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  • HIGH-MASS VS. LOW-MASS BOILERS – The Arguments

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

    New gas-fired appliances in particular have a technological efficiency milepost that must be considered. Specifically, the traditional tube/ribbon gas burner “dry base” boilers vs. the new condensing technology based low-mass boilers. They have a significant energy 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 are the most popular and have been doing reasonably well service-wise. Cast aluminum heat exchanger units on the other hand where supply water pH (acidity) has not been addressed at installation or in operation 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 condensing gas systems. They certainly do flag service and maintenance issues with their sophisticated sensor complements!

    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 even “out-of-the-box” or within weeks of installation. An installed system operating life of 15 years or less is being reported in trade journal articles. The British Market Experience is much deeper and lengthier than ours and their forums are full of user complaints of premature failures and high maintenance costs. Reported system economic lives are commonly less than 10 and up to 15 years on “upscale” products. Cast-iron boilers have been effectively regulated out of the EU market. Do we have a similar lesson coming?

    The common cause of failure for all condensing boilers 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 energy containment and management 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 the norm for the past two hundred years!

    Let us first disqualify any “dry base” welded-steel-fabrication (oil or gas) boilers from this discussion. Their poorer 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 now barely meet D.O.E. Energy Star Ratings as well, and we hope for not 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 by us, our trade service collaborators and 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! Always get customer referrals at a minimum (including for ourselves)!

    The “wet base” full-combustion-containment cast-iron oil boiler is the industry standard, with a solid performance history. They do however differ by both manufacturer and specific model design attributes that may affect life performance. If your oil boiler does not last at a very 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, material selection, application integrity and its performance under field conditions. The design safety factor of any cast iron boiler can be readily extended by simply providing more iron and more water capacity. “The more it weighs, the longer it stays” – our poor poetry. Works every time, but potentially adds cost to the boiler — as it should.

    We must now extend the preceding arguments to contemporary oil & gas appliance issues. There are three emerging problems well 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 will also negate this issue altogether.
    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 in particular circulation efficiency. It has virtually never been an issue in a cast-iron (a naturally magnetite scavenging material) boiler, but here it is with very low iron content condensing systems. It can only be addressed with an integral, full-time Magnetite Filtration System – a significant initial cost and recurring maintenance additive.
    3. HYDRONIC DISTRIBUTION. Contemporary practices of near-boiler and distribution (to radiation) piping are excessive, unwarranted and inefficient. (We refer to this as “The Plumber’s Playground”.) The seemingly excessive piping and controls witnessed in new installations are so, REDUCING system efficiency while increasing distribution power demands. Low-mass and condensing boiler systems are disproportionately affected. Again, correct application of Delta-T ECM Hydronic Distribution Technology is a must for overall system performance.

    If you’ve read from our “Heating Blog Library” on this site, you would certainly qualify us as “High-Mass Boiler” advocates. We have additionally optimized our system design and performance by fully incorporating “Delta-T ECM Hydronic Distribution Technology”. It smooths hydronic system thermal demands while significantly reducing operating costs. We slogan this as: “Putting an ‘Automatic Transmission’ on a Boiler”™ — to use an automotive analogy. In fairness this technology would certainly be very beneficial when applied to any low-mass boiler! In fact we consider it a prerequisite in any low-mass boiler installation. Please reference our new Packaged ΔT ECM Hydronic (FHW) Heating Appliance™ (Patents Pending – USA & Canada) on this site, interchangeably fired by OIL or GAS.

    Considering the aforementioned, we much prefer high-mass boilers with their “thermal damping” and reduced cycling characteristics. More iron mass and greater water content equals less burner cycling and ultimately longer component life. We have noted in fact (although we don’t necessarily advocate it) that ultimately a boiler 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 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”.)

    TRY THIS: Divide the Cost of a Boiler by its Shipping Weight to determine its approximate the Cost per Pound. The Weil-McLain UO is 1/2 to 2/3rds the Cost-Per-Pound of ANY equivalent U.S. or Foreign Boiler! Just what are you buying – durability or ….. ???

