Had a WF Envision ND049 w/vertical closed loop (4) system designed and installed 25 months ago with the following heat calc on a 30 year old home about 3100 sq ft, with LOTS of glass.  Old windows were dual pane air insul but just replaced every window with low-e triple pane w/argon fill (U-0.18 and SHGC-0.19).  Total of 15 casement windows and 8 sliding patio doors plus 2 skylites replaced.  Exterior of the house was wrapped w/Tyvek and resided with a fiber-cement product. 

Design Data:
Comfort Conditions:
Heating Load: 51,000 Btuh
Heating Setpoint: 68 °F
Heating Temp Diff: 68 °F
Cooling Setpoint: 74 °F
Cooling Load: 30,000 Btuh
Start Cooling Temp: 74 °F
Cooling Temp Diff: 17 °F
HW Temp Setting: 130 °F
Constant Fan: No
HW Users: 3 people
Design City: MINNEAPOLIS, MN

Annual Loads:
Winter Design: -12 °F
Heating: 109.9 million Btu
Summer Design: 92 °F
Cooling: 14.6 million Btu
Bldg Bal Temp: 56.7 °F
Hot Water: 17.3 million Btu
Internal Gains: 8,490 Btuh
HW Use - Daily: 55.0 gallons

Am I considered oversized now?

In my area we'd have considered you oversized then.......
3 pane windows likely dropped your load ~5%
j

Can you ballpark how many square feet of glass you have? What are the wall and ceiling insulation?
Double pane air filled is is around U 0.40, You are replacing it with 0.18 so you cut that portion of your heat loss in 1/2 Air infiltration is an unknown, but a good casement, with an exterior flange should be fairly good. If they taped the seams of the tyvek, that will help tighten up the house overall also. Depending on the ratio of wall area to glass, the improvement is probably somewhere between 5 and 15% very rough ballpark. Since you have a 50,000 btu design load in the 3,000 sf house with a design temp of -12,° the house has to be reasonably well insulated to start with, likely the windows had a disproportionate heat loss.

Cheers

Eric

By the way, so what if your heat pump is big for the job now? No compelling reason to do anything about it. When it's time for a new heat pump have them re-load (calc) the house and get a smaller unit if you can.
J

With a 25 month old heat pump, I'm not ready to replace it for another 15 or so years?

BTW, old gas furnace was rated @ 138,000 BTUH so that was definitely oversized (but that's what they did 15 years ago).  I believe the WF ND049 is rated around 58,000 BTUH so we're not horribly oversized.

Walls are 2x4 construction with foam sheathing so we probably realize perhaps R-13 or 15.  Whole house was wrapped with Tyvek with all seams taped.  Attic ceiling is likely R-38.  I'll have to calculate glass square footage but we do have several large windows.

We had an energy audit done about the time of the geo retrofit with blower door test and infrared.  The engineer said the house performed pretty well.

ND049 heat capacity is actually only around 36kBtuh with 30*F entering water, a reasonably forseeable That's well below the 51k design load. With cheap enough power, that still might be oversized in that the first cost of the extra ton (over an 038) exceeds the operating cost savings via reduced strip use.

While older double pane glass might have come in at U=0.4, the value for the window ASSEMBLY which includes the structure of the window may have been much higher, especially if the old frames were metal w/o a thermal break.