System vs Heat-Only Boiler: Which Do You Need?
Both system and heat-only boilers serve homes with hot water cylinders — the difference is what surrounds them. A system boiler runs a sealed, pressurised circuit with its expansion vessel built in, feeding an unvented (mains-pressure) cylinder in the modern setup. A heat-only (conventional) boiler traditionally partners loft tanks and an open-vented cylinder. Replacing like-for-like is cheapest; upgrading from tanks to sealed-and-unvented is the transformation — and which is right depends on your mains flow, your loft's contents and how far you want this replacement cycle to reach.
The actual differences
A system boiler integrates the sealed-system components — expansion vessel, pump — inside the case: cleaner installation, pressurised circuit (the familiar 1–1.5 bar gauge), no loft tanks for the heating side, and a natural partner for unvented cylinders delivering mains-pressure hot water to every tap. It's the default modern architecture for cylinder homes.
A heat-only boiler is the minimal appliance — burner and exchanger — with pump, tanks and expansion handled externally in the traditional open-vented arrangement: a feed-and-expansion tank in the loft for the heating circuit, a cold-water storage tank feeding a vented cylinder by gravity. It survives for good reasons in specific situations: direct swaps into sound open-vented systems, and homes whose mains flow can't support pressurised alternatives.
When each wins
System boiler territory: any full refurbishment of a cylinder home (sealed circuit plus unvented cylinder is the modern spec), homes wanting mains-pressure showers without pumps, lofts being converted or cleared of tanks, and heat pump futures — the sealed-cylinder architecture converts gracefully later. If you're changing more than just the boiler, this is the direction of travel.
Heat-only territory, honestly: the direct swap into a working open-vented system you're not otherwise touching (cheapest, least disruptive — a sound gravity system with a tired boiler doesn't demand wholesale conversion this cycle), and the poor-mains house where unvented pressure has nothing to feed on — vented storage plus shower pumps remains the pragmatic answer until the supply is fixed. Both are legitimate 'keep it simple' calls, not failures of ambition.
The unvented cylinder upgrade
The jump worth pricing whenever a cylinder home replaces its boiler: swapping the vented cylinder and tanks for an unvented unit — mains-pressure hot water at every outlet, showers that stop negotiating, the loft emptied. Costs £1,200–£2,500 for the cylinder work within a wider job; requires decent mains flow (measured, not assumed) and G3-qualified installation, which is a legal requirement and a network standard.
The combined move — system boiler plus unvented cylinder, replacing heat-only plus tanks — runs £2,800–£4,500 all-in and is the full modernisation. Against a £2,200 like-for-like heat-only swap, the £1,000–£2,000 difference buys the pressure, the loft and the future-readiness; whether that's this cycle's money is exactly the decision, and it deserves both quotes side by side.
Deciding without regret
The decision inputs, in order: mains flow rate (measured — it gates the unvented route entirely), scope appetite (boiler-only swap versus system modernisation), loft plans (conversions force the tank question anyway), and horizon (heat pump intentions favour the sealed-cylinder architecture now). A surveyor who checks all four before recommending is designing; pricing follows honestly from there.
The regret patterns to avoid: converting to unvented on unmeasured mains (the pressure that never arrives), and spending the full modernisation on a house being sold or gut-renovated soon (the next project would have absorbed it cheaper). Matched to your actual facts and horizon, either architecture serves well — which is this comparison's quiet conclusion: it's a fit question, not a quality contest.
| Factor | System boiler | Heat-only |
|---|---|---|
| Circuit | Sealed, pressurised | Open-vented (tanks) |
| Cylinder partner | Unvented (mains pressure) | Vented (gravity) |
| Loft tanks | None (heating side) | Feed & expansion + storage |
| Like-for-like swap cost | £2,000–£3,000 | £1,800–£2,600 |
| Full modernisation | £2,800–£4,500 with cylinder | — |
| Best for | Refurbs, pressure, HP futures | Direct swaps, poor mains |
Timing the change well
Decisions like this have natural moments: replacement time (when something has failed anyway), renovation time (when floors are up and trades are on site), and pre-failure time (when the incumbent is old enough that acting on your schedule beats reacting on its). The expensive moment is mid-winter, choosing under pressure from a cold house. If your system is ageing, deciding now — even if executing later — is cheap insurance.
Frequently asked questions
Should I convert from heat-only to a system boiler?
If you're modernising anyway — cylinder, loft, pressure ambitions — yes, it's the direction of travel. If the open-vented system is sound and you only need a boiler, the like-for-like swap is the legitimate economical answer this cycle.
What's the advantage of an unvented cylinder?
Mains-pressure hot water at every tap — transformative showers without pumps — plus the loft cleared of tanks. It needs decent measured mains flow and legally requires G3-qualified installation, both of which the survey confirms.
Can I keep my loft tanks with a new boiler?
Yes — a heat-only replacement into the existing open-vented system is the standard direct swap. It's the cheapest route and right where the system's sound and the scope is boiler-only.
Which suits a future heat pump better?
The sealed system with a cylinder: heat pumps need cylinder-based hot water, and the architecture converts gracefully. If a pump is on your horizon, weight this replacement toward the system-and-cylinder route.
If you'd rather hand this over
Plenty of readers reach this point, weigh the afternoon involved, and sensibly hand it to someone who does it weekly. No judgement — that's what the network exists for. Every engineer is Gas Safe registered (plus OFTEC for oil and MCS for heat pumps), insured and locally based; prices are agreed up front; and genuine no-heat emergencies are prioritised. Describe the symptoms you've identified here and you'll get an accurate price rather than a guess — informed customers reliably get better quotes.
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