A regular heat-only boiler heats water for your radiators and hot water cylinder, but nothing else — it relies on separate components to store water and manage pressure. It still makes sense in homes with an existing open-vented system, older pipework, and a hot water cylinder already in place, where switching to a combi or system boiler would mean significant disruption for little gain.
What a heat-only boiler actually does
A regular boiler — sometimes called a conventional or heat-only boiler — does one job: it heats water and sends it round the system. It does not store hot water or generate it on demand. That work is handed off to other parts of the setup.
In a typical installation, the boiler is paired with a hot water cylinder (usually in an airing cupboard) and two tanks in the loft. One is the feed and expansion tank, which tops up the heating circuit and gives the water somewhere to go as it expands when hot. The other is the cold water storage tank, which feeds the cylinder and, in many older homes, the cold taps and toilets too.
This arrangement is an open-vented system, meaning it stays at roughly atmospheric pressure rather than being sealed and pressurised. That is the key difference from the more modern sealed systems used by combi and system boilers.
Why older homes often keep them
A regular heat-only boiler heats water for your radiators and hot water cylinder, but nothing else — it relies on separate components to store water and manage pressure.
Many properties built before the 1990s were designed around this layout, and the whole house works with it. The pipework, the cylinder, the tanks and the boiler form one coherent system. Ripping that out to fit something newer is rarely as simple as swapping the boiler itself.
A few reasons these systems tend to stay put:
- High hot water demand. A stored cylinder can supply several taps or showers at once without the flow dropping. A combi boiler heats water on demand and can struggle when two outlets run together.
- Larger homes with more than one bathroom. The cylinder copes better with simultaneous use, which matters in family households.
- Older or non-standard pipework. Some older pipes run at lower mains pressure or use wider bores suited to gravity-fed flow. A combi expects good mains pressure and flow rate to perform well.
- Compatibility with immersion heaters. A cylinder can be heated by an electric immersion as a backup, which a combi cannot offer.
There is also the practical point that if the existing system is sound and the household is happy with how it performs, replacing only the boiler is usually cheaper and less invasive than converting the whole setup.
The role of loft tanks
The tanks in the loft are central to how a regular boiler works, and they are the part most people forget about until something goes wrong.
The feed and expansion tank — often a small black plastic cistern — keeps the heating circuit topped up and absorbs the expansion of heated water. As the water in the radiators warms and expands, the surplus rises into this tank rather than building pressure in a sealed loop. A ball valve refills the circuit if the water level drops.
The larger cold water storage tank supplies the hot water cylinder. Gravity from the height of the loft provides the pressure, which is why hot water flow on these systems can feel gentler than mains-pressure systems. In many homes it also feeds cold taps upstairs and the toilet, leaving only the kitchen cold tap on the mains.
Loft tanks do need occasional attention. Things worth checking include the ball valve working freely, the lid fitting properly to keep out dust and insects, and insulation around the tank and pipes to prevent freezing in winter. A frozen feed and expansion tank can stop the heating working until it thaws.
When to convert and when not to
There is no single right answer — it depends on the property, the household, and the state of the existing system.
Keeping a regular boiler often makes sense when:
- The home has two or more bathrooms and high simultaneous hot water demand.
- Mains water pressure or flow rate is poor, which would leave a combi underperforming.
- The existing cylinder and tanks are in good condition and the layout already works.
- The household values an immersion heater backup or wants to add solar water heating, which suits a cylinder.
Converting to a combi or sealed system can be worth considering when:
- The loft space is wanted for conversion and the tanks are in the way.
- Hot water demand is modest — for example a one-bathroom flat or small house.
- Mains pressure and flow are strong enough to give good performance on demand.
- The cylinder and tanks are old, corroded or leaking and would need replacing anyway.
It is worth remembering that conversion is not just a boiler change. Removing the tanks, draining and pressurising the system, and possibly upgrading pipework all add cost and labour. A heating engineer should assess mains flow rate before recommending a combi, because a poor result on a flow test is the most common reason a conversion disappoints.
If the current system is reliable and suits the home, replacing a tired regular boiler with a new heat-only model is often the most sensible route. Modern condensing heat-only boilers are far more efficient than older units while keeping the rest of the system you already have. Anyone weighing up the options should ask an installer to explain both paths clearly, including the disruption, the running costs, and how each would handle the household's actual hot water use.