When an old cast iron boiler fails, the decision arises as to whether a condensing boiler would be worth the added investment. Most older systems designed to operate with cast iron boilers were designed with water temperatures of 160 degrees F to 200 degrees F.
Perhaps a quick and dirty review of condensing boiler theory is in order. Condensing boilers achieve their rather remarkable efficiencies by operating at low temperatures (return water temperatures at less than 120 degrees F or so, depending upon manufacturer and model).
The low return water temperature works to remove a lot more heat from the hot combustion gasses, dropping them to temperatures below their dew point. The gasses contain a good percentage of water vapor, being that the fuel (natural gas or propane, typically) is a hydrocarbon, in which water vapor is formed during combustion, as the hydrogen molecules combine with oxygen. When the combustion gasses are lowered to the dew point temperature, the water vapor begins to condense into liquid water. And when that happens, an extra (roughly) 1,000 BTU per pound of condensed water is released to the hydronic heating system. The result is that condensing boilers operate with much higher efficiencies that non-condensing boiler.
Next, let’s focus on the system, the one that was designed to operate at 180 degrees. Conventional wisdom is that you cannot reap the benefits of a condensing boiler in such a system, as the return water temperature will never be low enough to force condensing to happen. Well, maybe. In fact, I have been involved in any number of such conversions and all were successful to some degree, in achieving condensing. How can this be?
Let’s look at some factors. First, realize that newer boilers almost always include hot water reset as a standard or standard option, so it is a simple matter to set up a varying hot water temperature schedule. Mild day…low water temperature. Cold day…high temperature. Most days are mild days, and depending on the system, possibly mild enough to allow condensing operation for a good number of hours annually.
Next would be oversizing. Many old boilers heating systems were very much oversized, so that a room needing 12′ of fin tube radiation might have 16′ or 18′ or…you get the picture. So maybe 180-degree water really is not required.
Lastly, the type of building and occupancy is a factor. The most successful condensing retrofit I have been part of was an office space with high internal loads. Relatively high occupancy, high lighting levels and each person had a computer feeding multiple servers. When occupied, we found that the building could heat successfully with 130-degree water on the coldest days (in Wisconsin!) and lower temperatures in mild conditions. Being well insulated, the building only lost a few degrees at night and on weekends, no more than normal night set back.
Certainly, there are many detailed considerations in the design of a condensing retrofit. For example, it may be necessary to add a system volume tank due to the small mass of many of today’s condensing boilers. And you certainly don’t EVER want to cause extensive condensing in a non-condensing boiler. I hope to write a course detailing some of these factors in the coming year. But, the key element of this post is that one should never assume that a condensing boiler could never pay for itself when installed in a system designed for non-condensing temperatures. A survey, including a good load and installed equipment analysis is the place to start. You might be surprised!