My Everchanging Garden

Gardening That Grows With Me

How Radiant Heat In A Greenhouse Is Different Than Force Air

Radiant heating significantly changes how a greenhouse is heated, and I’ve had to relearn plant placement. With forced air fans, the floor area was cooler as the cold came up through the ground. The middle was warm since that’s where the heaters were directed. And even though heat rises, it was coldest at the top at night because of massive heat loss through the glass peak. So, moving from the ground up, you get cold-warm-cold. With electric heat, growing on benches made sense, as that was the warmest spot.

With radiant heat, cold stratification is different. The floor is the warmest. The bench area has to rely on electric heaters at night. The top is still cold because of heat loss. With radiant heat, from bottom to top, you get warmest-warm-cold at night and warm-warm-cold in the day.

With radiant heat, putting plants on the floor will heat the root zone and plant canopy. You can see my recent tests from a cold snap to understand how well this works. So, my plan going forward will be to use tables most of the time (so the plants will get more sun) but put plants on the ground during extreme cold. I can also use the heated floor as a grow mat for seed starts that prefer to be warmer.

Some other updates …

I’ve changed the radiant heating system itself since my installation post. As I noted in that post, I wasn’t happy with the boiler’s performance. It was cycling on and off too quickly, limiting the amount of heat going through the lines. Part of the solution was to add a second circulation loop that fed some of the hot water into the return water. This loop pre-heats the return water slightly so the boiler does not have to run so high, necessitating a shut-off period. The installers also added an extra pump to help with water flow. Here is the new set-up.

new radiant heating boiler setup

The second change I made was to move the south electric heater from above the benches to below the bench. This should help push heat lower, keeping the heat in the greenhouse longer.

Next, I had someone else look at the settings on the boiler. They changed a few, including slowing down the water flow, and the results seemed better, and as good as I’m likely to get. In hindsight, I should have had more lines under the floor, insulation underground outside the building and perhaps a second zone of bench heating or wall heat. However, what I am getting works. The floor temperature is around 30C, even when an outside wall is 3.6°C.

FLIR image of floor with radiant heat

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