Combi vs System vs Conventional Boilers: Which Should I Fit?

Summary

The boiler type decision drives the whole heating system architecture — pipe routing, cylinder positions, expansion vessel locations, control wiring, and the flue location. Get it right at quote stage and the install runs smoothly. Get it wrong and you'll be re-designing mid-install or apologising to a customer about poor flow rates.

The three types differ in how they generate hot water and where the heating system gets its water:

• Combi (combination) — one appliance does heating AND hot water. Hot water is generated on demand from mains pressure flowing through a plate heat exchanger. No cylinder.
• System — appliance heats the central heating circuit and indirectly heats hot water stored in an unvented cylinder. Mains-pressure hot water from the cylinder.
• Conventional (regular / heat-only) — appliance heats the central heating circuit and indirectly heats hot water stored in a vented cylinder fed by an F&E (feed and expansion) tank in the loft. Atmospheric-pressure hot water.

For a heating engineer, the choice is often less about which is "best" and more about which fits the property's existing infrastructure, customer's hot water habits, and Part L energy efficiency requirements.

This article covers the selection decision, the sizing rules, the install differences, and the Boiler Plus 2018 control requirements that apply to all three types.

Key Facts

• Combi outputs (typical) — 24kW, 28kW, 30kW, 35kW, 40kW. Higher kW = better DHW flow rate
• DHW flow rate (combi) — typically 11–15 l/min at 30°C rise depending on kW. Mains-supply-limited
• System boiler outputs — 12kW, 15kW, 18kW, 24kW, 28kW. Sized to heat load, not DHW (cylinder buffers DHW)
• Conventional boiler outputs — 12kW, 15kW, 18kW, 24kW. Same as system
• DHW cylinder sizes — 120L, 150L, 180L, 210L (unvented); typical sizing 30–45L per occupant
• Mains pressure required (combi) — minimum 1 bar dynamic; ideal 1.5–3 bar
• Mains flow rate required (combi) — minimum 12 l/min at the boiler inlet for good DHW
• Boiler Plus 2018 — England Part L requirement: new boilers must have load/weather compensation, smart thermostat, or flue gas heat recovery
• ERP (Energy-related Products) rating — A-rated boiler minimum for new installs in UK (>92% seasonal efficiency)
• Combi flue position — must be 300mm from openable window, 600mm from boundary, 1.2m from another flue
• System / regular flue — same flue rules
• Standard combi physical size — typically 700mm × 400mm × 350mm
• System / regular physical size — typically 600mm × 400mm × 350mm (smaller than combi as no HEX for DHW)
• F&E tank (conventional only) — typically 25L–40L plastic in loft
• Expansion vessel (combi and system) — 8L–12L sealed, integral to boiler in most cases

Quick Reference Table

Detailed Guidance

When to fit a combi

The default for the modern UK 1–2 bathroom property:

• Frees up the airing cupboard (no cylinder)
• Frees up the loft (no F&E tank)
• One appliance to service
• Mains-pressure hot water at every outlet
• No standby losses (water only heated when needed)

Caveats:

• Limited by mains supply — poor mains pressure or flow gives weak showers
• Cannot run two showers simultaneously (typical 12 l/min DHW = one shower at full flow)
• If electric showers are present, customer expects mains-pressure showers — combi DHW is the same logic
• Larger properties (3+ bathrooms) genuinely need a cylinder; combi DHW won't keep up

When to fit a system boiler with unvented cylinder

The default for properties with multiple bathrooms or simultaneous hot water demand:

• Cylinder buffers DHW demand — multiple outlets work at full flow simultaneously
• Mains-pressure DHW from the cylinder (the unvented cylinder is pressurised by the mains)
• Boiler is sized to the heating load, not DHW (the cylinder does that work)
• Pump and expansion vessel typically integral to the boiler

Considerations:

• Cylinder takes space — typically in airing cupboard or designated cupboard
• Unvented cylinder installation requires G3-qualified installer (under Building Regs Part G3 — discharge of unvented hot water)
• Annual cylinder service mandatory (expansion vessel pressure, T&P valve test, anode inspection)

When to fit a conventional (regular / heat-only) boiler

Niche use cases:

• Like-for-like replacement in a property with existing vented system and customer doesn't want to remove the loft tanks
• Property has very poor mains pressure (sub-1 bar) and gravity-fed hot water from a tall cylinder gives better performance than mains
• Off-grid oil installs where the system architecture is established
• Solid fuel boilers or thermal stores in combination — conventional boiler integrates with these
• Heritage properties where pipework alterations to fit a combi or system would be disruptive

Decreasing in use — most replacements convert vented systems to system boilers + unvented cylinders, or to combis if the property suits.

