Multi(ple)-Boiler Systems (MBS) are the mainstay of larger commercial installations, managed by sophisticated controls and automation software. Less recognized is their potential in upscale residential, light commercial, high-risk, high-occupancy and “special demand” applications. MBS’s are the exceptions in practice however, typically constricted by system design and installation costs vs. the single, larger boiler alternative. (We have included below a photo and detail of a prior duplex boiler application utilizing the Weil-McLain Gold Series for reference.)

In the process of developing our Neo-Gravity Hydronic (FHW) Heating Appliance™ (U.S. Patent 10,690,356 & Canada Filing) it became very evident that our “boiler-of-choice”, the Weil-McLain Ultra Oil Series unit is in fact now a commercial grade boiler in a residential “wrapper”. A higher efficiency “three-pass”, very high-mass design, it outweighs its “Gold Series” predecessor significantly and all of its competitors by at minimum a third to near twice. Its robustness would indicate a service life of at minimum thirty years, given reasonable water quality maintenance. It is truly “The Heavyweight Champion” of residential boilers. (Reference our depiction of a Triplex Ultra Oil (UO) Series configuration and detail following below.)

The automatic backup feature of multi-boiler systems makes them particularly attractive for emergency services facilities, low or high occupancy rate, multi-unit structures. Examples are larger apartment houses, condominiums, upscale residential housing, municipal and private service facilities and high value content structures.

Example: Removal of combustion heating appliances (and hence fire risk) from motored vehicle equipment bays is particularly attractive to insurance underwriters. These conditions can warrant the moderately higher system cost of multiple boilers with significant offsets of reduced fuel consumption and risk of loss minimization.

The following attribute table is provided to assist in system applicability:

System TypeSystem ConfigurationSystem Control(s)System BackupSystem EfficiencySystem Cost
Single BoilerSingle Supply & ReturnSingle Boiler AquastatNot availableNormalMarket
Multi-Boiler (MBS)Multi-Boiler Loops to System Loop(s)Multiple Boiler Aquastats to System Manager (BCP-3)AutomaticSignificantly HigherAbove Market


  1. Multi-Boiler Systems must be individually specified and quoted due to the number of variables. Three Weil-McLain UO Series boiler size increments can be duplexed or triplexed to match application requirements.
  2. Boiler loops, hydronic interchange, distribution manifolds and logs are typically pre-built and hard-piped to suit site requirements. Piping sizes will vary with application demands.
  3. “Delta-T ECM Hydronic Distribution” is not applicable within individual or multi-boiler loops, but the recently introduced Taco® 007E ECM Circulator is a natural, cost-effective upgrade for these. (Refer to our “Handy Header™” based Delta-T ECM Distributors on this site.) NOTE: The Taco 007E ECM Circulator is now being supplied as standard equipment on all new Weil-McLain UO Series Boiler shipments!
  4. Our Delta-T ECM Hydronic Distributors (Patent Pending)  are specifically designed to be branched off any system distribution header(s).
  5. MBS Systems are typically augmented with a Site Temperature Management System to compliment their performance attributes.
  6. We do not currently have sufficient field data to predict energy savings performance from other than Taco at this writing.

MBS System Examples:

Our duplexed, two-pass heat-only 400kbtuh Multi-Boiler System with ODS (Outdoor Temperature Sensing) serving four existing Air Handlers. Application: Larger Public Library, less than 20% Occupancy Rate, featuring multi-staged temperature setbacks and Honeywell RedLINK managed.

Just completing its third full heating season. Temperature programming management is being finalized. Client advises that fuel savings thus far are “very substantial”. Will publish when requested records become available to us.

Duplex Weil-McLain WGO-6 (Oil) System, as installed.

A Duplex Three-pass, Heat-only 300kbtuh Multi-Boiler System, Delta-T serving two Down-flow Unit Heaters and a conventional Air Handler. Application: Rural Municipal Fire Station, unmanned w/ Generator-backup.

This is currently in a proposal form only.

 Depiction of a Triplex Weil-McLain UO (Oil) System

We are now offering duplexed Weil-McLain UO-3’s to triplexed UO-5′s giving us an effective system output range between approx. 200 to 500kbtu/h, adequately filling upscale residential and light commercial applications. They are partially pre-built, modular systems featuring all iron and cast construction for durability and service life, based on the Weil-McLain Easy-Fit® Primary-Secondary Piping System. A Weil-McLain BCP-3 Boiler Control Panel with ODS (Outdoor Temperature Sensor) is utilized for simple and effective system management. It offers significant boiler staging and exercising (rotation) efficiency advantages over a larger, single boiler while providing automatic system backup upon any unit failure.

The Weil-McLain UO Series boilers require only front and limited rear service access as installed, permitting very efficient hydronic coupling and space utilization. A duplex set is approx. 50″ Wide x 60″ Deep while a triplex is approx. 75″ Wide x 60″ Deep. The overhead system distribution header is front-to-rear and optionally located at the left or right of boiler loops, further compacting system piping.

Total system efficiency is further enhanced by utilizing the new Delta-T ECM (Electronically Commutated Motor) Hydronic Distribution Technology provided by Taco 00e Series Circulators and Zone Valving.

Now INTERCHANGEABLY-FIRED by Oil (#2 Fuel) or Gas (NG or LP) Burners. Swap the burner(s) and not the boilers! How about individual boiler firings by fuel types? Think about it.

Let’s have a look at your specific requirement.

Updated 12/04/2022 – P.D.M., Sr.