Have you ever gone to all the work of creating a building energy model in RETScreen, only to be left wondering how to convert those annual gigajoules into a specific equipment capacity?

This situation can occur when the capital cost of installing a greener heating system, such as a “geothermal” ground source heat pump, depends on the design heat loss, or required capacity. A geothermal system can be so expensive that on a lifecycle cost basis it’s one of the few heating systems where it’s good practice to equipment for less than what’s needed under design day conditions, by up to 30% less.

First, you need to make sure that you have completed a full building energy model. You should also be familiar with the design heat loss concept and how it is calculated. Beware the programmer’s motto: garbage in = garbage out. For existing buildings if at all possible, verify the model against actual energy bills and historical weather data. To use historical weather data, overwrite it in the climate data section on the Start worksheet.

On the climate data screen, make a note of the annual degree day information:

Annual heating degree days in RETScreen

If you have specific degree days to use here, by all means enter them. RETScreen uses 18 C for heating degree day values. Simply erase the whole column and put the annual degree days all into January:

Using a custom annual degree days value in RETScreen

Next, on the Energy Model worksheet make the following changes:

• Set all schedules for 24/7 and occupancy rate at 24 hours a day all year
• Set Temperature – space heating to 21.0 C
• Set Heating/cooling changeover temperature to 18.0 C
• Select Energy – base case in the Show field. Use GJ as the reporting number, as this will give you the best accuracy.

Preparing RETScreen Energy Model worksheet for design day heating calculation

Now you’ll need the reported heating energy required due to the building envelope.  Use the sum of Heating GJ numbers, or better still, add up only those that relate to space heat. Typically this means the Building Envelope, Ventilation and Lights. Exclude Hot Water because it goes down the drain.

Check to see if your Electrical Equipment is marked as having an effect on space heat or not. It’s up to you. Since I place lighting loads in the Electrical Equipment section, I include it as having an effect on space heat.

Next, open up a regular spreadsheet. Obtain a whole-building UA factor by multiplying your heating GJ by 1E9, and then dividing by: (heating degree days * 24 * 3600).

Multiply the result by your desired delta T in degrees Celsius. That is, your desired indoor design temperature minus desired outdoor design temperature. Use whatever you want or are required to under code.

The result is the instantaneous (steady-state) design heat loss in Watts. Divide by 1000 to get kW, and then multiply by 3412 to get the design heat loss in BTU/h.

That’s it in words, here it is with numbers.

Given: 4194 Heating Degree Days, heat energy 522 GJ, outdoor design temperature of -23 C, and indoor design temperature of 21 C.

Heat loss = (522 * 1E9 / (4194 * 24 * 3600)) * (21 – (-23)) / 1000

Heat loss = 63.4 kW

Multiply by 3412 to get BTU/h:

Heat loss = 216,267 BTU/h

Now take this number to your mechanical contractor and find out how much it would roughly cost to install this capacity to complete your financial analysis. If you are considering capital-intensive equipment at less than full capacity, you can use a Heating model in RETScreen to specify an undersized heat pump that still provides for over 90% of the annual heat load.

Beware, RETScreen rounds its values to the nearest GJ, which for small buildings can make the design heat loss number very uncertain.

Finally, keep in mind that RETScreen is based on metric calculations, and accuracy drops in some unit conversions.