Easy Greenhouse Heater

Anyone with a greenhouse knows that January is when to get the growing season started. To do this most will line part or all of the greenhouse with bubble wrap and apply heat. Heating in the UK may continue right through to the month of May!

In previous years Mrs rog has used a gas heater to keep things warm, last winter the heater started to fail, there would be days when the pilot light had failed and the greenhouse temperature had dropped too low. This year,having run an electricity supply, I installed a tube heater all seemed ok until we had a -5 deg C night and the thermometer showed that at some point the temperature inside the greenhouse had dropped to 1 deg C.

Not good enough! a bit of thought was needed to solve the problem.

I could have tried adding another tube heater, but I think the problem is that these heaters do not react quickly enough when the ambient temperature dips suddenly.

400/500 watt convector heaters are cheap enough but my feeling was that stirring the air would work better so I looked at fan heaters.

Most that I found were of the 2Kw variety which I felt was too much power and certainly too much money.

Then I had an idea........

Over recent months there has been a lot of advertising of plug in personal space heaters. They chuck out 400/500 watts of heat and have a fan to circulate the air. If you read feedback on the heaters you will find that:

• They are noisy in a living room (not a problem in a greenhouse)
• The design will not allow them to plug into sockets, low down, near skirting boards
• Do not heat a very large area.

On the plus side, lots of people bought them and are listing them for sale on places like Ebay so you can pick one up nice and cheap!

• 10mm ply wood
• Single non switch socket
• Backbox for socket (I used inset box, can be surface box)
• Plug in timer
• Frost thermostat 10 deg to -5 deg
• Length of 3 core flex with plug
• Some wood screws and glue

Cut the ply to dimensions shown on drawing, the gusset up the back is just an offcut. If you are using a recessed back box cut out the hole before assembling using glue and screws.

Screw the thermostat onto the back of the frame below the level of the plug socket.

The wiring is pretty simple, strip off a bit of the outer insulation partway from the end (measure up between socket and thermostat to get it in the right place). The neutral wire gets cut half way along, the earth gets cut where it goes back into the outer insulation, the line wire goes straight through to the common connection of the thermostat, the other blue wire connects between the normally open terminal on the thermostat and the line connection on the plug socket.

All that needs to be done is plug the heater in and connect up to the mains via the timer.

I will explain why you may need a timer. Even if you do not touch the timer function on the heater this particular type will turn off after 12 hours, to reset you need to turn the power off and on again. I used a mechanical timer as I had one, it is set to turn off for a quarter of an hour (minimum distance between the change over pins) every 8 hours. If you have an electronic timer the turn off time could be 1 minute.

Now you need to wait until it gets cold!

The heater is low down, as heat rises all I needed to do was have my digital thermometer where I wanted the temperature correct, it was just a case of adjusting the thermostat until it was right.

This heater has now been running for over 3 months, once I had fine tuned the temperature in the first week I have not had to touch it and the thermometer tells me that the temperature in the greenhouse has been at the correct level for the whole period.