Bosch Thermotechnology · Greenstar FS (floor-standing)
Bosch Greenstar KWB 42-3A
The Greenstar KWB 42-3A is the floor-standing combi model of the FS line ('Combi FS'): 36 to 151.6 MBH heating input with built-in domestic hot water at 3.61 GPM nominal flow (140 °F outlet), in a 164 lb floor cabinet.
Key figures
- Max input
- 151.6 MBH
- Max CSA output
- 137.5 MBH
- AFUE
- —
- Min input
- 36 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Bosch Greenstar KWB 42-3A in practice
The KWB 42-3A is the floor-standing combi of the WB kW class — Bosch's manual calls the combi floors 'Combi FS' and documents them in the same Greenstar FS/Combi FS I&O manual as the heat-only KBR models. It provides space heating and on-demand domestic hot water from one floor cabinet: 36 to 151.6 MBH natural-gas heating input with 3.61 GPM nominal DHW flow at a 140 °F outlet temperature.
The FS manual prints DHW output of 135,800 BTU/hr at a 140 °F outlet and lists the whole KWB cabinet family at 164 lb (74 kg) with max supply temperature of 190 °F (88 °C) and a 39 dB(A) sound rating. Bosch's CA pages market the Greenstar family at 95% AFUE and note that floor-standing models make boiler replacements quick and easy.
As a combi, the unit covers typical domestic hot water without a separate tank. It uses the same two-character Heatronic fault-code family as the wall models — the FS manual's Table 40 lists them — and its vent tables group this model in the class that governs the maximum run, so the vent route is checked against the FS manual before installation.
Best suited to
- Floor-standing replacements that combine heating and on-demand hot water
- Mechanical rooms where a wall-hung combi is not an option but a 164 lb floor unit is
- Homes wanting the FS cabinet with the KWB combi's built-in DHW module
Applicability and support
- Model number
- KWB 42-3A
- 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 | 36 MBH | 36,000 BTU/hr at 180/79 °F (82/26 °C), natural gas. |
| Input rate, maximum | 151.6 MBH | 151,600 BTU/hr at 180/79 °F, natural gas; altitude tables reduce the maximum above 2,000 ft. |
| Output | ||
| Output rate, maximum (104/86 °F) | 137.5 MBH | 137,500 BTU/hr at 104/86 °F water temperatures per the manual technical table. |
| Domestic hot water | ||
| DHW output rate (140 °F (60 °C) outlet) | 135.8 MBH | 135,800 BTU/hr per the manual technical table. |
| Nominal DHW flow rate (140 °F outlet) | 3.61 gpm | Nominal domestic hot water flow at a 140 °F (60 °C) outlet temperature per the FS manual. |
| Venting and air | ||
| Venting | 2" NG only; 3" or 3"/5" (80/125 mm) concentric for LPG | Per the manual: the 35/42 model class is limited to 25 ft of straight 2" concentric pipe with 2 elbows (NG), or 44 ft of 3"/5" concentric pipe with 1 elbow. Termination type changes the tables. |
| Physical and connections | ||
| Weight (without packaging) | 164 lb | 164 lb (74 kg) without packaging per the FS manual. |
| Dimensions (W x H x D) | 21-15/16" × 41-1/4" × 21-7/8" (557 × 1048 × 555 mm) | Per the manual technical data table. |
| Sound pressure level | ≤ 39 dB(A) | Per the manual technical data table. |
| Maximum supply temperature | 190 °F | 190 °F (88 °C) per the FS manual. |
| Maximum permissible operating pressure (PMS), heating | 30 psi | 30 psi (2.07 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. |
Frequently asked questions
Is the KWB 42-3A a heating-only boiler?
No — it is a combi. The KWB codes are the floor-standing combis with built-in domestic hot water; the heat-only floor models are the KBR codes.
What is the difference between KBR and KWB models?
Both are floor-standing Greenstar models in the same 557 x 1048 x 555 mm cabinet. KBR models are heating-only at 150 lb; KWB combis add the DHW module and weigh 164 lb.
How much hot water does this floor combi produce?
The FS manual prints 3.61 GPM nominal DHW flow at a 140 °F outlet for the KWB 42-3A, with DHW output of 135,800 BTU/hr at that outlet temperature.
Questions about the Bosch Greenstar KWB 42-3A?
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.
Compare and explore
- Bosch Greenstar KWB 35-3A vs KWB 42-3AComparison
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- Bosch Greenstar KBR 35-3ASame series
- Bosch Greenstar KBR 42-3ASame series
- Bosch Greenstar KWB 28-3ASame series
- Bosch Greenstar KWB 35-3ASame series
- Greenstar FS (floor-standing) series hub
- Buying guide
Sources and verification
- Bosch Home Comfort Canada — Greenstar Boilers · accessed September 7, 2026
- Greenstar Floor Boilers Installation & Maintenance Manual (6720810590) · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.