Making Sense Of The Numbers

Boiler ratings are a means of assessing whether the boiler (or any product with ratings) is capable of performing the function it is being selected for. They also establish a benchmark that all equipment is compared to. As is the case with most technical products (computers, smartphones, dishwashers, and so on), there is a standard by which these products are rated.

If everyone agrees to test to these standards, then a summary table of performance ratings of all products can be used to compare different products and evaluate the best choice for the application.

A great example of that is the typical heating boiler. All boilers from reputable manufacturers are submitted to a battery of tests to determine performance. The basic results are usually captured on the rating plate (see Figures 1 and 2) and can be reviewed on industry websites for proper comparison. I do not recommend the selection of boilers based solely on the ratings shown on a particular company’s website. The reason for this is that there are ways that some test results can be misrepresented to show the best possible performance.

A more accurate industry test that is used as a benchmark to determine the Annual Fuel Utilization Efficiency (AFUE) simulates a real world efficiency that can be expected by the end user. The test parameters are very tightly controlled and verified, so all boilers with an AFUE rating can then be compared. However, there are some companies that refer to other (higher) efficiency numbers like Thermal or Steady State, which may not have been verified by an independent testing lab and may not necessarily represent the efficiencies realized in the field.

These numbers could make the company’s products look better. Not everyone is following that practice, otherwise all would be stating the highest number they could justify. So instead, the industry benchmark of AFUE should be considered, since all residential boilers up to 300 000 Btu/hr have to be tested to this standard.

Other typical ratings that are shown on rating plates include both the input capacity and the output heating capacity, measured in Btus per hour (or Btu/hr). These two numbers show that a boiler with an input of 70 000 Btu/hr burns fuel (natural gas, propone or fuel oil) to produce 70 000 Btu/hr of thermal energy. Not all of this energy is transmitted into the water, resulting in the DOE (U.S. Department of Energy) output heating capacity number of 65 000 Btu/hr. About 5000 Btu/hr go up the chimney, or are standby losses off the boiler.

Following that, boiler manufacturers typically show the Net I=B=R (Institute of Boiler and Radiator Manufacturers)  rating. This is probably one of the most confusing numbers still in existence within our industry. It is the old school way of determining how much of the 65 000 Btus we can count on getting into the house. The rating goes back to the days when hydronic systems consisted of gravity circulation systems with big cast iron radiators. It assumed that the piping losses and pickup allowance would amount to about a 15 per cent loss, which would bring us down to 57 000 Btu/hr of usable energy to heat the building.

Today, these losses are often much smaller with distribution systems such as radiant floor heating and fan coils, different piping materials (PEX, CPVC), or insulation of the distribution piping. We are still required to follow the industry guidelines and these I=B=R ratings are published. With many modern hydronic systems the DOE output can be used to select the size of boiler required to satisfy the building heat load.

Beyond these numbers, you may still see the EDR rating (Equivalence of Direct Radiation). Depending on whether it is a steam or water boiler, this number states the amount of radiation a particular boiler can heat, in equivalent feet. The factor for steam is 240 and for water it is 170. So a water boiler with a Net rating of 57 000 Btu/hr divided by 170 would heat about 335 feet of baseboard radiation.

You may find EDR ratings for the old-style column radiators published in old manuals and catalogues. This is helpful when you are replacing an old boiler and you are uncertain of the amount of heat needed for the building.

In the old days, the boilers were always oversized, many times by a factor of at least two. Counting the existing radiation within the building and determining the EDR allows you to then relate back to the output capacity required to select the appropriate boiler. No sense in putting in a boiler that is bigger than necessary for the amount of existing radiation. As with all equipment, when boilers are oversized the overall system efficiency suffers resulting in discomfort in the space and reduced equipment life expectancy due to short cycling.

So, having an understanding of at least the basic ratings for a boiler and the radiation can very often help avoid oversizing on projects. All boilers may not be equal, but at least they can all be compared equally.

Other information is often provided by the manufacturer on a boiler’s rating plate, including the maximum water operating pressures and temperatures, content of fluid, minimum and maximum gas pressures (if gas-fired), heat exchanger surface areas, clearance to combustibles, power ratings and boiler serial number.

For newer projects, it should be safe to size using the DOE Rating, with the newer materials used for distribution piping (lower distribution losses) and building construction (lower external losses). If the building is older, the I=B=R rating may be the better sizing method. <>