Heating cost

The question of how to heat a greenhouse keeps coming up, so I thought you might be interested in my experience, having just worked out the cost.

I have a 10' by 8' wooden summerhouse, with a glass front, in which I overwinter plants. I use a small 2 Kwatt electric heater, but set to 1 kwatt, which is controlled by an external thermostat. I have overridden the heater's internal thermostat, which I suspect is very cheap and not very good. I also have a power meter in the circuit to measure exactly how much energy is used. The thermostat is set to keep the temperature above about 1C.

Last year it used 14 Kwh hours, and this year 27 Kwh hours. Which at 14.27 p/Kwh (my current rate) would have cost me £2.00 for last year's usage, and £3.85 this year. Of this about 15% is used just running the power meter and thermostat, rather than for heating.

Last summer, I built myself a 8'4" x 4'6" leanto greenhouse attached to the side of the summerhouse, with the same setup. This used 9 Kwh hours costing £1.28 over the winter. I was surprised the greenhouse used so much less energy that the summerhouse. Whilst the summerhouse is twice the size, it is much better insulated, with a double wood skin. But the greenhouse is in a more sheltered spot. It was bubble wrapped.

The cost, of course, would have been a lot more if I had tried to maintain a higher temperature. But for me it did the job of keeping my plants frost free. It goes to show that electricity need not be too expensive, given that the heat is only used when it is required as opposed to a parrafin heater, which must be on all night. The other advantage of electricity is that you can go away for 10 days knowing that all will be well.

 

That is some very interesting findings there Peter. I have had my greenhouse for two years but have resisted the lure of heating so far thinking it would be leaking money! turns out I was wrong in that case. I have a 6' x 8' greenhouse in a sheltered but sunny spot (ideal) but even so, this winter I seem to have lost my oleander which I was leaving in there for protection! I may look into the heaters for next year now after reading that! :gnthb:

 

Hi Peter
I`d be tempted to use the attached greenhouse to capture the heat and then direct it into the summerhouse via trombe style vents top and bottom.
The low mass greenhouse temp will rise fast and fall fast, so if you can shift the excess heat into the summerhouse you`ll hang onto it for longer than you would by leaving it in the greenhouse..if you see what i mean :wink:
It should reduce your heating bill a bit more.

 

Dave - for me - the secret was in getting a power consumption meter. They are not expensive, and are also useful for testing items in the house. I have got this one - Plug-In Power and Energy Monitor Meter: Amazon.co.uk: Electronics & Photo That way you are in control. The biggest problem I would have is not knowing how much it cost. My system is only set to be frost free - but I could afford to try a higher temperature for a couple of weeks as I would know exactly what it was costing.

Hex - I know exactly what you mean. However the times when it used the electricity most was during a cold spell when there was no sun and freezing temperatures during the day for several days, so there was no spare heat. Now if you could think of a way of storing the excess heat of summer to be used in the winter - you would be onto a winner. :D

 

Long term heat storage isnt feasible, short term storage (days rather than months) is possible if you have mad scientist tendancies :wink:

 

Problem I always foresee with electric heating is that we always get power Cuts on the coldest night of the year, one very cold night can kill plants that you have grown for years.

I have a couple of parrafin heaters just in case.

But would hate to have run around in the middle of night getting them going.:(

 

Pete - we haven't had a proper (old fashioned ) power cut for about 30 years. There are sometimes (very rarely) short term blips for an hour or so - for my types of plants I think that would not be a problem.

I understand that many tender plant can withstand several degrees of frost if it is immediately reversed. And Victoria's experience with Poinsettias withstanding frost in Portugal supports this. The big problem is long periods below zero, which tends to be a feature of the UK. Mind you, I believe you are growing some pretty tender things that may not like it.

Hex - please thank the mad scientist for his private message. :D

 

I was surprised at the low cost when I initially put in the system. It has done three complete years now, with the first year using slightly less units still. By recording the power meter figure every day, I could see exactly how many minutes the heater had been on the previous night.

The reason it uses so little power is that the summerhouse/greenhouse is naturally about 4C warmer than outside. So the power generally only comes on when the outside temperature falls below 4C, and this doesn't happen very often. The heater might be on for 20 minutes one night and then not come on again for another 10 days. The problem with any system that is not controlled by a thermostst is that the heat has to be on all night every night just incase it is needed for that 20 minute period.

The heater is really just removing the odd cold spikes rather than trying to heat the greenhouse all the time. If you raise the thermostat setting to say 10C, you risk having the heater on for long periods of time - and that would be expensive.