HIGH-MASS VS. LOW-MASS BOILERS – THE ARGUMENTS

Updated: 10/01/2018 PDM, Sr.

It comes down to economics, as do most things. The current trend is to market low-mass (lighter weight) boilers against the traditional heavier, high-mass cast-iron boilers. However, there are risks involved that must be assessed and quantified.

New gas-fired appliances have a technological efficiency milepost that must be considered, specifically the traditional tube/ribbon burner “dry base” boilers vs. the new condensing technology low-mass boilers. They have a significant efficiency increase of 10+% in AFUE Efficiency Rating. Along with this however is a marked increase in appliance costs and operational risks — both quite important, yet never publicized!

Stainless steel heat exchanger condensing gas units have been generally doing well service-wise. Cast aluminum heat exchanger units where supply water pH (acidity) has not been addressed at installation and at further maintenance points have had issues, to which we can personally attest. Given this reservation they have also performed reasonably well. Being quite sophisticated control-wise, one has to admire the engineered system integrity of these condensing gas systems. They certainly do flag service and maintenance issues with their sophisticated sensor systems!

However a disturbing pattern is emerging and the alarms are sounding in the trade journals and forums. Premature welded stainless heat exchanger leakage failures are being reported, some “out-of-the-box” or within weeks of installation. A reduced anticipated installed system operating life of 15 years or less is also reported. The common cause for all should be water condition issues, but it’s also become manufacturing process integrity in some cases. Regardless, manufacturers are countering with stringent water condition qualification requirements within their typically 10-Year Pro-Rated Warranties. Not only do water conditions have to be measured and recorded at installation, but subsequent annual service procedures must re-qualify and record these values. Violate your water control guidelines and void your warranties!

Oil-fired appliances on the other hand are a different animal, with combustion containment and management issues paramount in their design. Heating oil has over 60% more energy content per gallon than gases and can’t be technologically “modulated” (vary the firing rate) as with a gas appliance. Adjusting the output of an oil boiler is done by managing system temperature and/or by utilizing multiple boilers (MBS Systems) in larger installations. Therefore the oil boiler must be designed to perform at its maximum firing rate and within its design parameters. This is typically accomplished by using a cast-iron heat exchanger, and has been for the last two hundred years!

Let us first disqualify any “dry base” welded-steel-fabrication oil boiler from this discussion. Their 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! (They barely meet D.O.E. Energy Star Ratings now as well, and we hope for not too much longer.) Particularly disturbing to us is the poor field history of a nameless, highly publicized “high efficiency” gas-or-oil welded plate construction boiler. Their factory service record as emoted both directly to us and through our trade service collaborators by their customers is damnable! Independent servicemen hate to touch them. Parts are product-specific, pricey and their eventual structural fate is predictable. We regard them as a “black eye” upon our industry. Beware of the sales pitch and do your homework! Get customer referrals at a minimum (including for ourselves).

The “wet base” fully internal combustion-containment cast iron oil boiler is the industry standard, with a solid performance history. They differ however by both manufacturer and specific model design attributes that may affect life performance. If your oil boiler does not last at a minimum 30 years or more, something is very wrong. We have replaced 100+ year old cast iron boilers with still beautiful castings, but you just couldn’t afford to feed them! You are more likely to see a cast-iron boiler section seal corrosion failure in particularly older units or hard-freeze icing fracture neglect than a “worn-out” boiler. Either situation is economically catastrophic though, requiring unit replacement.

The high-mass vs. low-mass oil boiler argument ultimately comes down to design parameters related to cast-iron material selection, application integrity and its performance under field conditions. 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 it’s polymerization in plastics, embrittlement in metals, et al to a future point of failure.

We must now extend the preceding arguments to contemporary oil & gas appliance issues. There are three emerging problems documented in Heating Trade Publications that must be addressed:

  1. “COLD-SHOTTING”. A trade term that describes the condition where cold water is introduced into a hot boiler and thermally “shocks” the heat exchanger metallurgy. Single or cumulative events can result in catastrophic metal fracturing, seal and/or weld joinery failures. The “solution” is the additional piping and installation of a Boiler Return Water Temperature Control Valve to any condensing or low-mass boiler system. This is not an issue with a high-mass cast-iron boiler, nor has it ever been. Please note that Delta-T ECM Hydronic Distribution Technology, properly applied negates this issue altogether. (See further.)
  2. MAGNETITE ALLEVIATION. Magnetite is a magnetic, accumulating ferrous (iron) oxide (rust) present in boiler and supply water and attaches itself to internal boiler components, reducing particularly circulation efficiency. It has virtually never been an issue in a cast-iron (naturally magnetite scavenging material) boiler, but here it is in very low iron condensing systems. It can only be addressed with an integral, full-time Magnetite Filtration System – a significant cost and maintenance cost adder.
  3. HYDRONIC DISTRIBUTION. Contemporary practices of near-boiler and distribution (to radiation) piping are excessive, unwarranted and inefficient. The seemingly excessive piping and controls witnessed in new installations are so, REDUCING system efficiency and increasing operating costs. Low-mass and condensing boiler systems are disproportionately affected. Correctly applied Delta-T ECM Hydronic Distribution Technology is a must for overall system efficiency. (See further.)

If you’ve also read from our “Heating Blog Library” you would certainly qualify us as “High-Mass Boiler” advocates. We have additionally optimized our system performance by fully incorporating “Delta-T ECM Hydronic Distribution Technology” into our designs. They smooth hydronic system thermal demands while significantly reducing operating costs. In fairness this technology would certainly be very beneficial when applied to any low-mass boiler! In fact we consider it a prerequisite in a low-mass boiler installation. Please refer to our new Packaged ΔT ECM Hydronic (FHW) Heating Appliance™ (Patents Pending – USA & Canada) on this site.

Considering the aforementioned, we much prefer high-mass boilers with their “thermal damping” and reduced cycling characteristics. More iron mass and water capacity 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 dictated by the quality and amount of fuel oil passed through our Weil-McLain Ultra Series Triple-Pass Boilers than calendar cycles. Their heat exchanger passages are very open compared to prior generation two-pass units. 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”.)

Summarizing, weigh the operating characteristics of your particular hydronic system application before you select any boiler. In particular look at an intelligent hydronic distribution option such as the Taco® Delta-T ECM Distribution System. Just “plugging and playing” a low-mass boiler into your system may not play too long, nor too well.

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