Hot water cylinders — fitted, serviced and repaired properly.
The hot water cylinder is the tank that stores your hot water — not the boiler, though plenty of people call it that. I fit, service and repair them across Luton, Watford, St Albans and the wider Bedfordshire, Hertfordshire and Buckinghamshire area, including the sealed, mains-fed sort properly called an unvented hot water cylinder, which is controlled work I’m qualified (G3) to install and sign off. Whether yours is dripping, has gone cold, or you’re planning a new one, I can sort it — and explain it in plain English along the way.
First, what it actually is — and your options.
A hot water cylinder is simply a tank that stores hot water, ready for your taps and showers. Something else heats that water — usually a boiler or a heat pump, or electric heaters built into the tank — so the cylinder and the boiler are two different things, even though they often get muddled up.
You’ll hear cylinders called “vented” or “unvented”, and it’s worth knowing which you have. A vented cylinder is the older sort: it’s fed by a cold tank up in the loft and is open to the air through a vent pipe. An unvented cylinder is the newer, sealed sort: there’s no loft tank, and it’s connected straight to your cold mains. “Vented” or “unvented” just means open to the air, or sealed — that’s all the word is telling you.
One thing that’s widely misunderstood: an unvented cylinder doesn’t make your water pressure strong. It simply lets you use your mains pressure for hot water, so the hot runs at the same pressure as the cold. If your incoming mains is weak, your hot water will be weak too — a cylinder can’t fix that, and I’ll tell you straight if that’s the case rather than sell you something that won’t deliver.
There’s more than one way to get hot water in a home, and the right one depends on the property and how you live:
Unvented cylinder
Sealed, fed from the mains, with no loft tank. Hot and cold run at the same (mains) pressure, and it can serve several taps or showers at once — as long as your mains supply is good. It needs the safety devices and a yearly service.
Vented cylinder
The traditional setup, fed by a cold tank in the loft. Simple and reliable, but the pressure is gentler — especially upstairs, near the tank — because it’s fed by gravity rather than the mains.
Combi boiler
No cylinder at all — it heats water on demand as you use it. Great for smaller homes with one bathroom, and it frees up space, but it can struggle to run two hot taps or showers at the same time.
Electric, at the tap
A small electric heater fitted under a sink or basin, for a single outlet. Handy for a downstairs cloakroom, an office toilet, or anywhere a long way from the main system, where running pipes to it wouldn’t be worth it.
Two ways a cylinder is heated: direct and indirect.
If your cylinder is heated by a boiler or a heat pump, it’s an “indirect” cylinder: hot water from the boiler runs through a coil inside the tank and warms the water around it, a bit like a kettle element. The electric immersion heater is then just a backup.
If there’s no boiler or heat pump, and the cylinder is heated only by electric heaters built into it, it’s a “direct” cylinder — common in flats and all-electric homes. These usually have two heaters, which I’ll come back to in a moment.
A sealed cylinder needs safety features — here’s why.
Water swells as it heats. In an old vented cylinder that’s no problem — the extra water just nudges back up into the loft tank. But a sealed unvented cylinder has nowhere for that extra water to go, and it’s under mains pressure, so it needs a few devices to manage the expansion and to make sure the water can never overheat. That’s also why it’s controlled work under the building regulations (the “G3” rules), and has to be installed, signed off and serviced by someone qualified.
How the expansion is handled — and why it fails
It’s worth understanding this one, because it’s behind most cylinder problems. The room for that expanding water is made in one of two ways. Most systems use a separate expansion vessel: a small tank split in two by a rubber membrane, air on one side, water on the other. As the water heats and swells, it pushes against the membrane and squashes the air cushion, making just enough room and keeping the pressure steady. Some cylinders — the Megaflo type and similar — do the same job with a built-in air gap (an air bubble) trapped at the top of the tank, instead of a separate vessel.
