Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
The ZBR 35-3A and ZBR 42-3A are the two largest heating-only Greenstar wall boilers and share everything structural — the 103.6 lb cabinet, the 36,000 BTU/hr minimum fire, the 30 psi rating and the same 35/42-class vent limits. The 42-3A adds about 20 MBH of input ceiling and 14.7 MBH of output, with the sound rating rising from 44 to 45 dB(A).
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Bosch Greenstar ZBR 35-3A | Bosch Greenstar ZBR 42-3A |
|---|---|---|
| Input rate, minimum | 36 MBH | 36 MBH |
| Input rate, maximum | 131.9 MBH | 151.6 MBH |
| Output rate, maximum (104/86 °F) | 122.8 MBH | 137.5 MBH |
| Weight (without packaging) | 103.6 lb | 103.6 lb |
| Sound pressure level | ≤ 44 dB(A) | ≤ 45 dB(A) |
| Maximum supply temperature | 187 °F | 187 °F |
| Maximum permissible operating pressure (PMS), heating | 30 psi | 30 psi |
Both boilers fire down to the same 36,000 BTU/hr minimum and live in the same 103.6 lb wall cabinet with 187 °F maximum supply temperature and 30 psi heating pressure — so below the 35-3A's ceiling the two behave identically, including shoulder-season low-fire behaviour.
The differences start at the maximum: 131,900 BTU/hr input and 122,800 BTU/hr output at 104/86 °F on the 35-3A, against 151,600 BTU/hr input and 137,500 BTU/hr output on the 42-3A. The 42-3A also prints the family's highest sound rating at 45 dB(A) versus 44 dB(A).
Both sit in the 35/42 vent class with its shorter runs (25 ft of 2" concentric with two elbows on NG), so venting never separates these two — only the heat-loss calculation and output headroom do. Altitude derate tables in the manual apply from 2,000 ft upward on both.
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.
Marginally: the manual prints 45 dB(A) for the 42-3A against 44 dB(A) for the 35-3A, both well within typical mechanical-room levels.
Yes — both are in the 35/42 vent class with identical limits in the manual tables (25 ft of 2" concentric with two elbows on NG, 44 ft of 3"/5" concentric with one elbow).
Either could, but the 35-3A's 131,900 BTU/hr ceiling with the same 36,000 BTU/hr minimum fire usually wins unless the design-day calculation needs the 42-3A's extra headroom.
A consultation includes a heat-loss review so the size is right the first time.
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.