High bill and weak cooling: outdoor unit undersized
A high bill and weak cooling that two services could not fix usually look like a tired compressor. Here the fault was set on the day of install. The outdoor unit was never large enough for the three cassettes it fed.
By Team Snowflake | Updated 28 Jul 2026
Case summary
Mitsubishi Electric Cassette6 years oldCondoAng Mo Kio, Singapore
- Concern
- Two service visits had already been paid for with no lasting change. A full system replacement was now being considered.
- Found
- System 3 indoor units connected to System 2 rated outdoor unit capacity
- Key check
- Checked model nameplate ratings against system configuration and load test
- Result
- After the outdoor unit was replaced, cooling improved at once and the bill dropped 25 percent over the next two months. The system has run well since.
What we were told
The bill had run high for about a year, and cooling felt noticeably weaker. The system had been serviced twice with no lasting change. The owner worried the compressor was on its way out.
What we checked
The earlier visits had topped up gas and cleaned coils, the usual fixes for weak cooling. None held, so we did not repeat them. Instead we read the model nameplate on the outdoor unit and matched its rated capacity against the three indoor cassettes. That comparison is the quickest way to confirm a sizing mismatch, and the numbers gave the answer within minutes.
Three ceiling cassette indoor units, each rated 2.5 kW.
Outdoor unit model rated for System 2, approximately 5 kW total capacity.
Actual load when all three units run simultaneously: 7.5 kW.
Outdoor unit was running beyond its intended load continuously.
What we found
The installer wired three cassettes, each rated 2.5 kW, to an outdoor unit rated about 5 kW. That is a System 2 outdoor carrying a System 3 load. Combined indoor demand of 7.5 kW sat fifty percent above what the outdoor unit was built for. With all three cassettes running, the compressor could not move enough cooling to meet the demand. It ran flat out and never switched off, which is why the bill climbed. The cooling felt weak because one undersized outdoor unit was stretched across three rooms instead of two. The compressor was not failing. It had simply been asked to do more than it could, every hour of every day for six years.
What fixed it
The compressor still worked. It was just overworked. The fix was to swap the outdoor unit for a System 3 model sized to the three cassettes. No service visit could overcome a gap this large, so more cleaning would have wasted money. We suggested getting quotes from two or three contractors, each naming the outdoor unit they would fit. The right unit would pay back through lower bills and steady cooling in all three rooms.
Outcome
After the outdoor unit was replaced, cooling improved at once and the bill dropped 25 percent over the next two months. The system has run well since.
What this case teaches us
A high bill with weak cooling can mean the outdoor unit is too small
- When repeated servicing does not help, check the model numbers before paying for more. An undersized outdoor unit cannot be cleaned back to health.
- Add up the rating of every indoor unit and compare it to the outdoor unit. A System 2 outdoor cannot carry three indoor cassettes.
- An overloaded compressor runs flat out without ever cooling enough. The bill climbs and the rooms still feel warm.
Questions this case answers
Electricity bill climbing while the aircon cools less: what causes that?
Can three indoor units run off a System 2 outdoor unit?
Why did repeated servicing not fix the weak cooling?
Is my outdoor unit too small for three indoor units?
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Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.