Combi vs System vs Regular Boiler: Which Does My Customer Need?

Summary

Choosing the correct boiler type is the single most important decision in a domestic heating installation. Get it wrong and the customer ends up with poor flow rates, lukewarm showers, or an oversized system wasting energy. In the UK, approximately 70% of new domestic boiler installations are combis, but that does not mean a combi is always appropriate. The three types — combi, system, and regular (heat-only) — each serve a distinct property profile based on the number of bathrooms, mains water pressure, hot water demand pattern, and available space. This guide gives you the technical detail to make a confident recommendation every time.

Key Facts

• All new and replacement boilers in England must meet a minimum ErP efficiency of 92% (Boiler Plus 2018, Building Regulations Part L).
• Combi boilers heat water on demand from the mains — no cylinder or tank required.
• System boilers heat water and store it in an unvented hot water cylinder, fed directly from the mains.
• Regular (heat-only) boilers require both a hot water cylinder AND a cold water storage tank (typically in the loft), operating on a gravity-fed or pumped system.
• A combi boiler's DHW output is limited by incoming mains flow rate — if the mains only delivers 12 l/min, fitting a 35 kW combi is pointless.
• Boiler Plus (England, April 2018) requires combi boiler installations to include one additional energy-saving measure: weather compensation, load compensation, flue gas heat recovery (FGHR), or a smart thermostat with automation and optimisation.
• System and regular boiler installations under Boiler Plus must include time and temperature controls with a boiler interlock, but do not require the additional measure above.
• The Boiler Upgrade Scheme (BUS) grant of £7,500 is available for heat pump installations — worth flagging to customers as an alternative, but gas boilers remain the dominant replacement market.

Detailed Guidance

How do the three boiler types actually work?

Combi (combination) boiler: A single wall-hung unit that provides both central heating and instantaneous domestic hot water (DHW). When a hot tap opens, the boiler diverts from heating mode to DHW mode and heats mains water through a plate heat exchanger on demand. There is no stored hot water.

System boiler: A wall-hung or floor-standing boiler that heats water for central heating directly and also heats water stored in a separate unvented hot water cylinder (typically 150-300 litres). The expansion vessel, pump, and safety components are built into the boiler, simplifying installation compared to a regular boiler. The cylinder is pressurised from the mains.

Regular (heat-only / conventional) boiler: The traditional setup. The boiler heats water for the central heating circuit and sends hot water to a vented copper cylinder. Cold water is supplied to the cylinder from a cold water storage tank in the loft via gravity. Requires a feed-and-expansion tank (F&E) for the heating circuit as well.

Comparison table

What size boiler does the property need?

Sizing depends on two separate outputs: central heating (CH) demand and domestic hot water (DHW) demand. For combis, the DHW output is what dictates the boiler size because it is always higher than the CH requirement.

Combi boiler sizing by property

Critical point: These flow rates assume a 35°C temperature rise (10°C inlet to 45°C outlet). In winter, when mains water drops to 5-6°C, actual flow rates will be lower. Always check the manufacturer's published flow rate at a 35°C rise AND a 40°C rise.

System and regular boiler sizing

For system and regular boilers, the CH output is the primary sizing factor because the cylinder handles DHW storage separately. A rough method is 1.5 kW per radiator. For a typical 3-bed semi with 8-10 radiators, an 18-24 kW system boiler is sufficient. Cylinder size matters more than boiler output for DHW — typically 150L for 1-2 bathrooms, 200-250L for 3+ bathrooms, and 300L for large properties or high-demand households.

When should I recommend each type?

Recommend a combi when:

• The property has one bathroom (or one bathroom plus a separate downstairs cloakroom with basin only)
• Mains water flow rate is at least 12-15 l/min at the point of entry
• There is no requirement for simultaneous hot water at multiple outlets
• Space is limited — no room for a cylinder or loft tanks
• The customer wants simplicity and lower installation cost
• The property has fewer than 15 radiators

Recommend a system boiler when:

• The property has two or more bathrooms with regular simultaneous use
• Good mains pressure is available (at least 1.5 bar, ideally 2-3 bar)
• There is space for an unvented cylinder (airing cupboard or utility room)
• The customer wants to integrate solar thermal panels in future
• High demand for bath fills — a stored cylinder gives much faster bath fill times than a combi
• The property has 15+ radiators

Recommend a regular (heat-only) boiler when:

• Mains water pressure is too low for an unvented system (below 1.0-1.5 bar)
• The property already has a gravity-fed system with vented cylinder and loft tanks, and the customer does not want a full system conversion
• Building constraints prevent installation of an unvented cylinder (e.g. no discharge pipe route)
• The customer has an existing gravity-fed system that works well and wants a like-for-like replacement
• Listed building or conservation area restrictions apply

Do NOT fit a combi when:

• Mains flow rate is below 12 l/min — the customer will get a trickle from the shower
• The property has 3+ bathrooms with regular simultaneous demand
• The customer regularly fills a large bath — a 35 kW combi takes 10+ minutes to fill a standard bath
• The property is at the end of a long mains run with poor pressure

What about converting between types?

Regular to combi (most common conversion): Increasingly popular as homeowners want to free up loft and airing cupboard space. The old cylinder, loft tanks, and associated pipework are removed. Key checks: verify mains flow rate is sufficient, confirm the existing radiator circuit can handle sealed system pressure, and flush the system thoroughly (BS 7593). Budget for additional pipework modifications — the old gravity DHW circuit needs capping off.

