Air conditioning installation and replacement

Yes. Oversizing can cause short cycles, uneven comfort and poor humidity control.

Often. The ducts and blower must handle cooling airflow and the system must include proper return air and condensate management.

It depends on room loads and airflow, not simply square footage or room count.

It is the industry-standard method for calculating a building's heating and cooling loads room by room, published by the Air Conditioning Contractors of America. It accounts for insulation levels, window area and orientation, air leakage, occupancy, internal gains and local design temperatures: meaning it produces a number for your house instead of for a house of your size. It is the correct basis for equipment selection, and the alternatives contractors sometimes use instead, such as square-footage rules of thumb or matching the existing unit, are the main source of oversized systems. Asking for the Manual J output is a reasonable request.

In Vermont this question usually answers itself in favour of the heat pump. If you are installing new outdoor equipment and running new refrigerant lines anyway, a cold-climate heat pump does the same cooling job and also provides heating, often qualifies for utility rebates a cooling-only unit does not, and gives you a second heat source, which has real value in a state where winter power outages and fuel price swings both happen. A cooling-only condenser makes sense mainly where an efficient heating system is already new and staying.

A window unit or a portable, obviously yes. A split system, no: refrigerant handling requires EPA Section 608 certification, the lines must be evacuated to a deep vacuum with a micron gauge before the charge is released, and skipping that step introduces moisture and non-condensables that damage the system over the following years without producing an immediate fault. It also needs electrical work and a permit. This is a case where a poorly done installation still runs, which is precisely what makes it a bad DIY candidate.

It cools the air quickly, shuts off, and never runs long enough to remove humidity, so the house is cold and clammy rather than comfortable. Dehumidification requires run time, and an oversized system by definition has less of it. In Vermont's humid summers that trade-off matters more than the temperature reading suggests, and it is the usual explanation for a house that feels worse after a new air conditioner than before. Oversizing also means more start cycles, more wear, and a higher purchase price for a worse outcome.

Spring or autumn, both for scheduling and for quality of work. Installing in the middle of a heat wave means the busiest possible crews, the least time for careful commissioning, and the least negotiating room on price. There is also a practical constraint specific to a cold climate: a system cannot be fully charge-verified and commissioned in the way it should be when outdoor temperatures are very low, so a very early spring installation may need a follow-up visit once the weather allows a proper check.

Almost certainly, if anything is being mounted outside or penetrating the building envelope. In a condominium the exterior wall and often the area an outdoor unit would sit on are common elements governed by the declaration, and installing without approval can result in being required to remove it at your own cost. Many associations have a written process and some have restrictions on the location and noise of outdoor units. Get the approval in writing before you sign an installation contract, not after.

Compressor and parts warranties commonly run 10 years, conditioned on registration within a set window after installation: an unregistered unit can default to a shorter warranty, often just 5 years, for no reason other than a missed form.

Matching the new system to the old unit's tonnage rather than to an actual load calculation. An oversized system cools quickly but does not run long enough to remove humidity, leaving the house cold and clammy instead of comfortable.