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Bosch Greenstar Combi (wall)
Model Number: ZWB28-3/35-3/42-3
Wall-hung condensing combi in three sizes (100, 131, 151), up to 151,600 BTU/hr, 95% AFUE, heat plus on-demand hot water.
Technical Specifications
Source: manufacturer spec sheet · checked Sep 27, 2026
Prices depend on sizing, venting and piping in your home, so we quote every installation.
Sizing: the right size comes from a room-by-room heat-loss calculation, not the size of the old boiler or a rule of thumb. Oversized boilers short-cycle and waste fuel; steam systems are sized to the connected radiation instead. See how we approach boiler installation.
Home-Specific Operating Cost Review
No guaranteed annual savings or payback period
Historical Use
Fuel consumption and weather
Current Rates
Utility and delivered-fuel prices
System Design
Load, piping, controls, and temperatures
Full Specifications
- Model Number
- ZWB28-3/35-3/42-3
- Efficiency (AFUE)
- 95%
- Boiler Type
- Combi Boiler
- Fuel
- natural gas/propane
- Venting
- sealed combustion; concentric, 1-pipe or 2-pipe (PVC, CPVC, PP, stainless)
- BTU Input Range
- 25k - 152k BTU
- Domestic Hot Water
- yes, on-demand (stainless plate exchanger)
- In Production
- Yes
- Parts Warranty
- 5-year parts and labor
- Heat Exchanger Warranty
- 10-year limited lifetime
Specifications sourced from the manufacturer spec sheet, checked Sep 27, 2026.
Ideal For
Smaller homes and condos that want heat and hot water from one wall-mounted unit and to drop a separate water heater.
Expert Review: Bosch Greenstar Combi (wall)
The Bosch Greenstar Combi is a wall-hung, modulating condensing gas boiler that heats the house and makes on-demand domestic hot water from one cabinet. It condenses in both modes. Bosch builds it in three sizes: Combi 100 (ZWB28-3), 131 (ZWB35-3) and 151 (ZWB42-3). On natural gas, input runs from 24,600 BTU/hr at minimum on the 100 up to 151,600 BTU/hr on the 151, all at 95% AFUE.
Hot water is made through a stainless steel flat-plate heat exchanger and a three-way valve. The 151 is rated at 137,500 BTU/hr of hot-water output at 113°F. Bosch limits hot-water temperature to 140°F to slow scale build-up, and a 3.2-gallon expansion tank is built in.
The boiler ships for natural gas with a propane kit included, and uses the same venting options as the heat-only model: concentric, one-pipe or two-pipe, up to 100 feet on 3-inch PVC. The cabinet is about 33.5 by 17.3 by 13.9 inches and weighs roughly 110 lb. Bosch lists a 5-year parts-and-labor warranty and a 10-year limited lifetime warranty on the heat exchanger.
A combi works best where hot-water demand is moderate and the incoming water flow is good. Homes that run several showers at once are usually better off with a heat-only boiler and an indirect tank.
Key Features & Technology
Three sizes: 100, 131 and 151
95% AFUE; condenses in heating and hot-water modes
Stainless flat-plate hot-water exchanger
Built-in 3.2-gallon expansion tank
Hot water capped at 140°F to limit scale
Propane conversion kit included
Advantages
- Replaces the boiler and water heater with one unit
- Compact wall mount frees floor space
- Expansion tank already built in, so there is less piping
Considerations
- Hot-water output is shared with heating, so it is limited during heavy use
- Minimum input (24,600 BTU/hr) is higher than the heat-only 57
- Hot water tops out at 140°F
Sizing & Installation Notes
Combi boilers like the Greenstar Combi (wall) are sized on two loads at once: the home's heat loss and the peak domestic hot water demand. In most Long Island homes the hot-water load is the larger one, so we size to your household's simultaneous shower and tap use.
Every Bosch installation starts with a free in-home assessment covering the heat-loss calculation, venting and chimney condition, gas or oil supply sizing, and permit requirements for your town. Learn more about our boiler installation process, or see current rebate and incentive programs that may apply.
Warranty & Protection
Parts Warranty
5-year parts and labor
Heat Exchanger
10-year limited lifetime
Our Guarantee
100% Satisfaction
All Bosch boiler installations by Home+ Air & Heat include our 1-year workmanship warranty, covering labor and any installation-related issues.
Compare the Bosch Lineup
| Model | Efficiency | BTU Range | Fuel | Heat Exchanger Warranty |
|---|---|---|---|---|
| Greenstar Combi (wall) (this model) | 95% AFUE | 25k - 152k BTU | natural gas/propane | 10-year limited lifetime |
| SSB (Buderus SSB85/120/160) | 96% AFUE | 17k - 160k BTU | natural gas/propane | 10-year limited (heat exchanger, leak) |
| Greenstar (wall heating-only) | 95% AFUE | 13k - 152k BTU | natural gas/propane | 10-year limited lifetime |
| Greenstar FS Combi (floor) | 95% AFUE | 25k - 152k BTU | natural gas/propane | 10-year limited lifetime |
See the full Bosch brand page for lineup context, or browse the Best Combi Boilers 2027 guide to compare across brands.
Frequently Asked Questions
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