Building Energy & Services

The order the work has to run in

From weather file to equipment schedule

Each step below consumes the result of the previous one. Steps 1–3 establish the load; steps 4–6 turn that load into hardware; steps 7–9 check that the design does not fail in summer, at part load, or on the inside face of the envelope.

1. Fix the design conditions

   External design temperature θ_e comes from the national annex to EN 12831-1, not from an annual minimum — it is a statistical value tied to a return period. Internal design temperatures come from EN 16798-1 categories or ASHRAE 55. Both numbers propagate into every later step, so a 2 K error here is a 2 K error everywhere.

   No calculator — take these from the applicable annex before you start.

2. Establish fabric U-values

   EN ISO 6946 sums layer resistances between fixed surface resistances: R_si = 0.13 m²K/W for horizontal heat flow through a wall, R_se = 0.04 m²K/W outside. The layer arithmetic is easy; the part people omit is the thermal-bridge correction ΔU, which on a repeating-bridge wall is routinely 0.02–0.15 W/m²K — a meaningful fraction of a modern wall's total.

   Tools: Insulation / U-value / R-value · Wall heat transfer

3. Calculate the design heat load

   EN 12831-1 splits the room load into transmission, ventilation and — where intermittent heating is used — a reheat term: Φ_HL = Φ_T + Φ_V + Φ_RH Do this room by room if emitters are to be sized from it. A whole-building figure is adequate for generator capacity and useless for emitter selection.

   Tools: Building heat loss · HVAC BTU load

4. Size the emitters — and correct the catalogue rating

   Radiator outputs are published to EN 442-2 at a reference excess temperature of 50 K. Real systems almost never run there. Correct with

   Q = Q₅₀ × (ΔT / 50)ⁿ, n ≈ 1.3 for panel radiators

   At 55/45 °C flow/return into a 20 °C room the excess temperature is 30 K, so the factor is (30/50)^1.3 = 0.51. A radiator catalogued at 1 kW delivers about 515 W. This single exponent is why low-temperature retrofits need roughly double the installed radiator surface, and why heat-pump conversions fail when the existing emitters are simply reused.

   Tool: Radiator sizing

5. Set flows and balance the circuit

   Flow follows from the emitter duty and the design temperature drop, ṁ = Φ / (c_p · ΔT) which for 1 kW at ΔT = 10 K and c_p = 4.18 kJ/kgK is 0.024 kg/s, about 86 kg/h. Balancing then converts that flow and the pressure available at each valve into a K_v setting, K_v = Q / √Δp with Q in m³/h and Δp in bar.

   Tool: Radiator balancing wizard

6. Choose the generator — check which load actually governs

   For a house, space heating and domestic hot water are usually not the governing load at the same time, and in a combi installation DHW almost always wins. Ten litres per minute at a 35 K rise is 0.167 kg/s × 4.18 kJ/kgK × 35 K ≈ 24 kW which is why 24 kW combis are ubiquitous in dwellings whose design heat loss is 5–8 kW. Size the heat output to the heating load and the instantaneous output to the hot-water draw, then take the larger — or decouple the two with storage.

   Tools: Boiler size · Water heater / hot-water sizing

7. Set the ventilation rate

   ASHRAE 62.1 builds the breathing-zone rate from a per-person and a per-area term, V_bz = R_pP_z + R_aA_z; EN 16798-1 does the same with category-dependent values. CO₂ is the practical check rather than the design basis: a sedentary adult generates about 0.0045 L/s of CO₂, so holding 1000 ppm against a 400 ppm outdoor baseline needs 0.0045 / 0.0006 ≈ 7.5 L/s per person at steady state.

   Tool: Ventilation, airflow & ACH

8. Check summer overheating before glazing is fixed

   Overheating is a geometry and glazing decision, not a cooling decision, and it is settled long before plant is specified. Solar gain through unshaded south or west glazing can exceed the entire winter design heat loss of a well-insulated room. Assess against the EN 16798-1 adaptive criteria or CIBSE TM59 rather than a single peak temperature.

