Bosch Thermotechnology · Greenstar Wall (ZBR/ZWB)
Bosch Greenstar ZBR 28-3A
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.
Key figures
- Max input
- 100.8 MBH
- Max CSA output
- 93.8 MBH
- AFUE
- —
- Min input
- 24.6 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Bosch Greenstar ZBR 28-3A in practice
The ZBR 28-3A steps the wall line up to 100,800 BTU/hr maximum natural-gas input (the technical table's 29.5 kW figure) while keeping the same 24,600 BTU/hr minimum fire as the 21-3A, the same 103.6 lb cabinet and the same 440 x 850 x 353 mm footprint. It is heating-only: an indirect tank or separate appliance handles domestic hot water.
Output at 104/86 °F reaches 93,800 BTU/hr per the manual, max supply temperature is 187 °F and the heating circuit is rated to 30 psi. Bosch's family marketing on the CA pages claims 95% AFUE efficiency for the Greenstar line, with the manual itself printing the input/output tables rather than an AFUE row.
Like every -3A wall model it vents as Category IV direct vent with 2" pipe for NG only, and it shows the Heatronic two-character fault codes documented in the manual's display table. Codes should be read against the 11.2025 Wall I&O manual, which is the generation-specific reference.
Best suited to
- Detached homes whose heat-loss calculation lands between roughly 25 and 100 MBH
- Replacing a mid-size wall-hung boiler without enlarging the mounting footprint
- Heating-only systems paired with an indirect domestic hot water tank
Applicability and support
- Model number
- ZBR 28-3A
- Canadian availability
- available
- Production status
- current
- ROMA supplies
- Confirmed at consultation
- ROMA services
- Confirmed at consultation
- Warranty
- Not verified
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.
Specifications
Published specifications
Values as published by Bosch Thermotechnology. Units are shown with every value; notes flag conflicting sources or missing figures.
| Specification | Value | Note |
|---|---|---|
| Input | ||
| Input rate, minimum | 24.6 MBH | 24,600 BTU/hr at 180/79 °F (82/26 °C), natural gas. |
| Input rate, maximum | 100.8 MBH | 100,800 BTU/hr at 180/79 °F (82/26 °C), natural gas; altitude tables in the manual reduce the maximum above 2,000 ft. |
| Output | ||
| Output rate, maximum (104/86 °F) | 93.8 MBH | 93,800 BTU/hr at 104/86 °F water temperatures per the manual technical table. |
| Venting and air | ||
| Venting | 2" NG only; 3" or 3"/5" (80/125 mm) concentric for LPG | Per the manual: the 21/28 model class runs up to 52 ft of straight 2" concentric pipe with 2 elbows (NG), or 50 ft of 3"/5" concentric pipe with 1 elbow. |
| Physical and connections | ||
| Weight (without packaging) | 103.6 lb | 103.6 lb (47 kg) without packaging per the manual. |
| Dimensions (W x H x D) | 17-21/64" × 33-15/32" × 13-57/64" (440 × 850 × 353 mm) | Per the manual technical data table. |
| Sound pressure level | ≤ 39 dB(A) | Per the manual technical data table. |
| Maximum supply temperature | 187 °F | 187 °F (86 °C). |
| Maximum permissible operating pressure (PMS), heating | 30 psi | 30 psi (2.07 bar). |
| Nominal water capacity (heating) | 0.925 gal | 0.925 gal (3.5 l). |
| Permissible inlet gas pressure | NG 3.5–10.5 in. W.C.; LPG 8–13 in. W.C. | NG is the factory default; LPG only after successful gas conversion with the included conversion kit. |
| Operating limits and other | ||
| Fuel | Natural gas or propane | Separate NG and LPG ratings in the manual; LPG requires the gas conversion kit included with the appliance. |
Frequently asked questions
How does the ZBR 28-3A differ from the ZBR 21-3A?
It shares the cabinet, weight, 24,600 BTU/hr minimum fire and 30 psi heating rating; the maximum input rises from 79,200 to 100,800 BTU/hr. Both are heating-only models.
What does the 28 stand for?
The 28 is the kW-class size label Bosch applies to the 100 MBH-class wall model; the engineering code ZBR 28-3A is what the I&O manual's technical tables and code-plug key use.
Does this boiler produce domestic hot water?
No. It is a heating-only ZBR model. Domestic hot water is handled by a separate indirect tank or appliance on the system.
Questions about the Bosch Greenstar ZBR 28-3A?
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.
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Sources and verification
- Bosch Home Comfort Canada — Greenstar Boilers · accessed September 7, 2026
- Greenstar Wall Boilers Installation and Service Manual (6721853955) · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.