I have read countless entries on this site (thank you all for the guidance) and wanted to gather your thoughts on my current situation.
I have an addition to a house I recently purchased. 3.5 of its walls are exposed to the exterior. It is 19’x13′ with 10′ ceilings and on a concrete slab attached to an entryway. It was discovered that this room had issues cooling in the summer and heating in the winter. I believe there is minimal insulation in the addition. I am located outside Boston and the loadcalc for the room came to be around 17k BTU for heating and 9k BTU for cooling. This was confirmed with loadcalc and coolcalc.
Due to the calcs, I was advised to get a Mitshubishi Hyper Heat MUZ-FS15NAH with a MSZ-FS15 head. It was designed 1:1 to help maximize turndown. It was sized to primarily help with heating in the winter months to offset the high oil prices.
To my situation –
During the end of summer, it started to raise the humidity in the room. This is when the temps were low 70s outside. I realize this is a by-product of an oversized unit. I minimized this issue by disabling the fan when the set point is reached.
Now that it is the heating season, the unit seems to be cycling and does not really modulate down to the ~500W when at its 5.2k BTU rating. The room is set to 65 with an outside temp of 30 at night and the unit will heat for 20+ minutes until it reaches 3 degrees above the set point. It will then stop the compressor unit outside and run the blower fan very slowly, but the unit does not modulate down. It goes from 1.2kW to 0, then waits, then spikes back up.
The original plan with the installer was to have this unit heat/cool the entryway it is attached to (a 120sqft area that is over a dirt crawlspace and two exterior walls), but I cannot seem to get enough supply into this area before the unit senses the temperature increase and kills the compressor.
Is this behavior normal? I do not have a full understanding on how the modulation works. Is it a product of being in a shoulder season? I want to have an efficient system that works properly and saves me money vs the installed hydronic system with an oil furnace. Should I have gone with the FS09NAH unit?
I appreciate any insight or advice on this matter! I have attached a graph for the power output with the unit. Again, the indoor unit was set to 65 and the outside temp was low 30s (there was ice buildup on the bottom of the unit in the morning)
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