Either way, the air cushion slowly disappears over the years — it leaks away on a vessel, or gets absorbed into the water on an air-gap cylinder. Once it’s gone, there’s nowhere for the expanding water to go, so the pressure climbs as the cylinder heats. What usually happens then is that the pressure relief valve on the cold inlet opens first — it’s set to a lower pressure than the temperature valve on the cylinder — and lets a little cold mains water out through the tundish. So it’s normally wasted cold water, not heated water (hot water only escapes if that valve sticks shut, or the temperature valve opens too early). It’s wasteful all the same, and a clear sign the vessel needs recharging, or the air gap restoring — a routine part of a service.
Backup, booster, and cheap night-time heating.
The immersion heater is an electric element inside the cylinder, like a giant kettle element. If your cylinder is heated by a boiler or a heat pump, the immersion is a backup — there for when the boiler’s down, or to top the tank up — not the main heater.
With a heat pump it earns its keep in another way. A heat pump heats water to a gentler temperature than a boiler, so the immersion is used now and then to give the tank a hotter cycle that keeps the stored water safe, as well as being the backup. It matters that it works — if it has failed, you can lose that safety top-up without realising — so I always check it on a heat-pump system.
On a direct (electric-only) cylinder there are usually two immersion heaters. The top one heats just the upper part of the tank for a quick boost of hot water during the day, when you don’t want to wait for a full tank. The bottom one heats the whole cylinder, and ideally runs overnight on a cheaper night-time electricity rate — an Economy 7 tariff or similar — so you heat a full tank cheaply while you sleep, with the top heater there as a daytime top-up if you run short.
The secondary return pump — comfort, and what it costs.
In a larger home, or where a tap is a long way from the cylinder, you can wait an age for hot water to arrive — running cold water down the drain while you wait. A secondary return fixes that: a small pump keeps hot water gently circulating around a loop of pipe, so it’s hot the instant you turn the tap.
That comfort isn’t free, though. The pump uses a little electricity, and the circulating pipe loses heat the whole time it’s running, so the cylinder has to reheat more often. Left running around the clock, it quietly adds to your bills.
The sensible answer is to run it on a timer — on for the mornings and evenings when you actually want instant hot water, off the rest of the time — and to insulate the loop pipework well. Done that way, you get the comfort without the constant cost. If the pump fails you’ll simply be back to waiting for hot water; and if it has been left running constantly, switching it to a timer is an easy saving I can set up.
What usually goes wrong — and what it’s telling you.
Most cylinder problems come back to a handful of causes, and the symptom usually points straight to the part.
Water from the outside pipe or tundish
Usually a waterlogged expansion vessel (or a lost air gap) letting the pressure climb as the water heats — the cold inlet valve then lets a little water out. Common and fixable. But if the water is hot and it’s the temperature valve releasing it, that’s an overheating fault and needs prompt attention.
Cold or only lukewarm
Usually the immersion heater or its thermostat, the boiler or heat pump not heating the coil, a tripped cut-out, or a control fault — rather than the tank itself.
Poor flow at the hot taps
Often a blocked strainer in the inlet valve, the inlet pressure set too low, or a weak incoming mains — and sometimes scale narrowing the pipework over the years.
Banging or rumbling
In a hard-water area, scale on the immersion or coil makes the cylinder rumble as it heats, the same way a kettle does. It also slows the reheat and pushes up the bill.
Weeping fittings or immersion
Joints, the immersion seal or a valve can weep over time. A genuine leak from the tank itself is rarer, but it needs finding quickly before it does damage.
Keeps tripping
A cut-out that keeps tripping is doing its job — it’s telling you the thermostat or heater has failed and is overheating. It shouldn’t simply be reset and ignored.
Why a yearly service is worth it.
A yearly service isn’t just a box-tick — it’s how you know the cylinder is actually working as it should, and working safely. Take the expansion vessel (or the air gap on a Megaflo-type cylinder): when it fails, the pressure relief valve on the cold inlet quietly lets mains water trickle out through the tundish — usually cold water, often without you noticing — while your water meter keeps ticking. A service catches that, and the other small faults, before they cost you.
