Heating and cooling choices

The load comes first, and it is calculated

Before any equipment is chosen, somebody has to establish how much heating and cooling the building actually needs. That is a calculation performed on the specific house: its dimensions, its orientation, its window areas and specifications, its insulation levels, its air tightness and the local design conditions.

The alternative, and it is still widespread, is a rule of thumb based on floor area. That approach was approximately serviceable when houses were leaky and poorly insulated and every house of a given size lost heat at roughly the same rate. It is now wrong in a specific direction: it consistently oversizes equipment for a well-built envelope, because it cannot see the envelope at all.

Ask for the load calculation as a document. It is a normal deliverable, it names its assumptions, and reading those assumptions tells you whether it describes the house you are actually building or a generic one.

Why oversizing is a fault and not a margin

Oversized equipment sounds like insurance. It behaves like a design defect, for reasons that are mechanical rather than aesthetic.

A system with far more capacity than the building needs satisfies the thermostat quickly and shuts off. Then it starts again. Short cycling wears equipment, wastes the energy spent bringing a system up to operating condition each time, and produces uneven temperatures because the system never runs long enough to distribute conditioned air evenly.

On the cooling side the consequence is worse than comfort. Removing humidity depends on air passing over a cold coil for a sustained period. A system that cools the air quickly and stops has not run long enough to dehumidify, so the house reaches its set temperature and stays clammy. In a humid climate this is the single most common complaint about a new cooling system and its cause is almost always the size of it.

Correct sizing is not a smaller margin of safety. It is the condition under which the equipment does the job it was designed for.

What separates the options

Heat pumps move heat rather than producing it, which is why their output can exceed the energy they consume. They provide heating and cooling from one machine. Cold-climate performance has improved substantially and continues to be the main axis on which models differ, so the question in a cold region is not whether a heat pump works but which one and with what supplementary arrangement.

Combustion equipment produces heat by burning fuel. It is well understood, its output does not fall as it gets colder, and it brings requirements of its own: a fuel supply, venting, and combustion air. In a tight house those requirements interact with the envelope, which is why the sealed-combustion arrangement is the standard answer.

Resistance electric heating is the simplest and, per unit of heat, generally the most expensive to run, which puts it in supplementary and small-load roles rather than as a primary system in a cold climate.

Distribution is a separate decision and it is often the one that determines comfort. Ducted systems need duct space designed into the building, and ducts outside the conditioned envelope lose a large fraction of what passes through them. Ductless systems avoid that entirely and condition zones rather than a whole house, which suits some plans and not others. Hydronic distribution is quiet and comfortable and does not provide cooling on its own.

The interactions people miss

Three, and each of them changes the equipment decision rather than merely complicating it.

The envelope sets the load, so improving the envelope shrinks the equipment. Money spent on air sealing and insulation reduces the capital cost of the mechanical system as well as the running cost, and the two are usually budgeted separately as though they were unrelated.

Ventilation is a separate requirement, not a by-product of the heating system, and in a tight house it must be provided deliberately. [2]

And combustion appliances in a tight house need an arrangement that does not depend on drawing combustion air from the living space. That is a design decision for a qualified mechanical professional, made with knowledge of the tested air tightness of the building.

It is worth being explicit about why that matters, because the reasoning is usually left implicit and the consequence is severe. An appliance short of the air it needs can pull the products of combustion back into the room rather than sending them up the flue, and those products include carbon monoxide, which cannot be seen or smelled. That is why sealed combustion is the standard answer in a tight house rather than an upgrade to it, and it is why carbon monoxide alarms are required in most jurisdictions.

If an alarm sounds, everyone in the house goes outside first, and the call to the fire service or the fuel utility is made from there. Nobody stays inside to look at the appliance.

What to ask for

Four documents, all of which a competent contractor produces as a matter of course.

The load calculation, with its assumptions visible. The equipment selection showing how the chosen capacity relates to that calculated load. The duct design, if the system is ducted, including where the ducts run relative to the conditioned envelope. And the ventilation strategy, stated as a separate item rather than assumed.

The requirements those documents have to satisfy come from the adopted mechanical and energy codes, which vary by jurisdiction, edition and amendment. [1] If any of the four does not exist, the system has been sized by habit, and the reasoning on The Build Sheet about specifying rather than assuming applies here more than anywhere else on this site.