Extrapolating Manual J to Different Conditions
In an attempt to make HVAC equipment comparisons a bit more data-driven, I’m trying to do a rough first-order “simulation” of equipment performance under actual conditions. I can get hourly weather data (1) and equipment performance specifications under varying conditions (2), and I have a Manual J load calculation. The weather data is simple enough to work with if I only consider outdoor dry bulb temperature, and the equipment performance data is easy enough to do a linear interpolation between the reported datapoints. However, I’m not certain how to extrapolate the Manual J data.
As an example, if my Manual J heating load is 20k BTU/h at an indoor setpoint of 70F and an outdoor dry bulb of 10F, the temperature difference is 60F, giving a naive 333BTUper degree-hour below 70F (difference between indoor and outdoor). However, that doesn’t intuitively seem right — I’m certainly not running the heat on a summer night when it finally drops to 65F, or on a sunny spring or fall day — in short, this method of calculating the load per degree ignores any internal heat sources or thermal inertia. In Dana’s equipment sizing article (3), he mentions the concept of a “balance point” temperature, which is quoted as typically between 60 and 65 (lower for better levels of insulation). If I re-calculate using a 60F “balance point” rather than a 70F indoor setpoint, I get 400BTU per degree-hour below 60F (difference between balance point and outdoor).
Both of these methods have the same load at the outdoor design temperature, but the outdoor design temperature is by its nature atypical. In this example, at a quite common outdoor temperature of 30F, the first method gives a heating demand of 13320BTU/h, while the second gives a demand of 12000BTU/h. At an outdoor temperature of 40F, the difference is almost 2k BTU/h. These differences aren’t huge in an absolute sense, but do add up, it’s 10-20% off and when equipment COP at varying outdoor temperatures is taken into account, can amount to 500kWh/year difference
I suppose the real answer is that there’s no such thing as an accurate rough estimate, and “it depends” based on lots of other external factors (wind, sun, internal loads, “thermal mass”, seasonal temperature tolerance variation, etc.) — but does anyone have insight into a good way to extrapolate Manual J loads into a BTU/degree-hour number and a temperature baseline(s) that can be used to estimate load at various conditions?
(1) Historical and average weather data: https://www.ncei.noaa.gov/maps/hourly/
(2) Example equipment performance specs: https://ashp.neep.org/#!/product/32101
(3) Equipment sizing article: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Note: Why am I doing this? Because simply comparing SEER / HSPF numbers isn’t nerdy enough, and is also not representative of performance under conditions different than the test conditions. As an example, with the numbers I was playing around with last night, a unit with the highest HSPF simulated worse than a unit with a lower HSPF (though the lowest HSPF unit did stay in last after simulation).
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