# run time of current system

• 10-12-2011, 09:30 PM
electricblue
run time of current system
Manual J says I need a 2.5 ton. I currently have 4 ton. Is there a way to time the cycle of my current system with a set point and outside temp to tell if I am currently over sized?
• 10-12-2011, 09:59 PM
catmanacman
on the hottest day of the year did your unit cycle on and off several times
• 10-12-2011, 10:17 PM
hvacrmedic
There's a way to ball park it. Assume that your unit is maintaining an indoor/outdoor temperature difference of 10°. You clock the run times and find that the unit is running about 20 minutes out of the hour, or 33.33% of the time. Assuming that the heat load is proportional to the indoor/outdoor temperture difference, the unit would theoretically be able to maintain an indoor/outdoor difference of

10/(33.33/100) = 30°

if it were to run nonstop.

In reality the capacity of the system drops off as the ambient temperature increases, however the load isn't strictly propotional to the temperature difference either. The heat load does not increase at as great a rate as the temperature difference, so these two factors tend to cancel each other out to some extent.

This will be nowhere near exact, but if you find that your system is grossly oversized using this method, then it probably is grossly oversized in reality.
• 10-12-2011, 10:37 PM
btuhack
Good luck.

• 10-12-2011, 11:00 PM
hvacrmedic
Quote:

Originally Posted by btuhack
Good luck.

I remember that thread. A lot of good points raised. It would probably be more accurate to clock the runtimes on the hottest days. The percent runtime method would, for some not so obvious reasons, be more accurate at higher indoor/outdoor temp differences. I probably wouldn't try it with a temp difference below 15°.
• 10-13-2011, 07:01 AM
timebuilder
Quote:

Originally Posted by electricblue
Manual J says I need a 2.5 ton. I currently have 4 ton. Is there a way to time the cycle of my current system with a set point and outside temp to tell if I am currently over sized?

Is it more likely that a manual J is incorrect, or a rule of thumb used to size your current system?
• 10-13-2011, 12:24 PM
HVACTechNC
Quote:

Originally Posted by timebuilder