Questions from the mailbag:

Can you explain why the heating load changes so much when going from the design reports to hourly loads, while the cooling load does not change nearly as much? Is this normal?


My Answer:

Yes! It's very normal.

First thing: the heating design simulation yields a big number. The heating design simulation assumes it's dark and usually assumes all lights are off, all people are absent, all plug loads are off, there are leaky windows AND it's the coldest hour of the year. Basically, it's like a homeowner goes on vacation in January and turns everything off (well, like the homeowner went on vacation and flipped all the circuit breakers off - except for the furnace)

In cooling design, the model indeed assumes a worst case, but there are several reasons that it isn't as large of a difference. Firstly, we must remember that that all loads = heat. Thus, when you add realistic receptacles/lights in heating mode - it adds heat to the building and the heating load gets smaller. On the flipside, when you add realistic receptacles/lights, it still makes the cooling load bigger - just not by "design conditions". Since all internal gains yield heat, as you tally them, the heating load approaches zero (and farther away from design conditions) but in cooling mode, the load moves farther away from zero (and closer to design conditions)

Another item is that the design cooling load [in many cases] mostly comes from the sun. Solar load peaks are pretty consistent over the duration of summer months. In many cases, this means you are starting at about 65% of the peak cooling load anytime the summer sun is shining at 3 pm or so. If you add 50% of your receptacles and lights you will already approach 80% of the design conditions.

In heating, the main design load is often ventilation, which in your case, 65% of the peak heating load is due to ventilation: as you add realistic loads for lights and people, that number shrinks to 50%. Add in a little bit more loads from fans, motors, etc and that number approaches zero heating load. As a result, it's not uncommon that the observed heating load is often only 30% of the design heating load.

A simple example: it's easy to heat a building by turning all the lights on and filling it with people. In winter for example, a building may use zero mechanical heating if the internal loads are maximized. However, in summer, even removing all of the internal loads will not nullify the cooling load.

Here's a common output comparison:





How'd I do? Let me know in the comments