Heating systems for commercial and industrial buildings: boilers, burners, heat pumps and controls (Fluids, HVAC and Building Services) | READOS

Heating is easy to take for granted until it fails on a cold morning, or until the gas bill arrives. In many Australian commercial and industrial buildings, heating is a smaller load than cooling, but it can still be a large energy cost, particularly in southern cities, at higher altitudes, in buildings with high ventilation rates and in processes that need hot water or warm air. Heating systems also carry hazards that cooling systems do not: combustion, flue gases, carbon monoxide, high water temperatures and pressure.

Most heating systems combine a few basic elements: a heat source, such as a boiler, furnace or heat pump; a way of distributing heat, by water or air; terminal units that deliver heat to spaces; and controls that decide how much heat is produced and where it goes. Efficiency depends on all of them working together. A high-efficiency boiler can perform poorly if the water returning to it is too hot, and an efficient heat pump can disappoint if its controls fight against it.

This article explains, in general terms, the main types of heating systems, how boilers and burners work and are kept safe, why heat pumps are increasingly used, how controls and water treatment affect performance, and what owners and managers should check. It is general information; gas, pressure equipment and electrical work must be designed and carried out by appropriately licensed and qualified people.

Start with the load

A building loses heat through its fabric, by conduction through walls, roofs, floors and glazing, and through air, by ventilation and infiltration of cold outside air. Processes may add heat demands of their own, such as hot water for washing or warm air for drying. The design heating load is the heat needed to hold indoor conditions on a cold design day, and it sets the size of the plant.

Reducing the load is often the cheapest improvement. Sealing gaps, closing doors with strip curtains or fast-acting doors, insulating roofs and pipework, and controlling ventilation to match occupancy can reduce heating demand substantially before any equipment is changed. The HVAC for commercial and industrial buildings article covers loads and system choices across both heating and cooling.

Ways to deliver heat

Hydronic systems

Hydronic systems heat water in a boiler or heat pump and pump it through pipes to radiators, fan-coil units, air-handling unit coils or pipes embedded in floors. Water carries a lot of heat for its volume, pipes are compact, and hydronic systems can serve many zones with good control. Underfloor heating uses low water temperatures and suits high-efficiency heat sources.

Warm-air systems

Ducted warm-air systems heat air in a furnace or coil and distribute it through ducts. They respond quickly and can be combined with cooling and ventilation, but ducts take space and air carries far less heat than water.

Radiant heating

In warehouses and workshops with high ceilings, heating the whole air volume is wasteful because warm air rises to the roof. Radiant heaters, such as gas-fired radiant tubes or electric radiant panels, heat people and surfaces directly, much as the sun does, allowing comfortable conditions with lower air temperatures. They suit spaces with large doors that open frequently.

Heat pumps

A heat pump uses a refrigeration cycle to move heat from outside air, water or the ground into a building. Because it moves heat rather than generating it, it delivers several units of heat for each unit of electricity: the ratio is the coefficient of performance, commonly around 3 to 4 for air-source units in suitable conditions. Reverse-cycle air conditioners are heat pumps, and air-to-water heat pumps can supply hydronic systems and domestic hot water. Performance falls as outdoor temperatures drop and as the required water temperature rises, so heat pumps work best with low-temperature distribution.

Electric resistance

Electric resistance heaters convert electricity directly to heat with a coefficient of performance of one. They are simple and cheap to install but expensive to run for large loads, so they are best suited to small, occasional or spot heating.

How boilers work

A boiler transfers heat from burning fuel, usually natural gas or LPG in Australia, to water. In fire-tube boilers, hot gases pass through tubes surrounded by water; in water-tube boilers, water flows through tubes surrounded by hot gases. Smaller commercial systems often use compact modular boilers, several of which can be combined to match varying loads.

