This measure can be claimed if the boiler used for delivering hot water heating has annual fuel utilization efficiency greater than the base case, as set out in the Key Assumptions for the Base Case in the Design section. Actual efficiency must be input for the selected equipment if this measure is selected. This measure does not apply to electric water heating. The base case default selection for the ‘Fuel used for hot water generation’ is ‘Electricity;’ if this measure is selected, the fuel selection must be changed to ‘Natural Gas’ for gas boilers, or LPG or Diesel as appropriate.
The specification of a highly efficient boiler for water heating reduces the energy required to satisfy the hot water demand for the building.
In order to qualify, the high efficiency boiler must be able to demonstrate efficiency greater than the base case of 80%. There are several different methodologies for calculating the efficiency of a boiler. 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 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/certified-boilers/results.
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 should be calculated. The weighted efficiency takes into account the size of the boilers and the expected run time.
For tankless water heaters, if the AFUE rating is not available, Thermal Efficiency (TE) can be used instead. AFUE accounts for factors such as stand-by losses and performance at part-loads in addition to thermal efficiency but tankless water heaters do not have appreciable standby losses, so their Thermal efficiency is much closer to AFUE compared to Boilers.
Even the most efficient boilers have a maximum efficiency around 98%, because some energy (heat) is lost via the flue gases and through the main body of the boiler itself; also, lack of maintenance can reduce a boiler’s efficiency. The following table shows a set of solutions related to hot water boilers.
Table 41: Types of High Efficiency Hot Water Boilers
|Condensing Boiler||The only boilers likely to achieve efficiency level of at least 90%. They extract latent heat from the waste gases’ water vapor that is generated by the combustion process. To minimize the cost of a boiler installation, hot water demand should be minimized before sizing the system.|
|Combi Boiler||This is a type of condensing boiler that provides both heating and hot water without needing a separate tank.|
|Low Temperature Hot Water (LTHW) Boilers||Produce hot water at around 90ºC, which is then distributed via pipework to a hot water storage tank. These boilers commonly run on Natural Gas, but may also be LPG.|
|High Efficiency Boiler||These generally have lower water content, larger heat exchanger surface areas and greater insulation of the boiler shell. They are suitable to applications where a higher water temperature is required, such as kitchens, laundry and showers.|
|"Staged" Multi Boiler System||Reduces the amount of time in which a boiler is running at less than peak load, since only few boilers run depending on the demand. So, in the peak times more boilers are in use while during the off-peak times only the boilers to cover a small demand will be active.|
|Modular Boiler Systems||Series of boilers linked together to meet different demands; they are suitable for buildings or processes with a significant variable hot water/heating demand. Modular systems are generally composed of several identical boiler units although a mix of condensing and conventional boilers could be used.|
Relationship to Other Measures
The hot water demand that the boiler is sized to supply is affected by the rate of hot water use. Hot water use should be minimized first by selecting low-flow taps in washbasins and low-flow showerheads. If this measure is selected the main fuel for hot water must be selected as ‘Natural Gas’ in the key assumptions for base case on the Design page. This measure reduces both ‘Water Heating’ and ‘Other’ due to the reduction of pumping requirements for water.
The base case efficiency of the hot water boiler is assumed to be 80%. The default value for the efficiency of the improved case is 90%, which can be overwritten.
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:
At the post-construction stage, the following must be used to demonstrate compliance:
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