Bosch Thermotechnology · Greenstar FS (floor-standing)
Bosch Greenstar KBR 21-3A
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.
Key figures
- Max input
- 79.2 MBH
- Max CSA output
- 74.7 MBH
- AFUE
- —
- Min input
- 24.6 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Bosch Greenstar KBR 21-3A in practice
The KBR 21-3A is Bosch Canada's floor-standing heating-only model of the BR kW class, part of the Greenstar FS line the 11.2025 I&O manual documents alongside the KWB combis. It fires 24.6 to 79.2 MBH natural-gas input, delivers no domestic hot water itself and stands in the FS cabinet shared by the whole floor family: 21-15/16 x 41-1/4 x 21-7/8 in (557 x 1048 x 555 mm) at 150 lb (68 kg).
Bosch's Canadian pages market the Greenstar family at 95% AFUE and describe floor-standing models as the option that makes boiler replacements quick and easy. The FS manual's technical tables rate max supply temperature at 190 °F (88 °C), 30 psi heating pressure and a sound level of 33 dB(A), with input/output rows printed instead of an AFUE figure.
Installation follows CAN/CSA B149.1 in Canada, as the manual states, and the floor models use the same two-character Heatronic fault-code family as the wall line (Table 40 'Faults shown on the display' in the FS manual). Codes and vent runs are checked against this FS manual, whose vent tables group the 21/28 and 35/42 classes separately.
Best suited to
- Replacements where a floor-standing boiler drops onto an existing footprint without re-hanging a wall unit
- Basements and mechanical rooms that suit a 557 x 1048 x 555 mm floor cabinet
- Heating-only systems paired with an indirect domestic hot water tank
Applicability and support
- Model number
- KBR 21-3A
- 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 | 79.2 MBH | 79,200 BTU/hr at 180/79 °F, natural gas; altitude tables reduce the maximum above 2,000 ft. |
| Output | ||
| Output rate, maximum (104/86 °F) | 74.7 MBH | 74,700 BTU/hr at 104/86 °F water temperatures per the manual technical table. |
| 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) | 150 lb | 150 lb (68 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 | ≤ 33 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 KBR 21-3A a combi boiler?
No. KBR codes are the floor-standing heating-only models; domestic hot water comes from an indirect tank or separate appliance. The floor-standing combis are the KWB models ('Combi FS').
How heavy is this floor-standing model?
150 lb (68 kg) without packaging, per the FS manual technical tables. The KWB combi versions of the same cabinets weigh 164 lb.
How is this model different from the wall-hung Greenstar of the same class?
It is floor-standing, 150 lb, rated 190 °F max supply in the FS manual, and its vent tables belong to the FS manual. The heating-side input/output figures are essentially the same family figures as the wall models'.
Questions about the Bosch Greenstar KBR 21-3A?
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 KBR 21-3A vs KBR 28-3AComparison
- Bosch Greenstar KBR 28-3ASame series
- Bosch Greenstar KBR 35-3ASame series
- Bosch Greenstar KBR 42-3ASame series
- Bosch Greenstar KWB 28-3ASame series
- Bosch Greenstar KWB 35-3ASame series
- Bosch Greenstar KWB 42-3ASame 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.