Four HVAC quotes, four different sizes - who's right?

Why oversizing is the Florida failure

A system that is too large cools the air quickly, satisfies the thermostat and shuts off before it has run long enough to remove humidity. You end up with a clammy house at 74 degrees, condensation on vents, and a real mould risk. Bigger is not safer here; it is worse.

Ask each bidder for the load calculation

A Manual J takes the house as it is: orientation, glazing, insulation, infiltration, ceiling heights, and the actual occupancy. It produces a number that has a reason behind it. Ask to see it. A contractor who has done one will hand it over; one who has not will explain why it is unnecessary.

The two things that get ignored

• Duct work. Leaky or undersized returns waste a large fraction of a new system's capacity, and no equipment upgrade fixes an undersized return.
• Condensate routing. Where the drain runs, whether there is a secondary drain and a pan, and whether it is accessible to flush — this is the number one cause of ceiling damage in Florida houses.

What to get in writing

1. 1The Manual J result and the equipment selected against it.
2. 2What is happening to the ducts and the returns.
3. 3Air handler location, drain pan, and how the condensate is routed.
4. 4Permit and inspection, in the contractor name.
5. 5Start-up readings: airflow and temperature split, recorded on the day.

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What to ask about humidity specifically

In this climate the system is a dehumidifier that also cools. Ask each bidder how their proposal handles humidity: variable-speed equipment, a longer run time at lower capacity, and correctly sized ducts all pull more moisture out than a larger unit does.

If a house has been humid with an oversized system, the answer is rarely a bigger one. It is usually a correctly sized system, sealed ducts and a return path that lets it run long enough to work.

The warranty and who honours it

Equipment warranties are usually from the manufacturer and depend on registration within a set period, while labour is from the installer. Ask who registers the equipment, what the labour warranty is, and what happens if the installer is no longer trading.

Ask about maintenance requirements too. Most manufacturer warranties expect annual servicing and proof of it, which is another reason to keep the folder of documents.