On a service I recharge the expansion vessel, test the safety valves, check the tundish and discharge pipe, the thermostat and cut-out, and the condition of the tank and immersion. Most manufacturers also require a yearly service to keep the warranty valid, and if you let property it’s part of keeping it safe for tenants. Stored hot water brings a Legionella duty too; see my Legionella risk assessment page.
Scale, and why a softener pays for itself.
This is a hard-water area, and scale is hard on a cylinder. It builds up on the immersion heater and on the boiler coil — the hottest surfaces — where it acts like a blanket: the water takes longer to heat, you use more energy, and the immersion and coil wear out sooner. Scale also collects on the valve seats and strainers, which is a common cause of dripping valves and poor flow.
Softening the incoming water prevents it. A water softener keeps the immersion, the coil and the valves clean, so the cylinder heats quickly, lasts longer and stays trouble-free — and I can fit and set one up alongside the cylinder work.
Fitting or replacing a cylinder, sized for your home.
When you need a new cylinder — a first one, a replacement, or a change from an old vented system to a sealed unvented one — I size it to your home and how much hot water you actually use, fit the right tank and controls, and sign it off to the G3 building regulations with the paperwork you need. I’ll check your mains supply can feed it properly first, so it performs the way it should. I work on all the common makes, from Megaflo and Heatrae Sadia to Telford, Gledhill, Joule and OSO.
Most good cylinders have two coils inside, which makes them ready for a heat pump later as well as a boiler now. I connect the cylinder so a heat pump can use both coils, which gives faster reheat and the large heating surface a heat pump likes — and keeps a future heat pump job simpler and cheaper. If that’s on your mind, see my heat pump and solar hot water pages.
What you can safely check yourself first.
One important safety note: a sealed cylinder holds hot water under pressure, with a lot of energy stored in it. Never touch the safety valves, the expansion vessel or the temperature controls, and never block or cap the discharge pipe — those are the safety devices, and adjusting them is qualified G3 work. The checks below are the safe ones; anything past them needs an engineer.
If a check points to a valve, the expansion vessel, the immersion or a leak, that’s where I come in. Not sure what you’re looking at? Send me a few photos and a short description through my contact page, and when I get a chance I’ll take a look and give you a steer — whether it’s something simple or it needs a visit. There’s no charge for that, and no rush to book anything.
Straightforward about price.
Services by the hour; bigger jobs quoted clearly.
A service is charged at an hourly rate and usually takes an hour or less. If I find it needs more than a routine service — a part replacing, or a fault that will take longer — I’ll explain what’s needed, agree a plan and a price with you first, and not just run the clock. For a new cylinder or a replacement I’ll carry out a survey and give you a written quote up front. Either way you’re dealing with one G3-qualified engineer who does the work, signs it off, and backs it with a 24-month workmanship warranty.
Luton, Watford, St Albans and across the home counties.
I’m based in Luton and cover Bedfordshire, Hertfordshire, Buckinghamshire and north London as standard. Distance is rarely the issue; if you need a cylinder fitted, serviced or fixed, I’ll come to it.
- Bedfordshire — Luton, Dunstable, Houghton Regis and the surrounding towns.
- Hertfordshire — Watford, St Albans, Harpenden, Hemel Hempstead, Welwyn Garden City, Hitchin and nearby.
- Buckinghamshire & north London — Leighton Buzzard, Aylesbury, Barnet, Finchley and the neighbouring areas.
- Further afield — larger or specialist jobs anywhere worth the journey, with travel agreed up front.
Tell me about your hot water.
Whether your cylinder is dripping, has gone cold, the pressure’s poor, or you’re planning a new one, tell me what’s happening and I’ll explain what’s likely going on and what it’ll cost to put right — in plain English. The easiest way is to send a few photos and a short description through my contact form.
Call 07864 757929
Send photos & details
Email office@bson247.co.uk
Building Services ON 247 — Luton. Hot water cylinder installation, servicing and repair — including unvented (G3) — across Bedfordshire, Hertfordshire and Buckinghamshire.