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Published by Mo's Plumbing and Heating
What Is a Heat Only Boiler?
A heat only boiler, also called regular or conventional, is the traditional arrangement: a boiler that only heats water, a separate cylinder that stores it, and usually a tank or two in the loft feeding the whole thing by gravity. It is the oldest of the three types still commonly in service, and if you have one the real question is whether to keep it.
The Short Answer
A heat only boiler heats water and nothing else. It has no pump or expansion vessel built in, so those sit elsewhere in the system, and it works on an open vented circuit fed from a small tank in the loft rather than being sealed and pressurised. Hot water is stored in a cylinder fed by a second, larger cold tank. It is not obsolete and it is not unsafe, but it has more components in more places than the alternatives, and each of those is somewhere a fault can occur.
What Makes It Different
Two things, and everything else follows from them.
First, the boiler does one job. It heats water for the radiators and for the cylinder coil, and that is all. The pump that moves the water, the expansion arrangements and the controls all live outside the appliance, typically in an airing cupboard. On a system boiler most of those are inside the unit.
Second, the circuit is open vented rather than sealed. A small tank in the loft, the feed and expansion tank, keeps the heating circuit topped up by gravity and gives the water somewhere to expand into when it heats. That is why a heat only system has no pressure gauge and no filling loop: there is nothing to pressurise.
Alongside it there is usually a larger cold water storage tank, also in the loft, feeding the hot water cylinder and often the bathroom taps. That is why the hot water pressure in an older house is frequently weaker upstairs than downstairs: it is being delivered by gravity from a tank a short distance above, not by the mains.
So the giveaway signs are: a cylinder in a cupboard, one or two tanks in the loft, no pressure gauge on the boiler, and a pump you can hear somewhere other than inside the boiler.
The Advantages People Forget
Heat only systems get written off as outdated, and there are genuine reasons some households are better off keeping one.
Simultaneous demand is handled well, because hot water is stored. Several outlets can draw at once without either weakening, and a big cylinder in a busy household is a real comfort advantage over a combi.
The immersion heater is a proper backup. If the boiler fails, an immersion in the cylinder can still heat water electrically, which is worth a great deal in the middle of winter when a repair cannot happen the same day.
The components are simple and largely accessible. A pump, a motorised valve, a cylinder thermostat and a programmer are all separate items that can be replaced individually and relatively cheaply, rather than being integrated into an appliance. Many heat only faults are inexpensive repairs on parts that are still widely available.
And the boiler itself is not doing the complicated work. There is no diverter valve and no plate heat exchanger, which removes two of the commonest failure points on a combi.
The Real Drawbacks
Also honestly stated, because they are the reason so many have been converted.
Tanks in the loft are a liability. They can freeze in an uninsulated roof space, they can overflow, and when either happens the water arrives through a ceiling. They also want occasional checking, which almost nobody does.
There is standing heat loss from the cylinder, so you pay to keep water hot that you may not use. Modern cylinders are far better insulated than older ones, but the loss is not zero.
It takes up space. A cylinder cupboard plus loft tanks is a lot of a property given over to plumbing, and in a flat that cupboard is often the only meaningful storage.
And more components spread over more places means more that can go wrong, and more places to look when something does. That is not a reliability problem so much as a diagnosis one.
Keep It or Convert It
The decision that actually matters if you have one, and there is no single right answer.
Reasons to keep: the household uses hot water at several outlets at once, the incoming mains flow rate is too modest to satisfy a combi, you value the immersion backup, or the system is working and a conversion would be a large disruptive job for benefits you do not particularly want.
Reasons to convert: you want the cupboard and the loft space back, nobody uses two outlets at once, the tanks are old and you would rather not have water stored above the ceiling, or you are having other work done that makes the disruption incidental.
The middle option that gets overlooked is converting to a system boiler rather than to a combi. That keeps the cylinder and the stored hot water, so simultaneous demand and the immersion backup survive, while removing the loft tanks and putting the circuit onto a sealed pressurised system. For a larger household in an older property it is frequently the better answer and it is rarely offered unprompted.
Replacing a heat only boiler with another heat only boiler is also entirely legitimate where the system suits the house, and it is the least disruptive option of all.
Living With One
Practical things worth knowing if you are keeping it.
Know where the tanks are and look at them once a year. You are checking that the float valve shuts off properly, that the tank is not full of debris, that the lid is on and that the insulation is intact. Five minutes.
Know where the immersion switch is and check it works before you need it, rather than discovering during a breakdown that it has never been used and the element has failed.
Insulate the pipework in the loft and the tanks themselves. Loft insulation keeps heat in the house and therefore out of the roof space, which makes the tanks colder, which makes freezing more likely. Insulating the tanks and pipes is the other half of that job and it frequently gets left out.
And if the system has never been cleaned, it is worth doing. Older open vented systems introduce oxygen more readily than sealed ones, which drives corrosion, so sludge tends to accumulate more than in a modern sealed circuit.
Back Boilers Behind the Fireplace
The version of this most particular to Glasgow is the back boiler, and it is useful to know about because it changes what replacement involves more than any other factor. A back boiler sits behind the fireplace, usually with a gas fire on the front of it, heating water for a cylinder and the radiators from a position built into the chimney breast.
They were fitted in enormous numbers through the city's tenements and inter-war housing, because they used a chimney that already existed and needed no external flue at all. A good number are still in service, and they are almost always part of a heat only system with a cylinder and loft tanks.
Removing one is a substantially bigger job than swapping a wall-hung boiler. The unit is built into the fireplace opening, so taking it out means opening up the chimney breast, and what is behind it varies from tidy to alarming. The fireplace then needs making good, the chimney needs capping or ventilating correctly, and the new boiler needs an entirely new position with a new flue route through an external wall, plus a condensate route that never existed. That is why quotes for back boiler removal sit well above a straight swap and why the day count runs longer.
It is also why some are still there. Where a back boiler is working and the household is content with the system, replacing it is a considered decision rather than an obvious one. Where it has failed, the honest position is that the replacement is a bigger project than people expect, and that is much better known at the survey than discovered halfway through.
When to Call Someone Out
Worth a conversation if you have a heat only system and are weighing whether to keep it, because the answer depends on your household and the property rather than on the type being old. The middle option, converting to a system boiler and keeping the cylinder while losing the loft tanks, is frequently the right one for larger older properties and is rarely offered unless asked for. Worth calling too for anything involving a back boiler, since removal is a considerably larger job than a wall-hung swap and the price and the day count both reflect that. Mo and Muhammad are Gas Safe registered for domestic work. The survey is free on installations and replacements.
Boiler Installation in GlasgowCommon Questions
Yes. Heat only, regular and conventional all describe the same arrangement: a boiler that only heats water, a separate cylinder storing it, and usually tanks in the loft on an open vented circuit. Different installers and manufacturers favour different names for it and they mean the same thing.
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Read Next
Full guide
What Is a System Boiler?
A system boiler heats your radiators and heats a separate hot water cylinder, which stores hot water ready to use.
How Does a Combi Boiler Work?
A combi heats your radiators, and when you open a hot tap it stops heating the radiators and diverts all its output into heating the water flowing through to that tap instead.
What Is a Condensing Boiler?
A condensing boiler extracts extra heat from its own exhaust gases by cooling them until the water vapour in them turns back into liquid.
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