My home is about 3,100 sq ft on two floors with a single open staircase at the front of the house. I done a Manual J using HVAC-Calc inputting my current configuration and then re-running it after making changes to the home like more insulation and reducing infiltration. Therefore, I have an “as is” and an “as will be” analysis.
The current “as-is” results show that the home has heat loss/gain as follows:
Heat loss: 49,877 First floor
41,967 Second floor
91,920 Total Heat Loss
Heat Gain: 26,565 (20,506 Sen) First floor
21,099 (18,310 Sen) Second floor
47,965 (38,830 Sen) Total Heat Gain
The “as will be” results are:
Heat loss: 43,680 First floor
33,586 Second floor
77,324 Total Heat Loss
Heat Gain: 24,217 (18,440 Sen) First floor
17,568 (15,475 Sen) Second floor
41,801 (34,923 Sen) Total Heat Gain
My current equipment is as follows
Oil fired hot air furnace rated output 86,000 BTU
Cooling with 2.5T York A/C (single stage)
York 3T heat pump (single stage) strips used only for defrost
Here is what I am trying to figure out from the Man J calculation.
My current “as is” configuration calls for nearly equal heat loss for both the first and second floors at the design temp of 0*. But, the real world configuration shows that the first floor oil furnace is providing the majority of both the first and second floor requirements. In making this statement I am assuming that the heat pump has output of around 10K BTU when the outside temp is around 5*, the low that we have had this winter. I should note that the house has even temps from room to room and maintains the set point when the OD temp is 5*.
Similarly, while the Man J calls for more cooling on the first floor than the second, the reality is the second floor heat pump is larger than the first floor A/C and, in fact, provides the majority of the cooling for the entire house. When at or near the summer design temp of 96* the heat pump is running continuously but the first floor A/C is cycling – albeit not a lot. As during the winter months the home is generally comfortable during the summer with consistant room temps and the set point being maintained.
I understand the physics of heat rising in the winter and cool air falling during the summer, but when it comes time to replace equipment how should it be sized? If the equipment were sized for cooling based on the Man J, it appears as though from real world experince the second floor is going to be warm while the first floor unit will constantly short cycle. Also, it looks like the heat requirements would not be met if the Man J is followed as the heat second floor unit would short cycle while the first floor unit could never meet demand.
It seems like when there are upper and lower zones, each with separate equipment and controls, then the burden of the heating and cooling must shift accordingly. I’m interested in real world experience and comments.