Converting a combi back to a cylinder means fitting either a system boiler with an unvented cylinder, or a regular boiler with tanks in the loft. Go for a system boiler if the incoming mains has the pressure and flow the cylinder manufacturer asks for and you want the loft free. Go for a regular boiler if the main is weak or shared, or if the vented cylinder, feed tank and vent pipe are still in place.
System boiler with an unvented cylinder or regular boiler with loft tanks: what differs?
Both routes back from a combi restore stored hot water, but they differ in where the cold water comes from. One feeds straight off the mains, the other from a tank in the loft.
| Criterion | System boiler with unvented cylinder | Regular boiler with vented cylinder |
|---|---|---|
| Loft tanks | No cold water tank and no feed and expansion tank, so the loft stays free | Needs a cold water storage tank plus a feed and expansion tank above the cylinder |
| Hot water pressure | Cylinder runs at mains pressure, so showers match the incoming main | Pressure comes from the height of the loft tank, so upstairs showers can be weak |
| Mains supply needed | Must meet the cylinder maker's minimum pressure and flow at the stop tap | Copes with a weak or shared main, because the tank stores and feeds by gravity |
| Who may install | Gas Safe engineer for the boiler, plus a person competent for unvented work under Part G3 in England | Gas Safe engineer for the boiler, with no Part G3 unvented competence involved |
| Safety discharge | Needs relief valves, a tundish and a discharge pipe ending somewhere visible and safe outside | Open vent pipe over the loft tank relieves the system instead |
Water pressure decides most conversions. A survey measures pressure and flow at the stop tap before any cylinder is specified. If you are weighing the move in the other direction, system to combi conversion sets out what you gain and lose.
When a system boiler with an unvented cylinder suits a combi conversion
A system boiler with an unvented cylinder suits a combi conversion when the incoming mains can supply it. Unvented means the cylinder is fed straight from the mains, with no tank above it. Heatrae Sadia specifies at least 1.5 bar working pressure for its Megaflo Eco cylinders. Flow requirements vary by range, with around 20 litres per minute on standard models and 50 litres per minute on Megaflo Eco Plus.
At those minimum figures the manufacturer notes that performance can feel weak when two outlets run together. More pressure and flow give a better result.
- Two bathrooms that get used at the same time, where one combi has to split the flow.
- A loft conversion, or no usable loft, so tanks have nowhere sensible to go.
- An airing cupboard that still has the space where a cylinder once stood.
- A shower that should run at mains pressure without a separate pump.
The cylinder holds a fixed volume, so sizing matters far more than it did with a combi. What size hot water cylinder you need explains how bathrooms and occupants feed into that choice.
When a regular boiler with loft tanks suits a combi conversion
A regular boiler suits a combi conversion where the mains cannot feed an unvented cylinder, or where the old vented pipework is still in place. A regular boiler, also called heat-only, needs a cold water storage tank and a small feed and expansion tank above the cylinder. Gravity provides the pressure, so a weak or shared incoming main matters much less.
- Flow at the stop tap falls short of the cylinder manufacturer's stated minimum.
- The loft tanks, vent pipe and cylinder cupboard survived the earlier combi swap.
- No safe, visible route exists outside for an unvented discharge pipe, which Approved Document G3 requires in England.
- The house already has gravity-fed outlets and a shower pump you want to keep.
The trade-off is pressure upstairs. A tank sitting only a short height above the shower head gives a modest flow. Loft space also has to stay usable and protected from frost.
What do the rules in England require when you convert a combi to a cylinder?
Three sets of rules apply when a combi is converted back to a cylinder in England. Gas work on the boiler may only be carried out by a Gas Safe registered engineer, under the Gas Safety (Installation and Use) Regulations 1998 as amended, which apply across Great Britain. Replacing a boiler is notifiable work to building control in England and Wales. Where the installer is registered with a competent person scheme, they self-certify and you receive a building regulations compliance certificate.
An unvented cylinder brings in Part G3 of the Building Regulations 2010 in England. A hot water storage system with no vent to atmosphere may only be installed by a person competent to do so. It needs at least two independent safety devices, the stored water must never exceed 100 °C, and the discharge has to run somewhere visible and safe. Who can install an unvented hot water cylinder covers that competence in more detail.
The Water Supply (Water Fittings) Regulations 1999 in England and Wales add a notification to your water supplier. Any unvented vessel over 15 litres needs a temperature control plus a temperature or combined temperature and pressure relief valve. Approved Document L in England also covers the boiler swap, since time and temperature control and a boiler interlock count as part of the installation.
What changes day to day after converting a combi to a cylinder?
Hot water runs to a schedule again once a combi is replaced by a cylinder. You set a time programme for hot water alongside the heating, because the cylinder heats up and then holds the water. HSE guidance says hot water should be stored at 60 °C or above and distributed above 50 °C. Legionella bacteria multiply between 20 °C and 45 °C and do not survive above 60 °C.
Where a vulnerable person uses the bath or shower and the scalding risk is significant, HSE points to a type 3 thermostatic mixing valve limiting the outlet to 44 °C. That valve controls what comes out of the tap, while the cylinder still stores hot.
Reheat time depends on the output of the coil inside the cylinder, not on the volume alone. As a worked example, Heatrae Sadia puts a 125 litre Megaflo Eco with about 12 kW input at roughly 70 per cent of its volume in about 30 minutes. Treat that as the manufacturer's own figure rather than a prediction for your house.
Opening the boiler case, the gas supply, the flue, primary pipework, motorised valves, the tundish and the discharge pipe are work for a Gas Safe registered engineer, and for unvented storage a person competent under Part G3. What you can check yourself afterwards is the pressure gauge, the thermostat and programmer settings, the reset button, and bleeding a radiator.
Which conversion should you choose: system boiler or regular boiler?
Choose a system boiler with an unvented cylinder if the mains pressure and flow at your stop tap meet the manufacturer's minimum. It also wants a cylinder position indoors and a safe, visible discharge route outside. Choose a regular boiler with a vented cylinder if the main is weak or shared, if the loft tanks and vent pipe are still there, or if no discharge route exists.
Both give you stored hot water, so two bathrooms can draw at the same time. The deciding numbers are pressure and flow at the stop tap, plus the space for the cylinder. A free home survey measures those, and you get a written fixed price before any work starts, as part of new boiler installation.
Sources
- HSE – Domestic gas: frequently asked questions
- Gas Safety (Installation and Use) Regulations 1998 (SI 1998/2451)
- Approved Document G: sanitation, hot water safety and water efficiency (with 2024 amendments)
- GOV.UK – Building regulations approval: when you do not need approval
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2
- Heatrae Sadia – Megaflo Eco SystemFit installation manual
- Heatrae Sadia – Recovery time vs size of a cylinder
- HSE – Legionella: hot and cold water systems
- Ideal Heating – The difference between combi, heat only and system boilers