I am modeling a project that has air-to-water heat pumps as the main plant. There is a natural gas back-up boiler for those few hours in the year where it is too cold for the heat pumps to meet the heating load.

I am not sure how to interpret 11.3.2.(j).(2) - For systems with mixed fuel heating sources, the system or systems that use the secondary heating source type (the one with the smallest total installed output capacity for the spaces served by the system) shall be modeled identical in the building design and the primary heating source type shall be used in Figure 11.3.2 to determine the budget system type.

In this case, as the heating water loop serves all systems in the building - all systems receive heat from the secondary heating source type at some point in the year - the language of this clause leave me wondering what baseline system to use?

My interpretation is to use System 9 ignoring the back-up natural gas heat source?

Christopher R. Jones, P.Eng.
Technical Specialist
Sustainability & Energy

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