How to Bleed and Refill Your Unvented Cylinder After a Cold Snap

Why Cold Snaps Cause Unvented Cylinder Problems

A sudden drop in temperature puts unvented hot water systems under significant stress. Pipes feeding the cylinder can partially freeze, causing pressure fluctuations that trigger the pressure relief valve (PRV) to discharge. The expansion vessel — which absorbs the pressure rise as water heats up — can also lose its pre-charge pressure in cold conditions, leaving the system unable to manage thermal expansion correctly.

The result is typically one or more of the following: the PRV dripping or weeping continuously, the system losing pressure overnight, or the cylinder failing to heat up properly. This guide walks through the correct procedure to diagnose, bleed, and refill the system safely.

Important: Unvented cylinders operate at mains pressure. Under UK Building Regulations Part G, work on unvented systems beyond basic maintenance must be carried out by a G3-qualified engineer. If you are not G3 qualified, use this guide to diagnose the fault and identify the parts needed before calling an engineer.

Step 1 — Check the Pressure Relief Valve (PRV)

After a cold snap, the first thing to inspect is the PRV and its discharge pipe (tundish). A PRV that has lifted — even briefly — may not reseat cleanly and will continue to weep water.

• Locate the tundish (the visible air-gap fitting, usually near the cylinder). Check whether water is dripping from the discharge pipe below it.
• A small amount of discharge during heating is normal. Continuous dripping is not.
• If the PRV is weeping continuously, it has either lifted due to overpressure (check the expansion vessel first — see Step 2) or the valve seat has been damaged and the valve needs replacing.

Do not cap or block the discharge pipe. This is a critical safety device.

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Step 2 — Check and Repressurise the Expansion Vessel

The expansion vessel is a small pressurised tank (typically 2–8 litres on domestic cylinders) connected to the cold feed. It contains a rubber diaphragm pre-charged with nitrogen or air, which absorbs the increase in water volume as the cylinder heats up. If the vessel loses its pre-charge — or the diaphragm fails — there is nowhere for the expanding water to go, so the PRV lifts instead.

How to check the expansion vessel pre-charge:

• Isolate the cold water supply to the cylinder and open a hot tap to relieve system pressure.
• Locate the Schrader valve on the expansion vessel (it looks like a tyre valve).
• Using a tyre pressure gauge, check the pre-charge pressure. For most domestic unvented cylinders, this should be 3.0 bar (check your cylinder manual to confirm).
• If the pressure reads zero, the diaphragm has failed and the vessel must be replaced — it cannot be repaired.
• If the pressure is low but not zero, repressurise using a foot pump or tyre inflator to the correct pre-charge pressure.

Shop the Part — Expansion Vessels

• Heatrae Sadia 95607066 12 Litre Primary Expansion Vessel — Megaflo Genuine Spare
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• Heatrae Sadia 95607067 Expansion Vessel Hose — Genuine Megaflo Spare
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Step 3 — Bleed and Refill the Cylinder

If the system has lost water through PRV discharge, or if pipes were drained to prevent freezing, the cylinder will need refilling correctly.

1. Close all hot taps and ensure the cold supply isolation valve to the cylinder is closed.
2. Check the expansion vessel pre-charge is correct before refilling (Step 2 above).
3. Slowly open the cold supply isolation valve to allow the cylinder to fill. You will hear air escaping — this is normal.
4. Open the nearest hot tap to allow air to purge from the system. Run until a steady flow of water (no spluttering) is achieved, then close.
5. Check for leaks at all connections, the PRV, and the expansion vessel hose before restoring power to the immersion heater.
6. Restore power and allow the cylinder to heat fully. Monitor the PRV discharge pipe for the first heating cycle.

Step 4 — When to Replace Rather Than Repressurise

Expansion vessels have a service life of approximately 5–8 years. If your vessel has lost pre-charge pressure more than once in a season, or if the diaphragm has failed, replacement is the correct course of action — not repeated repressurisation.

Similarly, a PRV that has lifted and reseated may have debris on the valve seat preventing a clean seal. If it continues to weep after the expansion vessel is correctly pressurised, replace the PRV.

Related Guide

If your cylinder uses an immersion heater and you're unsure which element length to order after a cold-weather service, see our Immersion Heater Length Guide: 11", 14" & 27" explained.

Order Genuine Parts from Heatraq

All expansion vessels and relief valves listed above are genuine Heatrae Sadia spares, stocked in the UK. Unsure which part fits your cylinder model? Contact the Heatraq team with your cylinder model number and we'll confirm compatibility before you order.