Unvented Cylinder – Sizing Guide
When deciding on the size of an unvented cylinder, some believe it depends on the number of people living in a home. It is recommended to select a unit size that can provide sufficient hot water for each person.
However, the main consideration is knowing the peak demands of the cylinders and being assured of the correct cylinder capacity and reheat time. Having the right size and an optimised system should determine a cylinder’s sizing.
What size hot water cylinder do I need?
The size of hot water cylinder you need depends on the water demand, the number of occupants, and the frequency of baths/showers.
The information below is an estimation of size ranges that may be applicable for your home. However, we strongly advise you to seek out the opinion of a plumber/heating installer before deciding which size cylinder would best suit your home. These sizes assume standard usage patterns. Properties with multiple bathrooms in simultaneous use may require larger cylinders or multiple units.
Furthermore, the Hot Water Association has a useful calculator that can assist you with finding the correct size.
| Installation | Indirect Hot Water Cylinder | Direct Hot Water Cylinder |
|---|---|---|
| 1 Bedroom + Bath/Shower | 75 - 120 Litres | 120 - 150 Litres |
| 2 Bedrooms + Bath/Shower | 150 Litres | 180 Litres |
| 3 Bedrooms + 2 Baths/Showers | 180 Litres | 210 Litres |
| 4 Bedrooms + 2 Baths/Showers | 210 - 250 Litres | 250 - 300 Litres |
| Larger Properties | 300+ Litres | 300+ Litres |
Unvented Cylinder Size Guide
Use the calculator below to find the approximate volume for any height and diameter combination, or filter the full size table to find your specification.
Volume Calculator
Select a height and diameter to calculate the approximate volume.
Standard dimensions shown. Contact us for bespoke specifications.
Size Reference Table
Filter by height and diameter to find your cylinder’s volume.
Select a height and diameter above to see the volume.
| Height (mm) | Diameter (mm) | Volume (L) |
|---|---|---|
| 900 | 375 | 92 |
| 900 | 400 | 104 |
| 900 | 450 | 129 |
| 900 | 500 | 161 |
| 900 | 600 | 220 |
| 900 | 650 | 254 |
| 900 | 750 | 308 |
| 900 | 900 | 487 |
| 900 | 1100 | 670 |
| 900 | 1200 | 700 |
| 900 | 1300 | 657 |
| 1000 | 375 | 103 |
| 1000 | 400 | 116 |
| 1000 | 450 | 145 |
| 1000 | 500 | 180 |
| 1000 | 600 | 248 |
| 1000 | 650 | 287 |
| 1000 | 750 | 352 |
| 1000 | 900 | 550 |
| 1000 | 1100 | 765 |
| 1000 | 1200 | 813 |
| 1000 | 1300 | 790 |
| 1100 | 375 | 114 |
| 1100 | 400 | 128 |
| 1100 | 450 | 161 |
| 1100 | 500 | 200 |
| 1100 | 600 | 276 |
| 1100 | 650 | 320 |
| 1100 | 750 | 396 |
| 1100 | 900 | 614 |
| 1100 | 1100 | 860 |
| 1100 | 1200 | 926 |
| 1100 | 1300 | 922 |
| 1200 | 375 | 124 |
| 1200 | 400 | 141 |
| 1200 | 450 | 176 |
| 1200 | 500 | 219 |
| 1200 | 600 | 304 |
| 1200 | 650 | 353 |
| 1200 | 750 | 440 |
| 1200 | 900 | 677 |
| 1200 | 1100 | 955 |
| 1200 | 1200 | 1039 |
| 1200 | 1300 | 1055 |
| 1300 | 375 | 135 |
| 1300 | 400 | 153 |
| 1300 | 450 | 192 |
| 1300 | 500 | 239 |
| 1300 | 600 | 332 |
| 1300 | 650 | 386 |
| 1300 | 750 | 484 |
| 1300 | 900 | 741 |
| 1300 | 1100 | 1049 |
| 1300 | 1200 | 1152 |
| 1300 | 1300 | 1187 |
| 1400 | 375 | 146 |
| 1400 | 400 | 166 |
| 1400 | 450 | 208 |
| 1400 | 500 | 258 |
| 1400 | 600 | 361 |
| 1400 | 650 | 419 |
| 1400 | 750 | 528 |
| 1400 | 900 | 804 |
| 1400 | 1100 | 1144 |
| 1400 | 1200 | 1264 |
| 1400 | 1300 | 1319 |
| 1500 | 375 | 157 |
| 1500 | 400 | 178 |
| 1500 | 450 | 224 |
| 1500 | 500 | 278 |
| 1500 | 600 | 389 |
| 1500 | 650 | 452 |
| 1500 | 750 | 572 |
| 1500 | 900 | 867 |
| 1500 | 1100 | 1239 |
