the worst problem stage is when your seedlings need heat and light 99% of the time
OK I would love to have LED's but I have had my system for so long (teens of years) and I don't particularly want to go to the expense of changing my set up.
From the outset I have had three forms of heat, four if you include my cold frames, and what is being discussed here is my reason for having them.
Initially I use a a Propagator for 'germination' purposes, then once the seed has germinated I place my trays on to a hot bed in the open air of the greenhouse. If needs be I have cheap tray propagating lids which I use to cover the trays of seedlings to gradually accustom the seedlings to cooler temperatures.
Having said that I find that I only use the propagator when I want germinating heat of 60°F (16°C) usually with veg I just sit the trays on the hotbed with a propagator lid over them.
I keep the greenhouse at a maximum temperature of 40-45°F (4-8°C) for frost prevention ( It would be too expensive to heat the whole greenhouse for germinating purposes.)
Similarly I have an overhead "strip" light set up with a timer to supplement light above the seedlings if I deem it necessary.
Once I am satisfied that the seedlings are progressing quite nicely I put the trays on high level shelves where the get maximum light and as the greenhouse is set at 40-45°F (4-8°C) they are warm enough.
Then after a week or to the go out into my coldframes.
I hope this helps!
Here is a blog including pictures of my system showing what I mean!http://www.thegardenersalmanac.co.uk/Data/Greenhouse%20heating/Greenhouse%20Heating.htm