What has to be true before it makes sense
These are the checks that decide whether a conversion is worth doing.
- Mains pressure and flow rate good enough to supply several outlets at once — flow matters as much as pressure
- A suitable route for the safety discharge pipe to terminate safely and visibly
- Room for the cylinder and its expansion vessel, which may differ in size from the old tank
- Pipework to the bathrooms that will take mains pressure; very old joints may need attention
- An installer qualified for unvented hot water work, because the safety devices are there for good reason
What changes, side by side
| Vented (loft tank) | Unvented (mains fed) | |
|---|---|---|
| Pressure at taps | Depends on tank height | Close to mains pressure |
| Loft tanks | Needed | Usually removed |
| Safety devices | Open vent pipe | Temperature and pressure relief valves, expansion vessel |
| Annual check | Good practice | Recommended for the safety components |
| If mains is weak | Unaffected | Performance drops |
Why flow rate is the deciding factor
Pressure is how hard water pushes; flow is how much arrives. A house can show decent pressure on a gauge yet only deliver a modest flow once two taps are open. An unvented cylinder can only give out what the mains gives it, so a weak flow means disappointing showers whatever the cylinder costs.
That is why a flow test at the kitchen tap is the first thing a sensible installer does. Our guide to water pressure versus flow rate explains the difference in more detail.
What the survey looks at
A proper survey before a conversion covers more than the cylinder cupboard. The installer will test mains pressure and flow at the incoming supply, look at the size and age of the supply pipe, check where a discharge pipe can terminate safely, and look at the route pipework takes to each bathroom. They will also ask how many people live in the house and how often showers and baths run at the same time, because that decides the cylinder size. Loft tanks that come out free up space and remove a source of freezing risk in winter, which some households count as a benefit in its own right. If any of those checks come back weak — a narrow old supply pipe, poor flow at peak times or no sensible discharge route — the honest advice may be to improve the supply first, or to look at a different hot water arrangement altogether. A good installer will tell you that before quoting for a cylinder.
Start with a flow test, not a brochure. If the mains can deliver, a conversion can transform a house's showers; if it can't, the money is better spent elsewhere.