ProTech HVAC

These days there's much ado about wall-hung, cast aluminum, high-efficiency, condensing, gas-fired boilers. The awareness of the technology has reached the buying public, the HVAC consumer. The majority of boiler manufacturers have added these high-tech models to their list of product offerings. It's well known that these installation friendly, often esthetically pleasing, appliances have replaced their asbestos slathered ancestry.

Well, what about that ancestry?

Cast iron has historically been used for quality boilers, rather than steel that less quality and less efficient boilers have been made from. The thing that makes cast iron more flexible as a construction material is the fact that it is “cast” that is to mean molten gray iron can be poured into molds of indefinite shapes. The shapes of these molds allow for designs that can extract a greater amount of heat from the burned fuel than a steel boiler design can. Innumerable small finger-like projections protrude into the flue gas stream, absorbing the heat and transferring it into the boiler water on the other side of the casting.

Here is a picture of cast iron boiler sections that were made in the 1970s. Notice the fingers that catch the heat from the flue gas as it flows between the sections. (This boiler was being demolished to make way for a newer model of similar design.)

This is the boiler (most of it anyway) before it was completely disassembled.

This is the new boiler by the same manufacturer – Weil McLain. I turned the boiler around 180 degrees and moved it back to better fit the space, the existing piping and line up better with the all important chimney thimble.

Steel boilers have steel spiral shaped baffles that insert into vertical or horizontal tubes to slow the passage of flue gas (and heat) so the steel water jacket can absorb the heat and transfer it to the unsuspecting water on the other side of the steel.

At this time I do not have a picture of a steel boiler.

While cast iron boilers of yesterday were a better value in the long run than steel, because they transferred a greater amount of heat into the water and typically lasted far longer, these designs were of single pass technology – meaning the flue gas travelled once through the boiler before exiting out the smokepipe, or flue. Terribly, for the environment and for the boiler’s owner, much heat still went up the chimney. A typical cast iron boiler design from yesteryear had a stack temperature of between 400 and 600 degrees. This means that amount of heat went up the chimney, which is to say that if you put your gas oven outside in the winter, set it for 500 degrees and opened its door the same heat loss would result.

Steel boilers were even worse! The stack temperature of a steel boiler was even higher due to its inability to efficiently restrain the heat in the flue gas long enough to put that heat into the heating system water. Instead, it would go up the chimney.

Fortunately, a Scottish mariner invented a boiler design that is known as a scotch marine boiler. These cast iron boilers force the flue gas to travel forward, then backward, then forward again before exiting out the smoke pipe. This design is also called triple pass. Buderus, a German manufacturer, has been making cast iron heaters and boilers since the early 1700s and still operate in the same foundry. Their North American headquarters is in Londonderry, New Hampshire. This company only makes hot water boiler, no steam boilers, which is what is in the above pictures.

Why would the oldest continually operating manufacturer of the finest cast iron boilers in the world not make steam boilers as well as hot water boilers? The answer is simple: there are virtually no steam heating systems in Germany, or Europe as a whole. Why? Steam heating is terribly inefficient. One must heat water to at least 212 degrees to produce steam and that is with perfectly clean boiler water. As steam boiler water becomes dirtier, and it does as a rule, the impurities make the water harder to heat to the point of steam production and require the water to be heated to as high as 230 degrees. On the other hand, hot water boilers typically heat their water to between 100 degrees and 180 degrees. Europeans have long been more energy conscientious than Americans. Americas are consumers, while Europeans think of themselves in a more conservative light, at least when it comes to fuel consumption. This is why not a single American boiler manufacturer makes a triple pass boiler, at least none that I am aware of and that are available through the suppliers in my area. Perhaps there is also a special relationship between American boiler manufacturers and the fuel industries.

This is a single pass boiler that I replaced with a triple pass boiler.

This is the triple pass Buderus boiler replacement that I installed in December, 2009.

Here is the Buderus boiler block with swing-out door and insulation only. Notice this boiler also has baffles – the best of both worlds for heat capture!

So what about cast aluminum boilers? Now that’s a horse of a different color altogether! It was necessary to explain cast iron versus steel, so you can begin to understand why cast aluminum came about.

