Most unvented cylinders need at least 1.5 bar of mains pressure and 20 litres per minute of flow before they work at all. That figure is a floor, not a comfort level: Worcester treat 1.5 bar with 25 l/min as the minimum in a domestic installation, and Vaillant ask for 2 bar static pressure with 20 to 25 l/min for full performance. Flow rate is the volume of water your cold main actually delivers in a minute at an open tap. This page sets out the manufacturer figures, how you can measure your own flow with a jug, and why mains pressure on its own tells you very little.
What is the minimum mains flow rate for an unvented cylinder?
Cylinder manufacturers put the minimum mains supply for an unvented cylinder at roughly 1.5 bar and 20 litres per minute. Below that the main cannot push water through the cylinder fast enough to serve a tap properly. The published figures differ a little by brand and by cylinder size.
- Megaflo Eco SystemFit: minimum supply 0.15 MPa (1.5 bar) and 20 l/min. At those values outlet flows may be poor when several taps run together.
- Baxi Assure: the same minimum of 0.15 MPa (1.5 bar) and 20 l/min, with the same warning about simultaneous use.
- Worcester Greenstore: works well from a dynamic working pressure of 1.5 bar. In a domestic installation, 1.5 bar and 25 l/min should be seen as the minimum.
- Vaillant uniSTOR: at least 2.0 bar measured static pressure with 20 to 25 l/min for full performance. Below 1 bar, do not install an unvented cylinder at all.
- Larger cylinders: Megaflo Commercial units of 400 to 2500 litres ask for 1.5 bar and a minimum of 100 l/min.
Reaching 20 l/min means the cylinder will function. It does not mean two showers will run well at the same time. Judge your mains against the demand in your home, not against the manufacturer's floor.
Why high mains pressure does not guarantee enough flow for an unvented cylinder
Pressure and flow are two different measurements, and a strong pressure reading does not promise a usable flow rate for an unvented cylinder. Pressure is the push in the pipe when nothing is running. Flow is what arrives at an open tap once the service pipe, the stopcock and the internal pipework have taken their share. Heatrae Sadia state that high mains pressure will not always guarantee high flow rates.
Worcester go further and warn that a static pressure test is not a reliable guide. The supply should be checked when demand on the street main is high. For hot water performance, the flow figure is the one that decides how the shower feels.
A cylinder cannot create flow. Stored hot water leaves only as fast as the cold main pushes in behind it. A data sheet quoting a maximum flow of 70 l/min, as the Megaflo Eco Slimline Indirect does, describes the ceiling of the cylinder rather than your house. If the main gives 15 l/min, that is what the shower gets. Fitting a shower pump to a mains-pressure cylinder is a separate question with its own rules.
How can you measure your own mains flow rate before fitting an unvented cylinder?
You can measure your mains flow rate yourself with a one litre jug and a watch. Close every other tap and switch off the washing machine and dishwasher. Open the kitchen cold tap fully and time how long the jug takes to fill. Viessmann give the worked example: about seven seconds for one litre is roughly 8.5 litres per minute.
Repeat the test twice, once in the morning and once in the afternoon. Viessmann note that pressure can drop between 7am and 8am, when demand on the street main peaks. Two readings show whether 20 l/min is even within reach, and whether the supply sags at the time your household wants a shower.
The jug is as far as a householder should go. A pressure test at the inlet control set, work on the expansion vessel, the pressure relief valve, the tundish or the discharge pipe is work for an engineer qualified in unvented hot water systems.