   Tool: Room overheating & solar shading

9. Verify surface temperatures, not just air temperatures

   Mould grows at surface relative humidity around 80%, well before liquid condensation appears. EN ISO 13788 handles this with the temperature factor f_Rsi = (θ_si − θ_e) / (θ_i − θ_e) compared against a critical value commonly taken as 0.75 for dwellings in national guidance. A wall that passes on U-value can still fail here at a reveal, a lintel or a balcony slab.

   Tools: Window condensation & mould risk · Dehumidifier sizing

Reference

Which document governs which step

Preliminary tools are only defensible if you can say what they are approximating. This maps each step to the document a reviewer will expect, with the European and North American routes side by side.

Failure modes

Where preliminary building-services work goes wrong

These three account for most of the corrections made between a screening calculation and a design that survives review. None of them are arithmetic errors.

Selecting emitters from a screening heat load

A W/m² rule of thumb is a legitimate way to establish generator capacity to about ±20% — the honest spread runs from roughly 25–35 W/m² for a current-regulation dwelling to 90 W/m² and above for an uninsulated solid-wall building. It is not a legitimate way to size the radiator in a specific room, because the screening figure averages over orientation, exposure, glazing ratio and the number of external walls. The corner room with two external walls and a large window can carry two or three times the per-area load of an internal room on the same floor. Emitters need the room-by-room calculation.

Using the catalogue radiator output directly

The ΔT₅₀ correction in step 4 is the most consequential single factor on this page and the one most often skipped. Taking a catalogue figure at face value on a 55/45 °C system under-sizes the emitter by about a factor of two. The symptom is a system that performs acceptably in mild weather and cannot reach set point on the coldest days — which is then misdiagnosed as a generator or control problem.

Sizing the heat generator before the hot-water peak is known

Space-heating load and DHW load scale with completely different things: one with fabric and weather, the other with the number and type of outlets. In dwellings the hot-water draw usually governs the instantaneous rating; in a building with storage and a reasonable draw-off profile it usually does not. The decision of which governs has to be made before the generator is chosen, not after, because it also determines whether storage is needed at all.

Scope

Where these tools stop

Everything on this page is a steady-state, single-zone screening calculation. That is a deliberate choice — it is the right level of effort for feasibility work, option comparison and sanity-checking someone else's output. It is the wrong level of effort for the following, and no amount of care with the inputs will change that:

• Compliance and regulatory submission. Building-regulation calculations run through a nationally approved methodology with prescribed inputs. A steady-state hand calculation does not substitute for it, even when it agrees.
• Dynamic and thermal-mass behaviour. Intermittent heating, night setback, solar gain through heavyweight construction and peak-shifting all need a transient model (EN ISO 52016-1 or a simulation tool), not a steady-state one.
• Interstitial condensation and moisture transport. The condensation tool here checks the inside surface. Moisture accumulating within the construction over a season is a Glaser or hygrothermal problem.
• Commissioning and as-built performance. Measured infiltration, actual emitter output and real control behaviour routinely differ from design. Design calculations predict; they do not verify.
• Anything safety-related in a ventilation system. Combustion air, flue provision, hydrogen dilution and smoke control are governed by prescriptive requirements, not by comfort airflow rates.

Each individual page states its own assumptions and the point at which its method stops being valid. Read that section before using a result for anything that will be built.

Sources

• EN 12831-1, Energy performance of buildings — Method for calculation of the design heat load.
• EN ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods.
• EN ISO 13788, Hygrothermal performance of building components — Internal surface temperature to avoid critical surface humidity, and interstitial condensation.
• EN 442-2, Radiators and convectors — Test methods and rating.
• EN 16798-1, Energy performance of buildings — Indoor environmental input parameters.
• ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality, ventilation rate procedure.
• ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy.
• CIBSE TM59, Design methodology for the assessment of overheating risk in homes.
• ASHRAE Handbook — Fundamentals, chapters on heat transfer, fenestration and residential load calculation.