The next step in answering questions related to seasonal performance is to combine the performance information for the heat pump and house with climate data for Boston.
Because the heating capacity of an air-to-water heat pump is highly dependent on outdoor air temperature, its seasonal performance will depend on the time during which the heat pump operates at different outdoor temperatures. Data for long-term averages of hourly outdoor temperature is readily available for hundreds of locations in North America. Sources for this data include:
• ACCA manual J
• ASHRAE Weather Data Viewer software
Figure 5-4 shows an example of “bin” temperature data for Boston, MA. In this case, the “bins” are 5ºF wide. Each vertical bar gives the number of hours that the outdoor temperature falls within the specified range, based on long-term average values for those temperatures. On any specific year, the number of hours in each bin may be higher or lower than the long-term average.
The outdoor design temperature in Boston is 9ºF. Notice how many hours correspond to outdoor temperatures significantly higher than this design temperature. This implies that most of the heating occurs under partial load conditions.
If internal heat gains are minimal, and thermostat setbacks are not used, space heating loads can be modeled as being approximately proportional to the difference between the indoor setpoint temperature and the outdoor temperature. This allows the heating load to be calculated as a percentage of design heating load using the average outdoor temperature for each bin. Figure 5-5 shows a spreadsheet where this has been done, based on the Boston bin temperature data shown in Figure 5-4.
The right side column in Figure 5-5 gives the hours during which the space heating load equals or exceeds each given percentage of design load. Plotting the last two columns of this spreadsheet up to outdoor temperatures of 65ºF yields a “heating duration curve,” as shown in Figure 5-6.
The yellow shaded area under the heating duration curve is approximately proportional to the total space heating energy needed over an average heating season. This is based on the assumption that internal heat gains alone supply the home’s heat loss at outdoor temperatures of 65ºF or higher.
The relationship between the area under the heating duration curve and the total seasonal space heating energy required can be very useful when combined with other building and heat pump performance data. Figure 5-7 shows one example.
The graph of heat pump heating capacity and building supply water temperature (on right) is shown next to the heating duration curve for Boston. The graphs have been scaled so that the design heating load of 75,000 Btu/hr on the right graph aligns with the 100% of design load on the left graph. A line has been drawn horizontally from the balance point on the right graph to the vertical axis of the other graph. The green shaded area under this line represents about 94% of the total area under the heating duration curve. This implies that, on an average year, the selected heat pump alone supplies about 94% of the total space heating energy while operating at or above the outdoor balance point temperature. The remaining space heating energy is supplied through the combined operation of the heat pump and a supplemental heat source.
The same horizontal line can be extended to the supply water temperature scale on the right graph. This shows that for 94% of the heating season, the required supply water temperature is at or below 103ºF, a temperature that allows modern air-to-water heat pumps to run at relatively high COP.
The low-ambient air-to-water heat pump assumed in this example could operate down to an outdoor temperature of -5ºF. On an average year in Boston, there is only 1 hour of outdoor temperature lower than -5ºF. Thus, the heat pump would remain in operation over essentially the full range of outdoor air temperature. The required supply water temperature corresponding to an outdoor temperature of -5ºF would be about 132ºF. The low- ambient heat pump used in this example would have a COP of about 1.7 under this condition.