Combi to system: Less common but sometimes necessary when a property extension adds bathrooms. Requires fitting a new unvented cylinder, running new DHW pipework, and the installer must hold a G3 qualification for unvented hot water storage.

Regular to system: A worthwhile upgrade — removes the loft tanks while retaining stored hot water. The cylinder is replaced with an unvented unit fed from the mains. Again, G3 qualification is required.

How do I check mains water pressure and flow rate?

Before recommending any boiler type, always measure:

1. Static pressure: Fit a pressure gauge to the kitchen cold tap (nearest the incoming main). Acceptable range is 1.0-4.0 bar. Below 1.0 bar, a combi will struggle. Above 4.0 bar, a pressure reducing valve (PRV) is needed.
2. Dynamic flow rate: Open the kitchen cold tap fully and measure fill time into a known-volume container. Calculate litres per minute. Minimum for a combi is 12 l/min; ideally 15+ l/min for a 30 kW+ unit.
3. Check at peak times: Mains pressure drops during morning and evening peaks. If borderline, test at 7-8 AM.

Frequently Asked Questions

Can a combi boiler run two showers at once?

Not effectively. A 30 kW combi delivers roughly 13 l/min — a decent shower needs 8-10 l/min. Running two simultaneously halves the available flow to each, giving a poor experience. If the customer regularly needs two showers at the same time, fit a system boiler with an appropriately sized unvented cylinder.

Is a bigger combi always better?

No. Oversizing wastes energy through increased cycling (the boiler fires up and shuts down frequently because it reaches temperature too quickly). A 42 kW combi in a one-bed flat will cycle excessively on the heating side. Match the boiler to the actual demand. For central heating, the boiler rarely needs more than 15-18 kW even in a large detached house — it is the DHW flow rate that drives combi sizing upward.

My customer has low mains pressure — what are the options?

Three routes: (1) Fit a regular boiler with a gravity-fed vented cylinder and loft tanks — works independently of mains pressure for DHW. (2) Fit a system boiler with a mains-boosted unvented cylinder using an accumulator or pump set. (3) Contact the water company — they have a legal obligation under the Water Industry Act 1991 to maintain minimum pressure (0.7 bar / 7 metres head at the boundary stop tap). If they are not meeting this, request a pressure survey.

Do I need a G3 qualification to fit an unvented cylinder?

Yes. Any installer fitting or servicing an unvented hot water storage system over 15 litres capacity must hold a current G3 (Unvented Hot Water) qualification. This applies to system boiler installations with unvented cylinders. It does NOT apply to combi boilers (no cylinder) or regular boilers with vented cylinders.

What about hydrogen-ready boilers?

Most major manufacturers (Worcester Bosch, Vaillant, Ideal, Baxi) now offer hydrogen-ready or hydrogen-blend-ready boilers. These can run on natural gas now and be converted to 20% hydrogen blend or potentially 100% hydrogen in future. While the government's hydrogen heating decision is still pending, fitting a hydrogen-ready boiler does not add significant cost and future-proofs the installation to some degree. It should not be the primary factor in choosing a boiler type.

Regulations & Standards

• Boiler Plus (2018, England only): All new gas boiler installations must have a minimum ErP efficiency of 92%. Combi installations must include one additional energy-saving measure (weather compensation, load compensation, FGHR, or smart thermostat with automation and optimisation). All installations require time and temperature controls with boiler interlock. Scotland and Wales have separate but similar requirements.

• Building Regulations Part L (Conservation of fuel and power): Requires minimum boiler efficiency of 92% ErP, thermostatic controls on all radiators (TRVs), system flush to BS 7593 on every installation, and commissioning in accordance with manufacturer's instructions. The 2021/2022 update introduced a maximum design flow temperature of 55°C for fully replaced wet heating systems.

• ErP Directive (Energy-related Products, 2015): EU-origin regulation retained in UK law. Requires energy labelling (A to G scale) on all boilers and hot water products up to 70 kW. Minimum efficiency standards for products placed on the market. Package labelling when a boiler is sold with controls, cylinder, or solar.

• Gas Safety (Installation and Use) Regulations 1998: All gas work must be carried out by a Gas Safe registered engineer. Notification to local authority building control (or via a Competent Person Scheme) is required for all boiler installations.

• BS 7593:2019 (System flushing): Mandatory system flush and treatment with inhibitor on all new boiler installations and replacements. A filter (e.g. magnetic system filter) should be fitted.

• G3 Qualification: Required for any work on unvented hot water storage systems over 15 litres. Applies to system boiler installations with unvented cylinders.

• Boiler Plus Factsheet — UK Government (BEIS)

• Boiler Plus Compliance — City Plumbing

• Boiler Plus — Worcester Bosch

• Building Regulations: Boilers and Heating — Planning Portal

• Part L Building Regulations — John Guest

• ErP Directive — Drayton Controls

• Boiler Sizing Guide — The Heating Hub

• Flow Rate Requirements: Sizing Your Combination Boiler — Heating and Plumbing World

• What Size Boiler Do I Need — British Gas

• What Size Boiler Do I Need — Viessmann UK

• Boiler Plus Guide 2026 — iHeat

• Boiler Plus — CIPHE

• pipe sizing — Pipe sizing for heating systems

• unvented cylinders — G3 qualification and unvented cylinder installation

• underfloor heating — UFH system design and installation