Modelling for New Heating Systems

Comparing heating systems by headline efficiency alone misleads, because efficiency depends on operating conditions. A condensing boiler achieves its rated efficiency only when return temperatures are low enough for condensing, a heat pump's efficiency falls as flow temperature rises and as outdoor temperature falls, and an electric resistance heater is efficient at the point of use but expensive in energy terms where electricity prices are high. A model that compares systems should therefore show performance at the flow temperatures, load patterns and control strategies the home will actually use, not merely nameplate figures.

Fabric comes first in any system comparison. A heating system sized and modelled for an uninsulated home will be oversized and inefficient once insulation is added, while a system modelled for an insulated home will struggle if insulation is never carried out. System comparisons should therefore state their fabric baseline clearly, and where fabric work is planned, the comparison should be made on the post-improvement fabric, with the system sized accordingly. Comparing systems on different fabric baselines is not a comparison at all.

Controls and zoning shape real performance as much as the heat generator. A system with a programmer, room thermostat, thermostatic radiator valves or weather compensation, used well, will use less energy than the same generator with crude controls, because it heats only the rooms and times needed. A model should state its control assumptions, and a comparison that assumes ideal controls for one system and poor controls for another is biased. Occupant behaviour, such as preferred temperatures and heating hours, further modifies real use, and a model should note where standard patterns differ from the household's likely pattern.

Part-load performance matters because heating systems rarely run at full output. Most of the heating season is mild, and systems spend most hours at part load, where cycling, standby losses and control quality dominate. A system that is efficient at full load but cycles wastefully at part load may use more energy over a season than a system with a lower headline figure but better part-load behaviour. A model that shows seasonal performance, rather than a single efficiency point, captures that distinction.

Before comparing systems, ask for a comparison table on the same fabric baseline, with flow temperatures, control assumptions and seasonal performance stated for each option. Check whether the fabric baseline matches the home as it will be when the system is installed, and whether emitter changes, such as larger radiators for low-temperature operation, are included in each option's scope and cost. A comparison that omits emitter work for one option understates its true cost and disruption.

Also ask about fuel, carbon and running cost assumptions, and their dates. Fuel prices and carbon factors change, and a comparison based on outdated prices or factors can favour a system that would not be favoured on current figures. A good comparison states its price and carbon assumptions and shows sensitivity to plausible changes, rather than presenting a single cost figure as certain.

By place, heating system modelling follows national methods and installer schemes. In the UK, SAP-based modelling and MCS guidance shape system comparisons and certified installations, while in EU member states, national methods and schemes govern. Fuel availability also varies locally: gas grid access, electricity capacity, biomass supply and district heating availability all constrain which systems are feasible for a given home, regardless of modelled performance.

Heritage, space and noise constraints further narrow options. A listed building may restrict flues or external units, a small home may lack space for a cylinder or a heat pump unit, and a heat pump's noise and placement need planning consideration. Those constraints should be checked before modelling effort is spent on an infeasible option.

A common mistake is to choose a system on headline efficiency or on a neighbour's recommendation, without a same-baseline comparison for this home. Another mistake is to install a new system before fabric work, leaving it oversized for the improved home and inefficient for its whole life.

A further mistake is to ignore controls in system choice. A sophisticated generator with poor controls, or controls that occupants cannot use, will underperform a simpler system that is well controlled and well understood by its users.

Keep the system comparison, the fabric baseline, the control assumptions and the quotations together. That file supports installation, commissioning and future system changes.

Questions to ask include: on what fabric baseline is each system compared, at what flow temperature, and with what controls. Those questions make a heating system comparison honest.