Water coming out of a vent pipe into a loft tank is a fault, not normal running. The vent pipe is the open end of a vented system and should carry nothing but air. Which cistern is filling tells you where the trouble sits. A small feed and expansion cistern points at the heating circuit. The large cold water storage cistern points at the hot water cylinder. This guide sets out what the rules in England require, why a warmed cistern matters, and the four checks you can safely make before calling an engineer.
Why is water coming out of the vent pipe into the loft tank?
Water coming out of a vent pipe into a loft tank means the system is forcing water up a pipe that should only stay open to the air. A vent pipe is the open end of a vented heating or hot water system. It holds that part of the system at atmospheric pressure and finishes over a cistern in the loft, so anything it releases is caught. Water arriving there every time the system heats up points to the installation or the controls.
Two different pipes get mixed up here. One is the open vent from the heating circuit, which ends over the small feed and expansion cistern. The other is the vent from a hot water cylinder, which ends over the large cold water storage cistern that feeds taps. The pipe above the cistern that is taking water tells you which side of the system to look at.
Water running from a vent pipe into a loft cistern is a fault to have inspected, not a quirk to live with. Heating or hot water is reaching a pipe that should carry nothing but air, and the stored cold water warms up with it.
Feed and expansion cistern or cold water storage cistern: which loft tank is taking the water?
Most lofts on a vented system hold two cisterns, and the one receiving water shows which side of the system is at fault. The small cistern serves the heating circuit; the large one serves the hot water cylinder and, in many homes, the cold taps.
| What to compare | Feed and expansion cistern | Cold water storage cistern |
|---|---|---|
| Size and job | Small cistern that fills the heating circuit and leaves room for the water to expand as it heats | Larger cistern that supplies the hot water cylinder and often the cold taps |
| Pipe above it | Open vent from the heating circuit; boiler makers specify at least 22 mm bore and no valve in the pipe | Vent pipe from the top of the cylinder; Approved Document G sets a minimum 19 mm bore in England |
| What discharge suggests | Heating water being pumped over the vent instead of circulating, drawing air into the system | Hot water rising up the vent from the cylinder and warming the stored cold water |
| Rule that applies | Manufacturer instructions require no pumping over and no air entry at the open vent | In England and Wales a primary circuit vent must not end over a cistern of wholesome water |
Checking which cistern is filling is a look, not a repair. The float valve, the pipework arrangement and the pump stay with the installer.
Is the water in the loft cold water cistern still safe once hot water has run into it?
Hot water running into a cold water storage cistern lifts the stored temperature into the range that HSE guidance on water systems tells duty holders to avoid. Legionella bacteria multiply between 20 and 45 degrees C, stay dormant below 20 degrees C and do not survive above 60 degrees C. HSE guidance asks for cold water storage and distribution to stay below 20 degrees C, with the cistern kept cool and insulated, and fitted with a lid and an insect screen.
That HSE guidance is written for people with health and safety duties, such as landlords and businesses, rather than for owner-occupiers. The risk explanation still holds in a house: a cistern warmed by a vent pipe is no longer cold storage.
In England and Wales, the Water Supply (Water Fittings) Regulations 1999 forbid a vent pipe from a primary circuit ending over a storage cistern that holds wholesome water. The primary circuit is the loop between the heat source and the coil in the cylinder, including the radiators. Heating water discharging into the drinking water cistern is therefore not a permitted arrangement there. The same regulations require each storage cistern to have an overflow or warning pipe and to exclude insects.
What Approved Document G requires of a hot water cylinder vent pipe in England
Approved Document G, which applies in England, sets out what the vent pipe on a vented hot water system has to do. It runs from the top of the cylinder to a point open to the atmosphere above the water level of the cold water storage cistern. For vented hot water storage the vent needs a bore of at least 19 mm.
Approved Document G also asks for more than a thermostat on a vented cylinder. Either the heat source or the storage vessel must have a device that stops the stored water going above 100 degrees C. On a directly heated cylinder that device is a non-self-resetting energy cut-out, which only a person can reset. On an indirectly heated cylinder it is an overheat cut-out.
