Two of the most frequent questions I receive are: won’t it be too hot and damp inside the greenhouse? And, what will it cost to heat and cool that much space? This post describes how I plan to address those two issues.
Normally in the Pacific Northwest, cooling is not a priority but with the solar gain from the greenhouse, I needed to find a cost-effective method for summer cooling and winter heating assist. This is where readers in the southern United States mutter ‘welcome to my world’. One difference between my situation and theirs, however, is that I have to decide on a heat-ventilation-air conditioning (HVAC) system for not just a house but also the greenhouse.
The first step was determining the volume of space to be ‘conditioned’. These are calculated as follows:
THE HOUSE HVAC
The house is approximate 500 square feet footprint with sloped ceilings so it averages 9′-10’. So the home’s interior equals approx. 5,000 cubic feet. With walls and ceiling insulated to code the home’s HVAC will need to provide 12,000-18,000 BTU.
Some constraints are there is little available wall space for radiators, baseboard, or wall-mounted convection heaters. There isn’t floor space or attic space for a furnace to supply a forced air system. Given the uncertainty of how well the greenhouse’s HVAC will control heat and moisture, it is prudent to have cooling capacity installed in the house. So the best option remaining is a combination of mini-splits for the primary heating and cooling with radiant floor in the bathroom.
THE GREENHOUSE HVAC
The greenhouse is 35’x60’ for approximately 2,100 square feet, less 500 square feet for the house, leaving a 1,600sf footprint. The sidewalls are 12′ rising to approximately 16′ at the roof peak, I’ll use 24,400 cubic feet as the greenhouse volume*.
My intent for the greenhouse is not to maintain ‘hot house’ growing conditions; I’m not planning to grow tomatoes in the winter. However, I want to be able to keep the home’s doors and windows open to the greenhouse as much of the year as possible, so that means some amount of environmental conditioning for both the house and the greenhouse.
These were my requirements for the greenhouse HVAC system:
1) it requires no, or minimal, energy assistance (either grid-tied electricity, propane, kerosene, or other fuel);
2) it is fully automated at installation or can be retro-fit soon after;
3) the greenhouse temperature can be maintained above freezing and preferably above 40 degrees Fahrenheit (4.4 Celsius). This is the equivalent of moving from my current USDA Hardiness zone 8b to zone 11; and
4) the greenhouse temperature can be maintained below 80 degrees Fahrenheit (26.7 Celsius) at ground level.
Regarding #3, I quickly learned that most greenhouses, plus night time row covers with no other heat, can create a 3-zone improvement. Although requirement #3 is for my personal comfort rather than to grow plants needing zone 11 temperatures, it was good to know that the greenhouse, without more, would provide year-round growing conditions.
What seemed the obvious place to start my search was with the form of HVAC commercial growers use, including any options available from the company I will order the greenhouse from. I learned that there is a great divide in the commercial growing industry with a large, installed and conservative base (including the greenhouse vendor) that use large fuel-burning furnaces to maintain the temperatures necessary for their particular crops. Then there is another faction, mostly younger, who are advancing more eco-friendly and passive (or passive assist) systems.
Searching among this second group, the terms that had the most ‘hits’ were all systems that take advantage of the fact that at some level below the surface, the earth’s temperature is constant around 50 degrees Fahrenheit (10 Celsius). These ground-based systems are:
- Geothermal heat exchange
- Ground-to-Air Heat Transfer (GAHT®)
- Climate Battery®
- Subterranean Heating and Cooling System (SHCS)
Are these all the same under different names? Or, what’s the difference?
Geothermal heat exchange systems are probably the best known of the earth-based systems. At the most basic level, these systems use liquid-filled tubes running either horizontally or vertically through the ground, to heat or cool the liquid and then the heat or cool is transferred to water-based HVAC units like conventional radiators, hot water tanks, or in-floor heating. Alternately, the heat or cool may be transferred to air-based HVAC systems like forced-air or ductless units.
These are very cost-efficient systems to operate since the only power they need are to run the heat pump which moves the liquid through the tubes and and to run a compressor for air-conditioning. These systems do not burn fuel to produce heat like a traditional furnace.
