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g/hvac • Posted by u/duct_doctor •

Manual J came back 2.5 tons, homeowner insists the old 5 ton was "barely keeping up" Install

2100 sq ft, 2019 build, decent envelope, blower door at 3.1 ACH50. Ran a full Manual J with actual measured infiltration, real window specs, and the true orientation. Comes back 27,400 BTU sensible at design. Call it 2.5 tons.

Existing unit is a 5 ton that the previous contractor installed in 2019. Homeowner says it "runs constantly in July and barely keeps up."

A 5 ton on a 2.5 ton load should short cycle so badly it never dehumidifies, which is exactly what "barely keeping up" feels like to a homeowner — the thermostat is satisfied and the house still feels swampy.

I am confident in the calc. I am less confident in my ability to sell a smaller unit to somebody who is certain bigger is better. How do you have this conversation?

💬 6 replies

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u/hvac_hero •

Stop arguing about tonnage and start measuring humidity. Put a data logger in the house for a week and show them the RH curve.

What they will see: temperature hits setpoint fine, RH sits at 58-62%. That is the actual complaint. "Barely keeping up" is almost never a temperature problem, it is a comfort problem, and comfort at 75°F and 60% RH feels worse than 78°F and 45%.

Once they see that the machine is satisfying the thermostat and failing to remove water, the conversation changes from "smaller sounds worse" to "why is my house damp". Then a properly sized variable-capacity unit is an obvious upgrade instead of a downgrade.

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u/duct_doctor OP •

Logger goes in tomorrow. That reframe is worth more than any amount of me explaining latent capacity to somebody who did not ask.

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u/bas_brianna •

Second the logger. I would also log runtime. A 5 ton on a 2.5 ton load will show cycle times of six to eight minutes. Show them that next to a manufacturer's chart of when a coil actually starts pulling latent — which is around the ten to twelve minute mark — and the picture completes itself.

The unit is turning off before it starts dehumidifying. Every single cycle.

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u/minisplit_mike •

Also consider whether a modulating unit lets you sidestep the fight entirely. A 3 ton inverter that can run down to 0.9 tons covers the load at every condition and the nameplate says 3, which is closer to the number they are emotionally attached to. Sometimes the path through is not winning the argument.

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u/trade_school_tina •

Learning question — the 3.1 ACH50 blower door number. Is that measured infiltration going straight into the Manual J, or do you convert it to natural infiltration first? My instructor mentioned a conversion and I did not follow it.

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u/duct_doctor OP •

Convert it. ACH50 is at 50 pascals which is not a condition your house ever experiences. There is a divisor based on climate zone and building height — the LBL method — that gets you to natural infiltration. Rule of thumb divisor is around 20 but it varies enough that you should use the real table. Feeding ACH50 straight into a Manual J will oversize you badly, which is a fun irony given this thread.

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