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Heating. Water. Air.

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Commercial building plant room at night: insulated heating pipework, valves and an air handling unit under amber work lights

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.

Start with heat loss See all calculators
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

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.

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Design the

heating

  1. Home
  2. Heating
Commercial condensing boiler cascade with brass and steel heating pipework, red isolation valves and temperature gauges

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 a steady-state fabric + ventilation loss (U·A·ΔT + 0.33·n·V·ΔT, after CIBSE Guide A and the principles of BS EN 12831-1). It is not the full EN 12831 method: it leaves out thermal bridging, temperature factors for unheated spaces, the ground-loss method and heat-up allowance. 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

ElementU-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 Table 1 maximum area-weighted average U-values (Um) for the building type you pick. Dwellings: pitched roof 0.16, flat roof 0.20, walls 0.18, floors 0.18, windows / doors / rooflights 1.4 W/m²K. Buildings other than dwellings: pitched roof 0.16, flat roof 0.20, walls 0.21, floors 0.21, windows / personnel doors / rooflights 1.65, curtain walling 1.8, vehicle and large doors 1.5 W/m²K. Underfloor heating: floor 0.15. Individual elements may be worse up to roof 0.3, wall / floor 0.6, openings 3.0 (both verified against the gov.ie PDFs).

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
Fabric loss
–
Ventilation loss
–
Total heat loss
–

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.

Pro

Calc sheet PDF for Heat Loss

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 A: Environmental Design — fabric performance & heat loss methods Guide B1: Heating — system design

Next: size the LTHW pipework pick a heat pump for this load

Design the

waterservices

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  2. Water Services
Polished copper water services pipework with brass compression fittings and chrome lever valves in a building riser

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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Further reading (CIBSE) Guide G: Public Health and Plumbing Engineering — water services design Guide C: Reference Data — pipe sizing spreadsheets & fluid properties

Related: heat loss for the load duct sizing (same engine, air)

Design the

ventilationservices

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  2. Ventilation
Galvanised steel spiral and rectangular ductwork with flanged joints and dampers hung in a commercial ceiling void

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.
–
Time to 95% saturation
–

Model: C(t) = Cout + (G/Q)×(1−e−ACH·t). G = occupants × activity rate. Q = ACH × volume. Assumes a well-mixed room that starts at the outdoor level, with constant emission and air change. CO₂ rates: Persily & de Jonge (2017), Indoor Air, Table 4, adult 30–40 years (use a lower rate for children). CO₂ is an indicator of fresh air per person, not a health limit: BS EN 16798-1 Annex B uses roughly 550 / 800 / 1350 ppm above outdoor for categories I / II / III, and the HSE treats a steady reading above 1500 ppm as poor ventilation. The Irish requirement is the air rate itself: TGD F 2019 §1.3.1.7, 10 l/s per person in buildings other than dwellings.

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
–
Supply air temp (after coil)
–
Sensible coil load
–
Latent coil load
–
Total coil load
–

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.

Related: find an AHU for this airflow pipe sizing (same engine, water)

Design the

heat pumps& AHUs

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  2. Refrigeration & Heat Pumps
Air-cooled chiller and monobloc heat pump units on a commercial rooftop plant deck at dusk

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.

Related: work out the heat loss first AHU coil load

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Engineer's desk at night with calculation sheets, a rolled services drawing, scale rule and pencil under a desk lamp

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