A new 93 percent efficient condensing boiler reduced natural gas consumption by 38 percent in 2011 and 35 percent in 2012 at Crosby Ironton's Cuyuna Elementray School.

High efficiency condensing boilers poised to deliver big savings

Will actual performance fall far short?

Just because it says “high efficiency” on the label does not necessarily make it so.

Condensing boilers are very efficient because the water vapor produced as a byproduct of combustion is condensed into liquid water, releasing an enormous quantity of heat for productive use instead of sending it up the chimney. For facilities with condensing boilers and for buildings considering the installation of such a boiler, this article will help clarify how to achieve maximum efficiency for your system and get the most value from your investment.

The labeled boiler efficiency will be realized only briefly during the heating season. The actual efficiency will be somewhat lower most of the time. The key to keeping the efficiency as high as possible throughout the heating season is to keep the system reset temperature as low as possible at all times.

Why condensing boilers are so efficient

Condensing boilers can take advantage of the energy available in the water vapor of exhaust flue gases. This is accomplished by plumbing the return water through a heat exchanger that extracts heat from the exhaust flue gases and uses it to pre-heat the water entering the boiler. As long as the temperature of the boiler return water is below roughly 130°F (the dew point), the moisture will begin to condense out of the flue gas, releasing additional energy that can be utilized. This condensed water is usually drained into a convenient floor drain. If the condensing boiler is operating properly, the plastic condensate drain pipe will have a generous flow rate whenever the boiler is firing throughout the spring and fall. This is well illustrated in an information piece posted by the Center for Energy and Environment.

Condensing boilers achieve their renowned efficiency by removing as much heat as possible from the flue gases, which means lowering the flue gas temperature enough to allow vapor to be converted to water. However, the flue gas can only be cooled to a temperature slightly above the temperature of the circulating water being returned to the boiler. Therefore, return temperature must be kept as low as possible to take advantage of the high efficiencies possible in condensing boilers. (see Figure 1).

Figure 1. This graph shows the importance of maintaining low inlet water temperatures to achieve high boiler efficiency. (Chart adapted from ASHRAE.)

Reset control settings are key

The outdoor temperature reset control is the critical piece of equipment to ensure that return water temperature is kept as low as possible (hence securing efficient operation of the condensing boiler). The “reset” schedule sets the temperature of the water supplied to the distribution system. It is a “schedule,” because as the outdoor temperature drops, the temperature of the water supplied to the distribution system needs to be increased to adequately heat the building. At moderate outdoor temperatures, the supply water does not need to be as hot. The reset schedule is different for each building because each building (even in the same climate) will have unique heating requirements based on the building’s levels of insulation, window performance, air leakage, mechanical ventilation needs, orientation, and other indoor conditions.

During the coldest part of the winter, the water temperature supplied by the boiler to the distribution system will typically be 30°F hotter than that returned to the boiler. At moderate outdoor conditions, the difference will ideally be just a few degrees. To function efficiently, the reset schedule of a condensing boiler needs to be completely different from that of the conventional boiler that was replaced. The reset schedule for a condensing boiler will be set up to supply water hot enough to just meet the building’s heating requirements, ideally resulting in return water temperatures below 130°F to allow for condensation of the flue gases to occur. Conversely, the reset schedule for a conventional boiler will be set up to supply water hot enough not only to meet the building’s heating requirements but also to provide an adequate safety margin to prevent condensation from occurring (typically 140°F or higher).

Figure 2 illustrates the difference between typical return water temperatures for a conventional boiler and a condensing boiler. At very cold outdoor conditions, both the condensing and conventional boilers have higher return water temperatures. As the figure illustrates, for condensing boilers this is typically well above the ideal 130°F for ensuring condensation of flue gases. However, if the temperature of return water to a condensing boiler is not reduced at warmer outdoor temperatures, it will operate in this noncondensing mode year-round. Note that the conventional boiler never operates in the condensing mode.

Figure 2. Typical return water temperature for condensing boilers (with proper reset control setting) and for conventional noncondensing boilers.

The reset control also needs to be programmed to facilitate morning warm-up after a night setback. During the warm-up period, the control is scheduled to bump up supply water temperature, resulting in return water temperature close to or perhaps even above the condensing threshold of 130°F. As space temperatures approach being satisfied, the supply (and concurrently the return) water temperature drops back down.

Avoiding control valve damage

In boiler systems with control valves, there is an additional reason to keep supply and return water temperatures as low as possible. When control valves are trying to operate with higher water temperatures, the valves will throttle down, potentially to near closing. This will cause wire-draw damage, which occurs when water grooves a channel in the seating material of a valve that operates near the valve’s shut-off position for extended periods. Such grooves allow small water flows when the valve is in the closed position, leading to imprecise or even problematic system control. With proper reset control, the valves will operate closer to full open most of the time and avoid such wire-draw damage.

The boiler reset controller must have the capacity to adjust to the three distinct time periods of morning boost, day occupancy, and night setback in order to maintain comfort, maximize efficiency, and avoid wire-draw damage to valves.