Canada-wide gas boiler information · On-site service in Greater Vancouver, BC
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Comparison
Verdict in brief
The Ultra 155 is where the Series 4 Ultra stops offering 2" venting: input rises from 105 to 155 MBH at the same 94.0% AFUE, but venting becomes 3"-only to 100 ft, water content grows to 1.17 gallons and weight to 234 lb.
Published manufacturer figures. Input and output are kept separate; blanks mean the manufacturer did not publish that value.
| Specification | Weil-McLain Ultra 105 | Weil-McLain Ultra 155 |
|---|---|---|
| Input, maximum | 105 MBH | 155 MBH |
| Input, minimum | 21 MBH | 31 MBH |
| DOE heating capacity | 94 MBH | 139 MBH |
| Net AHRI water rating | 82 MBH | 121 MBH |
| Efficiency (AFUE) | 94 % | 94 % |
| Weight | 207 lb | 234 lb |
| Boiler water content | 0.82 gal | 1.17 gal |
| Venting | 2" or 3" PVC, CPVC, polypropylene or stainless steel (Category IV) | 3" PVC, CPVC, polypropylene or stainless steel (Category IV); 3" pipe only on the 155 |
Both Ultras are 5:1-modulating, 94.0% AFUE Series 4 boilers with 1" NPT supply and return tappings and 1/2" NPT gas. The 155 steps input from 105 to 155 MBH, DOE heating capacity from 94 to 139 MBH and the net AHRI rating from 82 to 121 MBH, with low fire climbing from 21 to 31 MBH.
The practical change is venting: the 105 runs 2" or 3" to 100 ft, while the Series 4 manual allows only 3" pipe on the 155. A mechanical room planned around a 2" route therefore drives the choice down to the 105 unless the route can be reworked.
Water content and weight also step up — 1.17 gallons and 234 lb against 0.82 gallons and 207 lb — while the altitude allowance stays at 5,500 ft. Size from the heat-loss calculation first, then confirm the vent route fits.
Weil-McLain
The Ultra 105 adds 25 MBH of input over the 80: 105 MBH maximum, 21 MBH low fire, 94 MBH DOE heating capacity and 94.0% AFUE from the same nano-coated cast aluminum Series 4 platform.
Weil-McLain
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.
No. The Series 4 manual allows 2" only on the Ultra 80 and 105; the 155 is 3" pipe only, to 100 ft.
It ships at 234 lb against 207 lb and holds 1.17 gallons against 0.82 gallons; the low fire rises from 21 to 31 MBH.
Yes — the Ultra 105 and 155 are both rated 94.0% AFUE at 5:1 modulation.
A consultation includes a heat-loss review so the size is right the first time.
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.