The calculation behind the size
Ask for the Manual J load calculation summary and how the equipment was chosen under Manual S. Both are Air Conditioning Contractors of America (ACCA) methods, and the Department of Energy’s heat pump replacement checklist calls for a room-by-room Manual J and Manual S selection based on it. The summary should list heating and cooling loads in Btu per hour and the indoor and outdoor design temperatures used.
Tonnage alone says little. One ton is 12,000 Btu per hour of cooling, so a 3-ton unit is rated for 36,000 Btu per hour of cooling. That number doesn’t tell you how much heat the unit delivers on a cold January morning.
What the calculation assumed about your house
Load results depend on the insulation and air-leakage numbers entered. PNNL’s cold-climate sizing guide says to enter accurate data for both and calls a blower-door test the most accurate way to measure air leakage. Ask each bidder whether those numbers were measured or assumed.
If you plan to insulate or air-seal first, tell every bidder, because a DOE guide notes that reducing building loads can reduce the need for supplemental heat. Give all bidders the same information: renovation history, which rooms run hot or cold, and when. If two bids land on different sizes, compare their assumptions instead of picking the bigger one.
Winter output, summer comfort
Ask for the exact outdoor and indoor model numbers and the manufacturer’s expanded performance data for that pair. PNNL’s air conditioning guide says Manual S calls for performance at design conditions, not just the nameplate rating.
Sizing for winter tends to oversize for summer. An oversized system cools the air quickly and shuts off before it removes much moisture, which can leave the house clammy. Variable-capacity units help because they can run at lower output, so ask for the minimum output as well as the maximum. PNNL’s mini-split guide makes the same point for ductless systems. A good question is: “After you size this for winter, how does it handle our summer humidity?”
The backup heat plan
Every air-source heat pump has a thermal balance point: the outdoor temperature below which it can’t meet the home’s heating load alone. The DOE heat pump guide calls it a key number for sizing, backup heat and controls. Ask for the estimated balance point and whether the system is meant to carry the full design load or lean on backup.
If there is backup, ask what it is (electric resistance or gas), its capacity, and the outdoor temperature where it turns on. PNNL calls electric resistance backup inefficient and typically expensive and says to minimize it. Get the thermostat staging settings in writing.
Room-by-room delivery
A cold back bedroom can be an airflow problem rather than a size problem. The DOE checklist calls for inspecting and sealing exposed ducts, checking whether supply and return airflow are balanced and ducts are properly sized, and testing room-to-room airflow at commissioning. Ask which of those steps are in the price. For ductless systems, ask which rooms each indoor unit serves and its capacity versus that room’s load.
Philadelphia permit paperwork
Installing heating or air conditioning in Philadelphia needs a Mechanical Permit from L&I, and a licensed contractor has to do the work. Replacements in one- and two-family homes can use the no-plans EZ route. For residential work, the City requires an HVAC Equipment Design Form with the application and a Duct and Envelope Testing Certificate and HVAC Equipment Certificate for the field inspector. A bidder who plans to pull the permit should be producing design numbers anyway, so ask to see them.
The City lists a $100 filing fee credited toward the permit fee and, for appliances submitted separately in one- and two-family homes, $69 each. A heat pump also needs an Electrical Permit; our guide to Philadelphia electrical permits covers that. Ask whether both permits are in the bid.
Panel capacity is a separate question
Thermal sizing doesn’t tell you whether your panel can carry the heat pump plus any electric backup. PNNL’s panel calculator gives estimates only and says it doesn’t replace a licensed electrician’s evaluation. Ask the quote to separate confirmed electrical work from allowances. Our panel evaluation guide explains when a panel needs a closer look, and the contractor hiring checklist covers vetting the installer.