Bosch Thermotechnology · Greenstar Combi (100p/151p)
Bosch Greenstar Combi 100p (ZWB 28-3A)
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).
Key figures
- Max input
- 100 MBH
- Max CSA output
- 93.8 MBH
- AFUE
- —
- Min input
- 24.6 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Bosch Greenstar Combi 100p (ZWB 28-3A) in practice
Bosch Canada's 'Greenstar Combi 100P and 151P' I&O manual (6721853953, 11.2025) opens its technical data with the combi 100 p, engineering code ZWB 28-3A: a wall-hung condensing combi firing 24,600 to 100,000 BTU/hr natural-gas input for space heating while its built-in DHW module delivers up to 93,600 BTU/hr output at a 113 °F outlet (91,400 BTU/hr at 140 °F).
On the domestic side the manual prints a maximum DHW flow rate of 2.64 GPM and 2.03 GPM nominal flow at a 140 °F outlet temperature, with DHW pressure approved to 150 psi and a 3.17 gal expansion vessel. At 110.2 lb the combi is about 6.6 lb heavier than the heat-only ZBR models in the same 440 x 850 x 353 mm cabinet.
One documentation quirk: the Greenstar Wall I&O manual (6721853955) lists the same ZWB 28-3A with a maximum input of 100,800 BTU/hr (29.5 kW) while this combi manual prints 100,000 BTU/hr (29.3 kW) — a small edition-to-edition difference that Boilers.ca notes rather than resolves. Fault codes follow the combi manual's Table 33 display table for this record.
Best suited to
- Homes whose heat-loss calculation fits the 100 MBH input class with on-demand hot water
- Households replacing a wall-hung boiler where the combi saves a separate tank
- Projects matching the Combi 100p scope of Bosch's CA tech-documentation titles
Applicability and support
- Model number
- ZWB 28-3A
- 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 | 24.6 MBH | 24,600 BTU/hr at 180/79 °F (82/26 °C), natural gas. |
| Input rate, maximum | 100 MBH | 100,000 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) | 93.8 MBH | 93,800 BTU/hr at 104/86 °F water temperatures per the manual technical table. |
| Domestic hot water | ||
| DHW output rate (113 °F (45 °C) outlet) | 93.6 MBH | 93,600 BTU/hr per the manual technical table. |
| DHW output rate (140 °F (60 °C) outlet) | 91.4 MBH | 91,400 BTU/hr per the manual technical table. |
| Maximum DHW flow rate | 2.64 gpm | Maximum domestic hot water flow rate per the manual technical table. |
| Nominal DHW flow rate (140 °F outlet) | 2.03 gpm | Nominal domestic hot water flow at a 140 °F (60 °C) outlet temperature. |
| Maximum approved DHW pressure | 150 psi | 150 psi (10.3 bar); minimum water pressure 4.35 psi (0.3 bar). |
| Venting and air | ||
| Venting | 2" NG only; 3" or 3"/5" (80/125 mm) concentric for LPG | Per the manual: the 21/28 model class runs up to 52 ft of straight 2" concentric pipe with 2 elbows (NG), or 50 ft of 3"/5" concentric pipe with 1 elbow. |
| Physical and connections | ||
| Weight (without packaging) | 110.2 lb | 110.2 lb without packaging per the combi-100p/151p 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 | ≤ 39 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). |
| Expansion vessel total contents | 3.17 gal | 3.17 gal (12 l); pre-charge pressure 10.9 psi (0.75 bar). |
| 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. |
Manuals
Frequently asked questions
Is the Combi 100p the same boiler as the ZWB 28-3A?
Yes. The Combi 100P/151P I&O manual names the combi 100 p and gives its engineering code as ZWB 28-3A. Boilers.ca keeps one model record per code and places it in the Greenstar Combi (100p/151p) series.
Why does the Wall manual list a slightly higher maximum input?
The Greenstar Wall I&O (6721853955) prints 100,800 BTU/hr (29.5 kW) for the ZWB 28-3A while the Combi 100P/151P I&O (6721853953) prints 100,000 BTU/hr (29.3 kW) — a small edition difference between two official manuals for the same unit.
How much domestic hot water does the Combi 100p deliver?
The combi manual prints a maximum DHW flow of 2.64 GPM and a nominal 2.03 GPM at a 140 °F outlet, with DHW output of 91,400 BTU/hr at that outlet temperature.
Questions about the Bosch Greenstar Combi 100p (ZWB 28-3A)?
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.
Sources and verification
- Bosch Home Comfort Canada — Greenstar Boilers · accessed September 7, 2026
- Greenstar Combi 100P and 151P Installation and Service Manual (6721853953) · 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.