The main consideration is: how much heat will you lose as one layer of poly has a very low insulation (R) value. And this will depend on outdoor temperatures and how high you want the indoor temperature to be.

Since you already have the heater and hopefully an easy way to install it and get power to it, maybe just try it out and see. You may find you need another one in a greenhouse of that size and then it is down to cost of the heater and the electricity to run both of them.

I also have 1500 watt forced air heaters with fans, with three settings. Just thought I could use the infraread idea. Maybe air movement is better.

This small greenhouse heat requirement calculator- heat calculator - will give you the answers to your questions.

Melon. Try adding some black steel barrels of water to absorb that heat during the da6 and then dissipate it back to the house when the sun goes down. That type of latent heat will work great with your heater. I have 10 barrels in a 10 X 12 HFGH in MA and it works great. Let me know if you have any questions.

Air movement is pretty important. Water helps slightly but I find in the dead of winter it doesn't play much of a role .

You’re right air movement is important. I have a small 6” clip on fan mounted near the top of the roof set on low speed. It helps to move the heat around that has risen to the top.

Water barrels alone will help a little when they absorb the day’s sun. Their value increases when a heater is involved and increases their latent heat. For example, right now it is 34* out. The barrels are at 63* and the greenhouse is at 52*. Of coarse it’s not in the dead of winter but I’ll take free heat anytime I can.

have a good day.