The downsides are these systems require careful engineering and installation to prevent risk of the liquid freezing or leaking or the risk of mold and mildew from condensation on the radiator. Geothermal heat exchange systems are also expensive to install as the vertical systems must go deep or the horizontal systems require more ground space than I have available given the foot print of the existing buildings and septic system. (See How Geothermal Heat Pumps Work)
Subterranean Heating and Cooling System (SHCS), Ground to Air Heat Transfer (GAHT®), and Climate Battery®
Like Geothermal, Subterranean Heating and Cooling Systems (SHCS) use the earth’s constant temperature for heating or cooling. At its simplest level, a SHCS uses a bunch of 4” or 6” diameter perf-pipe run underground that connect to a manifold at each end that then connect to riser tubes that come above ground inside the greenhouse. Doing my best to understand this stuff on my own as an independent consumer, this is where it gets confusing for me. I think Subterranean Heating and Cooling System (SHCS), Ground to Air Heat Transfer (GAHT®), and Climate Battery® are all systems that move air through underground tubes to either heat or cool the air to the earth’s temperature. The distinction appears to be that SHCS is the generic name (i.e. “tissue”) versus GAHT® and Climate Battery® are each different companies’ trademarked designs and calculations (i.e. “Kleenex”).
Unlike geothermal, which uses a pump to move liquid through tubes, a SHCS uses fans to move air from the greenhouse, through the tubes, and back to the greenhouse. As the air passes through the earth it gives off heat and moisture (in the warm months and day time), or collects heat (in the cold months and night time). The “battery” aspect of a Climate Battery® refers to ‘charging’ the ground with the daytime heat from the greenhouse where it is stored until it is needed to warm the greenhouse during the night time. It appears from their websites that both GAHT® and Climate Battery® systems contain the tubes within the greenhouse and its insulated foundation. In comparison, a generic SHCS may draw air from outside the greenhouse through underground tubes that either heat or cool the air before it enters the greenhouse. (See an examples of using air through underground tubes to grow oranges in Nebraska in the winter! He explains the SHCS starting at approx 2min)
When SHCS draw warm and moist air underground, the air is cooled and the atmospheric moisture condenses into water. These systems use perforated pipe that allow this water to drain into the soil so SHCS also provide some measure of dehumidification.
SHCS have several advantages over geothermal but the main 3 for me are: 1) They are very cost-efficient to operate since the only power is to run the air circulating fans and the automation electronics which require less energy than geothermal’s heat pumps that circulate liquid. 2) They are also relatively inexpensive to install because they’re comprised of tubes and connectors that are readily available at hardware and construction supply stores. 3) They take up less ground than horizontal geothermal and don’t need to go as deep as vertical geothermal – both of these are key advantages for my site since between the new and existing structures there is little clear space left.
The decision is to install a SHCS but it has not been engineered yet so I don’t have confirmation on a couple key decisions: 1) I do not know if we will be able to contain all the tubes within the greenhouse foundation at only a depth of 4 feet, or whether we’ll need to augment the system with tubes running from outside the greenhouse. If tubes are run from outside the greenhouse, can they be run in the same trench as the water or power lines? 2) I don’t know what the power draw will be to determine whether it can be solar powered at installation or whether adding the solar system will need to be a separate project down the line.
I’ll post a follow-up once the engineering and cost estimates have been done.
*This is how the greenhouse air volume and the number of fans were calculated:
Greenhouse air volume calculation:
- Greenhouse footprint 35’x60’= 2,100sf
- Average height 14’ 2,100sf = 29,400cf
- Minus house volume 500sf * 10’ tall = 5,000cf
- Greenhouse air volume = 24,400cf
Circulating fan calculation:
- The climate battery fan should have high enough airflow ratings to move the entire volume of the greenhouse space 5x per hour (5ACH).
- 24,400cf * 5ACH = 122,000cfh / 60 minutes = 2,033cf to move per minutes
- 6” fan ~ 483cfm. 2,033cfm / 483 ~ 4 fans needed