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
Comparison
Verdict in brief
The Combi 100p (ZWB 28-3A) and Combi 151p (ZWB 42-3A) are the two current Greenstar wall combis of the Combi 100P/151P manual: the 151p more than doubles DHW output (135,800 versus 91,400 BTU/hr at a 140 °F outlet), lifts nominal DHW flow from 2.03 to 3.61 GPM, and raises the heating ceiling from 100,000 to 151,600 BTU/hr — at the cost of a higher 36,000 BTU/hr minimum fire and a 45 dB(A) sound rating.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Bosch Greenstar Combi 100p (ZWB 28-3A) | Bosch Greenstar Combi 151p (ZWB 42-3A) |
|---|---|---|
| Input rate, minimum | 24.6 MBH | 36 MBH |
| Input rate, maximum | 100 MBH | 151.6 MBH |
| DHW output rate (113 °F (45 °C) outlet) | 93.6 MBH | 137.5 MBH |
| DHW output rate (140 °F (60 °C) outlet) | 91.4 MBH | 135.8 MBH |
| Maximum DHW flow rate | 2.64 gpm | 3.963 gpm |
| Nominal DHW flow rate (140 °F outlet) | 2.03 gpm | 3.61 gpm |
| Weight (without packaging) | 110.2 lb | 110.2 lb |
| Maximum approved DHW pressure | 150 psi | 150 psi |
Both are wall-hung condensing combis in the same 440 x 850 x 353 mm cabinet at 110.2 lb, rated to 187 °F supply and 30 psi, with a 150 psi DHW-pressure approval and the same 3.17 gal expansion vessel. The combi-100p/151p I&O manual documents both, so the spec figures here are single-source.
The heating gap is modest relative to the DHW gap: 100,000 BTU/hr maximum input on the 100p against 151,600 BTU/hr on the 151p, with minimum fires of 24,600 and 36,000 BTU/hr respectively. On domestic hot water the 151p's DHW output at a 140 °F outlet is 135,800 BTU/hr versus 91,400 BTU/hr, with nominal flow of 3.61 GPM against 2.03 GPM.
The trade-offs are the 151p's higher minimum fire — in a low-load home the 100p tracks mild weather more closely — and its sound rating of 45 dB(A) against 39 dB(A). Both use 2" vent pipe on NG with class-dependent limits: the 100p allows 52 ft of 2" concentric with two elbows per the combi manual, the 151p 38 ft in the same table.
Bosch Thermotechnology
The Greenstar Combi 100p is the ZWB 28-3A wall-hung combi: 24,600 to 100,000 BTU/hr heating input with DHW output to 91,400 BTU/hr at a 140 °F outlet and 2.03 GPM nominal DHW flow, documented in the Combi 100P/151P I&O manual (6721853953).
Bosch Thermotechnology
The Greenstar Combi 151p is the ZWB 42-3A wall-hung combi — the largest Greenstar combi: 36,000 to 151,600 BTU/hr heating input, DHW output to 135,800 BTU/hr at a 140 °F outlet and 3.61 GPM nominal DHW flow, covered by the Combi 100P/151P I&O manual.
The 151p: 3.61 GPM nominal DHW flow at a 140 °F outlet and 135,800 BTU/hr DHW output, against 2.03 GPM and 91,400 BTU/hr on the 100p, per the Combi 100P/151P I&O manual.
Only if the heat-loss calculation and the household's hot-water draws genuinely need the larger side. The 151p's 36,000 BTU/hr minimum fire is higher, so in a low-load home the 100p condenses more steadily.
Both combis share the wall cabinet and the same 110.2 lb shipping weight; the 42-class heating block fits the same chassis with a higher firing range.
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.