Maximizing efficiency in greenhouse heating systems: Why now is the time to upgrade

One area that is frequently overlooked when considering energy conservation is the heating system. Energy audits show that many existing heating units are operating at 60% to 75% efficiency. With the high cost of fuels, now may be the time to replace these with new condensing-type boilers and heaters with more than 90% efficiency.

Although only natural gas and propane-fired units are currently readily available, development of fuel oil technology is progressing in both the U.S. and Europe. Currently, natural gas and propane fuels are competitive on a million Btu basis to fuel oil in many parts of North America.

How is high efficiency achieved compared to conventional units that typically operate at less than 75%? When fuel oil, propane and natural gas are burned, water as steam is one of the by-products. Burning 1 gallon of fuel oil will produce about 0.98 gallons of water; burning 1 gallon of propane will produce about 0.82 gallons of water; and burning 1 ccf of natural gas will produce about 1.16 gallons of water.

In a non-condensing boiler or furnace, this 500 to 600 °F water vapor (along with the other by-products) normally goes up the stack and is exhausted into the atmosphere. This high temperature is needed to avoid excessive flue condensation or damage to the boiler from the acidic condensate.

A condensing boiler incorporates an extra heat exchanger in the flue gas exhaust system so that the water vapor condenses back to a liquid. This process captures up to 8,000 Btu of heat per gallon of condensate, which is as much as 13% of the original fuel energy. The condensate heats the incoming air (if an air-to-air heat exchanger is used) or pre-heats the water (if an air-to-water heat exchanger is used).

After the heat is removed, the low-temperature condensate water and flue gases are exhausted through a corrosion-resistant pipe (usually PVC), eliminating the need for a chimney. In some areas, a neutralizer may need to be added to the condensate to lower the pH. In condensing boilers, air for combustion is supplied from outside the greenhouse rather than using the heated inside air.

Condensing boilers need low temperature return water from the radiators to work best. With water at 130 °F, 15% of the water vapor will condense, whereas at 60 °F, over 90% of the water vapor will condense.

A root zone system is a good example of where it will work well, with supply temperatures usually at 100 to 110 °F and return water at 80 to 90 °F. Efficiency is also affected by the humidity of the air. Under the right operating conditions, condensing boilers can achieve efficiency of 95% or more.

Most boilers with condensing capability have modulating burners that are capable of multiple firing rates. By using embedded control, the firing rate is matched to the heat load needed to give best performance. At low heat needs, the unit is most efficient. At high firing rates, the efficiency may decrease but never be less than a conventional boiler. Variable speed boiler water pumps are frequently installed to increase efficiency.

Today’s boilers are much smaller than the old standard units, as they have much less water in the jacket. They are self-contained so that much of the wiring and plumbing is done before you receive it. This reduces installation time considerably.

Design considerations: Avoid too large of a boiler

Oversizing a boiler can reduce efficiency, as the object is to have it run continuously. This is done by adjusting the firing rate: low fire when minimum heat is needed and high fire for the coldest weather.

Cycling decreases efficiency from startup and shutdown of the operation, as well as increases wear on the boiler. To get the greatest benefit from a condensing unit, it is better to size the boiler for an average winter temperature, not the normal coldest temperature frequently used by engineers. The more continuous operation will keep up with the heat needs better than a unit that cycles.

Condensing hot air unit heaters

These operate with the same basic principle of the boiler that captures excess heat from the exhaust flue gases. Most are designed with a single firing rate. To provide more uniform temperature in the greenhouse, multiple units are frequently installed. This will usually save fuel.

HAF air circulation should be incorporated in the system to keep a uniform temperature and to provide air movement within the crop area.

Although condensing boilers and heaters are more expensive than conventional units, the greater efficiency can offset the additional cost. Check with the USDA Natural Resources Conservation Service for potential incentive payments for replacing existing inefficient boilers and heaters.

Growers with this type of condensing-type technology have found the PVC exhaust system gives greater flexibility to where it can be installed. They also report there are less maintenance needs and middle-of-the-night emergency alarm system calls.