    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.

    Additionally, a very timely read is our commentary on the just published 2019 Annual Boiler Report. “Reading between the lines” we extract the admonition by several manufacturers that despite their pursuit of highly engineered condensing product offerings, a well configured cast-iron boiler system is still economically favorable. It appears, as we advocate, that a proven high-mass, triple-pass cast-iron boiler system fully integrating Delta-T ECM Distribution Technology prevails. The resultant is “SIMPLE, DURABLE, EFFICIENT HYDRONIC (FHW) HEATING ….. PERIOD!  Judge for yourself …..

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

    Updated: 04/03/2020 PDM, Sr.

     


  • THE WEIL-MCLAIN ULTRA OIL BOILER — “IT’S A HEMI!” (™ – CHRYSLER)

    While recently taking delivery of a large Weil-McLain Ultra Oil Boiler, our distributor’s driver commented: “I hate to deliver these boilers, they are so (mild expletive) heavy compared to the Buderus and the other ones.” (This large distributor markets several popular product lines.) The driver’s comment became evident when three of us were needed to tip up the crated boiler dolly in the truck, then four to move it into the building. (The driver normally handles it himself in the truck and then another or two assist him to transit into location.)

    This experience prompted us into a little research that confirmed the driver’s observation. Yes, the Ultra Oil “high-mass” is much heavier than its “low-mass” competitors — by 100 to 250 lbs at minimum for the equivalent firing rate (capacity). As Heating Engineers we also appreciate the design integrity of this new, FEA (“Finite Element Analysis”) designed boiler “block”. It is a “beautiful beast” in a very complimentary fashion, both in performance and serviceability.

    As a lifetime “motor head” (car performance enthusiast) the analogy came immediately. “It’s a Hemi!” The original Chrysler Hemi(spherical) Engines of the 1950’s and beyond dominated Drag Racing and other racing forms. Originally designed as an Industrial & Marine Engine, it was big, heavy, durable and ultimately very powerful. A modified 180HP automobile motor readily became a 1500+HP racing motor.

    So what does this have to do with boilers? Boilers are heat engines in the purest sense of the term. They ignite and burn fuels to efficiently generate hot water (or steam) to warm our living environment. In an automobile heat not converted into mechanical power is wasted. The boiler meanwhile is all heat generation and tempered distribution.

    Harnessing heat energy also entails risk. (Boilers are necessarily constructed of “sections” with seals between them. Picture a loaf of bread made up of “slices” with “crusts” at each end.) Controls manage this process but may not ultimately protect the boiler from damage under some circumstances, namely:

    1. Water contains minerals and contaminants that generate an acidic sludge in the bottom of boilers (and piping). (Open your boiler drain valve to confirm, if you dare.) Sludge corrodes, eating cast iron and in particular steel plate boiler weldments, reducing boiler life and heat exchange efficiency. Doesn’t help the seals either.
    2. A damaged combustion chamber may permit direct firing onto heat exchanger surfaces creating “hot spots” that create thermal shocking and failure conditions.
    3. Introducing cold water into a hot boiler thermally shocks it. It can occur even during normal operation when large, cold-water heating zones are suddenly demanded into a hot boiler. The metallurgy of the heat exchanger changes, embrittling it to ultimate failure by cracking, warping the section(s) and causing internal seal failures. These situations can be catastrophic.
    4. Using anti-freeze compounds. Many manufacturers, particularly of older boilers, prohibited their use. Newer seal materials have limited the risk. However, check your Warranties! Even using the recommended anti-freeze increases leakage potential in the system. (Servicemen generally dislike its use.)

    Summarizing: Cast Iron and Water Volume = Increased Boiler Life and Performance (Simple Physics)

    The boiler operates at a lower average temperature and accommodates load and surge conditions more readily with no efficiency penalties.

    May we suggest that a high-mass boiler is also a better value?