| 1500 | 1200 | 1377 |
| 1500 | 1300 | 1452 |
| 1600 | 375 | 168 |
| 1600 | 400 | 191 |
| 1600 | 450 | 240 |
| 1600 | 500 | 297 |
| 1600 | 600 | 417 |
| 1600 | 650 | 485 |
| 1600 | 750 | 616 |
| 1600 | 900 | 931 |
| 1600 | 1100 | 1334 |
| 1600 | 1200 | 1490 |
| 1600 | 1300 | 1584 |
| 1700 | 375 | 179 |
| 1700 | 400 | 203 |
| 1700 | 450 | 255 |
| 1700 | 500 | 317 |
| 1700 | 600 | 445 |
| 1700 | 650 | 518 |
| 1700 | 750 | 660 |
| 1700 | 900 | 994 |
| 1700 | 1100 | 1429 |
| 1700 | 1200 | 1603 |
| 1700 | 1300 | 1717 |
| 1800 | 375 | 190 |
| 1800 | 400 | 216 |
| 1800 | 450 | 271 |
| 1800 | 500 | 336 |
| 1800 | 600 | 473 |
| 1800 | 650 | 551 |
| 1800 | 750 | 704 |
| 1800 | 900 | 1058 |
| 1800 | 1100 | 1523 |
| 1800 | 1200 | 1716 |
| 1800 | 1300 | 1849 |
| 1900 | 375 | 201 |
| 1900 | 400 | 228 |
| 1900 | 450 | 287 |
| 1900 | 500 | 356 |
| 1900 | 600 | 501 |
| 1900 | 650 | 584 |
| 1900 | 750 | 748 |
| 1900 | 900 | 1121 |
| 1900 | 1100 | 1618 |
| 1900 | 1200 | 1828 |
| 1900 | 1300 | 1982 |
| 2000 | 375 | 212 |
| 2000 | 400 | 241 |
| 2000 | 450 | 303 |
| 2000 | 500 | 375 |
| 2000 | 600 | 529 |
| 2000 | 650 | 617 |
| 2000 | 750 | 792 |
| 2000 | 900 | 1184 |
| 2000 | 1100 | 1713 |
| 2000 | 1200 | 1941 |
| 2000 | 1300 | 2114 |
Safety Features
Due to the sealed nature of unvented cylinders, multiple safety features are required to prevent excessive pressure, overheating, or a general water system failure.
| Safety Feature | Function | Prevents |
|---|---|---|
| Pressure Relief Valve (PRV) | Releases water when pressure is too high | Cylinder damage from pressure build-up |
| Temperature and Pressure Relief Valve (TPRV) | Releases water if temperature or pressure is too high | Overheating and cylinder explosion/rupture |
| Expansion Vessel / Air Bubble | Absorbs extra water volume when heated | Excessive system pressure |
| Thermostat | Controls heating to maintain safe temperature | Overheating and scalding risk |
| High-Level Thermal Cut-Out | Cuts heat supply if thermostat fails | Uncontrolled heating |
| Check valve (Non-Return Valve) | Prevents contaminated water from flowing back into the mains | Water supply contamination |
| Discharge Pipe and Tundish | Directs excess water safely | Internal flooding and water damage |
Expanding Water
As water can not be compressed, when it is heated in a sealed, and pressurised system like an unvented hot water cylinder, the water particles expand. To prevent excessive pressure from building up, the use of an expansion vessel is used to store the excess water. The expansion vessel absorbs the pressure of the expanding water, preventing any stress on the cylinder or the system’s pipework.
Alternatively, some unvented cylinders are designed with a built-in air gap at the top of the cylinder. Like an expansion vessel, the air gap bubble compresses to accommodate the increased water volume. However, because the air can dissipate over time, occasional maintenance can be needed for this option.
At Newark Cylinders, we only provide expansion vessels with our unvented cylinders.
Conclusion
To have a reliable supply of hot water, while maintaining efficiency and safety, choosing the right size unvented cylinder is essential. By considering peak demand, reheat times, and the appropriate safety features, you can select a system that meets your household’s needs without excessive energy consumption or unnecessary stress on the cylinder.
Newark Cylinders is committed to delivering both performance and safety. Our unvented cylinders come with high-quality expansion vessels to ensure reliability and efficiency. Need help choosing the right size? Our expert team is ready to assist you.