If you've got a traditional hot water cylinder in an airing cupboard fed by a cold water tank in your loft, you'll know the limitations: slow recovery, low shower pressure, and the constant risk of the tank freezing or leaking in winter. An unvented hot water cylinder — the Megaflo is the best known brand, though there are several good alternatives — solves most of these problems in one go.
Here's what you need to know before deciding whether one is right for your property.
What is an unvented hot water cylinder?
A conventional vented hot water system works by gravity. Cold water sits in a tank in the loft and feeds the cylinder and taps by gravity pressure — which is why the pressure from an upstairs bathroom in an older Hertfordshire house is often disappointing.
An unvented cylinder works differently. It connects directly to the mains water supply, storing hot water at mains pressure. When you open a tap, you get mains-pressure hot water — the same pressure as your cold supply. No loft tank required.
The technical requirement that makes these systems different is that they must be installed by a qualified engineer holding a G3 unvented hot water systems qualification. This is a legal requirement under Building Regulations Part G — it's not a recommendation, it's the law. An unvented cylinder installed by someone without G3 certification is potentially dangerous and won't be covered by insurance.
What are the benefits?
Mains pressure hot water throughout the property. This is the main reason people install them. Power shower performance from a standard shower head. Multiple outlets — kitchen, bathroom, ensuite — without the pressure dropping when someone uses another tap. For larger Hertfordshire homes with multiple bathrooms, this makes a significant practical difference.
No cold water storage tank in the loft. Eliminates the risk of the tank freezing, cracking, or becoming contaminated. Frees up valuable loft space. Removes the risk of a loft tank leak causing major water damage to the rooms below.
Faster hot water recovery. Unvented cylinders are typically better insulated than older vented tanks and have more powerful heating elements or coils. Recovery times are noticeably faster.
Quieter operation. Without the ballcock refilling a loft tank after every shower, the system is significantly quieter.
Better for solar thermal systems. If you're adding or planning to add solar thermal panels, an unvented cylinder with a twin coil is the most efficient setup for maximising the solar gain.
What are the downsides?
Cost. A quality unvented cylinder — Megaflo, Heatrae Sadia, Vaillant uniSTOR — plus G3-compliant installation will cost between £1,500 and £3,000 depending on cylinder size, property complexity and whether any associated pipework needs upgrading.
Annual servicing is required. Unlike a simple vented cylinder which you can largely ignore for years, an unvented system must be serviced annually by a G3 qualified engineer. The service checks the pressure relief valve, expansion vessel, and temperature control devices — all of which are safety critical. The service costs around £100 + VAT. Factor this into the ongoing cost of ownership.
Mains pressure dependency. If your mains pressure is low (below about 1.5 bar), you may need a pressure booster pump to get full benefit. This adds cost and complexity. We'll assess your mains pressure before recommending a system.
Space for the cylinder. Unvented cylinders are typically slightly larger than their vented equivalents to account for the expansion vessel. Make sure you have adequate space in the airing cupboard or plant room.
Is your property suitable?
Unvented cylinders work particularly well in:
- Older Hertfordshire houses with gravity-fed systems that have low shower pressure
- Properties with multiple bathrooms where pressure drops are a daily frustration
- Homes where the loft tank is ageing, has been problematic, or is in a difficult position
- Properties where a loft conversion is planned (removing the tank frees up the space)
- Larger homes where a combi boiler alone wouldn't provide sufficient hot water volume
They work less well in properties with genuinely poor mains pressure — though a pressure booster can solve this — or where space for the cylinder is extremely limited.
What size cylinder do I need?
As a rough guide:
- 1–2 bedrooms, 1 bathroom: 150 litres
- 3 bedrooms, 1–2 bathrooms: 180–210 litres
- 4 bedrooms, 2+ bathrooms: 250–300 litres
- Larger properties, high demand: 300+ litres, or twin cylinders
These are starting points — actual sizing depends on your boiler output, heating schedule and typical demand patterns. We'll carry out a proper heat and hot water load calculation as part of the quotation process.
What does installation involve?
In most cases, installing an unvented cylinder means:
- Removing the old cylinder (and often the loft tank and associated pipework)
- Installing the new cylinder with all required safety devices (PRV, tundish, expansion vessel, thermostatic blending valve)
- Connecting to the mains supply and boiler
- Building Regulations notification (Part G) — which we handle as part of the installation
- Commission testing and handover of documentation
For a typical 3-4 bedroom Hertfordshire home, the installation takes 1–2 days.
Our G3 qualification
We're G3 qualified and fully insured for unvented cylinder installation and servicing across Hertfordshire and North London. All installations comply with Building Regulations Part G and come with a 12-month workmanship warranty.
To discuss whether an unvented cylinder is the right solution for your property, call us on 0208 092 1359 or send details via WhatsApp for a free initial assessment.