One other question:

There are CO2 generators that burn LP or NG. Do they produce as much heat as an actual heater suitable for the same size space would?

Changing CO2 levels won't cause any stress. 1000, 100, or 400 won't stress the plant any more than a passing cloud that suddenly changes light intensity.

You might consider a compost pile in the greenhouse for both heat and CO2. That would be my approach and very practical compared to a CO2 generator.

Here is what I was referring to in the link below.

This CO2 generator is rated for a 15' x 15' room. The BTU rating of this thing is similar to what would be recommended for an actual heater for the same size GH, around 9k - 11k.

So what that tells me is, in any climate with enough sunshine to where the photosynthetic rate would truly benefit from CO2 supplementation, the combo of solar heat gain plus the heat generated by the CO2 generator itself, would mean sky high temperatures and a need for ventilation, then blowing out all the CO2 just generated!

However, doing my calculations for using CO2 tanks, you have to buy so many tanks over the course of even just a month the cost becomes astronomical. Not to mention the actual logistics of moving a tank around every couple of days.

Here is a link that might be useful: CO2 generator