Just another data point. We run with two heating set points. But the
criteria for which one is in effect is not the time of day, but the amount
of light in the greenhouse. Nights and cloudy days get the lower set point
temp and the brighter light (above 1000 foot candles) is given a set point
of about 5 - 8 degrees higher. In most cases the normal warming of the sun
is enough to warm the greenhouse to the light set point. But on really cold
days when the heat lost is greater than the solar gain, the second set point
kicks in. For my greenhouse in Va, the second set point becomes important
when outside daily high temps are somewhere in the low teens or single
digits (the wind plays a big factor). Some of my greenhouse are computer
controlled and they take care of themselves. In the other greenhouses, I
just bump the thermostat up for three or four hours in the middle of the
bright very cold days. The need for the light set point is just not needed
very often here and I am in a colder location than Ray. But if I was
putting up a greenhouse in say MI, this could be much more important.

Pat

"Ray" wrote in message
...
Ah HAH! Where you are living plays no role in my analysis of the
situation, but that "minor detail" of the HPS lighting makes a big
difference!

As to the light versus temperature thing, you're right that at some point
it's just too cold, but growing most plants on the cool side of their
normal temperature range with high light levels will actually give
fantastic results. Not that I am capable of that on a regular basis, mind
you...

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Books, Artwork, and Lots of Free Info!
.
wrote in message
...
Ray,

Thanks for the comments.

Here in Vancouver BC if I would let the solar heat do my heating for
me during the winter I would be looking at growing just Cymbidiums and
even then marginally. As my gh is on the north side of the house, in
winter partially shaded by the house, on a property that is the second
highest point in Vancouver, you can see why I need to raise my
temperatures to some realistic figures Also I use HPS lights during
the winter. I agree with you that light is more important than
temperature except when the temperature is too low. If the plants are
too cold then no amount of light will make them grow.

The reason I want a programmable is that so I can set the max, but
more importantly the minimum temperature so in case it gets very cold
I can have three heaters running instead of just two as I do now using
single point thermostats and timers.



On Tue, 26 Oct 2004 17:34:14 -0400, "Ray"
wrote:

After more than 30 years of orchid growing, I still vote with
"old-timers"
that taught me way-back-when, and support NOT artificially raising the
daytime temperature of greenhouses, but letting solar heat gain do the
work.

Light is a more important factor to healthy plant growth than is warmth.
Artificially warming the GH on gray days will result in leggy, soft
growth
that in extreme cases cannot support its own weight, let alone that of
added
blossoms.

Save your money and buy a single-setpoint thermostat, set it to the
minimum
temperature you'd like your plants to experience, and let it go at that.