* On Buying the Right Boiler ... technical stuff but, if you heat your home, maybe worth a look...

This article was written for our local newspaper, after I was called in to evaluate problems several local people were having in their new heating systems recently.

Over the past few years there have been remarkable developments in gas fired hydronic heating system boiler technology. These new condensing boilers cost about twice as much as an old-style cast iron boiler but, when installed in the proper type of heating system, can reclaim the most of the heat energy formerly lost in the water vapor component of boiler exhaust gasses. This capability, coupled with computerized burner microcontrollers that modulate or adjust the burner to the level needed to suit inside and outside temperatures, can achieve remarkable efficiencies and energy cost savings over time, savings which can quickly cover the higher cost of this new type boiler. As always, there are laws of physics which must be obeyed in order so that these boilers to "work their magic" and achieve these savings, and not all hydronic or water-based heating systems will allow these new boilers to save you a dime.

There are several types of hydronic heating designs but they all fall into one of two groups; low temperature or high temperature water systems.

In-floor radiant heating sometimes employs a “high mass radiator,” a concrete subfloor slab that is insulated from the Earth below, within which hydronic heat pipes are placed in order to heat the floor and therefore the room above, and these slabs are perfect for use with new condensing boilers, since they require very low temperature heating water. Today there are also newer, manufactured low-mass sub-floor hydronic heat radiator systems which use grooved, metallic-coated subflooring products to conduct heat from the pipes into the rooms above rapidly. These "warm board" systems work well in multi-level homes where heavy slabs might be difficult to install on upper floors.

While not as efficient in heat delivery as the above in-floor systems, contractors still often use staple-up under-floor hydronic heat systems, in which the hydronic pipes are literally stapled up on the underside of a standard wooden sub floor, since these systems are inexpensive to install in new homes. Personally, I don't like staple-up designs, for a couple of reasons.  They require the hottest of low-temperature range water, since the radiator pipe is insulated from the heated space by the sub-floor as well as the floor covering(s) and because of this, don't deliver the efficiency of other radiant heating systems.  What's saved up front, as always, is paid for over time, with interest.

Even newer to the US, but used for years in Europe, are several lines of efficient low temperature hydronic wall-mounted convector-radiators, all which use low temperature hydronic water.

All of these heating delivery systems fall into the low temperature group, and can use new condensing boilers at maximum efficiency, since all function well when the boiler temperature is set between 120 and 130 degrees Fahrenheit. After the boiler water transfers some of its heat into the living space by circulating through the hydronic under-floor or other low temperature radiators, the water returns to the boiler 10 to 20 degrees cooler than it was when it left the boiler. This relatively cool boiler environment allows a modern condensing boiler to deliver its rated 95%+ efficiency in combustion and operation.

Anyone who wears glasses or ski goggles is familiar with condensation and knows you cant easily fog up your lenses on a hot summer day since it requires colder temperatures for condensation to take place. Since there is significant heat contained in a boilers exhaust gas water vapor, today’s highly efficient boilers reclaim most of that heat by passing the exhaust gas through a special heat exchanger inside the boiler water jacket, where the vapor condenses back into a liquid, losing its heat in what is called a phase change. Here’s the critical design factor: If the hydronic water surrounding the heat exchanger is warmer than 130 degrees, the dew-point of the exhaust gas water vapor, the vapor will not condense and release its latent heat into the hydronic water. Instead, the water vapor and heat will just go up the chimney and the expensive new boiler will perform no better than a cheap, conventional iron boiler.

Since at our altitude, the overall efficiency of energy transfer from burning gas into a cast iron boiler’s hydronic water jacket is about 65% and a well designed condensing boiler system will operate at 93% to 95%, the monthly energy cost savings or loss of them with an improper design and improper matching of the boiler to the heating system - are significant.

High temperature hydronic systems, typically zero-mass finned-tube baseboard radiator-convectors and forced hot air fan coil systems, require much hotter hydronic water than in-floor and other low temperature radiators, by design. Developed before boilers could safely condense and high temperature water was mandatory, baseboard fin-tubes and forced air systems require 180 degree water to operate at their designed heat transfer rates. After some of the heat energy is transferred to the living space, the water returns to the boiler, but this return water is still quite hot, generally no lower than 160 degrees Fahrenheit. This is far too hot to allow a condensing boiler to do its job.

The difference in the water temperatures required by the two types of heating systems is vital to note when the buying and matching a boiler and the radiating portion of the homes heating system. Since a modern condensing boiler will cost twice as much as an old fashioned cast iron boiler, using one in the wrong heating application, where its performance is no better than the old fashioned boiler, is a huge waste of money.

Can a regular iron boiler be operated at lower temperatures with low temperature radiators? Yes, but when exhaust gas condenses inside an iron boiler and metal chimney flue, the resulting liquid will quickly dissolve and ruin the boiler, since the exhaust condensate water is highly acidic. Condensing boilers have been designed using corrosive-resistant stainless steel heat exchangers and they drain the condensate water away through a lime or other alkaline neutralizer before the acid-neutralized condensation water is placed into the home’s sewer system. Conventional cast iron gas boilers must be operated at temperatures considerably above the exhaust gas dew point of 130 degrees, to expressly prevent condensation within the boiler and the chimney flue.

I have been recently asked to look at several heating systems recently that had been upgraded with new, expensive condensing boilers but were delivering no energy cost savings.

In one case, an old iron boiler had been replaced with an expensive new condensing boiler feeding a typical high temperature baseboard / forced air heating system which, since it required 180 degree water, could not take advantage of the boilers efficient capabilities. At another home, an expensive, new condensing boiler had been installed with a new hot water baseboard heating system; resulting in a low temperature condensing boiler trying to work in a high temperature heating application. Each system would have been better served with a conventional, cheap, iron boiler since the expensive boiler was delivering the same low heat transfer efficiency as an iron boiler, which would have cost less than half as much to purchase, for the same monthly utility bills.

If you’re building or need to replace your homes heating system, look into low temperature hydronic radiator designs; in the floors or integrated into or even mounted on the walls, since these types of heating systems will allow that new 95% efficient boiler to actually deliver its rated savings. Another huge plus of low temperature radiant heat systems is the ease with which their heat supply can be augmented using simple solar hot water collectors, which operate optimally in the low temperature 120 -130 degree hydronic heat temperature range and, properly incorporated into a low temperature radiant system, can easily cut monthly heat bills in this climate by another 50%.

If you’re installing hydronic baseboard fin-tube radiator or hydronic fan coil / forced air heat, skip the condensing boiler entirely, since it will never perform better than a conventional cast iron boiler and, when your monthly heat costs are 25%-35%  higher than your neighbor who uses a low temperature system, the $2500 or so savings on the iron boiler will pay for a few of those higher monthly bills.