E26: High Efficiency Condensing Boiler for Space Heating

Requirement Summary

This measure can be claimed if the boiler used for delivering space heating is a condensing boiler with an annual fuel utilization efficiency greater than the base case, as set out in the Key Assumptions for the Base Case in the Design section. By default, the base case system efficiency is 80% if gas is selected as the heating fuel.

Intention

The specification of a condensing boiler for space heating can reduce the energy required to satisfy the heating load for a building, if the fuel selected for heating is gas. Condensing boilers can be up to 97% efficient.

Approach/Methodologies

To qualify, the condensing boiler must be able to demonstrate an efficiency level greater than the base case of 80%. Different methodologies can be used for demonstrating the efficiency of a condensing boiler, for example, manufacturers might quote the gross efficiency, net efficiency, seasonal efficiency, or the annual fuel utilization efficiency (AFUE), each of which uses a different methodology to calculate the percentage.

EDGE uses AFUE as the measure of efficiency as it provides the most consistent methodology. AFUE is calculated by comparing the seasonal thermal energy output to the calorific value of the fuel used. AFUE data is available from the Energy Star website at http://www.energystar.gov/productfinder/product/certifiedboilers/results.

If the AFUE is not available, Thermal Efficiency may be used as a proxy.

Modular systems usually use multiple boilers of the same size and type. However, where boilers with different efficiency ratings are specified, the weighted average efficiency must be calculated accounting for boiler size and the expected run time.

Potential Technologies/Strategies

An efficient boiler is one that turns as much of its fuel into useful energy as possible. Condensing boilers are most likely to achieve the highest level of efficiency. They utilize the latent heat in the waste gases’ water vapor which is generated by the combustion process. Condensing boilers have a larger heat exchanger that recovers more heat and sends cooler gases up the flue. Additional heat is extracted from the water vapor from combustion; the heat extraction converts the vapor into liquid or “condensate”. This condensate is removed through the drain or the flue. The types of condensing boilers available in the market are as follows:

Table 39: Types of Condensing Boiler

Type/Method	Description
Heat-only Boilers	Conventional boilers Provide both space heating and hot water Hot water storage cylinder and cold-water top-up tanks are required, plus a loft tank for feed and expansion
System Boilers	The pump and expansion vessel are built-in, does not need a loft tank.  Designed to generate space heating and service hot water, the latter stored in a separate hot water storage tank.
Combination (Combi) Boilers	Combines a high-efficiency water heater and a central heating boiler in a compact unit Heats water instantaneously on demand Does not need a loft tank or storage cylinder  Good water pressure, as water is directly from the mains  Economical to run
Modulated Control Boilers	New generation More efficient because of the modulated controls

To achieve the best results, care must be taken not to oversize the boiler, since maximum levels of efficiency are achieved at a full load. In larger buildings with a centralized plant, like an education building, a modular system made up of an array of smaller boilers may be appropriate. Smaller boilers can be used so that when the system is under partial load, individual boilers within the array can still operate at full load.

To minimize the cost of a boiler installation, the heat loads must be minimized before sizing the system.

Relationship to Other Measures

The heat demand that the boiler is sized to supply is affected by the balance between heat gains and heat losses. Heat losses must be minimized during initial construction as the cost of improvements to the building fabric implemented during the initial construction is typically lower than the cost of installing a larger plant. If this measure is selected, natural gas must be selected as the main fuel for space heating in the Key Assumptions for Base Case on the Design page. Only “Heating energy” is reduced with this measure.

Assumptions

The base case for the efficiency of the space-heating boiler assumes 80% efficiency. The default efficiency for the improved case is 90%, which can be overwritten. Actual efficiency must be input for the selected equipment if this measure is selected. The base case is assumed to use Electricity as the fuel for heating; if this measure is selected, the main fuel must be changed to “Natural Gas.”

Compliance Guidance

To demonstrate compliance, the design team must provide the following documentation to support their claims.

Design Stage	Post-Construction Stage
At the design stage, the following must be used to demonstrate compliance:  Manufacturer’s data sheet for the condensing boiler(s) specified  System schematics showing the brand and model of boiler(s), and  The calculation of the average weighted efficiency if multiple condensing boilers with different efficiencies are specified	At the post-construction stage, the following must be used to demonstrate compliance: Updated system schematics Photographs of the installed condensing boilers  Manufacturer’s data sheet indicating AFUE for the condensing boiler(s) purchased, and Purchase receipts and delivery notes for the condensing boilers

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