Greenhouse heating

[Troy]

I'll be heating (frost protection) my greenhouse this winter with a 2 Kw electric heater. The thermostat switches the heater on when the temperature drops below 5° C. The heater has 2 settings :

Full - 2Kw Half - 1Kw

Obviously the first setting would use twice as much power as the second but presumably would be on for less time. As a matter of interest would they both use the same amount of power over a period?

TIA.

[Christian McArdle]

Approximately, although it may vary according to the thermal characteristics of the greenhouse and the thermostatic control. The variance will be about how perfectly the heater spreads its heat and how perfectly the thermostat measures it and could go either way. This assumes that the weaker power setting is sufficient to maintain temperature.

Christian.

[Andy Hall]

I suspect that the higher setting would use a little more.

This is because the typical bimetal thermostat will tend to operate after the temperature has overshot the set point. There is also something of a time inertia.

The greater the heat supply, usually the more the overshoot. Approximately, the heat loss is proportional to temperature, so the more the temperature overshoot, and the time of that, the greater the energy requirement.

Of course all of this will vary with the particular thermostat and the outside temperature.

Realistically, you would need to measure electricity consumption on two different nights of the same minimum and profile of temperature to be sure.

This is an expensive hobby, though, even with bubblewrap in the greenhouse.

.andy

To email, substitute .nospam with .gl

[Colin Wilson]

Philips used to sell heaters known as "thermotubes" that might be worth considering - they don`t use a lot of power, but will probably do the trick for keeping frost at bay.

[Andrew Gabriel]

I'm not a gardener and have never had a greenhouse, but even 1kW strikes me as excessive -- it's only to keep the frost off, not to get it up to room temperature. I would also doubt that the plants, when temperature is not far off zero, will like to have a hot draft blowing round them every few minutes as the thermostat cycles on and off (if this is a fan heater).

[Mary Fisher]

I grow most of our own vegetables in our small garden and have a small (8' x

6') greenhouse.

Last year I bought a fan heater, designed for greenhouse use, and used it on

1kw on a 4 degrees C setting.

It wasn't warm in there unless the sun shone, it certainly didn't put the plants under any strain, but it did mean that I could start seeds growing weeks before I could otherwise as well as keep other crops growing (albeit slowly) and therefore have an almost continuous production of vegetables.

I recommend it.

Mary

[Andrew Gabriel]

When my uncle was living in Canada, he had a greenhouse/conservatory in which there were a couple of lemon trees, and they used to comment on how nice it was to stoll out and pick a fresh lemon for their gin and tonics. One day, they actually worked out the cost, and it was something like £10 per lemon when you took the power requirements into account...

[Mary Fisher]

When I first started keeping bees my first pot of honey had cost about £500.

But there's more to it than that. If we took that argument to its logical conclusion we'd be talking directly to people instead of using internet, we'd be walking instead of driving cars, keeping sheep, spinning and weaving and sewing our own clothes ...

Growing your own vegetables doesn't have to be more expensive than buying them, especially when quality and freshness are considered. When you consider the pleasure gained no price is to high to be paid. What do most people spend on 'entertainment'?

We don't even have a telly.

I'm not advocating the simple life for everyone, far from it, but the cost of a lemon or a cabbage has to be balanced against many other factors.

Mary

[The Natural Philosopher]

M

We don't even have a telly.

Indeed. There is as pharse 'opportunity cost' which applied to e.g. veggie grow8ng would say something like

'I have a spade, a fork, and can shit in a bucket, and 1/4 acre of land, am a psnsioner, and couldn't get a job if I tried, that paid more than £5 an hour 2 days a wek, and I'd pay tax on that anyway'

'So if I simply dig my plot and plant £20 worth of seeds, and shit on them regularly, I will get a fine crop of veggies that go in teh supermarket for a few hundred quid every year'

'and my general health and attitude will be improved as well'

Obviosuly employing union labour at £30 an hour and uisng industrial garered fertiliser, and jumping through EEC hoops would make it totally uneconomic to supply teh local supermarket, but, its better than sitting watching 'Trisha' every afternoon innit?

[Mary Fisher]

I bow to your greater experience.

But not your spelling :-)

Mary

[Troy]

Thanks to everyone for your helpful replies. Looks like I'll be doing a bit of experimenting. One thing's for certain - I'm not going back to paraffin heating. I mistakenly bought a 2Kw one of those last year - what a nightmare - not cheap to run either.

Thanks again.