Is the size on your quote based on your house?

Chosen when: How did the quote you hold choose the size: Nobody said

A size with no stated reason is a guess about your house, and an equipment size is a decision you live with every January. Ask the contractor for a Manual J, which is a calculation of how much heating and cooling your specific house needs, built from its size, insulation, windows, air leakage and location. Ask to see the result on paper, along with the equipment size it led to. If the answer is that they have always installed this size, you have learned something about the quote. Michigan's winters make the heating side worth the same scrutiny as cooling: Detroit Metro airport averages about 6,067 heating degree days a year, Grand Rapids about 6,408 and Traverse City about 7,104 (NOAA 1991 to 2020 normals, base 65 F).

Try first: Write down the size on the quote, who gave it, and ask by email whether a Manual J was done for your address.

Stop and call someone if: The contractor will not show a load calculation and still wants a deposit.

Furnace replacement · Do you need a permit to replace a furnace in Michigan?

Chosen when: How did the quote you hold choose the size: A square-foot rule

Floor area is one input to a load calculation and not a substitute for one. Two houses with the same square footage can need different equipment because of insulation, air leakage, windows, ceiling height and how much sun reaches the glass. We found no government or standards-body source for a square-feet-per-ton rule, so this tool prints none. Ask what the contractor's number assumed, then ask for the Manual J that checks it. Keep the size on the quote and the floor area they used together on one page, so the two can be compared later. Michigan counties span two climate zones: Wayne, Kent and Ingham are IECC zone 5, and Grand Traverse is zone 6, which is one more reason a single rule does not travel.

Try first: Ask which square footage the contractor used, and whether it included the basement and any addition.

Stop and call someone if: The size changes between visits with no new information about the house.

AC installation · Do you need a permit to replace a furnace in Michigan?

Chosen when: How did the quote you hold choose the size: It matched the old unit

The old equipment was sized by someone, for the house as it stood then, and it may have been too large or too small from the start. Matching it carries that forward, and it ignores what has changed since: new windows, added insulation, an addition, a finished basement. Ask the plain question: has anyone calculated the load for the house as it stands now? Request the Manual J, and ask the contractor to explain any difference from the old size and why they chose it. The permit side is a separate matter, but the paperwork overlaps: for a replacement the contractor files the permit with the office that enforces the mechanical code at your address, and plans are required for a residential home when total heating input exceeds 375,000 BTU/H (LARA's mechanical FAQ).

Try first: Find the old unit's data plate and photograph it, so the size and the quote can be compared.

Stop and call someone if: The reason for the size is only that it is what was there.

Furnace replacement · Do you need a permit to replace a furnace in Michigan?

Chosen when: How did the quote you hold choose the size: A Manual J load calculation; What are you sizing: Central heat pump

For a heat pump the load calculation is more than good practice. Lansing Board of Water & Light requires a Manual J, plus a pre-inspection through Hometown Energy Savers at 800-573-3503, for its central heat pump rebate, and Ann Arbor's A2ZERO rebate asks for a Manual J and an AHRI certificate. Cold matters too: ENERGY STAR cold-climate heat pumps are tested down to 5 F and ENERGY STAR advises pairing a heat pump with a backup heat source at lower temperatures, and Traverse City averages about 7,104 heating degree days a year. Ask how the calculation handled the coldest days and what backup heat the quote includes.

Try first: Ask for the Manual J as a document you can keep, and file a copy with any rebate paperwork.

Stop and call someone if: The quote promises the heat pump will heat the house in any weather with no backup and no calculation.

Heat pump installation · Heat pumps in Michigan winters: what the sources say about cold

Chosen when: How did the quote you hold choose the size: A Manual J load calculation; What are you sizing: Ductless mini-split

Each indoor unit of a mini-split serves its own room or zone, so the room-by-room part of the Manual J matters more than the whole-house total. Ask for the load of each room or zone, which indoor unit serves it and why. Ask what happens to a closed-off room that has no unit of its own. Mini-splits that replace electric heat can also qualify for utility rebates: DTE's page says its mini-split rebate must replace electric heating, and Consumers Energy's chart says its mini-split heat pump must replace primary electric heat (see the rebate and tax-credit finder, checked Oct 19, 2026).

Try first: List every room the quote is meant to heat and cool, and ask which unit covers each one.

Stop and call someone if: One indoor unit is expected to serve rooms that are separated by closed doors and no load figure is given.

Mini-split installation · Heat pumps in Michigan winters: what the sources say about cold

Chosen when: How did the quote you hold choose the size: A Manual J load calculation; What is the home like: Older, insulation unknown

An older home with unknown insulation is where a load calculation earns its keep, because the inputs are the part nobody knows. Ask the contractor what insulation levels and air leakage the Manual J assumed and how they found out. If the numbers are averages, ask whether the size would change if you added insulation or sealed air leaks first, since a smaller load can mean smaller equipment. Your city page shows the share of homes built before 1980 from Census data; that describes the age of the housing stock, not what is inside your walls. Detroit Metro airport averages about 6,067 heating degree days a year, so in this house the heating figure is the one to read first.

Try first: Look in the attic and note what you see, then tell the contractor before the calculation is done.

Stop and call someone if: The calculation lists insulation and windows the contractor never looked at.

Furnace replacement · Do you need a permit to replace a furnace in Michigan?

Chosen when: How did the quote you hold choose the size: A Manual J load calculation; How much sun and glass does the house have: A lot of sun and glass

Sunlight through windows is heat the air conditioner has to remove, so a house with a lot of glass facing south or west can carry a larger cooling load than its floor area suggests. A load calculation counts window area, direction and shading. Ask which windows were entered facing which way, and whether shade or window film would change the answer. Cooling is the smaller season in Michigan: Detroit Metro airport averages about 847 cooling degree days a year against about 6,067 heating degree days, but it is still the season that decides the air conditioner's size.

Try first: Walk the house on a sunny afternoon and note which rooms are hottest, then give the contractor the list.

Stop and call someone if: The calculation treats every window the same regardless of direction.

AC installation · Do you need a permit to replace a furnace in Michigan?

Chosen when: How did the quote you hold choose the size: A Manual J load calculation; What is the home like: Newer or air-sealed

A newer or air-sealed house loses less heat through leaks, which usually lowers its load. That is why matching an old size goes wrong most often here: equipment sized for a leakier house can be larger than the sealed one needs. Ask the contractor what air leakage the Manual J assumed and whether it was measured or estimated. For a place in Michigan, remember the climate zone: Wayne, Kent, Ingham and Kalamazoo counties are IECC zone 5, and Grand Traverse is zone 6, so winter design conditions still apply however tight the house is.

Try first: Ask whether the air leakage figure came from a test of your house or from an estimate.

Stop and call someone if: The size on the quote is larger than the old unit's with no reason given.

Mini-split installation · Heat pumps in Michigan winters: what the sources say about cold

Chosen when: Any other answers

A Manual J is the right thing to be holding. Check that it is for your address and describes your house: the floor area, windows, insulation and outdoor design temperatures. Then ask the contractor to show the equipment size that follows from it, and to explain any place where the quote differs from it. Keep a copy: Lansing Board of Water & Light requires one for its central heat pump rebate and Ann Arbor's A2ZERO rebate requires one for heat pumps. LARA's mechanical FAQ says plans are required for a residential home when total heating input exceeds 375,000 BTU/H; that is a plan-review threshold, not a sizing rule.

Try first: Compare the address, floor area and window count on the calculation with your house.

Stop and call someone if: The equipment on the quote is a different size from the one the calculation produced and no one can say why.

Furnace replacement · Do you need a permit to replace a furnace in Michigan?