Weil-McLain Ultra 155
The Ultra 155 is the first Series 4 Ultra to require 3" venting: 155 MBH input, 31 MBH low fire, 139 MBH DOE heating capacity and 94.0% AFUE from the cast aluminum condensing block.
Key figures
- Max input
- 155 MBH
- Max CSA output
- 139 MBH
- AFUE
- 94 %
- Min input
- 31 MBH
Input is gas energy consumed; output is heat delivered. Never size from input alone.
Weil-McLain Ultra 155 in practice
The Ultra 155 modulates from a 31 MBH low fire to 155 MBH maximum CSA input, producing 139 MBH of DOE heating capacity and a 121 MBH net AHRI rating at 94.0% AFUE. It holds 1.17 gallons of boiler water and ships at 234 lb, with supply and return tappings still at 1" NPT.
Venting is where the 155 changes the plan: it is the first Ultra size that cannot use 2" pipe — the manual allows 3" only, to 100 ft. With 5:1 modulation, no altitude change up to 5,500 ft and heating-only operation, it pairs with an indirect tank for domestic hot water.
Best suited to
- Homes whose heat-loss calculation reaches the 120-155 MBH band
- Projects committed to a 3" vent route from the outset
- Larger hydronic systems that want Ultra control and the aluminum block
Applicability and support
- Model number
- ULTRA-155
- 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 | 155 MBH | CSA input (maximum). |
| Input, minimum | 31 MBH | Low fire (31 MBH) per the Series 4 brochure's CSA input minimum column. |
| Output | ||
| DOE heating capacity | 139 MBH | DOE heating capacity at maximum input; the manuals also call this CSA Output. |
| Net AHRI water rating | 121 MBH | Net AHRI rating per the boiler manual ratings table. |
| Efficiency | ||
| Efficiency (AFUE) | 94 % | 94.0% AFUE. |
| Domestic hot water | ||
| Domestic hot water | Heating-only boiler; domestic hot water via an indirect tank or separate appliance | |
| Venting and air | ||
| Venting | 3" PVC, CPVC, polypropylene or stainless steel (Category IV); 3" pipe only on the 155 | 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 3" | |
| Physical and connections | ||
| Weight | 234 lb | Approx. shipping weight per the brochure. |
| Gas connection | 1/2" NPT | |
| Heating water connections | 1" NPT | Supply and return tappings 1"; minimum recommended pipe 1-1/4". |
| Operating limits and other | ||
| Turndown | 5:1 | 5:1 modulation (20-100% firing rate) per the brochure and CA page. |
| Boiler water content | 1.17 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
Why does the Ultra 155 need 3" venting?
The Series 4 manual's maximum-length table allows 2" only on the Ultra 80 and 105; the 155 is 3" pipe only, to 100 ft.
What are the Ultra 155's ratings?
155 MBH input, 139 MBH DOE heating capacity, 121 MBH net AHRI and 94.0% AFUE.
What is the Ultra 155's low fire?
31 MBH per the brochure's CSA input minimum column, which keeps it modulating at low part-load conditions.
Questions about the Weil-McLain Ultra 155?
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
- Weil-McLain Ultra 105 vs Ultra 155Comparison
- Weil-McLain Ultra 155 vs Ultra 230Comparison
- Bosch Greenstar ZBR 42-3A vs Weil-McLain Ultra 155Comparison
- Weil-McLain Ultra 80Same series
- Weil-McLain Ultra 105Same series
- Weil-McLain Ultra 230Same series
- Weil-McLain Ultra 299Same series
- Weil-McLain Ultra 310Same series
- Weil-McLain Ultra 399Same series
- Ultra series hub
- Buying guide
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.