--- title: "Combi vs System vs Regular Boilers Explained | Reactive" description: "Combi, system or regular boiler? A Gas Safe engineer explains the real differences in cost, hot-water performance and which type fits your home." lang: en-GB json-ld: | [ { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "Can I run two showers at once on a combi boiler?", "acceptedAnswer": { "@type": "Answer", "text": "Realistically no, not at full performance. A 28–35kW combi will deliver about 12–15 litres per minute total — split across two showers, neither will be satisfying. If you regularly run two showers in parallel, a system boiler with a properly-sized unvented cylinder is the right answer." } }, { "@type": "Question", "name": "Do I save money switching from a regular boiler to a combi?", "acceptedAnswer": { "@type": "Answer", "text": "Operating costs are similar (both are 90%+ efficient if modern), but a combi removes the heat losses from the cylinder and avoids re-heating water you don't use. Typical fuel saving is 5–10% versus a comparable regular setup. The bigger gain is space — losing the cylinder and tanks frees up an airing cupboard and the loft." } }, { "@type": "Question", "name": "Are system boilers more reliable than combis?", "acceptedAnswer": { "@type": "Answer", "text": "Slightly, because they do less work — the boiler heats water in batches rather than instantaneously, so the heat exchanger sees gentler thermal cycles. In practice both are reliable when properly installed and serviced annually. Manufacturer warranties (Worcester, Vaillant) are typically 10 years on either type." } }, { "@type": "Question", "name": "What if I'm tight on space?", "acceptedAnswer": { "@type": "Answer", "text": "Combi wins easily — wall-hung in a kitchen cupboard or utility room, no cylinder needed. The smallest combis (Worcester 2000, Ideal Logic Max) fit inside a standard kitchen cupboard." } }, { "@type": "Question", "name": "Can I keep my existing cylinder and just change the boiler?", "acceptedAnswer": { "@type": "Answer", "text": "Often yes — a system boiler swap can keep an existing well-maintained unvented cylinder. Vented (gravity) cylinders are usually best replaced, especially if they're 15+ years old, because the energy gains are significant and a new cylinder restores performance." } }, { "@type": "Question", "name": "Which brand should I choose?", "acceptedAnswer": { "@type": "Answer", "text": "We install Worcester Bosch (Greenstar) and Vaillant (ecoTEC plus) most commonly — both come with 10-year manufacturer warranties through accredited installers and parts are widely stocked. Ideal Logic is a solid mid-range choice. We avoid budget brands fitted by developers because warranty support and parts availability are weaker." } } ] }, { "@context": "https://schema.org", "@type": "Article", "@id": "https://reactiveplumbing.co.uk/guides/combi-vs-system-vs-regular-boilers#article", "headline": "Combi vs System vs Regular Boilers: Which Is Right for You?", "description": "Combi, system or regular boiler? A Gas Safe engineer explains the real differences in cost, hot-water performance and which type fits your home.", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://reactiveplumbing.co.uk/guides/combi-vs-system-vs-regular-boilers" }, "author": { "@type": "Person", "name": "Carl Eddershaw", "jobTitle": "Director & Lead Engineer", "worksFor": { "@id": "https://reactiveplumbing.co.uk/#localbusiness" } }, "publisher": { "@id": "https://reactiveplumbing.co.uk/#organization" }, "datePublished": "2026-05-01", "dateModified": "2026-05-01", "image": "https://reactiveplumbing.co.uk/favicon.png", "inLanguage": "en-GB" }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "https://reactiveplumbing.co.uk" }, { "@type": "ListItem", "position": 2, "name": "Heating & Gas Services", "item": "https://reactiveplumbing.co.uk/heating-gas-services" }, { "@type": "ListItem", "position": 3, "name": "Boiler Installation", "item": "https://reactiveplumbing.co.uk/boiler-installation" }, { "@type": "ListItem", "position": 4, "name": "Combi vs System vs Regular Boilers: Which Is Right for You?", "item": "https://reactiveplumbing.co.uk/guides/combi-vs-system-vs-regular-boilers" } ] }, { "@context": "https://schema.org", "@type": "Person", "name": "Carl Eddershaw", "jobTitle": "Director & Lead Engineer", "image": "https://reactiveplumbing.co.uk/favicon.png", "worksFor": { "@id": "https://reactiveplumbing.co.uk/#localbusiness" }, "knowsAbout": [ "Gas Safe boiler installation", "OFTEC oil heating", "LPG installations", "Plumbing & drainage" ] } ] --- Gas Safe Registered · OFTEC Registered · Checkatrade Verified 9 Rutherford Court, Corby NN17 4SJ [info@reactiveph.co.uk](mailto:info@reactiveph.co.uk) [![Reactive Gas Heating & Plumbing logo](/__l5e/assets-v1/51d713a4-18a4-4b5f-96fb-2fcce169e56c/reactive-logo.png)](/) ServicesAreasGuides[About](/about)[Our Work](/our-work)[Plans](/maintenance-plans)[Reviews](/reviews)[Contact](/contact) [24/7 Call 07915 906262 ](tel:+447915906262)[Fixed Boiler Quote](/boiler-quote) 1. [Home](/) 2. [Heating & Gas Services](/heating-gas-services) 3. [Boiler Installation](/boiler-installation) 4. Guide Guide · Boiler Installation # Combi vs System vs Regular Boilers: Which Is Right for You? Three boiler types, three completely different installations and bills. The right choice depends on how many bathrooms you have, how strong your incoming mains pressure is, and whether you've already got a hot-water cylinder you don't want to lose. 7 min read By Carl Eddershaw, Director & Lead Engineer Last updated May 2026 Quick answer **Most UK homes with one bathroom are best on a combi boiler** — no cylinder, no loft tanks, instant hot water, lowest install cost. **Homes with two or more bathrooms run in parallel are usually better on a system boiler** with an unvented cylinder, because a combi can't keep up with two showers at once. **Stick with a regular (heat-only) boiler only if you've got a low-pressure mains, a usable loft, and don't want the cost of removing tanks.