Fabric loss
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Building services engineering calculators
Heat loss, pipe and duct sizing, plant duty. Done properly, for free.
Standard CIBSE methods, laid out so you can see every step. Method and source on every sheet, so you can check it against your own code before you rely on it.
Designed byA 4th year Energy Engineering student, Galway
Method basisCIBSE Guides A, B, C, G · TGD F/L · BS EN 806-3 · switchable per sheet
UnitsMetric and imperial
Calculation register
Pick a sheet. Every calculator shows its method and cites its source.
Why this exists
The checks building services engineers run most often were never in one place. Now they are. Each one is the standard method, laid out so you can see exactly what it's doing.
What's in Pro
Every calculator stays free. Pro is a one-off €10 for the parts you'd put in a design file:
- PDF calc sheets with project, reference, revision and engineer on the header
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- Your own defaults for roughness, U-values and velocity limits
Heating · Sheet 01
Space Heat Loss
Fabric loss (U·A·ΔT per element) plus ventilation and infiltration loss, with the TGD L backstop check on every element.
Note: this is the standard simplified method (BS EN 12831). U-values shown are typical starting points - overwrite them with the actual build-up from your project specification or the Part L / TGD tables.
Fabric loss
| Element | U-value (W/m²K) | Area (m²) | Loss (W) | TGD L backstop |
|---|
The five standard elements are fixed - type the U-value and area for each. Backstops are the TGD L 2022 (Dwellings) Table 1 area-weighted values: pitched roof 0.16, flat roof 0.20, walls 0.18, floors 0.18, windows / doors / rooflights 1.4 W/m²K (verified against the gov.ie PDF, Sep 2026).
Add another element
Garage door, party wall, rooflight, conservatory glazing, exposed soffit - anything not in the list above. Name it, give it a U-value and an area, and it joins the table with its own loss and backstop check (matched on the words wall / roof / floor / window / door).
W/m²K
m²
Ventilation / infiltration loss
m³
°C
°C
Fabric check
Ventilation loss
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Total heat loss
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Method: Fabric: Σ(U×A×ΔT). Ventilation: 0.33 × ACH × Volume × ΔT (0.33 Wh/m³K ≈ ρ·cp of air). ΔT = internal − external design temp.
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Calc sheet PDF for Heat Loss
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Water Services · Sheet 02
Pipe Sizing
Flow rate in, velocity and Pa/m out — circular or rectangular, metric or imperial. Limits from CIBSE, the Irish TGDs, British Standards or your own company rules. Demand from BS 8558 loading units if you don't have a flow yet.
Method: Darcy-Weisbach + Colebrook-White (laminar below Re 2300 uses f=64/Re). Rectangular sections use hydraulic diameter for the friction calc. Roughness values are typical starting points — verify against your project spec.
Pro
Calc sheet PDF for Pipe Pressure Drop
Pro turns this into a signed-off calc sheet: project, reference, revision and engineer on the header, all inputs and results, method and source. Save it to the design file.
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Ventilation · Sheet 03
Duct Sizing
Flow rate in, velocity and Pa/m out — circular, rectangular or flat oval, metric or imperial. Limits from CIBSE, the Irish TGDs, British Standards or your own company rules. CO₂ build-up and AHU coil load below.
Method: Darcy-Weisbach + Colebrook-White. Rectangular ducts use hydraulic diameter (2ab/(a+b)) for the friction calc — a good approximation for most aspect ratios; very flat/elongated ducts want CIBSE Guide C's additional aspect-ratio correction, not yet included.
Pro
Calc sheet PDF for Duct Sizer
Pro turns this into a signed-off calc sheet: project, reference, revision and engineer on the header, all inputs and results, method and source. Save it to the design file.
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Further reading (CIBSE)
Guide B2: Ventilation and Ductwork
Guide C: Reference Data — duct sizing spreadsheets
CO₂ build-up (indoor air quality)
m³
ppl
/hr
ppm
Steady-state conc.
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Time to 95% saturation
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Model: C(t) = Cout + (G/Q)×(1−e−ACH·t). G = occupants × activity rate. Q = ACH × volume. Assumes a well-mixed room and constant emission rate. Target indoor CO₂ for good cognitive function is typically kept below 800–1000 ppm.
AHU thermal load (simplified)
Scope note: this is a simplified sensible + latent coil-load estimate using standard psychrometric approximations (Magnus formula for saturation vapour pressure) — it's a sanity-check tool, not a replacement for full psychrometric AHU selection software. Required supply temperature is capped at your min/max supply limits; if the load can't be met within those limits at this airflow, the tool flags it rather than silently exceeding your caps.
m³
/hr
kW
°C
%
°C
°C
°C
%
Ventilation airflow
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Supply air temp (after coil)
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Sensible coil load
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Latent coil load
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Total coil load
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Method: ventilation airflow Q=ACH×Volume/3600. Air after heat recovery: T₁=Tout+η(Troom−Tout). Required supply temp solved from the internal load at this airflow, clamped to your min/max. Sensible load=ṁ·cp·(Tsupply−T₁). Latent load uses humidity ratios (Magnus approximation) between outdoor-after-HR air and room target RH.
Refrigeration & Heat Pumps · Sheet 04
Heat Pump / AHU / Chiller Duty
Pick what you're sizing first — the right fields show up for it. Then match the load against real Irish supplier ranges.
Scope note: this doesn't model actual refrigerant properties (P‑h enthalpy data for R32/R410A/R290 etc. is proprietary/complex and varies by manufacturer selection software) — it works off duty and COP/EER, which covers most day-to-day sizing checks. If you want true refrigerant-side mass flow, that needs a manufacturer's selection tool.
What are you sizing?
AC Units covers chillers, VRF, and HVRF (hydro VRF) together — each row in the finder below is clearly labeled with which type it is.
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