Boston_John
New Member
Long time reader but first time poster. I'm hoping to get some of your excellent advice to talk with our a/c installer about what I think is an undersized condenser unit and a related question about a wall unit.
Part 1:
I have an MXZ-5C42NA2-U1 (42k BTU condenser) to support the following wall units: 2x MSZ-GL06NA-U1, MFZ-KJ09NA-U1, MSZ-GL09NA-U1, and MSZ-GL18NA-U1 (five total wall units).
Just adding the max internal consumption of each of the wall units, 6k + 6k + 9k + 9k + 18k = 48k total, which is over the outdoor capacity of 42k by 6k. Does this mean the outdoor unit is undersized?
I'm expecting that there are some rules of thumb and I don't want to oversize our system, but during the recent heatwave here in the Boston area our system could not keep up. All 5 units were turned on AUTO. The two smaller units ran fine. The three larger units were running with high fan and were unable to produce much cool air (it was cooler than room temperature, but not by much). Both lights were on for all three units, indicating that they were working to reduce temperature.
When I turned off the KJ09NA unit, the GL09NA and GL18NA both responded with cooler air, and within 30 minutes or so, the fans dropped down a notch automatically. By morning, we were back running with low fan/single light on.
This implies to me that the three larger units were probably calling for more cooling than they could receive (or rather, that the condenser couldn't serve up what was necessary). I haven't heard back from the installer but my suspicion is that the condenser is undersized. I totally understand that planning my system for the occasional Boston heatwave is a nice-to-have, my goal here is to understand if the basic math is off. What do you think?
Part 2:
The KJ09NA unit is located in a 750 sqft finished attic space at the top of an open stairwell from the second floor. That number includes closets and a bathroom, so the used area is much less, closer to 500 sqft. There is ceiling insulation and only two windows, but the house was built in the early 1900s, and more importantly, that space will absorb all of the heat rising through the house. Even if the volume should be de-rated because of the sloped ceilings, I think the open stairwell and older construction means it will be more likely to heat up. The installer suggested that if the KJ09NA unit is not keeping up, perhaps I need to add another unit. To me, that seems like the wrong step. If the condenser is undersized, adding another unit would just make that worse. Plus, why would I not just go up in size to either one of KJ12NA or KJ15NA units? For this part, I think I'm trying to understand if the unit is sized properly (and being underserved) or if it's too small for the application.
Thanks in advance for any insight you can give!
Part 1:
I have an MXZ-5C42NA2-U1 (42k BTU condenser) to support the following wall units: 2x MSZ-GL06NA-U1, MFZ-KJ09NA-U1, MSZ-GL09NA-U1, and MSZ-GL18NA-U1 (five total wall units).
Just adding the max internal consumption of each of the wall units, 6k + 6k + 9k + 9k + 18k = 48k total, which is over the outdoor capacity of 42k by 6k. Does this mean the outdoor unit is undersized?
I'm expecting that there are some rules of thumb and I don't want to oversize our system, but during the recent heatwave here in the Boston area our system could not keep up. All 5 units were turned on AUTO. The two smaller units ran fine. The three larger units were running with high fan and were unable to produce much cool air (it was cooler than room temperature, but not by much). Both lights were on for all three units, indicating that they were working to reduce temperature.
When I turned off the KJ09NA unit, the GL09NA and GL18NA both responded with cooler air, and within 30 minutes or so, the fans dropped down a notch automatically. By morning, we were back running with low fan/single light on.
This implies to me that the three larger units were probably calling for more cooling than they could receive (or rather, that the condenser couldn't serve up what was necessary). I haven't heard back from the installer but my suspicion is that the condenser is undersized. I totally understand that planning my system for the occasional Boston heatwave is a nice-to-have, my goal here is to understand if the basic math is off. What do you think?
Part 2:
The KJ09NA unit is located in a 750 sqft finished attic space at the top of an open stairwell from the second floor. That number includes closets and a bathroom, so the used area is much less, closer to 500 sqft. There is ceiling insulation and only two windows, but the house was built in the early 1900s, and more importantly, that space will absorb all of the heat rising through the house. Even if the volume should be de-rated because of the sloped ceilings, I think the open stairwell and older construction means it will be more likely to heat up. The installer suggested that if the KJ09NA unit is not keeping up, perhaps I need to add another unit. To me, that seems like the wrong step. If the condenser is undersized, adding another unit would just make that worse. Plus, why would I not just go up in size to either one of KJ12NA or KJ15NA units? For this part, I think I'm trying to understand if the unit is sized properly (and being underserved) or if it's too small for the application.
Thanks in advance for any insight you can give!