Heat Only Boiler: What a Regular or Conventional Boiler Is

Heat only, regular, conventional: the same thing

The names get used loosely, so it helps to be clear. Heat only boiler, regular boiler, conventional boiler and open vent boiler all describe the same arrangement: the boiler only produces heat, and everything else needed to move and store that heat is separate. Manufacturers usually list these models as heat only, while installers and homeowners more often say regular or conventional.

The other two types are the system boiler, which has the pump and expansion vessel built in and a sealed heating circuit, and the combi, which heats water on demand with no cylinder. Our guide on what is a system boiler covers the first, and how does a combi boiler work covers the second.

How a heat only boiler system works

The boiler heats water, and a pump, usually in the airing cupboard near the cylinder, pushes it around the heating pipework. Motorised valves then send it to the radiators, to the coil inside the hot water cylinder, or to both, depending on what the programmer and thermostats are asking for. The coil heats the stored water in the cylinder without mixing with it. A heat only boiler fitted since 2005 will be a condensing model; our guide on what is a condensing boiler explains what that means for efficiency and winter faults.

Up in the loft, the feed and expansion tank keeps the heating circuit full. As the water in the heating heats up it expands, and the extra volume rises back up into that small tank. An open vent pipe runs from the heating up over the tank, as a safety route if the system overheats. Because the heating is open to the air in this way, it runs at low pressure, set by the height of the tank above the boiler rather than by a pressure gauge.

The larger cold water storage tank in the loft feeds the bottom of the hot water cylinder. As you draw hot water from the top of the cylinder, cold water from the loft tank replaces it. The hot water pressure you get at the taps depends on how high that tank sits above them, which is why showers on the top floor of a house with a regular boiler are often weak. Our guide on cold water tank in the loft explains what that tank does and whether it can go.

How to tell if you have a heat only boiler

Check the loft and the airing cupboard. You are looking for these signs.

• Two tanks in the loft: a large one, often a black plastic or older galvanised tank, and a much smaller one nearby.
• A hot water cylinder, usually copper with foam or a jacket, in an airing cupboard.
• A pump fitted on the pipework in the airing cupboard or next to the boiler, often with a valve either side.
• No pressure gauge on the boiler, or a gauge that sits near zero and never matters.
• Older homes sometimes have a gravity hot water circuit, where the boiler heats the cylinder with no pump at all on that side. A towel rail or radiator on that circuit gets warm whenever the hot water is on.

If there is a cylinder but no tanks in the loft and a pressure gauge on the boiler, you have a system boiler with an unvented cylinder. If there is no cylinder at all, you have a combi. Our guide on unvented vs vented cylinder helps you tell the cylinders apart.

Pros and cons of a heat only boiler

The main strength is that it can supply several outlets at once from stored water, and the system is simple and forgiving. Because the heating is open vented and low pressure, it suits old pipework and radiators that might not cope with being pressurised. Swapping an old heat only boiler for a new heat only boiler is usually the least disruptive replacement, because the rest of the system stays as it is.

The drawbacks are the tanks and the pressure. The loft tanks take space, can freeze in a hard winter if they are not insulated, and can overflow when a float valve sticks. Hot water pressure is limited by gravity, so many owners end up adding a shower pump. There are more separate parts to fail: the pump, the valves, the tank float valves. And if something goes wrong with the heating circuit, it tends to show up as water in the loft rather than on a gauge. Our guide on header tank overflowing covers the most common version of that.

Common faults on a heat only system

Heat only systems have their own set of faults, and most of them are outside the boiler itself.

• No hot water but heating works: the hot water motorised valve, the cylinder thermostat or the programmer channel. Our guide on no hot water but heating works takes you through it.
• Hot water appearing in the small loft tank: pumping over, often from a partly blocked pipe, a pump set too high or a valve problem.
• Radiators hot when hot water is on: a valve stuck open, or a gravity circuit doing what it was designed to do.
• Weak hot water pressure upstairs: the height of the loft tank, or a partly blocked or undersized pipe from it.
• Airlocks after the system has been drained, because an open vented system relies on gravity to refill.
• Pump noise or a pump that has seized after the summer, often the first time the heating is switched on in autumn.

Should you keep it or change it?

If the heat only boiler has failed and the rest of the system is in good order, replacing it like for like is quick and keeps disruption low. It is also the sensible choice in a house with old pipework, where pressurising the heating could reveal weak joints.

If you want better hot water pressure and to clear the loft, the usual upgrade is a system boiler with an unvented cylinder, which gives mains pressure at every tap. A combi removes the cylinder as well, which suits smaller homes with one bathroom. Our guide on combi or system boiler with a cylinder compares the options, and converting to a combi boiler covers what happens to the cylinder and tanks. Mains pressure is the deciding factor either way: a good flow rate at the stopcock makes both upgrades work well, a weak one makes neither work.

Getting it looked at

Gas work on the boiler needs a Gas Safe registered engineer. The pump, valves, cylinder and tanks are plumbing, and that is where most heat only faults actually sit.

If your hot water or heating has stopped, or water is appearing in the loft, call us on 020 4652 5111 or send an enquiry with the make of boiler and what you are seeing. We have 20 engineers across London and we deal with heat only systems every day, from the loft tanks down to the cylinder. Our rate is on the pricing page and is agreed before we attend.