--- title: "Combi vs System Boiler: Which is Right for You? | MAMOSS" description: "Combi vs system boiler — pros, cons and which one suits your home. Honest advice from London Gas Safe engineers. 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Honest advice from London Gas Safe engineers. Read before you buy.", "datePublished": "2026-04-29", "dateModified": "2026-04-29", "author": { "@type": "Organization", "name": "MAMOSS Engineering Team", "url": "https://mamoss.co.uk" }, "publisher": { "@id": "https://mamoss.co.uk/#organization" }, "mainEntityOfPage": "https://mamoss.co.uk/guides/combi-vs-system-boiler" }, { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "Can I run a combi in a house with two bathrooms?", "acceptedAnswer": { "@type": "Answer", "text": "You can — but you'll feel a pressure or temperature drop when both showers run at once. For two-bathroom homes used by more than two people, a system boiler is usually the better long-term choice." } }, { "@type": "Question", "name": "Is a system boiler more efficient than a combi?", "acceptedAnswer": { "@type": "Answer", "text": "Modern A-rated combi and system boilers are very close on efficiency. Real-world running cost depends more on insulation, hot water usage and controls than the boiler type." } }, { "@type": "Question", "name": "Will I lose loft space if I go from combi to system?", "acceptedAnswer": { "@type": "Answer", "text": "No — system boilers don't need a loft cold tank. You only need a cold tank for old-style 'regular' or 'heat-only' boilers." } }, { "@type": "Question", "name": "Which is more reliable, combi or system?", "acceptedAnswer": { "@type": "Answer", "text": "Both are reliable when serviced annually. Combis have more components doing more work in a smaller box, so faults — when they happen — are typically on the hot water side." } } ] } ] --- [ Limited Offer: New A-rated Boiler Installed from £1,950 — 10-yr warranty included Claim Now ](/contact) [![MAMOSS – Multi-Trade Property Services London](/assets/logo-CrEg7qQf.png)](/) [Home](/)[About](/about) Services Areas We Cover [Our Work](/our-work)[Reviews](/reviews)[Contact](/contact) [020 3432 2349](tel:02034322349)[07377 788881](tel:07377788881) [Request Estimate](/contact) - [Home](/) - [Heating & Boilers](/central-heating-service) - Combi vs system boiler: which one is right for your home? Boiler buying guide # Combi vs system boiler: which one is right for your home? A combi boiler is best for flats and small-to-medium homes with one bathroom — it provides hot water on demand, takes up less space, and doesn't need a cylinder. A system boiler is better for larger homes with two or more bathrooms running simultaneously, because it stores hot water in a cylinder and can supply multiple outlets at once without a drop in pressure. For most London terraced houses with a single bathroom, a combi is the right call. For larger semis and detached homes with multiple bathrooms, a system boiler usually wins. Last updated: 29 April 2026 By MAMOSS Engineering Team — Gas Safe registered engineers ## How a combi boiler works A combi (combination) boiler heats your central heating and provides hot water on demand from a single unit. There's no hot water cylinder and no cold water tank in the loft — just one wall-hung unit, usually in the kitchen or utility cupboard. When you turn on a hot tap, the boiler fires up and heats mains-pressure water as it passes through. That makes it efficient (no stored hot water cooling down) and powerful at the tap. ## How a system boiler works A system boiler heats your central heating and a separate hot water cylinder. The cylinder stores a tank of hot water, ready to be drawn from any tap or shower at any time. Because the hot water is stored, you can run two showers or a shower and a bath simultaneously without the temperature dropping — something a combi struggles with. You don't usually need a cold water tank in the loft (those are for the older 'regular' or 'heat-only' boilers). ## When a combi is the right choice Choose a combi if: you have one bathroom, you have limited space (especially in a London flat or terraced house), you want to free up an airing cupboard, you have decent mains water pressure (at least 1.5 bar), and you don't run two hot taps simultaneously very often. Combis suit the vast majority of London flats and small-to-mid terraces. They're typically cheaper to install and take up less space. ## When a system is the right choice Choose a system boiler if: you have two or more bathrooms used at the same time, you have a larger home (4+ bedrooms), your mains pressure is poor and a combi would underperform, or you want the option to add solar thermal or a heat pump in future. Larger London semis and detached homes — and any property where teenagers tend to overlap shower times — benefit from the stored hot water of a system boiler. ## Cost comparison A combi swap is usually cheaper to install than a system boiler swap because there's no cylinder. A regular-to-combi conversion is also cheaper than a regular-to-system conversion overall. Long-term running cost is similar for both modern A-rated units. The bigger drivers are insulation, controls and the size of the boiler relative to your home — not the combi/system choice itself. Need this work done in London? See our full [boiler installation in london](/services/boiler-installation) service, or get a fixed quote today. [Get a Free Quote](/contact)[07377 788881](tel:07377788881) Related guides and services [Boiler installation in London](/services/boiler-installation)[Boiler installation cost London](/guides/boiler-installation-cost-london)[How long does a boiler install take?](/guides/how-long-does-boiler-installation-take)[Heating & Boilers](/central-heating-service) FAQ ## Frequently asked questions ### Can I run a combi in a house with two bathrooms? ### Is a system boiler more efficient than a combi? ### Will I lose loft space if I go from combi to system? ### Which is more reliable, combi or system? [![MAMOSS – Multi-Trade Property Services London](/assets/logo-CrEg7qQf.png)](/) Multi-trade property services across London. Gas Safe registered, CHAS accredited, fully insured. 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