Technical Ins and Outs of
Thirty years ago, the design of residential boilers was a mature technology. As far back as the Forties, boilers of 80-82% efficiency were installed and performed well with little maintenance or service for half a century and more. There was and is a lot to be said for these old workhorses, but the world has changed.
Thirty years ago, spiraling fuel prices and the negative effect excess energy use has on our lives and on our planet were already evident. The obvious path for our industry to take was toward maximum efficiency appliances with low carbon footprint and low emission of greenhouse gasses along with boiler selection and installation procedures that optimize both.
Fifteen years ago, boilers with efficiencies over 90% were available. Actually, some were available fifteen and twenty years ago. At Bay Hydronic, we tried them. The results were poor. The designs that existed then simply were not reliable. Maintenance was high. Service life was short. They were, frankly, an embarrassment.
Things have changed. 90%+ efficient boilers are evolving and a few are now excellent. Some are still not so good. The differences are mainly in the heat exchanger design. Broadly speaking, there are three significant approaches
THE HEAT EXCHANGER
The "heat exchanger" is at the heart of any boiler. This is a device that contains heating system water inside a metal jacket. On the other side of that jacket, hot gasses coming off the burner pass by, giving up their heat to the metal jacket and passing it to the water contained in that jacket.
Very high efficiencies have always been possible. The problem is that if efficiency exceeds about 87%, water vapor in the exhaust gasses will condense back to water as those gasses cool. That water is acidic because it absorbs carbon dioxide from the exhaust, producing a weak solution of carbonic acid.
Carbonated soft drinks contain a much stronger solution of carbonic acid, so the stuff is not a big deal in any sense except in its ability to corrode metal and eat away at concrete.
Condensed acidic water sitting on the surface of a heat exchanger corrodes most metals, the rate depending on how reactive the metal is.
Aluminum is very reactive. Steel and iron are less reactive than aluminum, but still susceptible to rapid corrosion. Copper may be slightly better.
Boilers used to be made mainly of steel, iron, and copper. Prevention of corrosion by acidic condensate was therefore a high priority, so boiler designers intentionally kept efficiency down to limit condensation. (It still happened during cold starts, but that usually didn't last long enough to be a serious problem.)
That was the state of things with almost all boilers (there were exceptions) even as recently as ten years ago. The avoidance of condensation kept real world efficiencies down at 75%% to 85%at their peak. Then came:
Beginning about forty years back, around the time of the first Arab Oil Embargo, interest in higher efficiency started to grow and led to the development of high efficiency boilers.
For truly high efficiency, condensation has to be not only accepted, but embraced. After all, for every pint of water condensing on the heat exchanger, 900 BTUs of useful heat are given up to help heat the house. (When water vaporizes, it has to absorb 900 BTUs of heat energy to make the phase change. When it changes phase back to liquid, it must give up that heat.)
This means the corrosive nature of condensation had to be dealt with. There are three ways:
1. Make the metal thick enough that it will take a long time to
corrode through and simultaneously design it so the corrosion
itself will slough off and not build up a thick insulating layer that
will reduce performance and block passage of flue gasses. This
is done with some aluminum and cast iron boilers.
2. Add a corrosion resistant layer (usually ceramic) to the metal
to protect it. This was done with the Hydrotherm Pulse Boiler
which used copper fire tube design.
3. Use an appropriate grade of stainless steel that can withstand
acidity without corroding.
The aluminum heat exchanger is a questionable option. I have seen serious corrosion buildup not only on the side of the heat exchanger exposed to condensation, but more importantly, on the water side.
Corrosion on the water side of the heat exchanger happens if there is oxygen available to oxidize the metal. Efforts are made to prevent that. A front line of defense in modern radiant systems using plastic tube is to add a non-permeable barrier to the outside of the tube. Without that barrier, all plastic tubes are permeable to oxygen. This means oxygen will be carried to the boiler heat exchanger. It that exchanger is made of aluminum, the oxygen will gobble it at an amazing rate.
Even if the tube is not permeable, a very small leak over a period of time can have the same effect. All leaked water is replaced with fresh, oxygenated water. Corrosion happens fast. Even something as simple as an expansion tank problem can cause serious corrosion of this type within a few months. I know from bitter experience.
