Hi all. Thanks for the many comments and ideas.
First, I think I need to use this unit more as the primary heat source for the space before I reach a definitive conclusion and do anything. I'm just surprised by the amount of energy I see it using WHEN it's used (which isn't all that often in the middle of winter) compared to a smaller unit in the same building. I've primarily used it only when we're fully recharged, have hours of sun remaining in the day, and I want to raise the temp in tue space by a couple of degrees (which seems small, but maybe that's the issue).
The line sets are only about 11' long, and all three units were installed late this past summer by a certified Mitsubishi contractor that does a lot of installs. It's a fairly small company but has experienced techs. A manual J was performed separately by both a local Mitsubishi wholesaler (my friend owns it) and me, and we came up with almost identical figures. The installer agreed with the sizing for the units, and all three of us agreed that the 18K unit was on the large size for the load, but we went with that because the space (our garage) would sometimes be subject to larger temperature swings than the other two units.
The unit in question is designed for shoulder season heating, and summer cooling, as it's in our garage area, which is primarily heated with a hydronic slab. However, it's not a typical garage. Walls are R43 (same as the home), the back wall has other rooms facing it (so no exterior exposure), and there are no windows other than small ones in the garage doors, which are rated R17. There are two insulated fiberglass "man doors" that are the same as the rest of the building's doors. The slab underneath has R10 insulation (again, hydronic slab). The space is 12' tall with R65 insulation between the garage and the living space above it and the indoor unit is mounted about 9' high, midway along a long wall of the space. All walls and ceiling are air-sealed with vapor-variable material applied by a professional insulating company. We do not struggle to heat the space with the hydronic slab and can easily keep it at 65F (without the heat pump on at all) even at -10F. You cannot hear vehicles running or people working in the garage, from the living space above it. I don't think we're losing much heat anywhere other than the garage doors, and R17 isn't shabby for those. The heating load is only about 14,500 BTUs/hour (not much more than the living space above it).
Other than possibly low refrigerant and the size difference compared to the 12K unit, the only thing that might be throwing me off is the need to make up a 2-3 degree difference after the garage doors have been opened, but that only happens a couple of times per day, most days, and we have a 3-degree night setback for the 12K unit in the living area, that doesn't have as drastic a consumption difference when it makes up that difference in the morning (yes, I'm well aware of the advice to "set it and forget it, but we like cooler sleeping temps).
So...... I'm going to go with "watchful waiting" once we turn off the slab heat in spring, and see what happens, especially on days when we're away and there's no activity in the building. Time (and the Emporia Vue data) will tell... I'm just surprised at how much energy the unit uses even after the temperature in the space has reached the setpoint, we're not opening doors, and it's just maintaining the temperature at a point maybe 2 degrees higher than we would keep it using the hydronic slab. I'll let the installers deal with it if there's an issue.
First, I think I need to use this unit more as the primary heat source for the space before I reach a definitive conclusion and do anything. I'm just surprised by the amount of energy I see it using WHEN it's used (which isn't all that often in the middle of winter) compared to a smaller unit in the same building. I've primarily used it only when we're fully recharged, have hours of sun remaining in the day, and I want to raise the temp in tue space by a couple of degrees (which seems small, but maybe that's the issue).
The line sets are only about 11' long, and all three units were installed late this past summer by a certified Mitsubishi contractor that does a lot of installs. It's a fairly small company but has experienced techs. A manual J was performed separately by both a local Mitsubishi wholesaler (my friend owns it) and me, and we came up with almost identical figures. The installer agreed with the sizing for the units, and all three of us agreed that the 18K unit was on the large size for the load, but we went with that because the space (our garage) would sometimes be subject to larger temperature swings than the other two units.
The unit in question is designed for shoulder season heating, and summer cooling, as it's in our garage area, which is primarily heated with a hydronic slab. However, it's not a typical garage. Walls are R43 (same as the home), the back wall has other rooms facing it (so no exterior exposure), and there are no windows other than small ones in the garage doors, which are rated R17. There are two insulated fiberglass "man doors" that are the same as the rest of the building's doors. The slab underneath has R10 insulation (again, hydronic slab). The space is 12' tall with R65 insulation between the garage and the living space above it and the indoor unit is mounted about 9' high, midway along a long wall of the space. All walls and ceiling are air-sealed with vapor-variable material applied by a professional insulating company. We do not struggle to heat the space with the hydronic slab and can easily keep it at 65F (without the heat pump on at all) even at -10F. You cannot hear vehicles running or people working in the garage, from the living space above it. I don't think we're losing much heat anywhere other than the garage doors, and R17 isn't shabby for those. The heating load is only about 14,500 BTUs/hour (not much more than the living space above it).
Other than possibly low refrigerant and the size difference compared to the 12K unit, the only thing that might be throwing me off is the need to make up a 2-3 degree difference after the garage doors have been opened, but that only happens a couple of times per day, most days, and we have a 3-degree night setback for the 12K unit in the living area, that doesn't have as drastic a consumption difference when it makes up that difference in the morning (yes, I'm well aware of the advice to "set it and forget it, but we like cooler sleeping temps).
So...... I'm going to go with "watchful waiting" once we turn off the slab heat in spring, and see what happens, especially on days when we're away and there's no activity in the building. Time (and the Emporia Vue data) will tell... I'm just surprised at how much energy the unit uses even after the temperature in the space has reached the setpoint, we're not opening doors, and it's just maintaining the temperature at a point maybe 2 degrees higher than we would keep it using the hydronic slab. I'll let the installers deal with it if there's an issue.
