Replies

1. GBA Editor
   Martin Holladay | Apr 18, 2016 06:16am | #1

   David,
   A rough guide to energy savings is to compare the SEER or HSPF rating of the older equipment with the newer equipment. The ratio of these numbers should give you a guide to energy savings.

   (For more information on SEER and HSPF, see: Know Your Terms - Heat Pump Efficiency Ratings SEER & HSPF.)

   If you are replacing a SEER 10 unit with a SEER 13 unit, the SEER 13 unit will use about 77% as much electricity as the SEER 10 unit, because 10/13=0.769.

   My guess is that it's more cost-effective to wait until the old equipment conks out than to replace a unit early, but I haven't done the math.

2. 
   charlie_sullivan | Apr 18, 2016 06:51am | #2

   Martin's answer is good, assuming you replace like for like. But there might be two opportunities for better choices as part of the replacement. One is that the system is most likely oversized. Replacing it with a smaller right sized unit will increase run times and decrease cycling, leading to higher efficiency and better dehumidification in the summer. Additionally, replacing it with a variable speed unit will further reduce cycling and will provide higher efficiency when it is run at low speed.

   These effects are likely to be bigger than the raw SEER improvement, but it is harder to estimate their magnitude. It depends on the degree of oversizing, and also depends whether the residents appreciate the better humidity control, and perhaps set the thermostat higher as a result.

3. 
   Reid Baldwin | Apr 18, 2016 07:04am | #3

   Instead of comparing the old one with a new one of the same size, you should compare the old one with a new one of the CORRECT size. While the old one is still working, monitor run times to assess whether it is correctly sized or not.

   Edit: Charlie posted the same thought at the same time, with more information than me.

4. GBA Editor
   Martin Holladay | Apr 18, 2016 07:44am | #4

   Charlie and Reid,
   This is a complicated issue. As I wrote in my article on Manual J (Saving Energy With Manual J and Manual D):

   "There are strong arguments against routine oversizing of HVAC equipment. The best argument is simple: oversized equipment usually costs more than right-sized equipment. ... Increasing evidence shows that energy experts have exaggerated the negative effects of equipment oversizing, however. Studies have confirmed that oversized furnaces don't use any more energy than right-sized furnaces. Moreover, newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing.

   "Although there are ample reasons to believe that oversized air conditioners are less effective than right-sized equipment at dehumidification, at least one field study was unable to measure any performance improvements or energy savings after replacing an existing oversized air conditioner with a new right-sized unit."

   For further details on this issue, see the discussion in the comments section at the bottom of that article.

5. 
   davidmeiland | Apr 18, 2016 10:00am | #5

   Thank you, gentlemen. Martin's answer seems safe to me, using the published numbers (although I can't reliably find data on older machines). There is usually a new air handler going in at the same time, which presumably has a more efficient blower motor. It's a much bigger task to factor in possible envelope improvements, which are almost always on the table too.