Wheat bag, hot water bottle or electric pad: what actually holds heat

Walk into any British house where somebody has a bad knee and you will find at least two of these in a drawer. Most people own several because none of them quite does the job, so they keep buying the next one.

Here is what each is good at, and where each falls down.

The hot water bottle

Costs a few pounds, warms up in the time it takes the kettle to boil, and holds useful heat for perhaps twenty minutes before it becomes a warm rubber bag. It does not strap to anything. On a knee it slides off the moment you shift position, so you sit still with a hand on it.

The real drawback is safety. Bottles perish. The rubber degrades from the inside where you cannot see it, and burns from split bottles are common enough that manufacturers recommend replacing them every couple of years. Check the daisy wheel stamped on the neck for the date, and never fill one with boiling water.

The wheat bag

Around eight to fifteen pounds, moulds to the joint better than a bottle, and stays warm for roughly ten minutes of genuine heat. It is the most common thing people with knee pain own, and the most common thing they complain about.

Two problems. First, you are tied to a microwave, which means it works at home and nowhere else. Second, overheating them is a real hazard. Wheat bags can scorch and, in rare cases, catch fire if they are heated for too long or reheated when already warm. Follow the timing on the label and let it cool fully between uses.

The electric heat pad

Fifteen to thirty pounds, and the first option on this list that holds a steady temperature rather than shedding it. That is a genuine advantage. The catch is the cable. You are sitting within a metre of a socket for as long as you use it, which suits an evening in a chair and suits nothing else.

Most are also shaped for a back, so on a knee they cover awkwardly and slide.

Rubs and gels

Ten to eighteen pounds a tube, and worth saying plainly: the warmth from a heat rub is a sensation on the skin rather than heat travelling into the joint. Some people find that sensation genuinely helpful and there is nothing wrong with using it. It is doing a different job from a hot water bottle, and it runs out, so the cost repeats every few weeks.

How they compare on the thing that matters

If you line them up by how long they stay usefully warm, the order is clear. Rubs give you a surface sensation for a while. A wheat bag gives you ten minutes. A hot water bottle gives you fifteen or twenty on a slow fade. Only mains or battery powered heat holds a set temperature until you switch it off.

That is the whole argument for anything powered. Not that it is hotter, but that minute twenty-five feels like minute five.

What to look for, whatever you buy

• An automatic shut-off. Anything that heats and can be left on wants a timer that ends the session for you.
• A temperature you choose. Hotter is not better. Being able to start gentle matters more than a high maximum.
• Something that stays where you put it. If you have to hold it in place, you will use it sitting down and only sitting down.
• Honest sizing. Check the measurements against your own leg rather than trusting the phrase one size fits all.

If you have diabetes, circulation problems, or reduced feeling in your skin, speak to your GP before using heat of any kind. Reduced sensation is what turns a warm surface into a burn nobody noticed.

We built a cordless wrap to solve the two failures common to everything above: heat that fades, and heat you have to sit next to. It holds the level you choose for thirty minutes and stops on its own.