Minimum supply or stronger supply: what changes for an unvented cylinder
The minimum figures keep an unvented cylinder working; a stronger supply is what makes it feel like mains-pressure hot water. Worcester name 3 bar as the optimum working pressure, regulated through the inlet control set, with delivery of up to 50 l/min.
| What it affects | Minimum supply: 1.5 bar, 20 l per min | Stronger supply: around 3 bar, 25 l per min or more |
|---|---|---|
| One shower on its own | Runs, but flow at the head stays modest | Flow closer to the cylinder's rated output |
| Two outlets at once | Manufacturers warn outlet flows may be poor when taps run together | More headroom, so a basin tap takes less from the shower |
| Incoming cold pipe | Often a 15 mm main that limits what reaches the cylinder | Usually a 22 mm main, the size manufacturers recommend |
| Cylinder output | Capped by the mains; the cylinder never gives more than the supply delivers | Worcester cite delivery up to 50 l per min at the optimum 3 bar |
| Adding a bathroom later | Little margin left for another outlet | Room to add an en-suite without starving the main shower |
Vaillant describe the middle ground honestly: under 2 bar a uniSTOR still works satisfactorily, but at reduced flow. Where several outlets are used at the same time, a correspondingly higher pressure and flow are recommended.
Why a 15 mm cold main often holds the flow rate below the minimum
A 15 mm cold main is the common reason a home misses the 20 l/min minimum for an unvented cylinder. Baxi recommend a 22 mm cold water supply and allow an existing 15 mm pipe only where it already delivers sufficient flow, with more flow noise as the trade-off. Heatrae Sadia recommend 22 mm for the main run and for distribution to outlets, with short smaller legs to individual taps.
Worcester suggest upgrading an existing 15 mm cold main to a larger size where the recommended minimum pressure and flow are not achieved. That upgrade helps only when the restriction sits inside the property. The bottleneck can also be the service pipe from the street, which belongs to the water supplier, not to you.
Measuring at the stopcock and at the kitchen tap shows which of the two is holding you back. That check belongs at the start of any plan for converting a vented system to an unvented cylinder, before a cylinder is chosen.
What the rules in England require when an unvented cylinder is installed
In England, installing an unvented hot water cylinder is work controlled by the Building Regulations 2010. Schedule 1, Requirement G3 requires hot water systems to resist the temperatures and pressures of normal use and reasonably anticipated malfunctions. Stored water must be prevented from exceeding 100°C, and any discharge from safety devices must be safe, visible and clearly marked.
Approved Document G, with its 2024 amendments, explains how that duty is met. An unvented system needs at least two independent safety devices in addition to the control thermostat. Installation must be carried out by a competent person, shown by registration in a competent person scheme for unvented hot water or a valid qualification card for unvented hot water systems.
The Water Supply (Water Fittings) Regulations 1999, Schedule 2 add their own requirement. Every unvented water heater over 15 litres needs a temperature control device and a temperature relief valve, or a combined temperature and pressure relief valve, fitted directly to the vessel. A weak mains flow changes none of this. It limits performance, not the legal duties. More on who can install an unvented hot water cylinder.
What a survey checks that a jug test cannot tell you
A survey measures pressure and flow at the stopcock under running conditions, not just at one tap. Baxi recommend estimating the maximum demand of the household first, then checking whether the supply can meet it. Three bathrooms with a rainfall shower and a family of five are a different question from one bathroom and two people.
The visit also looks at the size of the incoming pipe, the route for the discharge pipe and tundish, and the space the cylinder needs. If the figures fall short, options such as larger internal pipework or storage solutions are weighed against what the mains can realistically give. FM Power carry out a free home survey before a new installation and give a written fixed price before any work starts.
If your jug test suggests the mains is close to the mark, you can book that survey through our Megaflo and unvented cylinder service and have the numbers checked properly.
Sources
- Megaflo Eco SystemFit unvented cylinders: installation manual (Heatrae Sadia)
- Megaflo Eco unvented cylinders: installation manual (Heatrae Sadia)
- Worcester Greenstore indirect unvented cylinder: installation instructions
- Vaillant uniSTOR: installation and maintenance instructions
- Baxi Assure direct and indirect cylinders: installation and service manual
- How to increase your water pressure (Viessmann) – jug test method
- Building Regulations 2010, Schedule 1 (Requirement G3)
- Approved Document G with 2024 amendments
- Water Supply (Water Fittings) Regulations 1999, Schedule 2