What Size Heating and Cooling System Your House Actually Needs

By calculating your house, not by copying the nameplate off the unit being carried out. Section M1401.3 of the Residential Code of New York State says heating and cooling equipment is sized in accordance with ACCA Manual S from building loads calculated in accordance with ACCA Manual J, so one question sorts your bidders before anyone visits: will you run a load calculation on my house, and will you give me the worksheet?

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The question to ask on the phone, before anyone comes out

Ask every company you call whether they will perform a load calculation on your house and hand you the worksheet. The answer tells you which kind of contractor you are talking to, and it costs you nothing to ask. Section M1401.3 of the Residential Code of New York State says heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or another approved sizing methodology, based on building loads calculated in accordance with ACCA Manual J or another approved calculation methodology. New York adopts that text without amending it. The energy code carries the same requirement at section R403.7, where it is marked Mandatory, and the residential code repeats it at N1103.7. So this is not a premium service somebody adds to a bid. It is how the size is supposed to be arrived at. What you want to hear back is a yes, plus a list of what they need access to. What is worth noticing is a bidder who quotes you a size over the telephone, because the only house they have looked at is the one in the photograph on your old furnace.

Why your old furnace's nameplate is the wrong number to copy

It describes a house that may no longer exist. NYSERDA instructs contractors to conduct their own sizing calculation and product selection using an industry approved method, and to avoid rules of thumb or replacing a system with a like-sized system without completing that calculation. Its reasoning is worth repeating: even if the previous system size was appropriate when it was installed, the home may have had weatherization or other improvements since, and many fossil fuel furnace and boiler systems were oversized themselves when they went in. Copy the nameplate and you inherit somebody else's guess from 1994. The Department of Energy's Building America Solution Center puts the square-foot shortcut in the same place, stating that rules of thumb lead to excessively oversized systems, with increased cost, wasted energy and too-frequent on and off cycling. This bites unusually hard in this city. The median year a Syracuse house was built is 1948, and 85.5 percent of the housing stock predates 1980, so most of these houses have had insulation, windows or air sealing added at some point in the decades since the heating system was chosen.

Bigger is not a safety margin, and this is what it costs you

This is the thing most homeowners have backwards, and it is the reason a bigger number on a quote is not reassurance. NYSERDA states that the impacts on energy use and comfort begin when a system is sized at 120 percent of the building load, that severe impacts can begin above 150 percent, and that at a certain point a system may be entirely unable to achieve heating or cooling. What actually happens is low-load cycling: the minimum capacity of the equipment is higher than what the house is asking for, so it runs briefly, satisfies, shuts off and starts again. NYSERDA describes the results as inefficient operation, high energy bills and increased wear, and says constant cycling can lead to premature breakdown of components such as fans, the compressor or valves. It also explains the comfort failure people actually notice: an oversized system pushes hot or cold air toward the thermostat before it has mixed evenly into the room, the thermostat falsely senses that the setting is satisfied, and the house ends up with uneven temperatures from one room to the next.

The oversizing percentage you will see quoted elsewhere is not on this page

You will find trade articles giving a hard percentage above the calculated load that Manual S supposedly allows. We checked, and we are not going to repeat one. Manual S is a copyrighted ACCA publication that is not published in full at any public address, and New York's codes incorporate it by reference at M1401.3 and R403.7 without reprinting its selection limits, so there is no public text to cite. The percentages circulating in the trade press could not be traced back to ACCA or to any code text on the date we checked. The two figures used above, 120 percent and 150 percent of the building load, are NYSERDA's and are attributed to NYSERDA on purpose. That distinction matters when you are holding three quotes. If a salesperson tells you a specific percentage over the load is permitted, the useful reply is to ask where the number comes from, and then to ask what your calculated load actually is.

On the cooling side, oversizing is what makes a house cold and clammy

Air conditioning does two jobs, and only one of them is temperature. NYSERDA explains that dehumidification requires the system to run for an extended period, that in theory a larger system would wick more moisture from the air, but that oversized systems have shorter run times and may fail to dehumidify the space at all. So the oversized unit wins the thermostat and loses the room. You get a house that is cold and still feels damp, which is the complaint people describe as clammy and almost never connect to the size of the equipment. Syracuse makes this easier to get wrong, because the cooling season here is short and sharp rather than long: the normals give 58.3 days a year at 80F or above and only 6.8 at 90F or above, with June, July and August accounting for 483 of the annual 570 cooling degree days. A system chosen for the handful of extreme afternoons spends the rest of the summer cycling.

