The sizing decision must connect those exposures to room loads and then to vertical air delivery. Manual J credits shared boundaries and assigns roof and glass gains; Manual S checks whether the selected equipment can modulate low enough for the compact load; Manual D determines whether attic branches and returns can serve three stacked levels.

Count exposed surfaces before counting square feet

Floor area cannot show whether a wall faces outdoors, a garage, the ground, or another conditioned unit. In an attached home, each surface needs its real adjacent condition. Crediting the party wall can reduce the calculated whole-building load, while the exposed end wall and roof may concentrate demand in a few rooms.

Orientation and shade matter too. A west end wall and top-floor windows can peak after lower rooms have already cooled. If the thermostat or return is on a lower level, the system may stop before the roof-exposed rooms reach comfort. That pattern is a load-distribution and air-delivery question, not proof that the entire house needs more nominal capacity.

Both sensible and latent loads remain part of the calculation. Dry summer context points attention toward sensible solar and envelope gains, but occupants, ventilation, and infiltration still add moisture. The designer should calculate rather than erase the latent component.

Keep West Valley City distinct from Salt Lake City

The approved climate proxy is the NOAA/NCEI Salt Lake City International Airport 1991–2020 normals: July normal maximum and minimum temperatures of 94.0°F and 68.2°F and January normals of 38.6°F and 24.2°F. The station provides nearby valley context. It is not West Valley City itself, a parcel design condition, or an equipment-selection input.

Site elevation, solar exposure, surroundings, and building boundary still belong to the project. This article is not a second Salt Lake City page. Salt Lake City’s approved blueprint examines a retained furnace, coil, and outdoor-unit match; West Valley City examines an attached-home boundary and stacked-floor delivery.

The Utah AC sizing guide provides the broader internal context. It should not replace the municipal identity or the townhome’s individual exposure map.

A hypothetical three-level end-unit townhome

Consider a hypothetical West Valley City attached-home case: an approximately 1,650-square-foot, early-2000s three-level end-unit townhome. It has one party wall, an exposed west end wall, top-floor roof exposure, compact attic supply ducts, and a furnace-closet return. The lower level overcools while the upper level remains warm, and a single-stage replacement has been proposed from total area alone. This illustration is not a claim about citywide housing stock.

The load model should credit the conditioned party-wall adjacency but give the west wall, roof, and top-floor glass their correct exposures. The air model should trace supply and return paths across all three levels. A reduced whole-house load after party-wall credit does not mean every room is easy to serve; the top level may still require a larger share of delivery during peak sun.

The 1,700-square-foot sizing comparison is useful only as an area-based reference. It cannot decide the party-wall boundary, roof gain, vertical stack effects, attic duct losses, or minimum equipment output in this hypothetical home.

Manual J balances party-wall credit against top-floor gain

Party-wall boundary credit

ACCA Manual J should identify the adjacent condition for every wall, ceiling, floor, and foundation surface. The party wall receives the appropriate treatment for adjoining conditioned space; it should not be entered as a sun-exposed exterior wall. The west end wall, roof, glazing, shading, infiltration, occupants, and internal gains receive their own inputs.

Room-by-room output prevents the shared-wall credit from being spread evenly. Middle rooms near the party wall may have modest peak sensible demand, while roof- and west-exposed rooms carry more. That load map becomes the basis for duct airflow targets and any zoning discussion.

The climate station’s monthly averages do not establish the design temperature. The report should name the approved project weather source and state assumptions for orientation, shade, assemblies, and leakage. Unknown conditions belong in the survey, not in an arbitrary capacity cushion.

Manual S checks minimum output for a compact load

ACCA Manual S compares the corrected load with performance data for the exact outdoor unit, indoor coil, blower, and controls. Once party-wall credit reduces the whole-building total, minimum output and staging can become as important as peak capacity. A single-stage unit chosen from detached-house area may cycle while still failing to solve top-floor delivery.

Total and sensible capacity should be reviewed at the approved condition and supported airflow. The latent component may be smaller than in humid markets, yet it remains in the comparison. A variable or two-stage option can improve part-load fit only if the documented minimum range aligns with the compact building load and the distribution can operate at the associated airflows.

Equipment selection must not absorb duct losses as hidden tonnage. Attic leakage and conductive gain belong in the load/distribution model, while the matched-system table states coil output. Keeping those roles separate makes alternatives comparable.

Manual D divides airflow across stacked floors

ACCA Manual D turns each room load into a delivery design using the blower’s available pressure. The furnace-closet return, filter, coil, risers, attic trunk, fittings, branches, and registers all consume pressure. Three levels also create routing and balancing constraints that total square footage cannot describe.

The design should identify the critical top-floor path and the return route from closed rooms. Lower-level registers may need less air than their size currently receives, but balancing them should not create excessive pressure or noise. If the vertical system lacks capacity, changes to trunks, branches, returns, registers, or zoning may be more effective than enlarging the compressor.

DOE guidance on sealed and insulated ducts supports inspecting attic runs for leakage, insulation defects, compression, and poor fittings. Those losses should be corrected and tested where practical rather than estimated from the upper-floor temperature complaint.

Verify attic ducts and return paths at peak sun

Top-floor roof gain and vertical air path

Commissioning should include a period when the west wall and roof are meaningfully loaded, while avoiding the claim that one observation is a design calculation. Record blower setup, total external static pressure, return behavior, and representative room delivery. Door position and thermostat location should be noted so the results explain the actual air loop.

The West Valley City Building and Development page identifies the municipal inspection authority, and the city’s Building Inspections FAQ documents mechanical-contractor verification in the permit process. These are administrative sources. ENERGY STAR HVAC Quality Installation guidance supplies the complementary selection, airflow, and commissioning perspective.

Thermostat and sensor location deserve a final check in a stacked plan. A lower-level thermostat can see comfort before the roof-exposed rooms do, while a top-floor sensor may drive the lower level colder if distribution is unchanged. Control changes should follow the room-load and airflow evidence, not be used to disguise it. If zoning or remote sensing is proposed, the equipment’s minimum airflow and return arrangements must be tested for every active configuration.

The owner’s closeout package should show which wall was treated as shared, which surfaces were exposed, where the ducts run, and what blower and balancing settings were accepted. Those details are essential if a neighboring unit becomes vacant, the roof changes, or later service disturbs a damper.

The final West Valley City proof traces the thermal boundary from party wall to roof and then follows the supply-and-return loop from the furnace closet to each level. Every exposed surface should map to a room load, and every room load should map to a verified air path. When those drawings close, the equipment can be selected for the actual compact load instead of a detached-house assumption.