Clear guides for heating and cooling loads, duct sizing, plumbing and hydronics—written for engineers who want to understand the calculation and review the result.
These guides explain the assumptions behind common mechanical design calculations—not only the final equation. Each article connects inputs, units, boundary conditions and review checks to the decision an engineer must make on a project. Worked examples show how the calculation behaves, while scope notes identify where project codes, standards and professional judgement still govern the result.
Start with the discipline you are working on, then follow the internal links into the matching MEPFlow workflow or a free calculator. The library covers preliminary room-by-room HVAC loads, duct airflow and pressure relationships, domestic-water fixture-unit demand, and hydronic load-to-flow sizing. New articles extend those topic clusters without duplicating the same search intent.
Showing 20 of 20 guides
CFM ↔ size ↔ velocity ↔ friction
A ductulator gives a fast first size, but the correct result still depends on airflow, friction, velocity, fittings and the complete fan-pressure path.
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V = πD²L ÷ 4
Pipe volume is simple geometry, but using nominal instead of actual inside diameter can distort expansion-tank, glycol and fill-volume calculations.
Read the guideH = pipe + fittings + devices
Pump head is the pressure loss around the controlling closed-loop circuit—not the building height and not one representative pipe-friction value.
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Q = U × A × ΔT
Build a transparent winter heat-loss calculation from envelope surfaces, outdoor air and room conditions instead of relying on a single rule of thumb.
Read the guideSensible + latent + solar
Separate the gains that change temperature from those that add moisture, then determine when each room and the whole building actually peak.
Read the guideCOP = Qout ÷ Win
A high catalogue COP does not guarantee winter capacity. Read performance at the actual outdoor design temperature and plan supplemental heat deliberately.
Read the guideV = CFM ÷ A
Use airflow and free area to calculate FPM or m/s, then understand why an acceptable velocity still does not prove the duct system is properly sized.
Read the guideΔP = duct + fittings + devices
The longest duct route is not automatically the worst route. Add every straight, fitting and component loss along each complete fan-to-terminal path.
Read the guideACH = 60 × CFM ÷ volume
ACH is a useful room-volume relationship, but ventilation, exhaust and pressurization requirements still depend on the governing standard and space use.
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Empirical ↔ universal
Both methods estimate pipe head loss, but they do not use roughness or fluid properties the same way—and their coefficients cannot be interchanged.
Read the guidePstatic → Presidual
A strong static reading does not prove the remote fixture will have enough pressure when the building is flowing at design demand.
Read the guideHeat loss → flow → head
Recirculation flow replaces distribution heat loss; it is not the same as peak fixture demand or the building's domestic hot-water load.
Read the guideQ = Cv√(ΔP ÷ SG)
Valve line size is not valve control size. Use design flow, available pressure drop and operating range to choose a controllable valve.
Read the guideExpansion ÷ acceptance
The tank must accept the fluid's thermal expansion without allowing system pressure to fall below fill requirements or exceed relief limits.
Read the guideV = Q × t ÷ (500ΔT)
A buffer tank adds thermal mass when active system volume is too small to give the heat pump or boiler a stable minimum run time.
Read the guideρ × cp × flow × ΔT
Glycol provides freeze protection, but copying water-based GPM and head-loss values can undersize flow, pumps and expansion volume.
Read the guideQ = UAΔT + Qair + Qgains
A practical guide to turning floor-plan geometry, envelope data, weather, people and outdoor air into reviewable room-level HVAC loads.
Read the guideCFM • V • f • ΔP
Understand what a duct sizer is actually solving—and how to review a system after the first automatic size is selected.
Read the guideWSFU → demand → pipe size
A practical workflow for moving from connected plumbing fixtures to cold- and hot-water demand, main sizes and branch checks.
Read the guideBTU/h → GPM → diameter
A practical explanation of the load-to-flow calculation and the hydraulic checks that turn GPM into a reviewable pipe size.
Read the guideUse MEPFlow to trace a floor plan, connect systems, calculate results and keep design assumptions visible for review.
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