How big should the new system be? Ask to see the arithmetic

Every heating and cooling quote carries a size, in BTU or tons. That number should be the output of a calculation about your house. It should not come from the label on the old unit or from a rule about square feet.

This guide explains what to ask for instead of a copied size. Use it on the phone today.

The Residential Code of New York State, in Section M1401.3, says heating and cooling equipment must be sized under ACCA Manual S, using loads calculated under ACCA Manual J, or another approved method. The Energy Conservation Construction Code repeats the rule for equipment sizing.

Put plainly, the size is meant to follow from a measured load. Ask the contractor which method is used on your quote. If the answer is a rule of thumb, ask whether your town's building office accepts it.

The easy path is to read the label on the old furnace and order the same size. NYSERDA tells contractors not to do this, and to run their own calculation instead. Its reason is simple: the house may have changed since the old system went in. New windows, added insulation or air sealing all reduce what the house needs.

NYSERDA adds that many furnaces and boilers were oversized when they were installed. A copied size repeats the original mistake. The US Department of Energy's Building America team makes the same point about rules of thumb: they tend to produce oversized systems.

If your quote matches the old size exactly, ask what has changed in your house since. If nothing has, ask why the old size was right in the first place.

A load calculation takes inputs from your house and the outdoor climate. NYSERDA lists them: the area of walls, windows, roof and floor, estimates of insulation values, the direction each surface faces, and in some cases a blower door test that measures air leakage. A contractor should also ask what you plan to change in the next five to ten years, such as new siding or windows.

The calculation also uses a design temperature for your location. NYSERDA says it must always be appropriate for where the house stands. New York's residential code leaves that value to the local building official, who can adjust it for local weather.

Syracuse has its own property conservation code, which bases the heating equipment's capability on an outside temperature of minus 10 degrees. Ask your contractor which design temperature the worksheet uses and where it came from.

Do not expect a contractor to hand you the worksheet in the first minute. You can ask for it when you ask for the quote, and you can ask who did the calculation.

An oversized system satisfies the thermostat quickly, shuts off, and starts again soon after. NYSERDA describes this as cycling that wastes energy, wears fans, compressors and valves, and leaves uneven temperatures through the house.

The cooling case is harder to see. NYSERDA notes that cooling needs long run times to dry the air, and that oversized systems run shorter and may fail to dehumidify. A house cooled by a unit that is too big can feel cold and damp together.

For heat pumps the stakes are higher. NYSERDA says effects begin at 120 percent of the load and can become severe above 150 percent, and that oversizing is more of a problem for heat pumps than for a furnace or boiler. If you are weighing one, read our guide on heat pumps in a Syracuse winter.

Notice that none of this says smaller is better. An undersized system leaves you cold. The aim is a size that matches the calculated load, which is why the arithmetic matters.

Size is only half of the answer. The equipment has to move air or hot water through what you already have. A large unit cannot push air through a trunk that is too small, and a new boiler cannot send more heat than the piping and radiators can carry.

NYSERDA names ACCA Manual D alongside Manuals J and S. Manual D is the duct design method. Ask the contractor whether the ducts were measured, and whether the quote lists any duct changes by name.

If your house heats with radiators, the same question applies to the pipes and the radiators. Ask what each part can carry and what stays in place. Federal survey data shows that steam or hot water heats over a quarter of the homes in the three states of New York, New Jersey and Pennsylvania.

Picture a house from the 1940s. The furnace was fitted when the walls were bare and the windows were single pane. Since then the owners added attic insulation, new windows and weatherstripping. The house now loses less heat than it used to.

A new furnace chosen from the old label would be sized for a house that no longer exists. That is the reason NYSERDA gives for doing the arithmetic again. Tell the contractor what you have changed, with dates if you know them, and ask that the worksheet reflect it.

Tell the contractor about changes you plan too. If you expect to replace windows next year, say so now, because the worksheet should reflect it.

Some questions invite a rule of thumb. How many tons per square foot do I need? What size do most people in my street have? Is the next size up safer? Each one has an easy answer that avoids the calculation.

Better questions point to the worksheet. Which inputs did you use? Which design temperature did you assume? Did you measure the windows and the walls, or take them from a drawing? A contractor who did the work can answer in a few lines.

You do not need to read every number. Look for three things: your address, a design temperature, and a total at the bottom that matches the size on the quote. If the total and the quote disagree, ask why.

Ask for the file, or a printout, to keep with your papers.

A new job may change what the house needs. Cooling adds a summer load. A heat pump adds a question about the coldest night. Tell the contractor your plans before the calculation, not after, so that the worksheet reflects the whole job.

Start with one sentence on the phone: will you do a load calculation and give me the worksheet? Then listen to the answer.

• If the contractor says yes and describes what they will measure, ask when you will see it.
• If the contractor says the old size is fine, ask what changed in the house and why that does not matter.
• If the contractor says it is not needed, ask how the size will be chosen instead.

Then lay your quotes side by side. The ones that show a worksheet and name the ducts or pipes are the ones that can be compared. A quote with no worksheet leaves the size unexplained.

We do not do the calculation and we do not set the price. We give you the question and introduce a local contractor who does the work. You decide whether anyone visits.