Mechanical & Plumbing
Hot water heating renewal series: Boiler efficiency
By Mike Thomas
In the Canadian climate, heating is the biggest energy use in most buildings. And most of our heating plants are based on water heated by boilers. This monthly column discusses fundamentals of hot water heating plants, how they use fuel, and how heating energy use can be reduced when older plants are replaced. This month’s topic is boiler efficiency.
By Mike Thomas
Boilers are the appliances in a heating plant that burn fuel to produce heat and then transfer that heat to water. In an ideal world, a boiler would transfer 100% of the heat content of the fuel into the water stream, making it 100 per cent efficient. In reality, much of the heat is lost.
Most buyers consider efficiency as one of two key criteria when choosing a boiler (the other being price). For an engineer like me, this is an oversimplification, but it’s the way the market works. So the industry in North America has come up with a way to score the efficiency of boilers to help building owners and their advisors make buying decisions.
The North American rating method is the Annual Fuel Utilization Efficiency rating, or AFUE. This method looks at operation in a tightly-controlled laboratory environment. The laboratory conditions are quite different from constantly changing field conditions, and they are also set up in a way that makes boilers appear more efficient that they will be in real boiler rooms. This is likely a result of proactive lobbying by boiler manufacturers. Tellingly, the test bed conditions are far enough from reality that they would void the warranty of many boilers if they were actually experienced in the field.
Testing shortcomings wouldn’t be a big deal if AFUE testing created a number that could be used for a fair comparison among boilers. But it doesn’t. Building owners who have shopped for boilers in the past few years will have noticed that every new boiler has an AFUE rating of 80 per cent or more. We know from tracking utility bills that boiler performance actually varies widely. AFUE gives no consideration to system mass, boiler design and materials, off cycle cooling, jacket heat losses, method of installation, or the way in which the boiler is operated.
Now I have to get a bit technical (my apologies – please skip to the end of this article if this is not your cup of tea). With respect to low-cost boilers, the major AFUE rating concerns are as follows:
- The boiler combustion is typically tested at 12.5-13.0% CO2 which is the range where the equipment should operate for maximum heat release from the fuel. However, to reduce field problems, US manufacturers instruct the installers to run the boilers at 10.5-11.0% CO2. As CO2 is actually a measurement of flame (chamber) temperature, dropping the CO2 reduces the temperatures by 140-170ºC, while at the same time increasing the stack temperature by 85-110ºC. More heat is carried up and out the stack.
- Most U.S. cast iron boilers carry multiple boiler ratings for a single unit. The AFUE testing is typically done at the lowest firing rate, which drops the stack temperature, and enhances the results. In actual installations the higher firing rates are more typical, increasing the flue temperatures to 230-290ºC, and reducing actual operating efficiencies.
- Boilers are tested at “steady state”, without any variability of external conditions. As all of the US cast iron boilers are made from brittle cast iron, the US manufacturers require that the boiler operate in a zone above 60ºC, to ensure that the heat exchanger will not be exposed to thermal shock or acidic flue gas condensate. AFUE does not credit boilers able to accept more efficient control schemes or withstand low temperature operation.
- During AFUE testing, little consideration is given to off cycle cooling. A standard atmospheric boiler requires a chamber that is open to the chimney, and is subject to the negative draft created by the chimney. Particularly in very cold weather, the chimney vacuums the heat from the chamber, creating high stack temperatures during operation. During the off cycle the boiler essentially becomes a high-capacity radiator to outside.
- Also not considered is boiler design or mass. The typical “pin type” cast-iron boiler is a single pass design, with a mass weight of cast iron and water in excess of 650# for a residential unit, and a chamber flue passage length of approximately 2.5’-3’. Boiler design is in reality simple physics. The larger the mass, the more Btu’s you need to warm it up, and therefore the more fuel consumed each time the boiler starts. Also, the shorter the flue passes, the less heat exchange area and time you have for heat absorption.
- Finally, there are other issues not seriously considered in the AFUE rating system, such as jacket heat loss, which can exceed 5% of the available Btu’s, or the radiant cooling effect of the barometric damper, which is required for safe operation on many traditional boilers.
In the final analysis, a conventional boiler can demonstrate an AFUE rating of 82-84%, but the actual thermal efficiency of these units can be as low as 56%. The result is that truly efficient boilers, which cost more, are hard to distinguish on paper from low-cost units.
European regulators have paid more attention to this and have come up with a rating system that more accurately reflects efficiency differences among boilers. This explains the fact that European building owners expect to pay much more for new boilers, and that the relative mix of installed boiler technologies is completely different from North America.
I hope this article gives readers some insight into the factors that can enter into heating plant performance. In future articles in this series, we’ll be covering issues owners will encounter when renewing older plants, such as Plant Monitoring and Control, Building Code Concerns, and Plant Sizing for Best Value.
Mike Thomas, a partner with Efficiency Engineering Inc. in Cambridge, has been designing and managing heat retrofit projects for over 20 years. He can be reached at firstname.lastname@example.org.