Greenhouse Unit Heaters Redefined
The revolutionary tubular heat exchanger inside the Lennox LF-24 series of greenhouse heaters ensures you years of performance and high efficiency. Combustion is precise because of special "in-shot" burners and fan assisted exhaust. Equipped with a TrueLeaf Combustion Air Separator, it's bullet-proof.

Gas Fired Unit Heaters

GrowerTalks Magazine Technology Report


Perhaps the most common piece of equipment in commercial greenhouses...
...is the gas-fired unit heater.

These generic-looking devices are hanging around in great numbers...and almost every greenhouse operation has them. Of course many operations are heated with the more efficient hot water boiler systems, but gas-fired unit heaters are by far the most common heating appliance you will find in greenhouses in the United States. Although, this is an article about a specific product, it’s important to understand as much as possible about the world of unit heaters. This is especially true now that the robber-barons are back running the fuel markets!

Little boxes that roar!

Based on the 1999 USDA Floriculture Report there are 533 million square feet of enclosed production greenhouse space in this country, and approximately 70% of it is heated with forced air. With the average grower spending around $.60 per square foot on average annually to heat, we’re talking about an annual throughput of fuel dollars of around $223. million dollars! That’s almost 1/4 billion (with a “B”) dollars being used annually by unit heaters in our industry!

Wow. If they use up so much revenue, how come they seem so. well, innocuous? The answer lies somewhere in the habit of many American growers to not look at the long term when investing in heating equipment and concentrating mostly on initial costs. “Fuel is cheap, why do I need efficient heaters?”, they said. Many growers do not understand the huge differences in efficiency between the various types of unit heaters on the market. However, with the upswing in fuel prices, it’s a good bet that growers will do a bit more research when making purchasing decisions from here on out.

Are there really differences from one unit to the next? Plenty—read on. and be brave.

Understanding the differences:

Ok, you have a metal box, you put gas to it, it burns, and a fan disperses it into the greenhouse space. What’s the big deal?

Direct Fired Units: No heat exchanger

Direct-fired unit heaters have a burner, a flame, and a fan. They are extremely simple. They operate thermostatically by running the burner and blowing air (sometimes a mixture of inside and outside air) over, around, and through the flame, picking up heat along the way. These units send this heated air and products of combustion into the growing space. This is a mixture of carbon-dioxide, carbon-monoxide, and ethylene— with some traces of sulfur and a few other things that may not be desired in your plant production space. Still, they are popular in some types of production, like retractable greenhouses that may not be very tight, and some cut flower operations growing blooms that are not sensitive to the ill-effects of ethylene or too much CO2.

Note:
It’s really important to keep the safety of your staff in mind when considering a heater with no heat exchanger, as carbon-monoxide is extremely poisonous to humans.

Heat Exchanger Equipped Units:

Most growing operations must install units that separate the products of combustion from the distributed warm air with a heat exchanger. This is because we cannot afford to fill up our greenhouses with the toxic elements previously mentioned for fear of damaging our crops or endangering our staff.

A unit heater’s heat exchanger is nothing more than a metal barrier designed to separate hot combustion gases from greenhouse air. The metal surface of the heat exchanger transmits the heat of combustion efficiently to it’s surface. The unit heater’s integral fan then transfers the heat to the air.

A heat exchanger for a greenhouse unit heater must have plenty of surface area and be optimized for survival in a humid, jungle-like environment. Many crops have been lost, delayed, or damaged by unit heaters with leaky heat exchangers. Currently, there are two basic heat exchanger designs on the market, stamped sheet metal “clam-shell” designs, and seamless tubular “serpentine” designs. The latter has been offered for approximately 6 years, while the clam-shell design has been around many decades.

So, now we’ve got a hot piece of metal that we’re blowing air over— that’s the basic design of all the heaters you see in greenhouses everywhere. What are the areas that make heaters different from one another? How can one heater of the same BTUH size use less fuel annually than another? Why shouldn’t I just buy the cheapest one? After all, they’re just painted metal boxes, aren’t they?

So many questions— no wonder many growers just opt for the cheapest “250” they can get— at least prices are a solid comparison. Right? Well, not exactly.

Startling Truths Revealed!

There are huge differences between heaters being sold today. If every grower using unit heaters immediately upgraded to better technology, we’d slash that $1/2 billion (with a “B”) by as much as 25%! That’s a cool $60 million a year that could go right to the bottom line! That’s a big number to our industry, but each grower needs to weigh the economics and Return On Investment (ROI) for their own facility. It’s very common to see a ROI of less than 2 years when fresh upgraded unit heaters are placed in a facility that had been operating with older units. The smart money is being spent on heaters with the following features.

Induced Draft:

Simple as it sounds, adding a little fan on the exhaust of a unit heater and installing the chimney out the end of the house and pointing the termination down (please follow each manufacturers recommendations for proper venting), can eliminate 10-15% of fuel use. The reason is explained with simple physics. We all know that hot air wants to go up, right? If your unit heater is of the “gravity stack” variety— and most of them are, every time your thermostat turns off, all the hot air you just generated starts looking for the “easy way out”.