Manufacturer instructions repeat the same points in fitting terms. The Heatrae Sadia Megalife manual requires the vent to be connected to the outlet, to rise continuously, to discharge into the cistern and to have a minimum 19 mm bore. No valve may sit in the vent, and a pressure relief device must never be used in its place. Our guide to hot water cylinder vent pipe regulations goes through the detail.
Why pumping over into the feed and expansion cistern is a problem for the heating system
Pumping over is the pump pushing heating water up the open vent and out into the feed and expansion cistern instead of round the circuit. Boiler makers design against it. Worcester Bosch installation instructions state that the installation must ensure there is no pumping over of water, and no entry of air, at the open vent above the feed and expansion cistern.
The same instructions set the layout rules. The relative positions of pump, cold feed and open vent must follow the manufacturer's drawing. The open vent must be at least 22 mm, with no valve in the vent or in the feed and expansion pipe, and both pipes must rise continuously from the appliance. The cistern has to give at least 1 m of static head above the highest point of the system, with room left for expansion, and Vaillant's open vent instructions give the same 1 m figure.
Air drawn in at the vent carries oxygen into the heating water. Worcester Bosch guidance on system filters tells installers to clean a system where magnetite is found, to protect it with inhibitor or a magnetic filter, and to make sure no oxygen gets into the heating water. A system that pumps over keeps feeding in exactly what that guidance sets out to keep away. A filter catches the debris but does not cure the pumping over, as our guide on magnetic filters explains.
What you can safely check yourself when the vent pipe is discharging, and what needs an engineer
Four checks are safe for a householder when a loft cistern is taking water from a vent pipe. A classic open-vented system with loft cisterns has no pressure gauge and no filling loop, so the first check applies only to a sealed system with a gauge or display.
- Boiler pressure, sealed systems only: manufacturers quote roughly 1 to 1.5 bar on a cold system, with 1 to 2 bar as the normal range. Our guide on low boiler pressure explains the gauge.
- Thermostat and programmer: confirm heating and hot water are switched on, the time settings are right and the target temperature makes sense.
- Reset button: press it as the manufacturer's instructions describe. A boiler that locks out again has a fault, so call an engineer instead of resetting it repeatedly.
- Bleeding radiators: cold at the top and warm at the bottom means air. Turn the heating off, let the radiators cool, open the bleed valve a quarter to half a turn anticlockwise, and close it as soon as water appears. Keep a cloth and a container to hand.
The pump and its speed setting, the float valve in the cistern, the arrangement of vent and cold feed, flushing and inhibitor dosing, and the cylinder and boiler thermostats are all installer work. So is anything touching gas, the flue, seals, condensate inside the appliance or the boiler casing. Under the Gas Safety (Installation and Use) Regulations 1998 as amended, gas work in Great Britain may only be done by a Gas Safe registered business. The engineer's competencies are listed on the back of the Gas Safe ID card, and you can ask to see it.
Can water from the vent pipe mean the hot water cylinder is overheating?
Water pushed up a cylinder vent pipe can point to a control fault on the hot water side, including overheating. Heatrae Sadia notes that cylinders and water heaters carry safety cut-outs against overheating, and that the signs are usually obvious, with a failed thermostat typically bringing loud noises. HSE has issued a safety alert on scalding risk from domestic hot water systems, so hot water turning up where it should not belongs in front of an engineer.
Storage temperature sits in a narrow band. Vaillant and Baxi both point to a cylinder temperature of 60 to 65 degrees C: below 50 degrees C the legionella risk rises, above 65 degrees C the scald risk does. Our guide on what temperature a hot water cylinder should be set to covers the setting itself.
Testing a cylinder thermostat, an immersion cut-out or a boiler thermostat is engineer work, not a householder job. If an inspection shows the boiler and controls are near the end of their working life, you can book a free home survey and a written fixed price for a new boiler installation.
Sources
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2
- Approved Document G: Sanitation, hot water safety and water efficiency (with 2024 amendments)
- HSE: Legionnaires' disease - hot and cold water systems
- HSE: Scalding risk from domestic hot water systems - safety alert
- HSE: Domestic gas - frequently asked questions
- Worcester Bosch: Installation, Commissioning and Servicing instruction manual
- Heatrae Sadia: Megalife Vented Cistern Fed Water Heater installation manual
- Vaillant: Hot water tank temperature