Weil-McLain Ultra 399
The Ultra 399 is the largest Series 4 Ultra: 399 MBH input from an 80 MBH low fire, with the US table's asterisked 91.7% combustion efficiency and 364 MBH gross output — the manual's footnote says those figures are not an AFUE.
Key figures
- Max input
- 399 MBH
- Max CSA output
- 364 MBH
- AFUE
- —
- Min input
- 80 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Weil-McLain Ultra 399 in practice
The Ultra 399 tops the Series 4 line at 399 MBH maximum CSA input from an 80 MBH low fire, delivering a 317 MBH net AHRI rating. Its US spec table and brochure print 91.7% and 364 MBH with asterisks and the footnote "Combustion efficiency and gross output (not an AFUE)", so the 399 carries no DOE AFUE figure of its own — the CA page's "up to 94.1%" applies to the 230, not the 399.
The 399 shares the large Ultra cabinet with the 299: 4" venting only to 100 ft, 1-1/4" NPT supply and return tappings, a 3/4" NPT gas connection, 2.1 gallons of water content and a 297 lb shipping weight. It is heating-only and floor- or wall-mountable, with no altitude modification to 5,500 ft.
Best suited to
- The largest residential loads and light-commercial plants near 400 MBH
- Cascade Ultra systems anchored by 399 MBH units
- Projects sized for 4" venting and 1-1/4" piping
Applicability and support
- Model number
- ULTRA-399
- Canadian availability
- available
- Production status
- current
- ROMA supplies
- Confirmed at consultation
- ROMA services
- Confirmed at consultation
- Warranty
- Series terms
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 Weil-McLain. Units are shown with every value; notes flag conflicting sources or missing figures.
| Specification | Value | Note |
|---|---|---|
| Input | ||
| Input, maximum | 399 MBH | CSA input (maximum). |
| Input, minimum | 80 MBH | Low fire (80 MBH) per the Series 4 brochure's CSA input minimum column. |
| Output | ||
| DOE heating capacity | 364 MBH | Asterisked gross output per the US table and brochure — not a DOE heating capacity figure. |
| Net AHRI water rating | 317 MBH | Net AHRI rating per the boiler manual ratings table. |
| Efficiency | ||
| Combustion efficiency | 91.7 % | Asterisked 91.7% combustion efficiency per the US table and brochure — NOT an AFUE; the CA page lists "up to 94.1%" which is the 230's rating, not this model's. |
| Domestic hot water | ||
| Domestic hot water | Heating-only boiler; domestic hot water via an indirect tank or separate appliance | |
| Venting and air | ||
| Venting | 4" PVC, CPVC, polypropylene or stainless steel (Category IV); 4" pipe only on the 399 | Canada: install per CAN/CSA B149.1 or B149.2 with a ULC S636-compliant vent kit. |
| Max. equivalent vent/air length | 100 ft in 4" | |
| Physical and connections | ||
| Weight | 297 lb | Approx. shipping weight per the brochure. |
| Gas connection | 3/4" NPT | |
| Heating water connections | 1-1/4" NPT | Supply and return tappings 1-1/4"; minimum recommended pipe 1-1/2". |
| Operating limits and other | ||
| Turndown | 5:1 | 5:1 modulation (20-100% firing rate) per the brochure and CA page. |
| Boiler water content | 2.1 gal | |
| Altitude rating | 5,500 ft | No modifications from sea level to 5,500 ft; the control has a high-altitude selection above 5,500 ft. |
| Fuel | Natural gas or propane | Propane requires a conversion kit per the CA product page. |
Manuals
- Ultra Series 4 Boiler Manual, 550-100-440/1025550-100-440/1025 · 2025-10
Alerts documented for this model
- TEMP RISE TOO QUICKLY
- SUPPLY 58F > RETURN
- RETURN > SUPPLY
- TEMPERATURE SENSOR
- FLUE TEMP TOO HIGH
- TEMPERATURE SENSOR
- LOW WTR CUTOFF OPEN
- DISPLAY COMM
- LIMIT OPEN
- HIGH TEMP LIMIT
- FLAME FAULT
- BLOWER FAULT
- IGNITION FAULT
- GAS VALVE FAULT
- AIR PRESSURE SW FAULT
- GAS PRESSURE SW FAULT
- HW FAULT EXPANSION BRD
- GV FAULT EXPANSION BRD
- EXP BOARD COM FAULT
Frequently asked questions
Is the Ultra 399 rated 91.7% AFUE?
No. The US Series 4 table asterisks the 399's 91.7% figure as combustion efficiency with 364 MBH gross output — not an AFUE. Series 4 AFUE ratings run 93.5% to 94.1% on the 80 through 230.
What are the Ultra 399's ratings?
399 MBH input, 317 MBH net AHRI, 364 MBH gross output and 91.7% combustion efficiency per the asterisked US Series 4 table.
What venting does the Ultra 399 need?
4" pipe only, to 100 ft, with 1-1/4" NPT supply and return tappings and a 3/4" NPT gas connection.
Questions about the Weil-McLain Ultra 399?
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
- Weil-McLain Canada — Ultra · accessed September 7, 2026
- Ultra Series 4 Boiler Manual 550-100-440/1025 · 550-100-440/1025 · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.