If your passive solar home is properly designed and everything is optimized, one of the most cost-effective backup heating systems available may be wall-mounted space heaters. For homes which only need an occasional top-up of heat or for infrequently-used rooms, they are ideal.
- Wall-mounted space heaters do not need lots of ducts and piping– just a single gas pipe and maybe a vent pipe (though there are models that don't need the vent).
- They are cheap, widely available, easy to source, come in a variety of models to suit all homes, and are easy to maintain.
- They can boast efficiencies of up to 99.9%.
- They provide immediate heat.
- They run on a variety of fuels, the most efficient and eco-friendly of which are propane and natural gas .
- Many models come with external controls, so they can be automated in your absence.
- Some models take in external air for combustion, reducing the drag on internal air.
- They are great for zone heating.
- They are excellent for backup heating – two heaters for a couple of thousand dollars fully installed is better than a $20,000 radiant floor setup when all you need is a top-up!
- Wall-mounted space heaters only run on non-renewable fuels – electricity, propane, natural gas, and kerosene.
- They do not produce an even heat.
- They are not designed for large spaces. They're much better at heating a single, standard room.
- They produce carbon dioxide pollutants.
- They use combustible materials, so are a fire risk – the US Consumer Product Safety Commission estimates they cause 25,000 house fires in the country every year.
- They can cause carbon monoxide poisoning if not properly installed and maintained.
- They may even explode.
One final note on the dangers of wall-mounted space heaters: if they're the best option for your home, look for a model with an oxygen-depletion sensor which switches the heater off when there's a problem, a pilot safety valve to ensure the gas is cut off if the pilot light goes out, and a thermal shut-off which switches the heater off if venting is interrupted.
Make sure you get heaters installed by experienced, competent professionals. You may also want to invest in a guard if you have children or pets.