** ## How each boiler type actually works A **combi boiler** heats your water on demand. Cold water enters the boiler from the mains, gets heated as it passes through a heat exchanger, and comes out the hot tap. There's no stored hot water and no cylinder — the boiler does both heating and hot water from a single wall-hung unit. A **system boiler** heats water that's stored in an unvented cylinder (usually airing-cupboard sized). The boiler itself is sealed and pressurised — no header tank in the loft. When you turn a hot tap on, you draw stored hot water at full mains pressure. When the cylinder cools, the boiler refills it. A **regular boiler** (also called heat-only or conventional) is the traditional setup: a boiler downstairs, a hot-water cylinder in the airing cupboard, and a cold-water tank plus feed-and-expansion tank in the loft. Gravity-fed, low pressure at the taps, lots of pipework. Most of these systems are 30+ years old and were designed for a different era. ## Real-world performance differences On a combi, you get hot water the moment you turn the tap on — but only one tap at a time can run flat-out. Try to run a kitchen tap and a shower together on a 28kW combi and one of them will go cold or weak. Combi flow rates are typically 10–15 litres per minute on a good unit; an unvented cylinder on a system boiler will deliver 20–30 LPM at full mains pressure. Heating-only performance is roughly the same across all three types — they all run at the same efficiency band (90%+ on modern condensing boilers). The differences are entirely about hot water and installation complexity. - **One bathroom, two adults** — combi handles this easily. Most efficient choice. - **Two bathrooms, one used at a time** — combi still works if mains pressure is good (1+ bar dynamic). - **Two bathrooms run simultaneously** — system boiler with unvented cylinder. Combi will disappoint. - **Three or more bathrooms** — system or regular with appropriately-sized cylinder, no exception. - **Low mains pressure** (under 1 bar dynamic) — combi will struggle. Stick with stored hot water. ## Installation cost and complexity Combi-for-combi swaps are typically the cheapest install option in our area on most properties. Going from a regular boiler with tanks to a combi is more involved — we remove the cylinder, drain and remove the loft tanks, cap off the old pipework, and re-pipe to the new combi location, so the conversion sits in a higher band. System boiler installations cost similar to combi swaps if you're keeping an existing cylinder, but if the cylinder needs upgrading to unvented (which it usually does for proper performance), add a separate fixed-price line for a quality stainless steel unvented cylinder plus the G3 unvented commissioning. For a full breakdown of pricing in our area, see our guide to [how much a new boiler costs in 2026](/guides/how-much-does-a-new-boiler-cost). ## Combi vs system vs regular at a glance Bands here reflect typical relative cost in the Northamptonshire / Rutland area on a typical install — your home will vary based on access, flue routing, and whether pipework needs upgrading. Every install is quoted as a fixed price after a free survey. Combi System Regular Best for 1 bathroom homes 2+ bathrooms in parallel Existing low-pressure setups Hot water cylinder? No Yes (unvented) Yes (vented) Loft tanks? No No Yes (cold + F&E) Hot water flow rate 10–15 LPM 20–30 LPM at mains pressure Limited by gravity head Two showers at once No Yes Possibly, weak Typical install cost Lowest Highest Mid-range Mains pressure required 1 bar+ dynamic 1.5 bar+ dynamic Any (gravity-fed) Space needed Wall-hung only Boiler + cylinder Boiler + cylinder + tanks ## Other things worth thinking about **Mains water pressure is the silent decider.** Always test dynamic flow rate before committing to a combi — we run a flow check on every survey and won't install a combi if the pressure won't sustain it. Roughly 15% of properties we survey in older Corby and Wellingborough streets fail combi-suitable mains tests; for those, a system boiler is the safer answer. **Future-proofing for heat pumps.** If you might switch to an air-source heat pump in the next decade, keeping a hot-water cylinder (system or regular) makes that transition cheaper. Combi-to-heat-pump conversions are more disruptive because you have to add a cylinder back in. **Property age and pipework.** Pre-1960s homes often have undersized 15mm gas runs sized for back-boilers, not modern combis. Re-piping to 22mm is part of any honest install quote — if a quote doesn't mention gas pipe sizing, ask why. About the author ### Carl Eddershaw Director & Lead Engineer · Reactive Gas Heating & Plumbing Reactive runs from its workshop at Rutherford Court in Corby. Gas Safe registered, OFTEC registered for oil and certified for LPG, with daily on-the-tools experience installing, repairing and servicing boilers across Northamptonshire, Rutland and Leicestershire. Every guide on this site is written from real call-out notes — not generic copy. ## Frequently asked questions ### Can I run two showers at once on a combi boiler? ### Do I save money switching from a regular boiler to a combi? ### Are system boilers more reliable than combis? ### What if I'm tight on space? ### Can I keep my existing cylinder and just change the boiler? ### Which brand should I choose? Need a hand? ## Get a fixed-price quote on the right boiler for your home We survey free, calculate the right kW output for your property, test mains pressure, and quote fixed in writing — combi, system or regular, whichever genuinely fits. 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