Last updated: May 2026

Quick answer

Most electric towel rails arrive pre-filled, so you do not add any liquid, and dry rails have no fluid at all. Filling from empty only applies when you are converting a central-heating rail to electric-only use. In that case, fill it with a solution of roughly 85% deionised water and 15% ethylene glycol, tilt the rail as you fill to push air out of the horizontal bars, stop when the fluid reaches halfway up the top horizontal bar, then bleed it on first heat-up. The wiring should always be done by a qualified electrician.

First, check what type of rail you have

Before you reach for a funnel, work out which of these you are holding. Getting this wrong is the single most common mistake we see.

🔥 Dry electric rail

Has a built-in heating element and no fluid inside at all. It never needs filling. Out of the box, onto the wall, wired in.

✅ Pre-filled electric rail  Most common

Supplied already filled with thermal fluid, so you do not add any liquid. You do, however, need to fit the heating element into the rail before mounting and wiring. No filling required.

🔧 Central-heating rail converted to electric

A plumbed radiator that you are turning into an electric-only unit by fitting an element and a blanking plug, then filling it manually from empty. Almost any of our towel radiators can be converted this way. This is the only type that needs filling.

If yours is a dry or pre-filled rail, you do not add any fluid, so the filling steps below do not apply to you. The rest of this guide is for converting a central-heating rail to electric-only use.

What liquid goes inside an electric towel radiator?

The fluid is what actually gets hot. The element heats the liquid, and the liquid carries that heat through the bars to warm the metal surface and the room. Without it, the element would overheat and fail almost immediately — so the mixture matters.

The industry-standard fill is a pre-mixed solution of about 85% deionised water and 15% ethylene glycol. Each ingredient does a job:

  • Deionised water is free from the minerals in tap water that cause limescale, sludge and internal corrosion.
  • Ethylene glycol (the same family as car antifreeze) improves heat transfer stability and protects the rail from freezing in cold rooms.

You will need to mix the solution yourself from deionised water and ethylene glycol at roughly an 85/15 ratio. An average towel rail holds roughly 5 to 6 litres, so work out your volume before buying glycol (see the tip in step 3 below).

⚠️ A note on tap water and inhibitor mixes

Some installers use deionised water with a central-heating corrosion inhibitor instead of glycol. That can work where the manufacturer permits it, but the two approaches are different recipes — do not combine them. Plain tap water on its own is a false economy: it scales up, gurgles and shortens the life of the rail. Always follow the instructions supplied with your specific rail and element.

What you will need

  • Deionised water and ethylene glycol, mixed to roughly 85% water / 15% glycol
  • Your electric element, blanking plugs and a manual air vent (bleed valve)
  • A funnel or measuring jug
  • PTFE tape and an adjustable spanner
  • A torch, a towel or cloth, a bucket and a pair of gloves

How to fill an electric towel radiator, step by step

⚡ Important: bathroom electrics are never a DIY job. The steps below explain the filling process so you understand what is involved, but the wiring to a fused spur must be carried out by a qualified electrician.

1

Confirm you are converting a central-heating rail

Double-check the type. If it is a dry rail (built-in element, no fluid) or pre-filled, you do not add any liquid — stop here. Filling from empty only applies to a plumbed central-heating rail being converted to electric-only.

2

Isolate the power and dry the area

Switch off and isolate the supply at the fused spur. Make sure your work area is completely dry — water and electricity together are dangerous. The spur outlet must be positioned so it cannot be touched while using a tap, shower or bath.

3

Identify the top of the rail and leave both top inlets open

The top of the rail is the end where the bar grouping is smallest — in a 4-5-5 layout, for example, the group of 4 bars is the top; the bottom end has more bars in each cluster. The element inlet and blanking plug inlet sit at this top end. During filling, leave both top inlets completely open — do not fit the blanking plug or element yet. You will pour the solution into one inlet and use the second open inlet to shine a torch down and monitor the fluid level as you fill. Fit the bleed valve with PTFE tape at the top but leave the other two inlets clear until filling is complete. Tip: if you want to measure the exact capacity first, fill the empty rail with plain water using a jug, note the volume, then drain before adding your final solution.

