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 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 low cost alternative. The alternative has failed historically in both durability and efficiency. We have noted in fact a disproportionate number of steel boilers appearing locally in “upscale” newer homes. After all, contractors have to cut somewhere!
In the gas market we are moving to new materials to compliment both the cleaner and 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 the Thermal Conductivity Chart you can appreciate why copper is 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 store and manage 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 with very good corrosion resistance. Designers must therefore carefully define the stainless steel heat exchanger to effect performance while utilizing a substantially more expensive material. With much poorer thermal conductivity, material thickness and surface area become prime parameters.
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 initiations. 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. All condensing (and other) heating appliance manufacturers are detailing pH (acidity) and other water conditions in product documentation and in particular Warranties!
For the past ten years 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. 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 will likely vary within particular suppliers, but be assured 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.
You, the customer should must 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 henceforth.
- Qualifying that your serviceman performs the specified pH (Acidity) Test during maintenance cycles.
- Keep this maintenance record 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. Their replacement cost is also reasonable in comparison to an on-demand unit.
So, hard water can become hard times. Be prepared.
Continuing upon our conversations referenced above offered an enlightenment that won’t be found in print. Specifically, that the life of a condensing boiler is obviously limited by water conditions is a definite and somewhat measurable, but also that its average life is projected to be substantially less than its predecessors. In other words, the average replacement life from a marketing perspective on a condensing boiler, 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 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 our obvious question?
Note: A heat exchanger replacement cost can approach half that of the initial boiler.
No field history on Stainless Heat Exchangers is available to us yet, and again we must defer judgment.
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. The precipitated black goop settles to their bottoms and must be periodically flushed to avoid circulation stoppage. (Steamers typically have heavier castings to suit.)
There is now a hybrid stainless steel/cast iron unit available from Weil-McLain (of course). Their Model GV90+ Gas Boiler has a primary cast iron heat exchanger supplied by an external stainless steel exchanger. The combination provides an extremely longer-lived condensing boiler, readily serviceable and modularly replaceable with 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 their more sophisticated cousins. Do the numbers justify the hybrid’s 5% lower efficiency and 20% lower cost against a potential doubled 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 farmers saying: “We use everything from the pig except its squeal.” There are only 3 points of efficiency left to play with, most of which is likely technically unachievable.
Note: We have not discussed the oil and other fuel conversions to Natural Gas. LP (Propane) in our region remains a fuel of choice. Refer to our Blog: OIL TO GAS FHW HEATING CONVERSION – ALL OF YOUR OPTIONS for applicable detail.
So what conclusions can we offer?
- A Condensing Gas Boiler upgrade is a “no-brainer” economically.
- Material choice, considering your water condition, is paramount.
- There are actually three configuration options available: Aluminum, Stainless Steel and a Cast Iron/Stainless Steel Hybrid.
- Condensing Boiler Life is a real factor. Check the Warrantees 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.
