How to Heat a Workshop: How Many kW You Need and What It Costs

A 1,000 m² industrial building 8 metres high with moderate insulation needs around 293 kW of heat output, rising to 381 kW once the stratification factor set out in the standard is applied. Four HC50-3S water unit heaters at 49.8 kW each cover a quarter of the requirement. Official distributor since 2007.

The formula used at quotation stage links three quantities: Q = V × ΔT × K. The volume in cubic metres, the difference between design indoor and outdoor temperature, and a heat-loss coefficient for the building envelope.

The K coefficient runs from 0.6 to 0.9 for a well-insulated building, 1.0 to 1.9 for one with moderate insulation, 2.0 to 2.9 for a poorly insulated one, and reaches 4.0 for an uninsulated structure. The result comes out in kcal/h and converts to kW by dividing by 860.61.

The table below applies the method with an indoor temperature of 16 °C and an outdoor temperature of −5 °C, so a ΔT of 21 degrees.

These are first-approximation values, not a design calculation. Detailed sizing follows UNI EN 12831 and has to be signed off by a building services engineer, not least because the standard is written for rooms up to 5 metres high and refers to its own annex for taller spaces.

The table above ignores something that weighs very heavily in an industrial building: warm air rises, and the installed output has to make up for what collects under the roof.

UNI EN 12831 quantifies this penalty with a height correction factor that depends on the emission system. For underfloor heating the factor is 1.00, so no penalty at all. For a low-temperature radiant ceiling it is 1.15. For forced convection with cross flow from below, which is the typical unit-heater configuration, the factor is 1.30 between 5 and 10 metres of height and rises to 1.60 between 10 and 15 metres.

In plain terms: that 1,000 m² building 8 metres high with 293 theoretical kW needs 381 once the factor is applied. At 10 metres high, 366 theoretical kW become 586. Stratification can add 15 to 60 per cent to the output you have to install, depending on system and height.

It is the number that separates a serious quotation from one done by eye, and in Italian sales documentation it almost never appears.

Unreliable figures circulate about the vertical temperature gradient. Field measurements published in scientific journals paint a soberer picture.

Ahmed, Sistonen and Simson measured two real industrial buildings around 10 metres high in winter, one air-heated and one with radiant heating, finding a gradient of about 0.2 K per metre in both (Building Simulation, 2018). Karpuk and colleagues recorded between 0.6 and 0.7 K per metre in a technology hall in Wrocław, but in a spring and summer campaign (E3S Web of Conferences, 2017). Porras-Amores measured between 0.3 and 0.7 °C per metre in the warm months in warehouses 4 to 10 metres high (Energies, 2014).

The most useful outcome of the 2018 work is not the figure itself. The authors note that for air heating the real gradient is five times lower than textbook values. Anyone sizing from the traditional technical literature is working on a stratification that measurement does not bear out.

A methodological point that matters on site: the profile is not linear. Saïd measured, in two 48,000 m³ hangars, a steep gradient in the first 3 metres above the floor and a much gentler one higher up (Energy and Buildings, 1997). A single figure in °C per metre understates what happens in the zone where people actually are.

The water unit heater connects to an existing circuit, typically fed by a boiler or a heat pump, and transfers heat to the air through a finned coil and a fan. It does not produce heat, it distributes it.

The output figures below refer to the regime declared by the manufacturer: water 90/70 °C with inlet air at 0 °C, at maximum fan speed. At lower regimes output drops, which is why two unit heaters carrying the same commercial label can behave differently on a 70/50 system.

All models have 3/4" connections, 230 V single-phase supply, a three-speed induction motor and IP54 protection. Maximum fluid temperature is 120 °C, maximum pressure 1.6 MPa.

Throw is the parameter that decides the number of units, not output. An HC50-3S at 49.8 kW reaches 24 metres, an HC45-3S at 42.0 kW stops at 14. In a long building, two more powerful units badly placed leave cold zones that four smaller ones would cover. The full range is in the industrial heating collection.

A destratification fan is an axial fan installed at height that pushes the warm air collected under the roof back down. It does not produce heat and does not move it from one space to another: it mixes.