Guess what? It’s the chimney stack of your heater that is most likely terminated above the ridge of the house so, as soon as you make the heat, it starts leaving out the very same place it was introduced. Does this make any sense at all? No, of course not... but unless you are aware, you’d never know it unless you had infra-red scanners outside your greenhouse.

With an active (induced) venting system, when the heater stops firing, the venting system is one of the last places your precious heat will want to go because it’s pointed down. These little fans are sized to positively evacuate the products of combustion out of the heat exchanger. They do use an inconsequential amount of electricity, but you won’t notice the cost. The addition of an induced draft fan increases the cost of the heater, but it almost always decreases the cost of venting... and the fuel savings begin piling up.

Outside Combustion Air:

So now we understand a bit more about venting the products of combustion, but let’s look a little closer at the combustion process. Fuel and air must mix for combustion to be generated. This air is typically obtained from the surrounding air around where the unit is located. That’s right, from inside the green house. Let’s think about that for a while. We’re using heated air from inside the greenhouse to make more heat. And, greenhouse air can be very humid, and can be laden with chemical particulates. This does not seem very logical. Not only are we using what is probably the poorest quality air for combustion, which will reduce the life of the heat exchanger, but we’re also ejecting already heated air from the greenhouse through the chimney. Do the math and you’ll find out that this in itself is a giant loss of efficiency.

A typical induced draft 250,000 BTUH heater has an exhaust fan that runs at about 60 CFM, or 3600 cubic feet per hour (CFH). This is exhaust comprised of already heated air and the by-products of combusting fuel. A typical bay of a typical greenhouse (30’ x 96’ gutter house) has approximately 36,000 cubic feet of air volume. Every hour that your heater runs, 10% of the heated inside air is ejected. This means that your heater has to “re-heat” 10% or more extra make-up air every hour... that’s expensive and wasteful.

So, what’s the solution? Bring the combustion air to the unit heater in a separate duct from the outside. This way, you are heating only re-circulated greenhouse air and not being forced into “compulsory” venting. This also eliminates the problem of burning the tainted inside air. Burning cool, oxygen-rich outside air will make your unit heaters happy for years. Separated combustion is available on the market, but in most cases it exacts a hefty price penalty and a slower ROI which brings us to the featured product of this article.

Lennox-LF-24 heater with CAS technology:

The Lennox LF-24 series of heaters are based on a “clean-sheet” design. The engineers tossed out all the paradigms about what a unit heater is supposed to be and looked at the process.

Heat Exchanger:

The engineers looked at the problems with traditional heat exchangers... cracking at welds, metal fatigue from being stamped, and corrosion attacking welds and they decided on a different course. The heat exchanger design they settled on has been wildly successful. It is a simple design using a series of cold-formed aluminized steel or stainless steel tubes expanded into sheet metal end plates. The design is completely seamless which eliminates the problems associated with welded and crimped joints failing prematurely. These heat exchangers are designed to last... and last.

"In-shot" Burners:

The combustion process occurs over most of the length of the heat exchanger tubes. Fuel is mixed with combustion air and introduced through multi-ported disc-like burners positioned at the inlet to the tubes. Drawing the mixture and creating very complete combustion is the induced draft fan and internal turbulators that swirl the flame thereby extracting maximum energy from combustion. These burners are more akin to a fire-tube boiler burner than the normal unit heater with “atmospheric” combustion.

Ignition System: Instead of building a system on top of another system like other manufacturers, the engineers went with a “direct-spark” system that lights the burners directly, rather than lighting a pilot first, and then the burners.

Induced Draft: A small squirrel-cage fan is located on the outlet of the heat exchanger and the products of combustion are drawn through the heat exchanger and exhausted through the induced draft fan. This facilitates simplified and much more efficient venting as described earlier.

Internal Diagnostic System: This innovative system provides a very simple means of trouble-shooting. In the event of a failure, sequencing flashes from green LED’s communicate exactly the source of the problem, thereby expediting the repair process. Modular components are easy to remove and replace.

High CFM Distribution Fans: The volume of air moved and the throw distance of effective heating are more than the traditional units on the market. The features are extensive and the enhancement of CAS technology makes it an ideal choice for greenhouse applications. CAS stands for Combustion Air Separation. It’s an enhancement to the basic heater that provides a very cost effective solution optimized for greenhouse applications.

The Lennox LF-24 series of heaters are a modular approach to heating. They are unlike any other heater being sold. They can be purchased as a standard induced draft heater, and upgraded with CAS to have access to outside combustion air and all the good that that entails. This modular approach delivers a much higher value than other heaters on the market.

Other features available for the LF-24 series are Poly tube inflation adapters, blower-style fan upgrades, and aluminized steel or stainless steel heat exchangers. Also, growers that operate facilities at high elevations will be glad to know that no de-rating is required on these units until you get above 7,500 feet above sea level.

The LF-24 series unit heater enhanced with TrueLeaf’s CAS technology delivers a lot of bang for the buck in an efficient, modular package.