Sooner or later, everyone who has a polytunnel or glasshouse will think about a heated propagator. It lets you raise many plants earlier, and germinate some others that would be impossible in normal greenhouse conditions. Unheated propagators are basically plastic boxes with transparent lids that raise the temperature a bit higher than the rest of the greenhouse, but although cheaper than heated ones they are not as effective. The best (and most expensive) models have openable vents in the lid to reduce overheating in sunny weather and thermostats that switch off the heat when the preset temperature is reached. The smallest will take one or two full-size seed trays but half-size trays are easier to handle. Good headroom inside lets you keep potted plants and cuttings in there if you need to. You can build a cheap propagator with a soil warming cable and a cloche cover.
Above all else, take care with electricity. Most propagator heaters run on mains voltage electricity, which does not mix with water and dampness. All wiring and installation for outdoor and greenhouse uses should be done by a registered electrical contractor, using industrial type plugs and sockets (or a low-voltage system with a voltage transformer), armoured cables and a residual current device.
Check propagators daily for temperature (a recording thermometer helps) and adjust the vents for the expected weather. Check that seedlings and cuttings have enough water, but not too much. Keep the propagator clean and tidy and free from dead or dying plant material to control fungus diseases. Aluminium foil can be used to shade it on very sunny days, and bubble-wrap is useful for cold nights. Take seedlings out of the propagator before they get tall and spindly from lack of light.
P.s. from Polydome – we supply propagation equipment, please see our section on this link: http://www.polydome.ie/subcategories.php?maincatid=2&catid=14