What this calculator does

The widget designs and sizes a **closed-loop hot water heating system** (LTHW/MTHW touchpoints). You can split a plant into multiple segments (branches/loops) — for example, radiators, underfloor heating, and an AHU coil — and the tool will:

• Convert **heat load → flow rate**, or run in **reverse mode** (given flow → compute load).
• Pick a **pipe internal diameter** from standard tables (Steel Sch 40 / Copper Type L / PPR) using your **velocity limit**.
• Estimate **head loss** using a chosen **design gradient** and **equivalent length** (straight length + fitting allowance).
• Roll up **costs** (pipe + insulation + valves/fittings factor + wastage), with India/US/UK rate presets and currency.
• Rough-size the **expansion vessel** from system water content (informative, user to verify).
• Generate a **downloadable HTML report** with KPIs, a detailed table, assumptions and a change log.

Standards presets (EN/CIBSE/ASHRAE/IS) auto-populate velocity limits, ΔT hints and design gradients. A note reminds you to verify the clause/annex in your project specification.

How the calculations work (in plain English)

How to use the widget (step-by-step)

1. 1 Choose SI or Imperial, then pick your Market (India/US/UK) and Currency.
2. 2 Select a Standard (e.g., EN 12828 + EN 14336). The note shows: “Values derived from <Standard> (user must verify).”
3. 3 Click a Preset (80/60, 70/50, 55/45 or 180/160 °F) to fill supply/return temps.
4. 4 Add one or more Segments (e.g., Radiators, Underfloor, AHU coil). For each: enter load or flow, length, fittings %, velocity cap and choose a material.
5. 5 Review the KPIs (Total load/flow, worst-case head, vessel estimate) and the table of pipe picks, velocities, and costs.
6. 6 Click Download HTML Report to save a printable, shareable report with KPIs, detailed table, assumptions and a change log.

Sample project: Office floor LTHW system (worked example)

Let’s size three loops on a single floor: Radiators (east wing), Underfloor heating (meeting rooms), and an AHU heating coil. Use the defaults: Standard = EN/CIBSE, design gradient S = 200 Pa/m, water properties interpolated by the tool.

Given data (inputs)

Step-by-step calculations

1. Flow rate for each segment (SI):
   • Radiators: \( \dot{m} = 25/(4.186·20)=0.299\ \mathrm{kg/s}\Rightarrow \dot{V}=0.305\ \mathrm{L/s} \) (≈ 4.84 gpm).
   • Underfloor: \( \dot{m} = 15/(4.186·10)=0.359\ \mathrm{kg/s}\Rightarrow \dot{V}=0.363\ \mathrm{L/s} \) (≈ 5.75 gpm).
   • AHU coil: \( \dot{m} = 30/(4.186·20)=0.359\ \mathrm{kg/s}\Rightarrow \dot{V}=0.366\ \mathrm{L/s} \) (≈ 5.81 gpm).
2. Required internal diameter at velocity cap \(v_{max}\):
   • Radiators: required ID ≈ 16.1 mm → pick **¾″ Sch 40** (ID ≈ 20.93 mm); velocity ≈ 0.89 m/s.
   • Underfloor: required ID ≈ 19.6 mm → pick **PPR 25** (ID ≈ 20.4 mm); velocity ≈ 1.11 m/s.
   • AHU coil: required ID ≈ 17.6 mm → pick **¾″ Type L** (ID ≈ 19.99 mm); velocity ≈ 1.17 m/s.
3. Head loss using design gradient \(S = 200\ \mathrm{Pa/m}\) and \(L_{eq}=L\,(1+f\%)\):
   • Radiators: \(L_{eq}=45×1.3=58.5\,\mathrm{m}\) → Δp=11,700 Pa → head ≈ 1.22 m.
   • Underfloor: \(L_{eq}=60×1.3=78\,\mathrm{m}\) → Δp=15,600 Pa → head ≈ 1.61 m (worst-case).
   • AHU coil: \(L_{eq}=30×1.3=39\,\mathrm{m}\) → Δp=7,800 Pa → head ≈ 0.81 m.
4. Totals (plant view): load = 70 kW; total flow ≈ 1.035 L/s (≈ 16.4 gpm); worst-case head ≈ 1.61 m (≈ 5.28 ft).
5. Pump power (indicative): \(P = \dot{V}\,Δp/η\). Using total \( \dot{V}=0.001035\,\mathrm{m^3/s}\), worst-case Δp = 15,600 Pa and η = 0.45 → **≈ 36 W hydraulic** (select a small ECM circulator allowing margin & control head).
6. Costing (India rates, wastage 5%, valves/fittings +25%):

   Segment	Pipe rate	Insul. rate	Length (incl. wastage)	Segment cost
   Radiators (Steel Sch 40)	₹280/m	₹120/m	47.25 m	₹23,625
   Underfloor (PPR)	₹180/m	₹120/m	63.00 m	₹23,625
   AHU coil (Copper Type L)	₹580/m	₹120/m	31.50 m	₹27,563
   Grand total				₹74,813

7. Expansion vessel (informative): Pipe water content ≈ **57.9 L** (from chosen IDs & lengths). Add 15% for emitters/boiler → **66.5 L** system water. Volumetric expansion 10→80 °C ≈ **2.87 %** → expansion volume ≈ **1.91 L**. With 3.0 bar safety valve, precharge ~1.5 bar → required nominal vessel ≈ **30 L** (choose next catalog size up; verify on manufacturer chart).

Numbers above mirror what you’ll see when you enter the same values in the calculator.

Jargon buster

• LTHW: Low-Temperature Hot Water (typically ≤ 90–105 °C depending on code).
• ΔT: Temperature drop across the loop (Supply − Return). Bigger ΔT ⇒ smaller flow for the same heat.
• Design gradient (S): Target pressure drop per metre, e.g., 200 Pa/m.
• Equivalent length: Straight length plus allowance for fittings (elbows, tees, valves) expressed as extra metres.
• Velocity cap: Upper limit to control noise/erosion (e.g., 1.2–1.5 m/s in many comfort systems).
• Expansion vessel: Accommodates water expansion as it heats. Sized by system water content, temperature span and pressure margins.
• PSV: Safety valve setting (e.g., 3 bar for small LTHW plants).

Practical tips & next steps

• Use a **condensing preset** (e.g., 70/50 °C) and ensure emitters are sized for the lower return temperature.
• For **underfloor** loops, mind ΔT (often 7–10 K) and keep velocities low to avoid noise.
• When a **single pump** serves multiple branches, set flow to the **sum of flows** and head to the **worst-case branch**.
• Always confirm expansion vessel size on the **maker’s chart** and check **precharge** matches static height.
• Before procurement, export the calculator’s **HTML report** and attach to your design note for traceability.

Need background? See: Heating Load Calculator, Pipe Design & Head Loss Estimator, and Heat Exchangers in HVAC.