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.

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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.

Sheet	Calculator	Method
01	Space heat lossFabric U·A·ΔT per element plus ventilation and infiltration. TGD L backstop check.	BS EN 12831 simplified. CIBSE Guide A.
02	Pipe sizingVelocity and pressure drop for water, any bore, six materials. Loading-unit demand and sizing table.	Darcy-Weisbach with Colebrook-White. CIBSE Guide C and G, BS 8558.
03	Duct sizingCircular, rectangular and flat oval. CO₂ build-up and AHU coil load alongside.	Darcy-Weisbach, equal friction. CIBSE Guide B2 and C.
04	Plant dutyHeat pump, AHU and chiller: load, electrical input, and matching Irish supplier ranges.	COP / EER energy balance. CIBSE Guide B3.

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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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).

Element name

U-valueW/m²K

Aream²

Ventilation / infiltration loss

Room volumem³

Air changes/hour

Internal design temp°C

External design temp°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·c_p of air). ΔT = internal − external design temp.

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Calc sheet PDF for Heat Loss

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Further reading (CIBSE) Guide A: Environmental Design — fabric performance & heat loss methods Guide B1: Heating — system design

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.

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Calc sheet PDF for Pipe Pressure Drop

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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

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.

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Calc sheet PDF for Duct Sizer

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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)

Room volumem³

Number of occupantsppl

Occupant activity level

Air changes per hour (ACH)/hr

Outdoor background CO₂ppm

Steady-state conc.

–

Time to 95% saturation

–

Model: C(t) = C_out + (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.

Room volumem³

Ventilation ACH/hr

Internal heat load (+heating / −cooling)kW

Heat recovery efficiency (η)

HR transfers moisture (enthalpy wheel)

Room temperature°C

Room target RH%

Min supply temp°C

Max supply temp°C

Outdoor temp°C

Outdoor RH%

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=T_out+η(T_room−T_out). Required supply temp solved from the internal load at this airflow, clamped to your min/max. Sensible load=ṁ·c_p·(T_supply−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.

1 · Load

Load

Enter capacity in kW, available/required pressure in kPa (accounting for fittings, bends etc. in the run), or a flow rate — this feeds the finder below and, if in kW, the electrical calc.

3 · Find a real unit for this load (Ireland suppliers)

How this works: this isn't a live catalogue search - it's a reference table of five real ranges per category, sold and supported in Ireland, filtered against the load you enter. Every product page and datasheet link was opened and checked on 17 September 2026 and points at the manufacturer's own document. Ranges are the nominal figures on those datasheets; always confirm the exact model and its capacity at your design temperatures before specifying it.

Range	Product / Supplier	Match	Product page	Datasheet

None of these fit? Search for an alternative

Type a product type or brand. The buttons build a search that includes your load, so the results are already in the right size band.

Product type, brand or model

Method: Heating: electrical input = Q_heat/COP, source-side duty = Q_heat − input. Cooling: electrical input = Q_cool/EER(dimensionless), heat rejected = Q_cool + input.

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Further reading (CIBSE) Guide B3: Air Conditioning and Refrigeration

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Heat loss, pipe and duct sizing, plant duty. Done properly, for free.

Calculation register

Why this exists

What's in Pro

Fabric loss

Add another element

Ventilation / infiltration loss

Calc sheet PDF for Heat Loss

Calc sheet PDF for Pipe Pressure Drop

Calc sheet PDF for Duct Sizer

CO₂ build-up (indoor air quality)

AHU thermal load (simplified)

What are you sizing?

1 · Load

2 · Electrical input

3 · Find a real unit for this load (Ireland suppliers)

None of these fit? Search for an alternative

Calc sheet PDF for Plant Duty

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