CONCLUSION REGARDING ALUMINUM BOILERS: My judgement is they are not acceptable. There are just too many possibilities for severe and rapid corrosion, and the risk is too easily avoided by simply going to stainless steel construction.
EXCEPTION: I know of one product which is a combination boiler and water heater, that uses a highly siliconized aluminum heating block that is very resistant to corrosion. Water passages for domestic hot water and radiant heating are pure copper. In certain situations, it makes a lot of sense.
STAINLESS STEEL HEAT EXCHANGERS
Stainless steel is a good material for 90%+ boilers: Not perfect, but still very good. Most stainless steel condensing boilers use nearly identical gas valves and powered burners, and they are pretty good quality,so there is not a lot of differentiation here.
STAINLESS STEEL HEAT EXCHANGERS, however come in differing configurations. The most common one is the spiral tube style and most of those are from the Gianoni Company in France. It looks like this:
Flattened coils of stainless steel tube
are hung from headers. Water flows
through the tubes. A gas burner
is set in the middle of the coils. Hot
gasses from the flame exit through
the spaces between the tubes and
pass their heat into the water inside the tubes. Items
of concern are:
1. Sometimes the metal may not be thick enough for a long life. 2. Combustion gas contaminants that, in other designs, exit the boiler along with the rest of the exhaust, are caught between the tubes and remain inside the combustion chamber
where they build up and cause serious problems. How serious? Well, something that looks like this is not uncommon. It can get worse, too.
All manufacturers of all gas burning appliances recommend yearly servicing by a trained technician. In the past, the recommendation has been widely ignored: Boilers went untouched for thirty years and more and did remarkably well.
Those days are gone. Today's boilers must have closer attention. If you decide on a boiler with the Gianonni style heat exchanger, annual maintenance is critical. Give serious consideration to an annual maintenance agreement with a reputable service company. Reconcile yourself that this service will cost $300.00 - $400.00 each time, and that in some cases service will need to be twice annually.
CONCLUSION REGARDING GIANNONI STYLE BOILERS: I would consider them if they were the only high efficiency boilers available. Since there are good quality alternatives that do not have fouling problems, I find this kind of boiler a poor choice.
Here is a heat exchanger we like a lot:
This is known as a "fire tube" heat
exchanger because the hot gasses
travel downward inside the many
tubes, passing heat to the water
outside. There are no tight
clearances for contaminants to catch
on, and any that form fall right
out. Condensation forming inside
also washes down the inside of the tubes, keeping them
clean. We have seen this exchanger cut open, and the metal is impressively thick. This is the heat exchanger in our favored boilers.
RECOMMENDATION: This is a very good choice, although there are evolutions in modulation technology that are attractive
Back when boilers were big clunky things, they had a lot of cast iron and a lot of water inside. Systems, too, had big iron pipes and radiators and a lot of water and only one heating zone: The whole house. We in the trade would say those systems had a lot of "thermal mass."
When the burner went on in these systems, it took a good while to bring all that "thermal mass" up to its working temperature. It had what we call "long on-cycles". That was good for the boiler and for efficiency.
Short "on-cycles" are hard on the boiler because controls and valves have to operate more often (they "short-cycle") and because temperatures never have a chance to stabilize. Heating up and cooling down quickly stresses metals and can lead to premature failure.
Low thermal mass and systems broken up into small heating zones cause short-cycling. Yet, boilers made with expensive stainless steel need to be small to keep costs down, and the highest levels of home comfort are achievable only by breaking up the house into multiple heating areas, each with it's own thermostat: "Zoning" the house.
"Modulation" is a strategy that adjusts the rate the boiler produces heat to match as nearly as possible the actual amount of heat needed at the moment. This reduces cycling, increases efficiency and makes the exhaust cleaner.
Given the prevalence of systems broken up into small sections by use of "zoning"--a very good practice, by the way--- the need for return to simplicity and reliability, and the need for efficiency, the industry is very slowly turning toward use of appliances that are really water heaters to overcome the deficiencies of low mass boilers by providing high water volume to add thermal mass. Combined with a heat exchanger, this can be the ultimate combination system.
As you can see, there is a lot involved in choosing a heat source for your application. For advice, give us a call. We can answer your questions.
Call us 1-800-WARMERS (1-800-927-6377)
Bay Hydronic, Inc.
California Licensed C-4 Boiler Contractor #278694. Since 1972