Only during times when stage 2 kicks in are you getting any benefit from the oversized high end system. A single stage 2.5 ton would have would have worked just as good, and yielded the same electric bills. The only difference is you wouldn't have the reserve capacity to hold the 72 degree setpoint during outdoor design tempatures.
3.7 tons cooling load sounds high for 2400sqft above ground @ 91 outdoor temp. Either somebody fudged/miscalculated the cooling load or you really do have a leaky house. I'm at 800sqft per ton on my own house (2 tons, 1600sqft) and it will keep it 75F indoors when it's 100F outside. Typical 1999 construction, not super tight but not a sieve either. People tend to turn thier thermostats down to 72 to compensate for humidity issues, if the AC is running wide open to hold 72 it's too cold for most people.
I too question the low static values, did you take the readings with a wet coil?
You are assuming that high stage does not run very often, which is not the case on most days after mid-June around here. On milder days the low stage compressor handles the load nicely. A 2.5 ton may have covered the cooling load on many days, but during the heating season I would be using a lot of strip heat. My electricity bills went down a good bit compared to the older 10 SEER single stage 3.5 ton heat pump it replaced (especially during heating season). My goal was to have a system that could maintain the set point during worst case design conditions; that was achieved and I couldn't be more happy with the comfort the system provides really. Some people are fine with having the home rise to 76-78 deg during the hottest days but I am not. A 3 ton 2-stage also would have been undersized considering my heating load. While I understand your suspicions (I am just as against oversizing equipment), I'm confident in the load calc and system design. Static was measured with a wet coil (but clean filter) and is monitored/displayed by the equipment. 1600 cfm with a clean filter - .55" ESP. I replace the 4" filter every 4-5 months or so, before the ESP rises close to the blower's maximum capabilities. Keep in mind if the dehumidification mode is active then the blower will provide 1280 cfm on high stage. Better moisture removal and lower static.
Last edited by RyanHughes; 06-08-2013 at 04:37 PM.
Pics are a bit small for me but looks good from what I see. I have installed a few XL20i's and on one house we had a 3 ton and 2 ton heat pump close together and if the foundation had been much lower the HO would have had a eye full of trane top side louvers out bath room window. They are gigantic regardless of capacity. 3 ton is a few inches taller than 2 ton and has same perimeter dimentions. On the pad note, we always use plastic pads as the "concrete/styrofoam" pads dont look as good and tend to crack/break rather than a plastic flex a bit if ground is not packed/leveled well. Had a few break in half as the irrigation/electricians/plumbers digging trenches before setouts and dont fill in trench enough.