Signs of a properly sized furnace?
The most common theme I see when people talk about oversized furnaces is reduced runtime & short-cycling.
But what does short-cycling mean quantitatively?
a) 5 mins on, 5 mins off, repeat
b) 5 mins on, 10 mins off, repeat
c) 10 mins on, 5 mins off, repeat
According to the Manual J heat load, my furnace is only 5% oversized. When I set the stat to warm up more than a couple degrees, it does take a while (e.g., more than an hour). I’ve measured almost 100 degrees from my warmest supply register & 68 degrees at that room’s return (rise of 32deg).
However, with the stat at 65degF when it’s 20degF outside, I experience scenario (b) above where it’s only on for 5 minutes and then off for around 10 minutes. This is for maintaining the set-point. With the Manual J indicating a design temp of -4degF for a 70degF set-point, I would’ve thought the furnace would be off for longer or that it’d be on for longer than it is.
Is this short-cycling or some sign that the furnace is still not sized correctly?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part