The popularity and performance of condensing gas technology hydronic (FHW) boilers is both noteworthy and deserved. Kicking up heating gas fuel (Natural & LP) efficiencies from the prior generation average of 80-85% to 90-97% in one technological step is astounding. However like any new technology it has come at a price, both positively and negatively. Problem is the negatives are not discussed with the sales enthusiasm.
It doesn’t take a very sharp pencil to justify a condensing boiler upgrade from a prior generation unit, and particularly a much older one. That 10% or (much) more is significant itself, but coupled with an indirect water heater operating as a “cold start” system can yield 40% or more in our experience. So where are the issues?
Hydronic boilers have traditionally been constructed of cast iron or of welded steel plate as a lower cost alternative. The welded-plate alternative has failed historically in both durability and efficiency. We have noted recently a disproportionate number of steel boilers appearing locally in “upscale” newer homes. After all, contractors have to cut costs somewhere!
In the gas market we are moving to new materials to compliment both the cleaner and more controllable combustion afforded by Natural Gas and LP (Propane). These boilers can “modulate” (adjust) their firing rate up to 80% to accommodate heating demand, much like pressing on the gas pedal of your automobile from an idle to adjust power and speed. This is accomplished using a sophisticated sensor and control system. So now we can utilize materials that are more favorable, specifically aluminum and stainless steel, but for very differing attributes.
Referring to a Thermal Conductivity Chart you can appreciate why copper is so commonly used as a heat-exchanger material in baseboard with thin aluminum fins to compliment. Cast iron is so-so but stainless steel is very poor. So why use these particular materials?
Cast iron is typically used in larger, heavy sections with a generous amount of water as a “thermal mass” device to manage both combustion and energy distribution. (Refer to our Blog: “HIGH-MASS VS. LOW-MASS BOILERS – THE ARGUMENTS” for more detail.)
Aluminum would also seem to be very desirable in this regard excepting that it is very susceptible to chemical corrosion and must be alloyed and/or chemically surface treated for protection.
Stainless steel on the other hand is a very poor heat conductor, but with very good corrosion resistance. Designers must therefore carefully define the stainless steel heat-exchanger to attain performance while utilizing a substantially more expensive material. With much poorer thermal conductivity, material thickness and heat transfer surface area become prime design parameters. However, coupling this with the necessity of welding stainless steel components together for structural and process integrity and you have a metallurgical compromise.
Corrosion is the common denominator in all heat-exchanger materials, caused by oxygen and minerals naturally present in water. As such, appliance manufacturers must deal with their eventual effect in their design executions. It’s not IF, but WHEN chemistry wins. Thus the only way to predictably present a hydronic (or steam) boiler to market is to specify the water quality requirements of the system. Note: All condensing (and other) heating appliance manufacturers detail pH (acidity) and additional water conditions in their product documentation and in their Warranties!
For the past decade or so, manufacturers have been quietly honoring warranty claims against condensing boilers that are clearly the result of poor water conditions. Cast iron boiler durability on the other hand has always been manageable. We presume that honoring condensing boiler warranties was a calculated marketing effort to promote the new technologies and systems, but no more.
Weil-McLain (our flagship supplier) is renowned within the heating industry for its warranties (and leniency). We have dozens of stories to reinforce this supposition in fact. “The customer is (virtually) always right.” However, speaking recently with Weil-McLain Field Personnel has prompted this blog both reflecting the industry’s necessary strategy change and the ultimate effects upon the consumer.
The extent of water quality management and documentation may vary within particular suppliers, but be assured that it is happening! This will be very evident not only in new system documentation but in Warranty Claims on existing condensing boiler systems, the most susceptible and therefore the industry focus.
Therefore you, the consumer must now “have his ducks in a row” by:
- Verifying that the water condition requirements of your boiler are met upon installation and start-up by yourself or your installer.
- Documenting your water conditions then and thenceforth.
- Qualifying that your serviceman performs the specified pH (Acidity) Test during maintenance cycles.
- Keeping these maintenance records on file.
Note: A similar situation exists in degree within the On-Demand DHW (Potable Drinking) Water Heater market. We in fact are certified and install the premium brand unit. They strongly recommend annual flushing with white vinegar to maintain heat-exchanger integrity. (This requires suitable piping installed on the unit, pump, etc.) Immersion coil heaters within boilers have historically had this issue, but not to the degree of the on-demand units due to their design attributes (less restrictive fluid passages). Their replacement cost is also more reasonable in comparison to an on-demand unit.
