Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
Moving from the ZBR 28-3A to the ZBR 35-3A raises the input ceiling from 100,800 to 131,900 BTU/hr but also lifts the minimum fire from 24,600 to 36,000 BTU/hr, the sound rating from 39 to 44 dB(A), and drops the 2" vent reach from 52 to 25 ft — real trade-offs for a low-load home.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Bosch Greenstar ZBR 28-3A | Bosch Greenstar ZBR 35-3A |
|---|---|---|
| Input rate, minimum | 24.6 MBH | 36 MBH |
| Input rate, maximum | 100.8 MBH | 131.9 MBH |
| Output rate, maximum (104/86 °F) | 93.8 MBH | 122.8 MBH |
| Weight (without packaging) | 103.6 lb | 103.6 lb |
| Sound pressure level | ≤ 39 dB(A) | ≤ 44 dB(A) |
| Maximum supply temperature | 187 °F | 187 °F |
| Maximum permissible operating pressure (PMS), heating | 30 psi | 30 psi |
Both are wall-hung heating-only boilers in the identical 103.6 lb, 440 x 850 x 353 mm cabinet at 187 °F maximum supply and 30 psi. The 35-3A adds 31 MBH of input ceiling (131,900 versus 100,800 BTU/hr) and lifts output at 104/86 °F from 93,800 to 122,800 BTU/hr.
The price is modulation range and installability. The 35-3A's minimum fire steps up to 36,000 BTU/hr from the 28-3A's 24,600 BTU/hr, so a home whose winter load is far below 100 MBH would have the larger boiler cycling at its low-fire floor in shoulder seasons. Its sound rating also rises from 39 to 44 dB(A).
Venting separates the classes: the 35-3A sits in the 35/42 class, limited to 25 ft of straight 2" concentric pipe with two elbows against 52 ft for the 21/28 class (and 44 ft of 3"/5" concentric with one elbow against 50 ft). A tight or long vent route can therefore decide the model as much as the heat loss.
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.
Not entirely. It shares the cabinet, weight, pressure and supply-temperature ratings, but its minimum fire is 36,000 BTU/hr (against 24,600), its sound rating is 44 dB(A) (against 39) and its vent limits drop into the 35/42 class.
No. The 35/42 class allows 25 ft of straight 2" concentric pipe with two elbows; the 21/28 class allows 52 ft. Longer runs need 3" pipe or the 3"/5" concentric system.
When the heat-loss calculation fits under about 100 MBH of input: the 28-3A then fires lower in mild weather, runs quieter and allows longer 2" vent runs.
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.