The Ultimate Guide to Central Heating Systems for UK Homes: Combi vs System vs Conventional

Reviewed by our senior heating engineer. Serving Harpenden (primary), St Albans, Borehamwood, Radlett, and Welwyn Garden City.

Upgrading your heating or planning a new install? Choosing between a combi, system, or conventional boiler can feel like alphabet soup. This guide explains how each setup works, who it suits, and what it means for comfort, efficiency, and cost—so you can decide with confidence for your Hertfordshire home.

What is a Combi Boiler?

A combi (combination) boiler delivers heating and hot water directly from the unit—no cylinder, no loft tank. It’s compact and efficient, ideal for smaller to medium homes.

• Best for: Flats, terraces, and smaller semis with 1 bathroom
• Pros: Space-saving, fast hot water, high efficiency, lower install cost
• Cons: Can struggle with two showers at once; needs good mains pressure

Real-world example: In a Harpenden 2-bed terrace with 10–12 L/min flow at the kitchen tap, we swapped a tired conventional system for a 28kW combi.

Result: reclaimed the airing cupboard, quicker heat-up, and simpler maintenance.

What is a System Boiler?

A system boiler heats radiators and a separate unvented cylinder for hot water. Core components (pump, expansion vessel) are built in, delivering strong hot water to multiple taps.

• Best for: Three- to five-bedroom homes, busy households with two+ bathrooms
• Pros: Excellent hot water flow, supports simultaneous showers, works well with modern controls
• Cons: Needs cylinder space; slightly higher install cost and some standing heat loss

Local note: Families in St Albans often choose a 200L unvented cylinder with a 19–24kW system boiler to keep two morning showers comfortable.

What is a Conventional (Regular) Boiler?

Also called heat-only, a conventional boiler uses a hot water cylinder and cold-water tank (usually in the loft). Many older UK homes still have these, especially where mains pressure is low.

• • Best for: Larger or older properties with traditional pipework
  • Pros: Works with low mains pressure, supports multiple bathrooms, proven reliability
  • Cons: Takes the most space and has more components to maintain

When it makes sense: In period homes in Radlett with very low mains flow, keeping a conventional setup avoids major re-piping and maintains reliable hot water.

Key Decision Factors for UK Homes

• Bathrooms & usage: One bathroom? A combi may be perfect. Two+ simultaneous showers? Go system or conventional.
• Mains flow rate: Aim for 12–15 L/min for comfortable combi performance; otherwise consider storage solutions.
• Space: No loft or airing cupboard? A combi frees up space.
• Budget: Combi installs usually cost less; system setups cost more but deliver hotel-like hot water.
• Hard water: Hertfordshire is hard-water territory; protect systems with scale control and filters.

Efficiency, Controls, and Bills

Modern condensing boilers can significantly cut fuel use versus older models. For best results, add smart controls (Nest, Hive, Tado), enable weather/load compensation, fit TRVs, and service annually. In hard-water areas, magnetic filters and scale reducers keep efficiency high. See guidance from the Energy Saving Trust for extra savings ideas.

Upgrading from Your Current Setup

• Conventional to Combi: Frees loft/airing cupboard space and simplifies maintenance; expect some re-piping.
• Conventional to System: Keeps powerful hot water via an unvented cylinder—great for families.
• Combi to System: Solves hot water bottlenecks in growing households.
• Like-for-like replacement: Minimises disruption and costs when the current layout works.

Common Questions

Which is cheapest to install? Usually a combi. Cylinders and extra labour make system/conventional pricier.

Will a combi handle two showers? Only with strong mains flow. Otherwise, a system boiler with cylinder is a safer bet.

What about reliability? Correctly sized and maintained systems from leading brands are all reliable. Water quality and setup matter as much as the badge.

Does hard water really matter? Yes. In Borehamwood/St Albans, scale can reduce efficiency; simple protection extends lifespan.

Local Expertise You Can Trust

We specify and install the right system for homes in Harpenden, St Albans, Borehamwood, Radlett, and Welwyn Garden City. From survey to aftercare, we handle sizing, smart controls, filters, and water treatment.

Mini Case Study (Harpenden)

A 3-bed semi with 11 L/min mains flow and 10 radiators struggled with morning hot water. We recommended a system boiler with a 200L unvented cylinder. Outcome: two comfortable showers at once, faster reheat, and happier mornings.