4

Fill slowly with the rail tilted at a slight angle

Do not lay the rail fully on its side. Tilt it at a slight angle and pour the solution slowly into one of the open top inlets using a funnel. As the fluid rises, gently adjust the tilt to help it work into each horizontal tube — this encourages any trapped air to rise and escape. Keep the rail tilted throughout the entire fill. Trapped air in the horizontal bars is the most common cause of cold spots after installation, and tilting while you fill is the step that prevents it.

5

Stop when the fluid reaches halfway up the top horizontal bar

The correct fill level is halfway up the top horizontal bar. Shine a torch down through the second open inlet to confirm the level — do not guess. Stopping here leaves a roughly 25mm (one inch) air gap above the fluid, giving it room to expand safely when hot. Overfilling removes that expansion space and can cause pressure build-up, weeping joints or tripped safety cut-outs.

6

Seal, mount, power up and bleed

Wrap both remaining inlet threads with PTFE tape, then fit and tighten the blanking plug and the element. Fitting the wall brackets before the rail is fully assembled makes mounting easier — remember a filled rail is noticeably heavier. Once mounted and wired by your electrician, restore power and bleed any trapped air on the first heat-up so the element is fully immersed. Check every joint for leaks.

How an electric towel rail heats up (so you know what is normal)

Heat naturally rises, so the top bars and the collector holding the element will always feel hottest. The lowest one or two bars stay slightly cooler on purpose, to shield the controller from direct heat. On tall, narrow rails the very top bar can take a few minutes to warm through because of the air cushion at the top. None of this is a fault — it is simply how the physics works.

Frequently asked questions

Do all electric towel rails need filling?
No. Most electric rails arrive pre-filled, so you add no liquid (you do fit the element yourself), and dry rails have no fluid at all. Filling from empty only applies when converting a central-heating rail to electric-only use.
What liquid do you put in an electric towel radiator?
A solution of about 85% deionised water and 15% ethylene glycol. The deionised water prevents limescale and corrosion; the glycol improves heat transfer and protects against freezing.
How much fluid does an electric towel rail hold?
Roughly 5 to 6 litres on average, depending on size. Fill the empty rail with plain water using a jug to measure the exact volume, then drain before adding your final solution.
Why do you leave an air gap when filling?
Fluid expands when heated. Stopping at halfway up the top horizontal bar leaves a roughly 25mm air gap that gives the fluid room to expand without building pressure that could cause leaks or trip safety cut-outs.
Can I use tap water?
Not recommended. The minerals in tap water cause limescale, sludge and corrosion, leading to noise, poor heat output and a shorter lifespan. Always use deionised water.
Do electric towel rails need bleeding?
Yes, on first heat-up, so the element sits fully immersed. After that, frequent bleeding is not usually needed unless the rail has been drained or moved.
Who should fill and wire one?
A qualified electrician. The rail must be hardwired to a fused spur in the correct bathroom zone with the right IP rating. Never use an extension lead.
What if it leaks after filling?
Isolate the power at the fused spur and re-seal the element, plugs and vent. Wear gloves — the fluid can irritate skin, eyes and lungs — and wipe up spills immediately.

Ready to upgrade your bathroom?

Browse our full range of electric towel rails and heating elements. Our electric towel rails arrive pre-filled, ready for the element to be fitted before installation. If you have a question about a specific model, our team is happy to help.

Browse Electric Towel Rails →

 

Ibrahim Kalay - Founder of Elegant Radiators

Ibrahim Kalay

Founder, Elegant Radiators

Heating specialist since 2014. Ibrahim founded Elegant Radiators from our Coventry warehouse (CV7 9NH), supplying thousands of UK homes with quality electric towel rails and radiators. His hands-on experience with BTU calculations, installation requirements, and product specifications informs every article on this blog.