Here's what it all boils down to:
At Weil-McLain, we think you should have the straight facts on an often fuzzy issue: boiler efficiency. It's an important factor in selecting a space heating boiler, but what does "efficiency" really mean? For example, commercial boilers may be both very efficient and very inefficient at the same time. How is that possible? The answer is simple: There are at least four different kinds of boiler efficiency.
Efficiency: four definitions
- Combustion efficiency – how efficiently the boiler burns the fuel
- Thermal efficiency – combustion efficiency minus radiation and convection losses
- Steady-state efficiency – how efficiently the boiler uses the heat from combustion when operating under full load
- Seasonal efficiency – how efficiently the boiler uses fuel over the entire heating season
While each type is important, seasonal efficiency is the one that really counts because it determines how much the building owner will pay for fuel over the course of the heating season.
An easy way to understand the difference between steady-state and seasonal efficiency is to relate it to an automobile. On the expressway going 60 mph at 35 mpg, the car is using gas very efficiently. This is similar to steady-state efficiency, which assumes the boiler always operates under a full load. But as soon as the car reaches the city, the average mileage will drop to 22 mpg due to starting and stopping. The car is no longer using gas efficiently. This is similar to seasonal efficiency, which will not be as high as steady-state efficiency because of the start and stop cycling of the boiler during the heating season.
High seasonal efficiency requires good steady-state efficiency as well as good combustion efficiency. But a boiler with high steady-state efficiency – say 80 percent – can easily have a seasonal efficiency of only 65 percent! It all depends on downtime losses – the losses that occur when the boiler isn't operating. Downtime losses depend on boiler construction, type of application, and design of the system.
When a boiler shuts off, the heat in the boiler continues to radiate through the jacket. This is lost energy. In addition, boiler room air continues to flow through many non-condensing boilers after the burner shuts off. This air is drawn up by chimney draft, cooling off the boiler. When the boiler turns on again, it must reheat the chimney to produce the proper draft for efficient combustion. The more often the boiler cycles, the greater the downtime losses and the lower the seasonal efficiency.
Increasing efficiency with commercial boilers
The longer a boiler operates, the higher the seasonal efficiency. Therefore, a boiler that is smaller than required will more closely match the heating load of the building for a larger part of the season because of fewer on-and-off cycles. When the first boiler can no longer keep up with the heat loss, a second boiler picks up the extra load, and then a third boiler, if necessary.
Therefore, instead of just one boiler for a job, two boilers with half the required capacity – or even three boilers each with one-third the required capacity – should be used. Each boiler will cycle one-half to one-third less than a single boiler, thus increasing seasonal efficiency significantly.
However, the key to maximizing efficiency with two or three commercial boilers is to be sure that each boiler is completely isolated from the others so that non-operating boilers will not be hot with system water. This will eliminate most of the jacket heat losses from the non-operating boilers.
Another method of increasing efficiency of non-condensing commercial boilers is to use lo-hi-lo burners rather than on-off models. With a lo-hi-lo burner, you have the equivalent of a half-size boiler on lo-fire and full-size boiler on hi-fire. The operating efficiency gain with one boiler with lo-hi-lo burner would be similar to using two on-off boilers. And with two boilers, it would be similar to using four on-off boilers.
The key to maximizing efficiency with two or more boilers – either residential or commercial – is the Weil-McLain primary-secondary piping system. With this system, the primary piping circuit runs continuously to supply the radiation; each boiler has its own secondary circuit to add incremental amounts of heat into the primary piping. The Weil-McLain primary-secondary piping system:
- Assures precise water temperature control
- Isolates non-operating boilers to eliminate wasteful jacket heat losses
- Returns the coolest water to each operating boiler to maximize efficiency
- Reduces installation costs through the use of Weil-McLain's Easy-Fit™ and Easy-Up™ Manifolds
Increasing efficiency of an existing boiler
When an old, inefficient boiler still has many years of service left, and the building owner does not want to invest in a complete replacement, two or three smaller high-efficiency condensing modulating boilers can be installed to supplement the existing boiler. Using primary-secondary piping, the new boilers handle the load in milder weather at seasonal efficiencies as high as 95 percent; the old boiler fires in only the coldest weather. Even in a northern city as New York, the average winter temperature is only 43 degrees F, so the small boilers can handle the demand during most of the heating season.
There are other ways to increase seasonal efficiency in an existing system:
- Eliminate leaks. Cold replacement water must be heated. The lime that is deposited when the water is heated can coat the boiler surfaces, reducing the efficiency. Also, replacement water costs money.
- Treat the system water. Even a 1/16 inch layer of scale on the inner boiler surfaces can reduce seasonal efficiency. Also, a boiler can fail if heavily limed. A water filter is a wise investment.
- Use indoor/outdoor reset thermostats. They will control water temperature in the primary loop. The cooler the water (in the fall and spring), the lower the system heat losses.
- Widen the operating and limit control differentials. This will reduce the number of burner firing cycles. The fewer the cycles, the higher the efficiency.
Energy management control systems (EMCS)
Weil-McLain offers a wide selection of energy management control systems to automatically sequence the boilers to match heating load requirements for increased efficiency and reduced fuel usage. Control packages and prewired panels are available for both commercial boiler systems and multiple boiler systems. In addition, there are control systems for zoning with zone valves or circulators, as well as for combined space heating and service water heating.