After using a car radiator for heating and cooling a greenhouse for many years now, we thought it is time for an update.

To control the temperature in our Geodesic Dome greenhouse, we used a Water Tank for passive temperature regulation and also an old car radiator for active heat exchange. To use a gas or electric heater can be costly. This recycled car radiator did the trick for us.

Note: Here we share the idea, it is not a complete how-to. The idea is for tinkerers :). If you are one of them, you will appreciate this post, if you are not, just note that there is this possibility or become a tinkerer.

How does the Car Radiator Heating and Cooling work

A greenhouse can get very hot during sunny days. To cool it down you can add ventilation using automatic window openers, and shade cloth. That’s how we started, too.

Our climate’s uniqueness is the cool nights. It is not unusual to have just 8 °C (46 °F) at night in the middle of summer and even cooler in spring and fall, which is too cold for heat-loving plants. We wanted to find a way to store the excess heat during the day to heat the greenhouse during the night.

The radiator heating and cooling system’s simple heat exchanger absorbs the heat energy from the air during the day and stores it in water, at night it gives that heat back to the ambient air in the greenhouse.

Simply said, whenever the water is cooler than the air it cools and when it’s warmer it heats.

Since we already had a water tank as thermal mass and a pump to filter the water, all that was needed was to connect the radiator in sequence after the filter. So the water would be pumped through the radiator after being filtered and then flow back into the tank.

The radiator works in a very simple but effective way. Note that a filter is not needed for the system to work. The filter is there for the water quality in the tank.

The heating and cooling do not work if day and night temperature is about the same with no sun to warm up the greenhouse. During longer periods of overcast cold weather, we have to add supplemental heat to the greenhouse. We have done both, adding some hot water to the water tank or just adding a heater to the greenhouse.

Since we usually get many sun hours, the system works great for us most of the time.

Materials Needed for a Radiator Heating and Cooling System

1/4 HP submersible utility pump

Car radiator (cleaned)

Hose

Electric wiring with a switch

Fan (if not attached to the radiator)

Installing the Radiator Heating and Cooling System

Car radiators come in different sizes and shapes. Choose one according to the size of the greenhouse and water tank.

For our first greenhouse, we had a radiator from an old van with the original dual fan unit already attached. I installed it above the water tank. No, it’s not really a beautiful piece in the greenhouse, but the functionality outweighs that by far.

In the current greenhouse, we use a radiator from a full-size sedan which is a bit overkill but that’s what I had available. It did not come with a fan, so we added one on top. First I installed it in the middle of the greenhouse, it worked great as for the airflow and was somewhat out of sight.

However, we noticed that the plants underneath were shaded, and also lots of debris was accumulating on the radiator blocking the airflow. So I moved the radiator to the north side above the tank, but this time higher and again at an angle. It seems to work optimally and also looks exceptionally.

I even installed an air to air intercooler in our indoor garden room, it really is just about a 1×1 ft radiator, but it still pushes lots of air through, too.

Once the radiator is in place it needs to be connected with a hose to the water pump in the tank so water can circulate through the radiator. Make sure to use a cleaned radiator, otherwise, the water in the tank will get very dirty.

If the radiator does not come with a fan, attach a fan on top of the radiator so it pushes the air true the radiator creating the heating or cooling effect. We find that a flat classic floor fan works best.

Car radiator heating and cooling system in action

Here is a video that we made on a cool spring morning. The indoor and outdoor temperature difference is significant, but still according to the temperature of the water. It was 4 C (39F) outside and 13 C (55F) in the greenhouse, the water was 15 C (59F).

If you can’t see the video go here.

After seeing how well the system works in our first dome, we did not install any windows in our second dome. We wanted to store all the heat we get in the water. It works amazingly well even without any ventilation besides the door.

The car radiator heating and cooling system allows us to start a few weeks earlier in spring, spring start is more dependent on soil temperature than air. In the fall we can prolong the season 1-2 months, till the temperature drops to -10C (14F).

Questions and Answers:

Q. With the radiator in there, can you continue to keep the frost out during the winter?

A. We can not, we are in zone 3, our winter temperature goes as low as -40 degrees. Depending on how cold it gets in your area in the winter you might be able to run the greenhouse through the winter. The radiator in combination with the water tank does not produce any heat by itself, it is just storing the heat in the water. It works very well in a plastic covered greenhouse till outside night temperatures drop to about -10C (14F), if it gets lower than that there is a danger of water freezing in the tank and potentially damaging the system.

Q. Will this work during a cold and overcast period?

A. This active heat exchange only works in combination with solar energy. On cloudy days there might be not enough heat to hold through a freezing night. A supplemental heater will be needed.

Q. Could you use solar panels or a solar heater to heat the water during the day?

A. You could heat the water in the winter with some kind of aquarium heater using solar energy or even a solar heat collector to heat water during the day. Except there might not be enough daylight in the winter.

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