Bosch Thermotechnology · Greenstar Wall (ZBR/ZWB)
Bosch Greenstar ZBR 42-3A
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.
Key figures
- Max input
- 151.6 MBH
- Max CSA output
- 137.5 MBH
- AFUE
- —
- Min input
- 36 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Bosch Greenstar ZBR 42-3A in practice
The ZBR 42-3A is the largest of the current heat-only Greenstar wall models, firing 36,000 to 151,600 BTU/hr natural-gas input at sea level with altitude derate tables taking it through 7,000 ft. Output at 104/86 °F reaches 137,500 BTU/hr, and like every wall model it is capped at 187 °F supply temperature and 30 psi heating pressure.
Despite the capacity it stays on the family cabinet: 17-21/64 x 33-15/32 x 13-57/64 in (440 x 850 x 353 mm) and 103.6 lb (47 kg), with a 45 dB(A) sound rating. Venting follows the 35/42-class tables — shorter runs than the 21/28 class — so the vent route is a real planning input at this size.
The 42-3A is heating-only: an indirect tank or separate appliance covers domestic hot water. It uses the same two-character Heatronic fault codes as the smaller wall models, documented in Table 38 ('Faults shown on the display') of the 11.2025 I&O manual, which is the reference Boilers.ca reads codes against.
Best suited to
- Larger detached homes whose heat-loss calculation approaches 150 MBH of input
- Wall-hung replacements where a floor-standing or large output is needed but the wall footprint must stay small
- Heating-only systems that need the full 42-class output without DHW hardware
Applicability and support
- Model number
- ZBR 42-3A
- 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 | 36 MBH | 36,000 BTU/hr at 180/79 °F (82/26 °C), natural gas. |
| Input rate, maximum | 151.6 MBH | 151,600 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) | 137.5 MBH | 137,500 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 35/42 model class is limited to 25 ft of straight 2" concentric pipe with 2 elbows (NG), or 44 ft of 3"/5" concentric pipe with 1 elbow. Termination type changes the tables. |
| 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 | ≤ 45 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 much heating can the ZBR 42-3A deliver?
Up to 151,600 BTU/hr natural-gas input at sea level, with output at 104/86 °F of 137,500 BTU/hr. Altitude tables in the manual derate the maximum above 2,000 ft.
Can it vent in 2" pipe?
Natural gas can use 2" pipe, but the 35/42 class is limited to 25 ft of straight 2" concentric pipe with two elbows; propane must use 3" or the 3"/5" concentric system. The manual tables decide the run.
Is the ZBR 42-3A the biggest Greenstar wall boiler?
It is the biggest heating-only wall model in the current -3A line. The ZWB 42-3A combi shares the same 42-class heating side and adds built-in domestic hot water.
Questions about the Bosch Greenstar ZBR 42-3A?
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.