What Is an Open Coil (Bonnell) Mattress?
An open coil mattress uses hourglass-shaped springs wired together into one continuous network. Press anywhere and the whole surface responds. That is its weakness and also, in the right room, its point.
How it is built
Rows of coils are joined by helical wires running across the unit, and the whole thing is bordered with a heavier rod that gives the mattress its edge. Above it sit an insulator pad and a comfort layer; below, nothing but the base. This is the construction that furnished most of Britain for a century, and the reason it survives is that it works and it is cheap to make well.
Bonnell is the most common coil shape — hourglass, so the spring is stiffer the further you compress it. That progressive resistance is why open coil feels firm without feeling dead.
What you get
- Firm support for less money than anything else. No other construction gets close on price per centimetre of support.
- Airflow. A connected coil unit is mostly air. Open coil mattresses sleep cool.
- A predictable, traditional feel. If you grew up on a sprung bed, this is the feel you remember.
What you give up
- Motion isolation. The coils are wired together, so one person turning over moves the other. On a shared bed this is the deal-breaker.
- Contouring. The surface responds as a whole rather than following the shoulder and hip separately.
- Lifespan. Six to eight years in daily use, against ten or more for pocket.
Who it is actually right for
The industry pushes everyone up to pocket because the margin is better. Open coil is genuinely the correct choice for:
- Guest rooms. Twenty nights a year. Motion isolation is irrelevant and the mattress will outlive its usefulness before it wears out.
- Teenagers and children's rooms. Light load, firm surface, and it will be replaced when they grow anyway.
- Anyone who genuinely wants a firm traditional bed and is sleeping alone.
- Holiday lets and second properties. Robust, cheap to replace, cool in a room that is shut up for weeks.
What to look at when buying one
Because the unit itself varies less than a pocket unit does, the differences are elsewhere:
- Depth of the comfort layer. Four centimetres of foam over the coils is the difference between comfortable and not.
- The border rod. This is what stops the edge collapsing when you sit on it.
- The cover. Quilted cotton breathes; a cheap synthetic quilt undoes the airflow advantage.
Common questions
Is open coil bad for your back?
No. It is firm, and firm suits front sleepers and many back sleepers. It is a poor choice for a side sleeper who needs give at the shoulder.
Can I put a topper on it?
Yes, and it is the cheapest way to soften one. It will not add motion isolation, though — that is in the unit.
Why is it so much cheaper?
Fewer parts, faster to assemble, less material. Nothing is being hidden.