Designing the greenhouse temperature control is probably the hardest part of this project. Even though winters are mild here in the Pacific Northwest I don’t intend to grow plants that need supplemental heat. Summer temperatures are also generally mild and below the U.S. national average. However, the summer of 2021 we had, for the first time in my lifetime, multiple record-breaking high-temperature days so when I refer to “temperature control” that really means “heat control.”
After researching passive cooling options I settled on a ground-to-air heat exchange system as the best option for my requirements. See my requirements and evaluation of HVAC options at Planning the Heat, Ventilation, and Air Conditioning – Live In A Greenhouse.
Rather than sidewalls of flexible material that can be rolled up in the summer, my greenhouse will have rigid sidewall panels. There will be, however, operable window vents the entire length of the ridge. As hot air rises, these vents should move hot air out at the ridge through natural convection. The design challenge then becomes how to provide a sufficient volume of cooler air at ground level to feed the convection.
There are some U.S. firms that specialize in greenhouse HVAC design and I inquired with a couple about engineering my system. For various reasons I chose to not contract with any of these firms. This means that the design has come down to what I and my builder have stitched together from extensive Google and YouTube research.
The factors to consider include:
- How many feet of pipe to lay inside the greenhouse vs coming into the greenhouse from outside,
- How deep to lay the pipe given the ground in this area does not freeze too much below the surface,
- The diameter of the in-ground pipe,
- The diameter of the risers,
- How to leverage the cool-sink of the lap pool and regeneration ponds,
- Where to use solid vs perforated tube to ensure any water that enters the system can exit the system without supporting mold growth,
- How to add fans to the system where their operation won’t be too loud, and
- Whether to add an evaporative cooling wet-wall option offered by the greenhouse manufacturer.
The plan as it currently stands includes 4 independent systems:
The first system is based on tubes run into the greenhouse from outside. These can be run in whatever trenches we’re digging for other purposes like water, power, septic, etc. This idea comes, primarily, from this video Nebraska retiree uses earths’s heat to grow oranges in snow – YouTube @1:50 His main concern is heating rather than cooling, and he has the advantage that his entire greenhouse is sunken in the ground. However, the frost line in Nebraska is deeper than at my property so his tubes are deeper than mine will need to be. But it demonstrates that just running the tubes from outside to inside with a blower is enough to provide reasonable temperature control in hot Nebraska summers.
This video from a commercial farm in Colorado also focuses more on winter heat than in summer cooling but is helpful because they explain a mix of cost-effective systems they use. They run the pipes from outside to inside the greenhouse with the fan on the exhaust end to pull air through. The geothermal starts at @4:56 The GreenHouse Gallery! At the Living Farm – YouTube.
The second system is based on 2 layers of tubes that draw hot air down at one end and discharge cooler air on the opposite end. This system draws the hot air down on the north side and exhausts on the south side. For this system I relied, primarily, on a couple videos from Christopher Brandsdal. Although he’s located in Norway so his location is much colder than mine and his main concern is warming overnight, his videos are helpful because he built his system in 2 halves that allow comparison of the two methods; the first half he snakes 2 layers of single runs of tubing back and forth with gravel below and above GeoThermal Greenhouse Build | Part 2 – YouTube, The second half of his system he uses the layout with tubes running into manifolds on each end GeoThermal Greenhouse Build | Part 3 – YouTube @11:45. Another benefit of Christopher’s videos are that he installed monitors throughout the system. His main job he works as a software developer so he developed method to collect data from the monitors and display it in graphs on his website – Microsoft Power BI. He also covers installation of fans GeoThermal Greenhouse Build | Part 4 – YouTube @5:45 and @10:20. Later he reviews the data about the ground and air temperature change from running the fans GeoThermal Greenhouse Build | Part 5 – YouTube.
The third system is based on tubes run under the bottom of the pool. In this system the hot air is drawn down from the northeast corner and exhausts near the pool’s southwest corner.
The fourth system is based on tubes looped back and forth, manifold-style on both long sides of the pool. I’m not sure the best place for the intake or exhaust for these so they’re not drawn on the plan yet. We’ll make the final decision when excavating for the greenhouse and house foundations.
I took a stab at sizing the tubes based on some of the videos I’ve watched but we’re still refining the in-ground tube diameters between 4”, 6″, or larger and between 8″ or 10″ risers.