150 Litre Direct Unvented Cylinder
150 Litre direct unvented cylinder

150 Litre direct unvented cylinders in corrosion resistant duplex stainless steel for mains pressure hot water from Flexiheat UK
Our 150 Litre direct unvented cylinder is a competitively priced cylinder manufactured for mains pressure hot water and engineered in corrosion-resistant duplex stainless steel for high flow rates and longevity of performance via quality manufacturing processes.
So confident are we in the quality of our 150 litre unvented direct cylinder that for use in domestic locations, such as homes, flats, etc., they have a 25 year guarantee for these direct unvented cylinder models (the stainless steel hot water cylinder has a 25 year warranty; the ancillary components, i.e., the immersion heaters, control thermostat, etc., have a one year warranty.
An unvented water heating system delivers mains pressure hot water, enabling high flow rates for showers and baths without the need for a hot water booster or boost pump.
Patented unvented hot water expansion system
With a patented internal hot water expansion system, via a venturi valve to maintain an air pocket within the cylinder, it ensures an external expansion vessel is not required. This unvented system has a long product life because it has no moving parts, which reduces running and lifetime expenses.

Our internal air gap and venturi valve system used on our direct unvented hot water cylinders- no external expansion vessel is required and little to no maintenance is required for the unvented hot water system.
Due to this, our direct unvented cylinders require less space, pipework, and pipe fittings, which simplifies the installation and reduces install costs and time.
Features and benefits
- Enhanced Venturi valve technology which automatically replenishes the internal air gap within the unvented cylinder.
- 3 bars of water pressure at the outlet (subject to incoming pressure)
- Mains pressure hot water – enabling high flow rates
- No expansion vessel or moving parts, ensuring low maintenance
- Energy cut out thermostat for saftey
- Evolved design for improved performance at low flow rates
A 150L direct unvented cylinder is suitable for?
A rough guide is that 150 litre direct unvented hot water cylinders are suitable for flats or smaller homes with one or two bedrooms with one bath and/or shower.
However, people’s hot water usage patterns can vary from household to household. We prefer to give you the hot water usage data, and you can make the decision, as they say.
In the United Kingdom, we are required to keep DHW at 60°C since it eliminates the possibility of Legionella disease and is the minimum recommended water temperature.
If the stored water temperature is 60°C, the amount of required water is in red, to be blended down to 43°C (the maximum bathing temperature before scalding can occur).
Please use 10°C for England and Wales as the lowest cold-water wintertime temperature and use 5°C for Scotland and Northern Ireland.
If the incoming cold-water temp is 10°C is 5°C
A bath uses around 135 Litres of water at 43°C 89 litres 94 litres
A standard 8 Litre shower head x 5 minutes 30 litres 31 litres
And yes 5 minutes is the average showering time – say all the time and motion studies report.
A kitchen sink holds up to 20 litres of water 16 litres 16.5 litres
N.B Washing up water temperature is normally 50C to ensure that grease is removed from dishes and cooking utensils.
Toilet sink taps—standard type—are around 4 litres per minute; “spray” type taps are 2 litres per minute and tend to be on for a brief period.
Please see the recovery time data below for our 150l direct cylinder to establish if this is the right 150l electric water heater for your circumstances and demands.
This 150 Litre volume direct unvented cylinder is the most popular seller in our range, however, should you wish a smaller volume cylinder, then please see the following webpage – 120 Litre direct unvented cylinder, this is our second most popular direct unvented cylinder that we sell.
150 litre unvented direct cylinder dimension
Our 150 litre stainless steel direct unvented cylinder has the following dimensions:
For the 150 Litre capacity direct unvented cylinder manufactured in highly corrosion resistant duplex stainless steel, the dimensions are:
Download the installation & technical manual for our direct and indirect hot water cylinders
Being manufactured with high grade stainless steel and using quality manufacturing processes for the cylinder ensures long product life.
Hot water heat-up time and recovery rates for our direct unvented cylinder 150 litre volume
As standard our 150 litre direct pressurised hot water cylinders in high grade stainless steel are fitted with 2 x 3kW immersion heaters
Our 150 Litre unvented indirect cylinder has the following hot water heat-up time and recovery rates with standard 2 x 3kW immersions:
Upgraded immersion heater options
We have the option of upgrading the immersion heaters to larger outputs for rapid heat recovery –
We can supply this with 2 x 6kW immersion heaters which are be wired in either 1 Phase/ 240 volt (32 amps per immersion is required) or 3 Phase (10.66 amps per phase is required), this will half the recovery times stated above in the chart. We also must upgrade the standard Temperature and pressure relief valve to a larger output model
Or alternatively you can have 1 x 6kW and 1 x 3kW immersion heater fitted, for a total kW input of 9kW, the standard 10kW T & P valve will remain if this configuration is used.
All of our unvented cylinders have a pressure test, and all are tested to 10 bar pressure. The maximum working pressure is 5.5 bar.
Our unvented cylinders are available in direct and indirect configurations at a competitive price.
Components supplied with our stainless steel unvented cylinder: 150L direct unvented cylinder
2 x 3kW immersion heaters manufactured in incoloy (a super austenitic stainless steel—with high resistance to oxidation), with a control thermostat and high temperature thermal/energy cut-out thermostat.
Front access to the controls on our electric pressurised hot water cylinder
A temperature and pressure relief valve to comply with the unvented cylinder regulations.
A complete pressure reducing valve combination valve/6-bar expansion relief valve for the cold water mains inlet
A Tundish – A tundish has a clear window that allows you to see if water is flowing from the pressure relief valve (PRV). This helps you identify potential issues with the unvented hot water system.
Heat loss ErP rating for this direct unvented cylinder
This direct unvented cylinder, 150 litres in capacity, has an ErP level of “B” for the thermal insulation / to reduce heat loss and ensure running costs are kept minimum,which is a very good heat loss rating for an unvented direct cylinder.
The thermal insulation, which is a injected polyurethane foam is CFC/HCFC free, with an ODP=0 and a GWP=1 rating, being 60mm fire-retardant insulation. Similar products only have 40mm insulation to reduce heat loss , and thus the ErP insualtion ratings are lower than our direct unvented cylinder.
Unvented direct cylinder 150 litre approvals
Our 150 ltr direct unvented cylinder has the following approvals: CE, BBA, and WRAS independent approvals (Water Regulations Approval Scheme—a certification body for domestic hot water heating products that assists businesses and customers choose safe products that comply with regulations) and benefits from ISO, KIWA, and WRC approvals as well, proving the quality manufacturing process of our stainless steel cylinders.
Unvented cylinder delivery
We deliver these direct unvented cylinders on a small pallet delivery service that comes on a lorry, with your responsibility to offload. We can upgrade the delivery service to have a tail lift at request to get the stainless steel 150L direct unvented cylinder to floor level.
Contact Flexiheat UK
Please call us at 01202 822221 to answer any questions or use the contact form above to get more information, the current stock situation, delivery options, or a cost on the best 150 litre direct unvented cylinder/150Ltr unvented direct cylinder models on the UK market. These are made from high-grade duplex stainless steel (A high grade stainless steel) ,with a 25 year warranty, with WRAS independent approvals, have a built-in expansion system, and are a competitively priced cylinder.