Minus ten degrees, and why that number decides the heating size

A heating system is not specified against an average winter. It is specified against a design temperature, which NYSERDA defines as the building load at the 99th percentile hottest and coolest hour of the year for the home's climate, with ASHRAE and ACCA each publishing recommended design temperatures by county and weather station. For a Syracuse address there is a local number in black and white: section 27-54(a)(2) of the City of Syracuse Property Conservation Code states that the capability of heating equipment to maintain the required indoor temperatures shall be based on an outside temperature of minus ten degrees Fahrenheit. Worth knowing before you ask, because the state code will not answer this for you. Table R301.2(1) of the Residential Code of New York State, the table that carries the winter design temperature, is published with every column blank, and its footnotes hand the job of filling it in to the local jurisdiction. So ask which design temperature your load calculation used. It is a single figure on the worksheet and it moves the answer more than any other input.

What a real load calculation involves, so you can tell whether one happened

It is an hour or two of work in your house, and it looks like it. NYSERDA describes the process: measuring ceiling height, wall area, window area, roof and floor area, estimating insulation R-values and assembly details and window U-values, and capturing the direction each window and wall faces, because south and west facing surfaces gain more heat from the sun. Surfaces shared between two conditioned spaces are excluded, because they neither lose nor gain heat. A blower door test assesses how tight the building actually is. NYSERDA also advises asking the homeowner what they plan for the next five to ten years of the home, including insulation, re-siding, window replacement, air sealing, remodeling or additions, because a house you are about to insulate has a different load from the one standing there today. Someone who walked the basement, looked at the old unit and wrote a number on a pad has not done any of this, and the worksheet you asked for is how you know.

The two exceptions that let a bigger unit be specified on purpose

Going up a size is not automatically a defect, and the code says when it is a documented decision rather than a shrug. Section M1401.3 carries two exceptions. The first applies where the specified equipment uses multistage technology or variable refrigerant flow technology and the calculated loads fall within the range of the manufacturer's published capacities for that equipment, which is why modulating equipment is treated differently from a single-stage box that is either on or off. The second applies where the manufacturer's published capacities cannot satisfy both the total and sensible heat gains calculated by the approved methodology, and the next larger standard size unit is specified. Equipment comes in steps, and sometimes your load falls between two of them. The difference between those exceptions and an oversized install is entirely in the paperwork: in one case somebody calculated the load and then selected upward for a stated reason, and in the other nobody calculated anything.

Where the sizing rule reaches into a house that is already standing

People assume this is a new-construction rule. Read the existing buildings chapter and it is less tidy than that. Section R503.1 of the Energy Conservation Construction Code of New York State says alterations to any building shall comply with the requirements of the code for new construction, and that an alteration shall leave the existing building not less conforming than it was before. Section R503.1.2, headed Heating and cooling systems, says new heating, cooling and duct systems that are part of the alteration shall comply with section R403, which is the section holding the Manual S and Manual J requirement. Section R504.2 then lists what counts as a repair for the purposes of the code, and the list is glass-only replacements, roof repairs, bulb or ballast changes and exterior door replacements. Heating and cooling equipment is not on it. What we will not do is tell you how that lands on your particular job. Whether a specific replacement at a specific address is an alteration is a determination for your code official, not for a website.

What to do with the worksheet once you have it

Put the three of them side by side. They are calculations of the same building, so the design temperature used, the square footage, the window areas and the resulting load should be recognizably the same house, and a bidder whose number lands far from the other two should be able to say which input drove it there. Then read the model numbers against those loads. That comparison takes twenty minutes and it is the only part of this process where you hold all the information. One honest caveat: if your system is twelve years old, working, and you are here because someone suggested a replacement, this may be a repair conversation rather than a purchase, and that is a different question for a different company. If you are genuinely buying, tell us about the house and what you have been quoted, and we will put you in front of licensed, insured contractors who work from a calculation rather than a nameplate.