How to Deal with Two Main Heating Systems in RdSAP
Dominic Bowkett
Last Update 6 months ago
Some properties — particularly older homes or extended dwellings — use more than one heating system to supply space heating. In RdSAP 10, you can enter two main heating systems, but they must meet specific criteria. Accurately identifying and recording them is essential for a correct SAP rating.
In RdSAP, up to two main heating systems can be recorded — but only when the property has two independent, fixed heating systems that both meet the definition of main systems.
A main heating system is a fixed system that heats a majority of the dwelling's floor area, typically through radiators, underfloor heating, or air handling.
➡️ Important: Only if all heating is via room heaters, can one or more of them be treated as “main systems” (RdSAP Convention 4.09).
Two main systems can be recorded if all the following apply:
The two systems cover distinct areas or zones of the dwelling.
Each system heats a significant portion of the property (measured by % of floor area).
They are separate systems (i.e. not both served from one boiler or circuit).
They are not secondary heating types (room heaters, plug-ins, stoves, etc.).
Identifying System 1 vs System 2
If a property does have two valid main systems, then:
System 1 must serve the living area.
If both do, System 1 is the one that heats more floor area.
If tied, System 1 is the one that also heats water.
If still tied, System 1 is the cheapest to run (based on SAP fuel cost / efficiency).
(Source: Convention 4.09)
Also you should note Convention 4.01a below for when one system serves both radiators and underfloor heating:
Where one system is serving both radiators and underfloor, specify the same heat source as main Sept 2009 heating 1 and main heating 2 following the rules in convention 4.09 (except for heat pumps). If Heat Pump serves both radiators and underfloor, enter radiators when they are serving 50% or more of the total floor area.
Where the same [proportion] enter the emitters as underfloor. Where a cast iron range-style cooker boiler is present treat the cooker as a radiator (convention 4.09).
1. Two distinct heating systems
Two boilers (e.g. one for the main dwelling, another for an extension)
A boiler and an air source heat pump
Two types of storage heaters (e.g. older manual type and high heat retention models)
A back boiler in the main part of the house and a gas boiler in an extension
Each system is recorded as a main system, and the floor area served by each must be estimated and entered.
2. One system, two emitter types
A boiler supplying both radiators and underfloor heating in different areas (e.g. underfloor heating on the ground floor, radiators upstairs)
In these cases, the software allows you to enter the same heat source twice, with each entry specifying a different emitter type.
3. Individual systems to different emitters
When two separate heat sources supply different emitter types, each serving different areas. For example:
A boiler heating radiators in some parts of the house
An air source heat pump supplying underfloor heating elsewhere
Here, the two heating systems are recorded separately, and the assessor must estimate the floor area served by each.
Explains how to enter the number of percentage of floor area served by each main system when two are present.
Used by the software to allocate heat demand between systems.
If there is a dominant system (heating more rooms), it should be recorded as Main Heating 1, and the secondary system as Main Heating 2.
✅ Correct Examples
✅ A gas boiler serving 60% of the floor area + a back boiler serving 40%: Two main systems (record % of area served).
✅ One heating system providing radiators and underfloor heating to different zones: Record as two systems using same heat source (see Convention 4.09a).
✅ Multiple types of storage heaters (e.g. high heat retention + standard): Treated as two main systems if covering different zones.
Two Main Heating Systems & Hot Water Provision
If only one of the heating systems provides domestic hot water, then that system must be entered as Main System 1.
Convention 4.09 (May 2025):
“Main System 1 is the system that provides water heating, if only one does”
(This rule comes after considering which system heats the living area and more floor area.)
🔹 If Both Provide Hot Water?
If both systems provide hot water (e.g. two combi boilers or two heat sources each with a hot water cylinder), then:
Main System 1 = the system that heats the living area.
If both do = the one that serves the greater floor area.
If still equal = choose the cheapest to run (fuel cost ÷ efficiency) per RdSAP Table 32.
If neither system provides DHW (e.g. hot water is via a standalone immersion heater), then:
Choose Main System 1 based on:
Which one heats the living area.
If both do, the one that serves the greater floor area.
If tied, the cheapest to run.
Only one system is entered in RdSAP for water heating. If both heating systems can provide DHW, you must:
Select the system actually used for hot water (ask occupant or use visual evidence).
If both are used equally and cannot determine, default to the cheapest to run.
If they both serve the same area, choose the most prevalent as System 1.
Calculate heating energy demand based on how the dwelling is split between the two systems.
The performance of each system (efficiency, fuel cost, etc.) will influence the overall rating, so it’s important to get this right.
Tip for Assessors
If the heating is selected under “Manual > Room Heaters”, it’s probably not a main system — unless it is the only heating present.