So, hard water can become hard times! Be prepared.
Continuing with our representatives’ conversations, they further offered an enlightenment that won’t be found in print. Specifically, that the life expectancy of a condensing gas boiler is measurably more limited by water conditions and average life is projected to be substantially less than its less sophisticated predecessors. Their average heat-exchanger replacement life expectancy from a marketing perspective on a condensing boiler, considering the heat-exchanger construction material is:
- Aluminum — approximately six years.
- Stainless Steel — approximately ten years.
- Cast Iron — over twenty years.
We are in no position to qualify or disqualify these statements, excepting to state that we have had an aluminum heat-exchanger failure at six years. It was replaced at no charge (of course) by Weil-McLain. The water condition at this installation was poor, but had been treated by a salt-based softener. How effectively is the obvious question?
Note: A heat-exchanger replacement cost can approach half that of the initial boiler.
No field histories on stainless steel heat-exchangers have been published, so again we must defer to judgment.
In either case, the substantial differences in both initial condensing gas appliance costs and life expectancy must be considered in making a purchase decision.
Cast Iron is a different matter altogether. It has been the material-of-choice for hydronic and steam boilers from the onset of the Industrial Age. They live long and harsh lives, particularly as “steamers” where their iron is literally eaten by continual ingestion of fresh water (oxygen and minerals) to create and vent steam as the heating medium. A precipitated “black goop” settles in their bottoms and must be periodically flushed to avoid corrosion and a circulation stoppage. (Note: Steamers typically have heavier castings to suit.)
There is also a unique hybrid stainless steel/cast iron condensing gas boiler available from Weil-McLain (of course). Their Model GV90+ Gas Boiler has a primary cast iron heat-exchanger coupled to an external stainless steel “condensing” exchanger. The combination provides an extremely longer-lived condensing boiler, readily serviceable with modular replacement at a respectable 91-92% AFUE Efficiency. So now there is an alternative with a seemingly longer economic life, but at the penalty of a few points in efficiency less than its more sophisticated cousins. Do the numbers justify the hybrid’s 3-5% lower efficiency and its 20% lower initial cost for a potentially doubled system lifetime?
Before you “run the numbers”, consider this point. Presuming that the hybrid has a double life over the aluminum or stainless units, you will effectively buy a second unit with no economic incentive at all. At 97% AFUE we have, to quote the old farmer’s saying: “Used everything from the pig except its squeal.” There are only 3 points of efficiency left to play with, most of which is likely technically non-achievable.
Note: We have not discussed heating oil and other fuel conversions to/from Natural Gas. LP (Propane) in our region remains, and likely always will be a more expensive “fuel of choice”. Refer to our Blog: OIL TO GAS FHW HEATING CONVERSION – ALL OF YOUR OPTIONS for applicable detail. Be mindful also that the past year has become an economic crossover for Heating Oil vs. Natural Gas, particularly when the Crude Oil Per Barrel Cost stays under about $45. We have a recent Blog: OIL & NATURAL GAS AS HEATING FUELS EQUATE @ $45/bbl that can serve to further inform ….. and confuse! Further, Natural Gas is not tracking #2 Heating Oil well, the differential widening substantially. So $45 is probably a low number today …..
So what conclusions can we offer?
- An older Gas Boiler upgrade to a Condensing Gas Boiler is a “no-brainer” economically.
- Heat Exchanger Material choice, considering your water condition, is paramount.
- There are not two but three condensing gas boiler options available: Aluminum, Stainless Steel and a Cast Iron/Stainless Steel Hybrid.
- Condensing Boiler Life is a real factor. Check the Warranties and Conditions!
- Factor both efficiency increases and potential system life decreases into your calculations. Initial system cost is also a variable.
- Your water condition documentation is paramount.
Summarizing, the Condensing Gas Boiler is the contemporary appliance-of-choice for cost effective residential heating, where applicable. Hopefully providing you with all of the rules of the game will make you, the consumer, a better player.
Author’s Note: Recent HVAC Trade Journal Articles are beginning to document premature material and weld failures (leakage) in condensing boilers, some immediately upon or within weeks after installation. Some can be attributed to factory process control by manufacturer, but underlying is that basic metallurgical integrity factor. There is no field repair option yet available.
Updated 06/29/2017 P.D.M., Sr.