Converting from a vented to an unvented system

Measure first

Everything about this decision rests on one measurement, and it takes ten minutes.

1. Static pressure at the stopcock with nothing running.
2. Dynamic flow rate with the stopcock fully open, measured with a flow cup or a bucket and a stopwatch.
3. Compare against the cylinder’s requirement. Every unvented cylinder specifies a minimum flow and pressure for satisfactory operation.
4. Think about peak conditions, not the quiet afternoon when the test was done. What does the main deliver at seven in the morning?

When the mains is not adequate

Three genuine options rather than proceeding regardless:

Upgrade the supply pipe

If the incoming main from the boundary is an old narrow pipe, replacing it in a larger diameter raises the ceiling on everything downstream. It is real work – excavation from the boundary to the house – and it is sometimes the right answer, particularly if it also resolves poor cold pressure throughout. Your water company may contribute to replacing the section on their side, and it is worth asking before committing.

Fit an accumulator

A vessel that stores mains water under pressure, released during peak demand. It converts a supply that is adequate on average but insufficient at peak into one that performs. Genuinely elegant, less well known than it deserves to be, and it needs space for the vessel.

Keep the vented system and fit a pump

Frequently the best value answer. A correctly specified shower or whole-house pump solves the pressure complaint – which is usually what prompted the conversation – at a fraction of the cost and disruption. See shower types and water pressure.

What the conversion actually involves

1. Survey and measurementFlow and pressure, cylinder position, discharge route, and where the new mains feed will run.
2. Size the new cylinderFrom peak demand, with attention to coil area as well as volume – see sizing a hot water cylinder.
3. Plan the discharge pipeworkD1, tundish and D2 to a safe termination, with fall and equivalent length calculated. This frequently constrains where the cylinder can go, and occasionally determines whether the conversion is possible at all.
4. Run a new mains cold feedOf adequate diameter, from the incoming main to the cylinder position. On an older house this can be the largest single element of the work.
5. Remove the old cylinderAnd the loft tank and its associated pipework – feed, vent, and often a second small tank serving the heating.
6. Install the new cylinder and control setPressure reducing valve, expansion vessel, expansion relief, temperature and pressure relief, thermostats.
7. Alter the heating connectionsThe heating system may have been open-vented via the loft tank. Removing the tank means the heating becomes a sealed system, which needs its own expansion vessel, pressure gauge and filling loop. This is frequently overlooked in quotes.
8. Alter the outletsTaps and showers specified for low pressure may need changing – they will work at mains pressure but some are not rated for it, and some will be noisy.
9. Commission, notify and certifyFilled, vented, temperatures verified, benchmark completed, and the work notified under Building Regulations.

The consequences people do not anticipate

• The heating system becomes sealed. If the heating was fed and vented via the loft tank, removing the tank changes it fundamentally. It needs an expansion vessel, a pressure gauge, a filling loop and a pressure relief valve. This is real work and a real cost, and it belongs on the quote.
• Low-pressure taps and showers. Fittings specified for gravity may be noisy, may deliver far more flow than intended, and some are not rated for mains pressure at all.
• No water in a mains failure. You lose the stored buffer the tank provided.
• Cold pressure changes too, because everything now comes off the main. Generally an improvement; occasionally it reveals that the main is the limitation.
• The loft space is freed, which most people count as a benefit.
• Annual servicing becomes a requirement, where a vented cylinder was largely maintenance-free.
• The discharge pipework has to go somewhere, and on some cylinder positions there is genuinely no compliant route.

Is it worth it?

The honest summary: where the mains supports it and the cylinder is due anyway, conversion is frequently an excellent improvement. Where the mains is marginal, or where the real complaint is one weak shower, there are cheaper answers that work better.