REVIT Air Conditioning & Ventilation

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@Grok - explain users how loads can be calculated in MagiCAD (heat loads for cooling), provide the best practices. We assume we are going to design a project in the US

 

[Grok]

- explain users how loads can be calculated in MagiCAD (heat loads for cooling), provide the best practices. We assume we are going to design a...

Cooling load calc in MagiCAD (US project)

MagiCAD does not replace a full ASHRAE load study by itself. On a US job you typically calculate cooling loads in Revit (or an ASHRAE/ACCA tool), then push the resulting airflows / capacities into MagiCAD for duct, AHU and pipe sizing. MagiCAD’s own heating/cooling module is built around EN / national European methods, so treat it as a check or a first pass, not as the stamped load report.

1. Set the project up for US work first

• Project units: IP (BTU/h, CFM, °F, in. w.g., ft). Do this before you start rooms.
• Location: set the Revit location to the actual city so design DB/WB come from ASHRAE Fundamentals (0.4% or 1% cooling, 99.6% or 99% heating). Do not leave a default European climate.
• Indoor design: typical office/school cooling 75 °F DB / 50–55% RH; follow the owner’s basis of design, not MagiCAD’s European defaults (often 24–26 °C / 50%).
• Energy code: ASHRAE 90.1 or the local IECC edition for envelope, lighting power density and equipment. Ventilation from ASHRAE 62.1 (or 62.2 for residential).

2. Model that the loads can actually see

Loads are only as good as the rooms and envelope.

• Use Revit Spaces (not just Rooms). Every conditioned volume needs a Space, bounded correctly, with the right Upper Limit / Limit Offset so plenums and shafts are not “in” the occupied zone.
• Place Spaces from the architectural link; do not sketch them by hand unless the arch model is unusable.
• Assign a Space Type (Office, Classroom, Corridor, etc.) so occupancy, lighting and plug loads come from a consistent schedule.
• Build the envelope with real constructions: walls, roofs, slabs, glazing with U-factor, SHGC and VT that match the spec (NFRC values, not a generic “double glazed” type). Orientation and shading matter a lot for cooling.
• Define thermal properties on building elements. If the arch model has no materials, you must add them or MagiCAD/Revit will use dummy U-values and the solar gain will be garbage.
• Separate plenums, shafts, stairs and unconditioned rooms. Give them their own Spaces and the correct condition type (Unconditioned / Unoccupied / Indirectly conditioned).

3. Where to actually run the cooling load

Preferred US workflow

• Revit Analyze → Heating and Cooling Loads (or Insight / a dedicated ASHRAE tool such as HAP, TRACE, or IES if the job requires it).
• Method: Revit’s engine is a simplified RTS-style calc. For anything with real thermal mass, large glass, or a PE stamp, use HAP/TRACE 3D Plus / IES and import the results.
• Residential / light commercial: ACCA Manual J (Wrightsoft, Elite, etc.), not MagiCAD.
• When the loads are accepted, enter the supply airflow and total/sensible cooling on the Space or on the MagiCAD air terminal / AHU. MagiCAD Ventilation then sizes ducts, pressure and sound from those numbers.

If you still run MagiCAD’s own heating & cooling calc

• Open the MagiCAD heating/cooling calculation, set the standard as close as you can get (there is no native ASHRAE 183 / RTS option in most builds).
• Override indoor/outdoor temps, internal gains and ventilation to ASHRAE 62.1 rates. Do not leave EN 12831 / VDI 2078 defaults.
• Check that solar, lighting and people gains are in BTU/h and that latent load is not ignored. US cooling is often latent-driven in the Southeast and Gulf Coast.
• Treat the MagiCAD result as a cross-check against Revit/HAP, not as the official load.

4. Inputs that decide whether the cooling load is believable

• People: ASHRAE Fundamentals sensible + latent per person, with a realistic occupancy schedule and diversity (open office is not 1 person / 50 ft² at 100% all afternoon).
• Lighting: LPD from 90.1, not the fixture catalog wattage unless you are doing a lighting-specific model. Use the actual lighting schedule if you have it.
• Plug / process: nameplate × usage factor × diversity. Data rooms, kitchens and labs need their own Spaces and often a dedicated unit.
• Ventilation: ASHRAE 62.1 Vbz by Space Type and occupancy. Outdoor air is a huge cooling load; do not use a flat 1 CFM/ft² unless the BOD says so.
• Infiltration: only on perimeter Spaces, and only if the envelope is not assumed pressurized. MagiCAD/Revit defaults are often too high or too low.
• Solar / glass: SHGC, frame factor, interior/exterior shades, overhangs. West glass in Phoenix is not the same as north glass in Seattle.
• Schedules: cooling load is a peak, not an annual energy number. Use a design-day occupancy/lighting/equipment profile, not an annual average.
• Diversity at AHU / plant: sum-of-peaks ≠ block load. Apply zone diversity before you size the air handler and the chiller.

5. Practical MagiCAD workflow after the loads exist

• Write supply CFM, min OA, and total/sensible BTU/h onto each Space (Revit shared parameters or MagiCAD room data).
• Place air terminals in MagiCAD and let them inherit the Space airflow. Avoid typing CFM on every diffuser by hand.
• Size ducts in MagiCAD Ventilation (constant pressure, static regain, or equal friction — pick one method and stick to it). Use US velocity limits (e.g. 400–500 fpm on small runouts, higher on mains per the BOD/SMACNA).
• Check coil entering/leaving conditions: mixed air, LAT, SHR. If MagiCAD only sees total CFM and no LAT, the cooling coil and reheat will be wrong.
• For hydronic cooling, export the coil loads to MagiCAD Piping and size chilled water with US ΔT (typically 10–16 °F), not a European 6 K default.
• Re-run pressure drop and fan static after fittings, fire dampers and sound attenuators are in the model. First-pass MagiCAD runs almost always understate ESP.

6. Best-practice checklist (US)

• One source of truth for loads (Revit or HAP/TRACE). MagiCAD consumes those numbers; it should not invent a second set.
• Match Space names/numbers to the architectural room schedule so coordination and CA stay sane.
• Do not oversize cooling “for safety.” Extra tonnage without a matching OA/dehumidification strategy is how you get 80% RH and mold complaints.
• Separate envelope-dominated Spaces from internally loaded Spaces. They do not peak at the same hour.
• Document design DB/WB, indoor setpoints, ventilation standard, and diversity in the MagiCAD/Revit project info so the next person does not “fix” your 75 °F to 24 °C.
• Calibrate a couple of Spaces by hand (q = 1.08 × CFM × ΔT for sensible; 0.68 × CFM × ΔW for latent) before you trust a 200-room printout.
• If the architect changes glass or roof mid-project, re-run loads before you resize a single duct. MagiCAD will happily resize to the old CFM.
• Keep a spreadsheet or schedule that maps Space → load → terminal → AHU. That is what you issue, not a screenshot of the MagiCAD dialog.

Bottom line: for a US project, do the cooling load in an ASHRAE-capable tool (Revit loads for simple work, HAP/TRACE/IES when it matters), lock CFM and BTU/h onto the Spaces, then use MagiCAD for what it is actually good at — duct/pipe sizing, pressure, and coordination. If you only run MagiCAD’s native load calc with European defaults, your coils, OA and humidity control will not match US practice.