It ultimately comes down to economics. There is a current effort to market low-mass (lighter weight) boilers from a costing perspective against the traditional heavier, high-mass boilers. After all, less material is less cost. However, there are risks involved that must be assessed and quantified.

These arguments apply specifically to both oil-fired boilers and gas-fired units.

(Please refer to our other blogs and appropriate external sources for detail related to this discussion.)

In gas-fired appliances there is a technological efficiency milepost that must be considered, specifically the traditional tube/ribbon burner “dry base” boilers vs. the new condensing technology low-mass burners. The difference is a marked difference in efficiency, amounting to a 10 to 15% AFUE Rating differential. Along with this is a marked difference in appliance cost — quite significant indeed.

The condensing gas units are doing quite well maintenance-wise, overall. There have been issues with aluminum combustion chambered units where supply water pH (acidity) has not been addressed at installation and at further maintenance points. Given this reservation they perform well, very well. The stainless steel chambered units seem to be doing well thus far also. Being quite sophisticated control-wise, one has to presume the engineered integrity of these condensing gas systems. They certainly flag maintenance issues with sensors!

Oil-fired boilers on the other hand are a different animal with combustion containment and management issues paramount in their design. Oil has over 60% more energy per gallon and can’t be “modulated” (varying the firing rate) as with gas within the appliance. Adjusting the output of the oil boiler is done by managing system temperature or by utilizing multiple boilers (MBS Systems) in larger installations. Therefore the oil boiler must be designed to perform at its maximum firing rate while utilizing its control system parameters.

Let us first disqualify the “dry base” steel fabrication oil boiler from this discussion. Its overall field performance history in both thermal efficiency and longevity are well documented and recognized. They are sold on price alone. If you have one, you won’t have it for long — unless you were born under a lucky star.

The “wet base” full combustion-containment cast iron oil boiler is the industry standard with a solid performance history. (Steamers are somewhat excepted from this discussion. They just “live differently” — see our other blogs.) There are differing performance histories by both manufacturer and specific models however. This is important in that it emphasizes the need for design integrity. If your FHW Oil Boiler does not last at least 20-25 years, something is very wrong. We have replaced 100+ year old cast iron boilers with still beautiful castings, but you couldn’t afford to feed them!

The high-mass vs. low-mass oil boiler argument ultimately comes down to design parameters related to cast iron material selection and application integrity, and its performance under field conditions.  Much like an aircraft designer and subsequently the operator (pilot) you must “fly within the envelope” of your craft. Go outside of it and you’re in trouble. With the boiler you have to assure that your installation does not violate the design rules inherent to your unit.

The design safety factor of a boiler can be readily extended by simply providing more cast iron and more water capacity. Works every time, but potentially adds cost to the boiler — or it should. All materials age in use, whether its polymerization in plastics, embrittlement in metals, et al to a future point of failure. With cast iron it is embrittlement from repetitive heating cycles and in particular “thermal shocking” under extreme field conditions. Certainly alloy selection and design integrity can help, but the end is the same.

If you’ve read our blogs you would certainly qualify us as “High-Mass Boiler” advocates. Along with this we have attempted to minimize our field issues by advocating “Delta-T Circulator Systems” in future designs. They smooth hydronic system thermal demands well while significantly reducing operational costs. In fairness this technology would certainly be very beneficial applied to a low-mass boiler! In fact we would refer to it as a prerequisite in a low-mass boiler installation. Have we “shot ourselves in the foot”, so to speak?

Despite the aforementioned, we still prefer the high-mass boilers due to their “thermal damping” and lower cycling characteristics. More iron and water equals less burner cycling and ultimately longer component life. We have noted in fact (although we don’t advocate it) that ultimately the maintenance cycle is now more evidenced by the quality and amount of fuel oil passed through our Weil-McLain Ultra Series Triple-pass Boilers. Their heat exchanger passages are very open compared to any two-pass unit. Combined with the combustion quality of the Beckett NX Burner you generate very little ash and accumulation. The same cannot be said of a nameless foreign boiler that seems to have high ash generation and more frequent maintenance calls. (We refuse to service them, by the way — hire the mechanic with that “Mercedes”.)

Was that a “Sales Pitch”? Sorry.

Summarizing, weigh the operating characteristics of your particular hydronic system application before you select any boiler. In particular look at an intelligent distribution option such as the “Delta-T” System. Just “plugging and playing” a low-mass boiler into your system may not play too long, or too well.

Last Edit: 10/10/2012 pdm