Heating for the conservatory
by lily
(England)
Which electric wall heaters would be the cheapest to run per hour for using in my conservatory or greenhouse?
Answer from Green Energy Efficient Homes
As I explain in my main electric heating article, Energy efficient electric heaters, all electric heaters are 100% efficient at converting electricity into heat, so there isn't that much you can do to lower the heating cost of your conservatory just by switching from one electric heater to another.
The two ways I can think of that would reduce your heating costs would be: adding thermal mass to the conservatory, and using storage electric heaters.
Thermal mass
Thermal mass just refers to anything inside the conservatory - and not touching an outside wall - that can soak up heat when it's warmer and let it back out when it's cooler. Think of the typical daily heat cycle of an unheated greenhouse or conservatory: it's at its coldest just before dawn, when there hasn't been any sunlight to heat it up, and the night air has sucked heat out through the low-R-value glass. Assuming it's a sunny day, the sun warms the air inside the greenhouse, so that it sometimes gets too hot and you might actually have to vent some of that heat out even on mild spring or autumn days, let alone summer days. Finally, the sun goes down, and the greenhouse begins the process of letting all the built-up heat escape through the glass.
If you add a large thermal mass inside the greenhouse - whether water or a masonry wall - you will allow that mass to buffer the heat that is soaked up during the day, so that the greenhouse doesn't overheat while the sun is shining, and the thermal mass will slowly release that heat through the night, keeping the greenhouse warmer than it otherwise would be even as heat is leaking through the glazing.
Water is your best bet for a thermal mass. If your conservatory is just for forcing seedlings and is not decorative, you can just fill 2 liter pop bottles and stack them underneath any raised areas, or along a north wall (to increase their solar exposure). If you have areas that are not currently planted, put the bottles there (in sunlight if possible). You can increase their daytime heat absorption by dying the water a dark color with food coloring, although this will fade over time. For a masonry thermal mass you can raise a bed over top of a brick or concrete structure - build a masonry wall then fill it up with soil, for example.
For a decorative conservatory, a small fish or water lily pond might be a good way to add an attractive thermal mass.
Storage electric heaters
The other way to cut your heating costs is to use storage electric heaters, which take advantage of lower electricity rates at certain times of the day. These heaters can be programmed to heat up a ceramic brick heat storage container (essentially, a thermal mass insulated inside the storage heater) at times when electricity is cheap; then, when heat is needed, the storage heater just lets heat out of the thermal mass while supplementing that heat with additional heat from the electric element as required. The net effect is that you use the same amount of electricity to heat your conservatory but you buy more of that electricity at the cheaper rate. Of course, electricity tends to be cheapest at night except in areas where it gets very cold at night and there's a large percentage of people heating with electricity (for example, Quebec in February) but if the weather's gray and cold during the day you may still need heat at noon, and having bought it the night before on a cheaper electrical rate will mean the savings add up.
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