Will higher BTU A/C inverter not work in room that needs 18K BTU?

Question

I currently have a 12,000 BTU TGM A/C inverter in my room. Whenever a technician goes for maintenance, they tell me that a 12k BTU is too small, based on size. I haven't changed it because it worked good enough, but now it's too hot, and I need to upgrade ASAP. Right now the outdoor temperature's reaching 100F.

So... I find a brand-new Midea 24K BTU Inverter Mini-split unit at a lower price of the 18K inverter. I call the Midea distributor and he tells me that I should only buy the split-unit that my room requires (based on room size).

According to him, if I buy something bigger, it'll be even hotter than before because of the way the inverter measures the temperature and turns on/off. The walls will start to sweat and it'll be even hotter than with the 12K inverter.

The Midea model is MOCA30-24CFN1-MROW, and I'm including documentation on the model:

• Brochure (PDF)

My question: if a room requires an 18k btu inverter based on the measurements, is it true that I'll be worse off if I buy an inverter that's too big?

Answer 1

This answer only applies to inverter-style mini-splits.

Short answer: Yes, you can install a 24K BTU inverter-style mini-split even though the cooling load only calls for 18K BTU, provided that your wiring and breaker will support the additional current requirement.

The comments following the OP’s question are true and correct for traditional on/off-style air conditioners or heat pumps, whether central, window-mounted or mini-split. With those, too big a unit will cause the room to get cold and clammy without adequate dehumidification, and they will cycle on and off so frequently that temperatures will fluctuate. Their life may be shortened by so much on-off cycling if they are oversized.

The inverter-style mini-split is a completely different animal. Its output is throttled to produce just enough output to balance the heat load. Under a steady or slowly changing heat load they find a suitable RPM for their compressor to pump just enough refrigerant to keep the room at a steady temperature. They may run at a constant low speed for many hours at a time.

I don’t have specs on the TGM 24K unit, but looking at my Fujitsu 18K BTU unit, the specs say that it throttles all the way down to 3,100 BTU when necessary or up to 18,400 BTU if needed. In my situation, if I need less than 3,100 BTU, the outdoor unit cycles on and off. Otherwise, it runs continuously at a variable speed. That’s the beauty of inverter-style mini-splits.

So if 12K BTU is not enough and 18K is just right, using a 24K BTU inverter-style mini-split will give you an initial burst of cold, but the unit will quickly slow down – it will not turn off – and will modulate itself down to 18K BTU or whatever is needed to perfectly balance your heat load.

Answer 2

HVAC sizing is a wild guess

When trying to determine the correct size for heating or air conditioning, what matters the most is

• the quality of the insulation in the walls and ceiling
• the "solar gain" -- heat being absorbed from direct sunlight, which adds 1000 watts (3400 BTU) per square meter (100 W/square foot) of radiant heat square-on to the sun.

However, these things are difficult to model, and require actual mathematics. Who got time for dat?

So air conditioning people go with a simpler number: Floor area and sometimes ceiling height, just to obfuscate the matter. That's ridiculous. That has no bearing whatsoever on HVAC load. You can have two houses with identical floor area/height, but one is direct sun with no insulation, and the other is LEED 5, all that stuff, in shaded forest. They will have wildly different HVAC loads.

You know better than them

Because they are guessing, your direct experience with the unit is actually more accurate than they are. You would remember sweating through half the summer while the unit ran continuously, or having to use 4 blankets on winter nights while it ran continuously. If that's not happening maybe the unit is OK.

So you tell me. Is the unit you have now satisfactory to heat/cool this space?

Too big is bad? Maybe on old units.

I do not agree with the HVAC industry's obsession with "right-sizing" - partly because they can't estimate accurately (see above). The main reason they don't want to oversize is to sell you the smallest unit possible, so their bid price is lower, so they win the bid!

There is a traditional problem with a too-large unit, however that applies to the older "one speed, on/off" units. That is if the unit achieves its goal too quickly, less dehumidification happens. So you might need to watch your relative humidity inside the room and back it up with a dehumidifier if needed.

However that should not be a problem for an inverter heat pump as you say you have. The inverter drive means it can run at variable speed to suit conditions, and will run on "low" for an extended period, assuring that dehumidification happens normally.