Condensing boilers

Flue gases from burning natural gas contain water vapour. If the flue gases are cooled below their dew point, typically around 55 °C for natural gas, the vapour condenses and releases its latent heat, raising efficiency. Condensing boilers have heat exchangers designed to do this and to resist the mildly acidic condensate. They can exceed 90% efficiency, but only if the water returning to them is cool enough. A condensing boiler connected to a system with high return temperatures will run like a conventional boiler, wasting its main advantage.

Turndown and cycling

Heating loads vary through the day and the season. A burner that can only run at full output will switch on and off frequently at light loads, which wastes energy in purging and heating up and increases wear. Modulating burners with a good turndown ratio, the ratio of maximum to minimum firing rate, and multiple modular boilers that stage on and off help match output to demand.

Burners and combustion safety

A burner mixes fuel with air and sustains a stable flame. Too little air produces carbon monoxide and soot; too much air carries heat out of the flue and reduces efficiency. Technicians tune burners using combustion analysis, measuring oxygen, carbon dioxide and carbon monoxide in the flue gases.

Every gas-fired appliance of significant size has a burner management or flame safeguard system that controls the start-up sequence:

Pre-purge, which clears any fuel from the combustion chamber with air before ignition;
Ignition, with a pilot or direct spark;
Flame proving, using a detector to confirm a flame is present; and
Lockout, which shuts off fuel if the flame is not proven or is lost.

These interlocks prevent unburnt gas accumulating and exploding. They must never be bypassed. In Australia, gas installations are covered by standards and regulations, including the gas installations standard and requirements for industrial and commercial gas-fired appliances, and work must be carried out by licensed gasfitters. Pressure equipment, including many boilers, is subject to design, registration and inspection requirements that vary by state and territory.

Flues, combustion air and carbon monoxide

Every combustion appliance needs enough air to burn its fuel completely and a flue that carries the products of combustion safely outside. Blocked flues, damaged heat exchangers, inadequate ventilation in plant rooms and exhaust fans that pull air backwards through a flue can all cause flue gases to spill into occupied spaces. The greatest danger is carbon monoxide, a colourless, odourless gas that can cause illness and death. Plant rooms need permanent ventilation openings that are never blocked, flues must be inspected, and carbon monoxide detection is a sensible addition where combustion appliances serve or adjoin occupied areas.

Hot water and process heat

Many sites also need hot water for amenities, cleaning or processes. Hot water systems must keep stored water hot enough, and circulate it well enough, to control the growth of Legionella bacteria, while delivering water at outlets at temperatures that will not scald people; thermostatic mixing valves are commonly used for this. Process heat, such as hot water for washing parts or warm air for drying, can often be supplied more efficiently by recovering heat from compressors, refrigeration plant or exhaust air, or by heat pumps, than by burning fuel. A heat recovery review is worthwhile wherever a site both rejects and consumes heat.

Choosing replacement plant

When a boiler or furnace reaches the end of its life, a like-for-like replacement is the easiest choice but not always the best. Replacement is the moment to re-measure the load, which may have fallen after insulation or changes in use, to reconsider the system’s water temperatures and to compare heat pumps, hybrid arrangements and modular boilers on whole-of-life cost. Space, electrical supply capacity, tariffs, maintenance skills and the building’s future use all influence the answer.

Distribution, pumps and balancing

In hydronic systems, circulating pumps move water through the boilers, pipes and terminal units. Variable-speed pumps controlled to maintain a set pressure difference or temperature difference save electricity at part load. Systems must be balanced so that each circuit receives its design flow; unbalanced systems leave some areas cold while others overheat, and often lead people to raise set points everywhere. The pipe flow, pressure drop and pump selection article explains how pipe sizes and pump curves interact.

Controls that save energy

Controls often decide whether a heating system is economical:

Time scheduling that heats spaces only when they are occupied.
Optimum start, which starts heating just early enough to reach comfort at opening time, based on outdoor and indoor temperatures.
Weather compensation, also called outdoor reset, which lowers the flow temperature in mild weather. It improves condensing boiler and heat pump efficiency.
Zoning, so that each area is heated according to its own needs.
Thermostatic radiator valves and room sensors that respond to local gains from sun, people and equipment.
Interlocks that stop heating and cooling running against each other in the same space.

