How Long to Heat a Heat Pack (and When to Stop)

Nearly every heat pack that dies early dies the same way. Somebody gave it an extra minute.

Grain packs are forgiving right up until they aren't. Below are the times I give for each of my sizes, the rules that keep a pack going for years, and how to tell when one is finished.

Times by size

These are for an 1100 watt microwave, which is what's in most Australian kitchens. A stronger microwave needs less, so start at the lower end and add a little at a time.

If your pack came from somewhere else, start at roughly 90 seconds per kilogram and adjust from there.

The seven rules

Lay it flat and spread out, never folded or bunched. Stop at the time above, then add twenty or thirty seconds at a time if you want it warmer. Let it cool right down before you heat it again. Pop a cup of water in beside it. Wipe the microwave out first. Don't walk away while it's heating. And if it ever smells burnt, it's finished.

Here's why each of those matters.

Lay it flat. A folded pack heats unevenly and builds a hot core in the middle that you can't feel through the fabric. Spread out, the heat goes where it should.

Short bursts after the first go. The difference between a lovely pack and a scorched one is often forty seconds. Twenty or thirty at a time means you can't overshoot by much.

Cool it completely first. This is the one people break most, usually on a cold night when they want more warmth right now. A warm pack has already lost some of its moisture, so the second heating dries the grain much faster and can take it past scorching. Give it an hour, or keep a second pack.

A cup of water alongside. The water takes up some of the microwave energy and adds a bit of humidity, so the filling is less likely to dry out and catch. Cheap insurance.

A clean microwave. Old food on the turntable gets very hot and can mark or singe the fabric. Give it a wipe.

Ovens, and everything else

You can warm one in a conventional oven for about 20 minutes on a low temperature, wrapped in foil, if you haven't got a microwave. It's slower, harder to judge and easier to get wrong, so I treat it as the backup rather than the usual way.

What I'd never do:

• A fan-forced oven on high, or a grill.
• A clothes dryer.
• A stovetop, a heater, an open fire or a hot water bottle.
• Microwaving it in a bowl or wrapped in a towel. Both trap the heat.

How hot is too hot on the skin

Warm and comfortable, never so hot that you flinch. Always keep a layer of clothing or a thin towel between the pack and bare skin, and take it off after about twenty minutes rather than leaving it on all evening.

Hot applications on the same patch of skin, week after week, can leave a lasting mottled brown mark. It's avoidable simply by keeping the temperature moderate and using a layer in between.

One more thing I'd want you to read. Please don't use a heat pack on anyone who can't tell you it's too hot or move away from it. That includes babies and very young children, anyone with reduced sensation or nerve damage such as diabetic neuropathy, anyone with poor circulation, and anyone asleep or affected by alcohol or sedating medication. Not on broken or irritated skin, not on a fresh injury that's hot and swollen, and not over a numb area. If you're pregnant or being treated for something, ask your doctor first.

Four signs a pack shouldn't go back in the microwave

1. A burnt or scorched smell, even faintly, even when it's stone cold.
2. Any brown or blackened patch on the fabric, inside or out.
3. Hot spots, where one part comes out far hotter than the rest.
4. Filling that feels powdery or crumbles, or a pack that stopped holding heat as long as it used to.

Any one of those and it's done. This matters more than it sounds. Fire services around the country see house fires every winter caused by grain packs that were overheated, reheated while still warm, or kept long past their life, and a pack that has scorched inside can keep smouldering for hours after it comes out.

If you're in any doubt at all, put it outside on concrete well away from anything that can burn, and leave it there until it's completely cold before it goes in the bin.

How long a pack should last

Plan on about three years of regular winter use. The filling dries out a little every time it's heated, which is completely normal, and eventually it stops holding warmth properly. There's more on getting the most out of one in how to wash a heat pack and make it last.

Mine are made to order in Brisbane and filled with Australian lupin, which is close to odourless heated and leaves no wheat dust. Have a look at the range, or read why I use lupin rather than wheat.