Volume 22, number 1, march 2008

Energy efficiency and hot water boilers

Energy efficiency is at the centre of many concerns, including the choice of heating appliances for new and existing facilities. Since 2002, the interest in efficient boilers has boomed, mainly for higher efficiency boilers (> 90%), commonly known as condensation boilers.

Last October 1, Gaz Métro reviewed the eligibility criteria of all boilers, which previously were based exclusively on an appliance certification standard. The objective of reviewing the amounts was also to ensure that the choice of appliance is based on its technical features. To promote efficient facilities, the boiler selection criteria, in addition to favouring higher efficiency than the current standard, promote the use of materials adapted to boiler room operating conditions and the use of multi-stage or modulating burners.

All appliances are certified or approved on site. Gaz Métro’s energy efficiency programs target appliances certified according to CAN/CSA 4.9 Standard. Combustion or thermal efficiency can be known on the basis of this standard. The energy efficiency standards of the various levels of government dictate the minimum required, which is established at 80% for boilers. To favour energy efficiency among its clientele, Gaz Métro has set its energy efficiency standards higher than the government standards, establishing them at 85% and 90%. It thus relies on the results obtained with the certification standard as the primary selection criterion. It must be understood that the test conditions which allow calculation of efficiency are identical for all types of boilers, regardless of whether their efficiency is near-condensing or condensing. These tests are performed at full load (100%) with a water return temperature of 26.5°C (± 5.5°C) and an output temperature of 82°C.

Cast iron boilers can be found which only operate in non-condensation mode, but which can withstand the effects temporarily. Boilers made of copper and similar materials must protect the heat exchanger against excessively cold return water. However, some manufacturers offer these boilers equipped with a second heat exchanger made of a material that allows it to operate in condensation mode. Most of the time, a protection system (three-way valve) is built into the boiler to protect the primary exchanger. In other cases, the manufacturer offers flexibility regarding connection of the second exchanger, which can be used to produce sanitary hot water. In this case, it is important to note that to be able to benefit from the increased efficiency of the condensation phenomenon, the hot water needs must be consistent with the heating needs. Otherwise, this type of boiler becomes the equivalent of a mid-efficiency or near-condensing boiler. Moreover, the discharge system must be made of condensation resistant materials.

Condensation boilers, as their name indicates, are used under conditions where the return water is under 60°C. If the return water exceeds this temperature, these boilers nonetheless operate at more than 87% efficiency in most cases. The heat exchanger material is usually stainless steel, which has no lower temperature limit for return water.

While the type of heat exchanger material indicates the boilers possible operating conditions, the type of burner and its operating mode indicate a boiler’s seasonal performance. One of the new selection criteria for boilers thus is the type of burner in terms of adaptation of the power to the required load. The new requirement dictates that the burner have at least three stages, including a minimum of 33% maximum power, or a modulating burner with a modulation range of at least 50% to 100%. In addition, because excess air plays a major role in the boiler efficiency, the air-gas ratio must remain optimal at each stage and throughout the modulation range (see graph at right). As we can see, the burner becomes a key component of the appliance. Preference should be given to a burner offering the widest possible modulation range. It is difficult for an atmospheric burner to produce interesting efficiency at low charge, while fan assisted or premix burners can deliver similar efficiencies at each stage or throughout the modulation range.

As for the choice of efficiency to favour, if it is justified, a single condensation boiler equipped with a modulating burner is sufficient, unless redundancy is required. In that case, composite boiler rooms are a solution. Finally, the near-condensing boiler offers an alternative that meets the needs of most existing boiler rooms.

Comparison of the variation of efficiency of different burner types

Marc Francoeur, Eng., CEM
Technical Advisor