The selection criterion is the vertical air throw in metres, that is how far down the unit can push the flow. Not the floor area in square metres, which the manufacturer does not declare for any model.

Two warnings before ordering. The XS4 model is compatible neither with the HC series speed controllers nor with the TRO thyristor controller, so it needs different control gear. And not every tall building stratifies: a study of a distribution centre with sensors between 2.1 and 9.8 metres recorded average maximum differences of just 0.2-0.5 °C (Polander and others, Purdue IHPBC, 2018). Measure before you buy.

The choice of energy source weighs more than the machine. The prices below were recorded in summer 2026 and should be checked again at quotation time.

The gas price is the Italian ARERA reference price for vulnerable-customer protection, the heating oil is the MASE survey for July 2026 at 1,770.20 euros per thousand litres, and the electricity is the average final price for non-domestic customers given in the ARERA Annual Report 2026, 26.25 cents per kWh including taxes. The calorific values used are 9.5 kWh/Smc for gas and 10 kWh/litre for oil.

Heating oil costs almost three times as much as gas for the same heat produced. The heat pump stays competitive as long as the seasonal COP holds up, and in an industrial building with low flow temperatures that depends on the heat exchange surface installed.

Italy's D.Lgs. 81/2008, Annex IV point 1.9.2.1, states that the temperature in workrooms must be appropriate for the human body during working hours, taking account of the methods used and the physical effort required. It sets no numerical value.

In practice the guidance of the supervisory bodies is used. The Italian Veneto Region, in resolution 1887 of 1997, considers a temperature of at least 16 °C appropriate for manual activities with low energy expenditure. Guidance from Italy's INAIL places moderate physical work between 18 and 21 °C and office work between 21 and 23 °C.

That is why the sizing table uses 16 °C as the indoor temperature: it is the defensible minimum for a workshop, not office comfort. Raising the target to 18 °C increases the design ΔT from 21 to 23 degrees and the required output by about ten per cent.

Sandra Gaspar's advice

I always ask what regime the system runs at before recommending a unit heater. I have seen buildings with machines oversized on paper that did not heat, because the boiler was running at 70/50 and the declared kW referred to 90/70. The figure on the label says nothing without its conditions.

It depends on height and use. Below 8 metres with staff continuously present, water unit heaters combined with destratification fans are the most widespread solution. Above 10 metres, or with doors that stay open, radiant heating limits losses because it warms bodies rather than air.

Running cost depends on the energy source more than on the machine. At summer 2026 prices, gas costs about €0.069 per kWh of heat against €0.197 for oil. Before replacing the generators it is worth checking roof insulation and door sealing, where the largest share is lost.

You need a machine that carries the load calculated with the stratification factor, not the theoretical one. The practical constraint is flow temperature: HC series water unit heaters deliver their declared kW at 90/70 °C, while a heat pump works much lower, so more heat exchange surface is needed.

There is no independently measured, verifiable percentage: the figures in circulation come from manufacturers. What the standard does quantify is the opposite, namely what stratification costs: 15 to 60 per cent more output depending on system and height.

Italy's D.Lgs. 81/2008 requires an appropriate temperature without stating a value. Regional and INAIL guidance put the reference at 16 °C for low-effort manual activities and between 18 and 21 °C for moderate physical work.

Sandra Gaspar, founder of Krollit. Official Reventon distributor since 2007, more than 10,000 professional customers served in Italy and across Europe. LinkedIn profile.

Sources: Reventon HC-3S and TORNADO series technical datasheets; UNI EN 12831; D.Lgs. 81/2008 Annex IV; Regione Veneto DGR 1887/1997; ARERA, Annual Report 2026; MASE, fuel price survey July 2026; Ahmed, Sistonen, Simson and others, Building Simulation 11(2), 2018; Karpuk and others, E3S Web of Conferences 22, 2017; Porras-Amores, Mazarrón, Cañas, Energies 7(3), 2014; Saïd, Energy and Buildings 26, 1997; Polander, Harder, Junker, Fronk, Purdue IHPBC, 2018.