A common failure is a set of good controls that has been overridden, left on manual or never commissioned properly.

Water treatment and maintenance

Hydronic systems need clean, treated water. Oxygen, scale and debris cause corrosion, blockages and failed pumps and valves. Systems are typically filled with treated water containing corrosion inhibitors, fitted with air and dirt separators and tested periodically. Boilers and burners need regular servicing, combustion testing, safety device testing and inspection of flues and ventilation. Records of inspections and tests are part of compliance and help diagnose problems.

A worked example

This is an illustrative example. The building and figures are hypothetical, and gas and electricity prices vary.

A two-storey office and workshop building is heated by a condensing gas boiler serving radiators and air-handling coils. It uses about 1,200 GJ of gas a year. A review finds that the boiler flow temperature is fixed at 80 °C all year and the return water rarely falls below 65 °C, so the boiler almost never condenses. Its seasonal efficiency is estimated at about 84%.

The service contractor commissions weather compensation, rebalances the circuits and fixes two stuck bypass valves that were sending hot flow straight back to the boiler. Return temperatures now fall below 50 °C for most of the season, and efficiency is estimated at about 92%. Gas use falls in proportion: 1,200 × (1 − 84 ÷ 92), or about 104 GJ a year. At $18 per GJ, that saves roughly $1,880 a year for a few days of commissioning work.

The owner also compares an air-to-water heat pump for the future. The building needs about 1,200 × 0.84, or roughly 1,008 GJ of delivered heat, which is about 280,000 kWh. At a seasonal coefficient of performance of 3.2, a heat pump would use about 87,500 kWh of electricity, costing roughly $21,900 at $0.25 per kWh, close to the current gas cost of about $21,600. The comparison shows that tariffs, water temperatures and solar generation will decide the answer, and the lower flow temperatures already achieved would make a heat pump more effective.

How this applies to a small Australian business

Reduce heat losses first through sealing, insulation and door control.
Check that condensing boilers actually condense by measuring return temperatures.
Commission and use controls: scheduling, optimum start, weather compensation and zoning.
Keep burner safety systems intact and serviced by licensed technicians.
Treat and monitor hydronic water to prevent corrosion and blockages.
Consider heat pumps and radiant heating where they suit the building and tariffs.

Common mistakes

Fixed high flow temperatures that stop condensing boilers and heat pumps working efficiently.
Unbalanced systems answered by raising set points.
Heating and cooling fighting each other in the same space.
Bypassed or neglected safety interlocks.
Oversized plant that cycles constantly at light load.

Frequently asked questions

Why does return water temperature matter so much? Condensing boilers recover extra heat only when flue gases are cooled below their dew point, which requires cool return water. Heat pumps also run more efficiently at lower water temperatures.

Are radiant heaters efficient? In tall, draughty spaces they can be, because they warm people and surfaces directly instead of heating the whole air volume.

Questions to ask

What is our heating load, and how much could we reduce it by sealing and insulation?
What flow and return temperatures does our system actually run at?
Are our heating controls commissioned, scheduled and working?
When were our burners last combustion-tested and safety devices checked?
Would a heat pump or radiant system suit our building better?

Bringing it together

Heating systems combine a heat source, distribution, terminal units and controls. Hydronic, warm-air, radiant, heat pump and electric systems each suit different buildings. Condensing boilers need cool return water, burners need tuning and intact safety interlocks, pumps need balancing and controls need commissioning. Reducing losses first, lowering water temperatures, maintaining equipment and comparing heat pumps on real tariffs lead to safer and cheaper heating. The psychrometrics for HVAC article explains how humidity and air conditions interact with heating and ventilation.

Source: KEVOS editorial notes. Examples and figures in this article are illustrations. This article is general information and does not replace advice from a qualified mechanical engineer, licensed gasfitter or regulator.