gas input, 103.6 lb, in the same 440 x 850 x 353 mm cabinet as the rest of the wall line.
Bosch Thermotechnology
The Greenstar ZBR 35-3A is the 132 MBH-class wall-hung heating-only Greenstar: 36,000 to 131,900 BTU/hr natural-gas input in the standard 103.6 lb wall cabinet, with the shorter vent runs Bosch allows for the 35/42 size class.
Bosch Thermotechnology
The Greenstar ZBR 42-3A is the top wall-hung heating-only Greenstar: 36,000 to 151,600 BTU/hr natural-gas input (with altitude tables to 7,000 ft), rated to 30 psi at 187 °F max supply, still in the 103.6 lb wall cabinet.
Bosch Thermotechnology
The Greenstar ZWB 35-3A is the mid-size wall-hung Greenstar combi — 36,000 to 131,900 BTU/hr natural-gas heating input with DHW output to 118,700 BTU/hr at a 140 °F outlet and 3.17 GPM maximum DHW flow — documented in the Greenstar Wall I&O manual because the Combi 100P/151P manual does not cover the 35-3A size.
| Specification | Bosch Greenstar ZBR 21-3A | Bosch Greenstar ZBR 28-3A | Bosch Greenstar ZBR 35-3A | Bosch Greenstar ZBR 42-3A | Bosch Greenstar ZWB 35-3A |
|---|---|---|---|---|---|
| Input rate, minimum | 24.6 MBH | 24.6 MBH | 36 MBH | 36 MBH | 36 MBH |
| Input rate, maximum | 79.2 MBH | 100.8 MBH | 131.9 MBH | 151.6 MBH | 131.9 MBH |
| Output rate, maximum (104/86 °F) | 74.7 MBH | 93.8 MBH | 122.8 MBH | 137.5 MBH | 122.8 MBH |
| afue | — | — | — | — | — |
| turndown | — | — | — | — | — |
| vent-max-3 | — | — | — | — | — |
| vent-max-2 | — | — | — | — | — |
| Weight (without packaging) | 103.6 lb | 103.6 lb | 103.6 lb | 103.6 lb | 110.2 lb |
Read the full comparison: Bosch Greenstar ZBR 21-3A vs ZBR 28-3A →
Read the full comparison: Bosch Greenstar ZBR 28-3A vs ZBR 35-3A →
Read the full comparison: Bosch Greenstar ZBR 35-3A vs ZBR 42-3A →
Read the full comparison: Bosch Greenstar ZBR 35-3A vs ZWB 35-3A →
Read the full comparison: Bosch Greenstar ZBR 42-3A vs Weil-McLain Ultra 155 →
Error codes and alerts
Manuals
Where ROMA provides on-site service
On-site service is currently offered in Greater Vancouver, BC, including Vancouver, Burnaby, Richmond, Surrey, Coquitlam, North Vancouver, West Vancouver, New Westminster, Port Coquitlam, Port Moody, Delta, Langley, Maple Ridge, Abbotsford, Chilliwack, White Rock, Pitt Meadows, Tsawwassen, Anmore, Lions Bay, Belcarra, Bowen Island, Mission. Everything else on this site is reference information you can use anywhere in Canada.
Services for Bosch Thermotechnology boilers
It marks the current North American engineering generation: ZBR codes are wall-hung heating-only boilers (21, 28, 35 and 42 kW classes) and ZWB codes are the wall-hung combi versions of the same classes. Older Greenstar generations are not listed on the current CA catalog.
Bosch's CA manual library covers those two combi sizes in the separate 'Greenstar Combi 100P and 151P' I&O manual (doc 6721853953), and Boilers.ca documents them under the Greenstar Combi (100p/151p) series. The Greenstar Wall manual also lists them, which is why the model sets of the two manuals overlap.
The control displays two-character fault codes on a seven-segment display — examples in the 11.2025 manual run from A7 (DHW temperature sensor defective) through E9, EA and F0 to Fd. Each code is read against this generation's manual because reset behaviour and code meaning belong to the generation.
Describe what the boiler is doing and get a straight answer about the next step.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.
Sources and verification
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.