Car radiator, heater core or something else for greenhouse?

A friend of mine used rabbits to help keep his plants warm.He put the plant beds above the rabbit boxes,and used the rabbit droppings for fertilizer.He fed them with out dated produce thrown away by local grocers.In the spring,he sold rabbits(Easter Bunnies). Rabbits produce a lot of body heat and it was sufficient to do the job for him.I do not know your climate,so this may not be suitable.

You could also try circulating water through bales of hay,with the coils in the center.Composting material produces a lot of heat that can be utilized.

It can reach up to 160F degrees internally.

Chicken house sweepings or turkey house sweepings are also excellent.

The composted hay or sweepings can can then be used in your garden in the spring.

Maybe you could just run a duct from the house, well insulated of course, and a fart fan to regulate temperature in the greenhouse...

It seems like you both have and have no problem with a high relative humidity. I think of corrosion and mold but you seem happy so let's roll with it.

It seems like you are looking for a cheap and simple heat radiator in this high humidity environment. An "out of the box" idea would be to have a piece of metal, possibly the corrugated roofing metal, near the interior north wall of your greenhouse. Mount the metal with enough of an angle that water will not drip off, possibly only a few degrees off horizontal. Have a pipe with holes at the top and a collection tray at the bottom. When heating is needed pump heated water to the upper pipe and let the hot water run down the face of the sheet metal. The large surface area of the running water combined with the even larger surface area of the sheet metal should create a very good radiator.

Taking this one more step toward simplicity, are the 50 and 120 gallon water storage tanks a missed opportunity? That is a lot of thermal mass. If you have metal tanks and heaters in them then that is a lot of thermal mass that you can heat up. Depending upon the shape and material of the tanks they might radiate well.

If you have 170 US gallons water in there it stores 0.7 kWh of heat for every 'C [0.4kWh/'F] hotter.

You do not give height!! but if 10 feet, then heat loss of green house, supposing well double glazed is about 0.7 kWh per hour per 'C inside to outside.

A recycled domestic heating radiator, black paint on sun side under recycled window glass, covered on back with the [washed] aluminium internal coating "crisp" [or is it "chip"] packets thrown away by billions, glued up to have 0.5 inch inside air gap could thermo-siphon heat into water [needs a control valve sensitive to temperature difference]. Half inch air gap, with bright aluminium both sides to cut radiant transfer, has about same heat loss as a good double-glazed window. I have seen photos of black-painted hot water tanks on house roofs in Israel.

Even metal containers for water, painted black may absorb more sun-heat & help warmth over night.

I have read somewhere,don't remember where,that in Israel,a greenhouse was built that used water circulating between an outer and inner layer of glazing kept the greenhouse at a stable temperature.

A small greenhouse such as yours could be built with double glazed sliding door glass panels,with the gap filled with water and recirculated by convection with proper layout.The panels are approximately 3 feet by 6 feet dimensions.

Install a tap at the top and bottom of each window panel.

Connect all of the outlets in series,top to bottom, top to bottom,and then the final tap from the top to the first one at the bottom.

I would insulate the roof with spray foam.It will get very cold at night very quickly so it will not help keep the greenhouse warm,just the opposite. A well insulated roof will not drip condensate.

Rabbits produce more heat per pound than just about any other mammal.They will help contribute to the CO2 in the greenhouse which plants need.The plants will provide oxygen for the rabbits with minimal need for fresh air,a sort of symbiotic relationship.I mentioned this in my previous post.

I also read about another way to to keep the seed beds warm is to get a steel cable from the scrap yard,such as used in a crane,and loop about 100 feet of it around and under the bottom of the seed bed.Connect the ends to a small welder on lowest setting,and adjust setting to get the temperature you want.Install a thermostat for control on the input side of the welder to minimize the current.

Power consumption is minimal,and it is very efficient since the heat from the welder transformer is diffused into the greenhouse,not wasted.It is not free,but it is very economical compared to other means.

Soil heating cables are also available commercially.

Lots of options available.