The Weil McLain boiler above is a single pass natural gas-fired, atmospheric-type steam boiler and is about as inefficient as a ‘modern’ boiler can be. The next boiler (with the smoke pipe rising from the top then turning horizontal) is an oil-fired, gun-type burner that fires a single-pass forced hot water boiler. This is about the least efficient cast iron hot water boiler that exists.

The (blue) Buderus forced hot water boiler is a triple pass, oil-fired with gun-type burner - about the most efficient oil-fired forced hot water boiler available today, and the only brand of oil-fired forced hot water boiler I install.

Here is what a gas-fired, sealed combustion, high efficiency forced hot water boiler looks like.

This boiler, a Buderus GB142/24 is so efficient – 94.5% - that it can be packaged in a plastic box. These boiler types are so light, due to the cast aluminum, which is much lighter than cast iron, that they can be mounted on a wall, virtually anywhere. This boiler is referred to as low mass and holds about a gallon of water. It is so quiet that it is barely audible when it runs. It is computer controlled, as it needs to be in order to modulate the size of the flame within. As the water returns from the heating elements – baseboard, radiant floor tubing, fan convectors, etc. – the computer monitors its temperature and raises the supply temperature going out as the return water temperature drops. It does this by increasing the size of the flame, accordingly.

Unfortunately, there are so many bells and whistles with high efficiency gas boilers (they are not compatible with oil burners, by the way) that they have a far shorter lifespan than cast iron. A cast iron boiler, when properly maintained, can outlive you and me. A cast aluminum boiler on the other hand has a relatively short life expectancy – around 15-20 years. So much efficiency is demanded of their design that their envelope is pushed to near maximum. Anything that pushes its limits tends to be short-lived. Look at Dale Earnhardt, Jim Morrison, the Space Shuttle. You get the idea.

I would guess that you would need to buy at least 3 cast aluminum boilers for every cast iron boiler. Plus, parts for a cast aluminum boiler will cost much more. While cast aluminum boilers don’t require cleaning on an annual basis like cast iron oil-fired boilers do, they still require a routine ‘check-up’. Many cast aluminum boiler owners don’t realize this, and what often happens is leaks develop and if are allowed to perpetuate can ruin parts, if not the boiler itself. Another important thing to consider with cast aluminum is it is highly susceptible to the affects of incorrect water PH. Therefore, the PH that the manufacture recommends should be adhered to, unwaveringly. Poor system water PH can ruin a cast aluminum boiler within a very short period and void the warranty.

A nice thing about cast aluminum boilers is they can generally be vented through inexpensive PVC plastic pipe, through the wall or roof. They can also be installed in a closet or out in the open, they are that compact, safe and elegant, all things being equal.

A downside with cast aluminum boilers is they are typically twice the cost of a cast iron boiler. Therefore, one needs to evaluate not only the purchase price of the boiler, but the operating cost (fuel), the service costs, the replacement cost and weigh them against its cast iron cousin. These costs are (loosely) defined as life cycle cost. The lifecycle cost of a cast aluminum boiler, I suspect, is higher than that of cast iron, but bear in mind I am using the Buderus cast iron versus the Buderus sealed combustion gas as an example.

Another very important thing to consider when deciding what type of boiler to purchase is that of serviceability. I’m talking about the available talent in your area. Not all service personnel are professional and qualified and some will not properly set up or service a sealed combustion gas-fired boiler correctly. These boilers are not plug-and-play! The manufactures instructions need to be followed to a T, otherwise, your investment will be sacrificed.

An oil-fired cast iron boiler can always be converted to any type of gas, while a cast aluminum gas-fired boiler cannot be converted to oil, and some can’t even be converted to another gas type.

So what would I buy if I needed a new boiler for my home or business? The answer depends on several factors:

1. The available fuel source
2. The available flue venting options
3. The cost of various fuels in my area
4. The application (the space needing heat)
5. The available talent in my area to install and service the boiler
6. The available space to install the boiler in
7. Perhaps, other considerations

I really like the Buderus boilers and prefer their cast iron ones. I ascribe to the KISS principal: Keep It Simple Stupid! Just like too-many-cooks-spoil-the-soup, too many bells and whistles on a boiler (like on the cast aluminum ones) make installation, service and operation failures more likely. Cast iron has my vote, all things being equal.

This entry was posted on Sunday, October 4th, 2009 at 3:41 pm and is filed under Economy, Heating. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.