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Bosch Thermotechnology · Series
The Greenstar FS is Bosch's floor-standing condensing line for Canada — heat-only KBR 21-3A, 28-3A, 35-3A and 42-3A models plus the KWB 28-3A, 35-3A and 42-3A floor-standing combis ('Combi FS') in one 557 x 1048 x 555 mm cabinet, covered by the 11.2025 FS/Combi FS I&O manual.
Series facts
Bosch's CA product pages market 95% AFUE efficiency for the Greenstar family; the 11.2025 I&O manual technical tables do not print an AFUE row (the AFUE figure is published in the AHRI directory).
Bosch's Canadian floor-standing condensing family is documented in one manual, titled 'Greenstar FS/Combi FS' (doc 6720810590, 11.2025): the KBR 21-3A, 28-3A, 35-3A and 42-3A heating-only floor boilers and the KWB 28-3A, 35-3A and 42-3A combi versions. The K prefix marks the floor-standing appliance in the code-plug key of the manual, and Bosch's CA pages describe floor models as replacements that make boiler swaps 'quick and easy' versus re-hanging a wall unit.
All FS models share the same 21 15/16 x 41 1/4 x 21 7/8 in (557 x 1048 x 555 mm) cabinet; heating-only KBR models weigh 150 lb and the KWB combis 164 lb without packaging. Max supply temperature is 190 °F (88 °C) and natural-gas input runs from 79,200 BTU/hr on the KBR 21-3A up to 151,600 BTU/hr on the 42-class models, with altitude tables through 7,000 ft. Sound ratings are low — the manual prints 33 dB(A) for KBR models and 39 dB(A) for KWB combis.
Floor models share the Heatronic control and its two-character fault codes (the FS manual's Table 40 'Faults shown on the display'), so a code such as EA (no flame detected) reads the same way as on the wall models. Canadian installations follow CAN/CSA B149.1 as stated in the manual, and venting tables differ between the 21/28 and 35/42 model classes, so the vent run must be checked against the model column.
Highlights
Models
One canonical page per model with input, output and efficiency kept separate.
Bosch Thermotechnology
The Greenstar KBR 21-3A is a floor-standing heating-only Greenstar boiler: 24.6 to 79.2 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Bosch Thermotechnology
The Greenstar KBR 28-3A is a floor-standing heating-only Greenstar boiler: 24.6 to 100 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Bosch Thermotechnology
The Greenstar KBR 35-3A is a floor-standing heating-only Greenstar boiler: 36 to 131.9 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Bosch Thermotechnology
The Greenstar KBR 42-3A is a floor-standing heating-only Greenstar boiler: 36 to 151.6 MBH natural-gas input in the 557 x 1048 x 555 mm FS cabinet, rated 150 lb without packaging.
Bosch Thermotechnology
The Greenstar KWB 28-3A is the floor-standing combi model of the FS line ('Combi FS'): 24.6 to 100.8 MBH heating input with built-in domestic hot water at 2.03 GPM nominal flow (140 °F outlet), in a 164 lb floor cabinet.
Bosch Thermotechnology
The Greenstar KWB 35-3A is the floor-standing combi model of the FS line ('Combi FS'): 36 to 131.9 MBH heating input with built-in domestic hot water at 2.6 GPM nominal flow (140 °F outlet), in a 164 lb floor cabinet.
Bosch Thermotechnology
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.
| Specification | Bosch Greenstar KBR 21-3A | Bosch Greenstar KBR 28-3A | Bosch Greenstar KBR 35-3A | Bosch Greenstar KBR 42-3A | Bosch Greenstar KWB 28-3A | Bosch Greenstar KWB 35-3A | Bosch Greenstar KWB 42-3A |
|---|---|---|---|---|---|---|---|
| Input rate, minimum | 24.6 MBH | 24.6 MBH | 36 MBH | 36 MBH | 24.6 MBH | 36 MBH | 36 MBH |
| Input rate, maximum | 79.2 MBH | 100 MBH | 131.9 MBH | 151.6 MBH | 100.8 MBH | 131.9 MBH | 151.6 MBH |
| Output rate, maximum (104/86 °F) | 74.7 MBH | 93.8 MBH | 122.8 MBH | 137.5 MBH | 93.8 MBH | 122.8 MBH | 137.5 MBH |
| afue | — | — | — | — | — | — | — |
| turndown | — | — | — | — | — | — | — |
| vent-max-3 | — | — | — | — | — | — | — |
| vent-max-2 | — | — | — | — | — | — | — |
| Weight (without packaging) | 150 lb | 150 lb | 150 lb | 150 lb | 164 lb | 164 lb | 164 lb |
Read the full comparison: Bosch Greenstar KBR 21-3A vs KBR 28-3A →
Read the full comparison: Bosch Greenstar KBR 28-3A vs KBR 35-3A →
Read the full comparison: Bosch Greenstar KBR 35-3A vs KBR 42-3A →
Read the full comparison: Bosch Greenstar KWB 28-3A vs KWB 35-3A →
Read the full comparison: Bosch Greenstar KWB 35-3A vs KWB 42-3A →
Error codes and alerts
Manuals
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.
Services for Bosch Thermotechnology boilers
FS is the floor-standing heating-only line (KBR codes) and Combi FS is the floor-standing combi version (KWB codes). Both sit in the same cabinet and are covered by the single Greenstar FS/Combi FS I&O manual (6720810590).
Bosch's FS venting tables give the 35/42 class shorter maximum straight lengths than the 21/28 class — for example 25 ft versus 52 ft of 2" concentric pipe with two elbows. The model column of the manual tables decides the allowed run.
150 lb without packaging for the heating-only KBR models and 164 lb for the KWB combis, per the technical data tables of the 11.2025 FS manual.
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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.