Sizing a combi

Combi sizing is DHW-led, not heating-led. A small 2-bed flat needs only 6–10kW for heating but you'll specify a 24–28kW combi to get acceptable DHW flow. Rule of thumb: 1 bathroom = 24–28kW; 2 bathrooms = 30–35kW; 3+ bathrooms = consider system instead.

Heat loss calculation determines the maximum kW you need for heating — the combi's heat output side modulates down to this, so oversizing doesn't waste energy in modern modulating combis.

Sizing a system boiler

System boiler sizing is heating-led. Calculate the property's heat loss (typically 5–15kW for a modern insulated home, 15–25kW for an older property) and size the boiler to match — never larger than the heat load + 10–15%. The cylinder size is sized separately based on number of occupants and DHW pattern.

Cylinder sizing:

• 1 person, 1 bathroom: 120L
• 2 people, 1 bathroom: 150L
• 3–4 people, 2 bathrooms: 180–210L
• 5+ people: 250L+ or twin cylinder

Boiler Plus 2018 controls requirement

Since April 2018, England's Part L requires every NEW gas boiler installation in a dwelling to include ONE of:

• Time and temperature control to heating AND hot water (standard for all installs)
• PLUS one of:
  • Load compensation (boiler matches output to demand)
  • Weather compensation (boiler matches output to outside temperature)
  • Smart thermostat with automation/optimisation
  • Flue gas heat recovery (combi only)

Most modern combis are sold pre-configured with one of these features (typically load compensation or smart-thermostat compatibility). Choose accordingly when specifying.

Flue requirements

All three boiler types use the same flue routing rules per BS 5440-1:

• Minimum 300mm from any openable window
• Minimum 600mm from property boundary
• Minimum 1.2m from another flue terminal
• Minimum 1.5m above any opening if facing upward
• Plume management kit may be needed if the flue is near a customer's outdoor area or neighbour's window

Install differences for the engineer

• Combi swap (combi to combi) — 4–8 hours; like-for-like + flue change
• Combi from system/conventional — 8–14 hours; remove cylinder, decommission F&E tank, re-pipe DHW from boiler, isolate or remove old hot water draw-off
• System boiler from conventional — 8–12 hours; remove F&E tank, fit new unvented cylinder, add expansion vessel, T&P valve, discharge pipe
• Conventional swap — 4–6 hours; like-for-like

Frequently Asked Questions

Customer wants a combi but I'm worried about mains pressure — what do I check?

Test the mains flow rate at the kitchen tap with a measuring jug — should be 12 l/min minimum for a combi to give acceptable DHW. Test the dynamic pressure with a gauge — should be at least 1 bar with the tap fully running. Below either threshold, recommend system boiler with cylinder OR mains pressure boosting (rare).

Can I keep the existing cylinder when fitting a new combi?

No — combi doesn't use a cylinder. Either decommission the cylinder (cap pipework), remove it, or repurpose it (some installers convert an existing copper cylinder into a thermal store, but rare).

Are combi boilers more efficient than system boilers?

Marginally, due to no standby losses. The Seasonal Coefficient of Performance difference is typically 1–3% — meaningful over the boiler's life but not a primary selection criterion.

What's the difference between a "system" and "conventional" boiler?

System has built-in pump, expansion vessel, and auto air vent — and works with a sealed system. Conventional (heat-only) requires external pump, F&E tank, and vent pipe — and works with a vented system. System is the modern default; conventional is the legacy approach.

Do all new boilers need to be condensing?

Yes — England's Building Regulations Part L require condensing efficiency on all new and replacement boilers in dwellings (≥92% seasonal efficiency, A-rated under ERP). Non-condensing boilers cannot be installed except in very narrow exempt scenarios (e.g. open-flue replacement where condensing is not technically feasible — assessment required).

Regulations & Standards

• Building Regulations Part L (Conservation of fuel and power) — Condensing efficiency requirement, ERP labelling, Boiler Plus 2018 controls

• Building Regulations Part G3 — Hot water safety, including unvented cylinder discharge requirements

• Building Regulations Part J (Combustion appliances) — Flue and air supply requirements

• BS 5440-1 — Flueing for gas-fired appliances

• BS 5440-2 — Air supply for gas-fired appliances

• Boiler Plus 2018 (Domestic Building Services Compliance Guide) — Specifies controls requirements for new boiler installs

• Gas Safety (Installation and Use) Regulations 1998 — Gas Safe registration required for all boiler installs

• Approved Document L: Conservation of fuel and power — gov.uk

• Approved Document G: Sanitation, hot water safety — gov.uk

• Gas Safe Register technical resources — Gas Safe

• Domestic Building Services Compliance Guide (Boiler Plus) — gov.uk

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