For a Montgomery County HVAC replacement, equipment size should be based on a documented heating and cooling load rather than square footage alone. Montgomery County's residential mechanical permit process identifies ACCA Manual J calculations as part of the information required for applicable work; if the calculation is required and missing, the application may not be processed. The calculation is not paperwork to add after the equipment is chosen—it informs the choice.
What Manual J is meant to answer
A load calculation estimates how much heating and cooling the home needs under design conditions. It considers the building envelope, windows, insulation, orientation, air leakage, rooms, and local climate assumptions. The result should lead to equipment selection and distribution decisions, not simply justify a preferred model.
Why oversized equipment causes problems
An oversized air conditioner can satisfy the thermostat quickly without running long enough to remove humidity effectively. Short cycling can create temperature swings, noise, wear, and comfort complaints. An oversized furnace can also cycle more than intended. In the DMV's humid summers, moisture control is part of comfort—not an optional upgrade.
What to request from a contractor
Ask for:
- The heating and cooling loads by zone or room where applicable
- The design temperatures and assumptions used
- Existing and proposed insulation, windows, and air-sealing assumptions
- Equipment model numbers and rated capacity at relevant conditions
- Blower, duct, static-pressure, and airflow assumptions
- Manual S or equivalent equipment-selection documentation
- Manual D or equivalent duct-design review when ductwork changes
- A written explanation of any capacity mismatch
Replacement does not always mean like-for-like size
The old unit's capacity may reflect old construction, previous comfort problems, or a prior sizing error. If insulation, windows, air sealing, additions, zoning, or duct changes have occurred, the new load may be different. If the home has hot or cold rooms, ask whether duct balance, leakage, return-air capacity, or envelope improvements are part of the solution.
Montgomery County permit and inspection questions
The county requires a mechanical permit for many HVAC installations and replacements. A gas furnace may involve both DPS and WSSC inspections, while an all-electric heat pump follows a different inspection path according to the county's published process. Ask the contractor to identify the exact permit categories, applicant, equipment documents, load calculations, inspections, and closeout records for your address.
How to compare HVAC proposals
Do not compare “three-ton system” against “three-ton system” as though that is enough. Create a table for capacity, efficiency, fuel, refrigerant, indoor coil or air handler, thermostat, electrical work, line set, pad, condensate, duct modifications, permits, labor warranty, manufacturer warranty, startup testing, and commissioning. Ask what happens if measured airflow or static pressure does not meet the design.
Red flags
- Sizing based only on square footage
- No load calculation or an unexplained refusal to provide assumptions
- Equipment model numbers missing from the proposal
- A low price that excludes permits, duct corrections, startup, or electrical work
- A promise that a larger system will “cool faster” without discussing humidity
- No plan for condensate, filter access, airflow, or final testing
Use the Renovessa estimator
Enter the home's location, approximate size and layout, current equipment, comfort problems, fuel type, duct conditions, insulation or window changes, and replacement goal in the Renovessa estimator. The structured brief helps contractors respond to the load and scope questions that make Montgomery County proposals comparable.
Sources and limitations
This guide uses Montgomery County's residential mechanical permit process, DOE cold-climate heat-pump sizing guidance, ENERGY STAR quality-installation guidance, and Maryland program documentation. It is not an engineering design or permit determination. A qualified HVAC professional must perform the project-specific calculations and confirm current county requirements.