Flat and Strip Heater

Contact is everything, and contact means flat

A flat heater sends its heat into whatever it is touching. Where it is not touching, that heat has nowhere to go. A plate that is bowed by half a millimetre leaves an air gap in the middle, and that is exactly where the element will fail — not at the ends, in the middle, where nothing was carrying the heat away.

Before fitting, put a straight edge across the surface. If it rocks, machine or lap it flat. It is cheaper than a heater every six months.

Tighten from the middle, and leave the ends free to move

Flat heaters are fixed through slotted holes rather than round ones for a reason. The heater grows several tenths of a millimetre as it comes up to temperature, and if both ends are pinned it has to buckle somewhere. Bolt the centre firmly, let the end slots take the movement, and use spring washers so the clamping force survives the first few heat cycles.

Overtightening is its own failure. The case is thin, and a bolt pulled down hard dents it into the mica underneath, which is where an earth fault starts.

Watt density falls the moment it is not touching metal

The same heater has two completely different ratings. Clamped to a steel plate it can run at 2 to 5 watts per square centimetre. Standing in open air, radiating, it must come down to around 1 to 2 or its own insulation cooks. This catches people out when a heater is bought for a platen and later re-used inside a duct.

Heat spreads along the metal, not evenly out of the heater

A strip heater is a line of heat. The plate it is bolted to does the spreading, and thin plate spreads badly. If a wide surface has to be even in temperature, either use a wider plate heater or use two strips spaced apart, rather than one strip with double the power. Doubling the power of a single strip makes the strip hotter, not the plate more even.

Keep the terminals out of the heat and out of the way

The terminal end has no heat of its own but sits next to something that does. Route leads away from the hot surface, use ceramic beads or a nickel lead where they cannot be routed away, and fit the terminal cover — an exposed stud on a machine that vibrates eventually finds something to short against.

Reading a heater that has already failed

Before ordering a replacement, measure the old one. Resistance across the terminals tells you the wattage it really was, which is often not the wattage written on the machine. Resistance from a terminal to the case tells you whether it failed open or failed to earth. A heater that failed to earth usually points at a mechanical cause — a dent, a trapped lead, or moisture — and that cause is still there waiting for the new one.