The Hotline Newsletter

Summer 2003

Why Other Heaters are NOT Just Like Cambridge

"To recirculate or not to recirculate." That is the Question.

Direct gas-fired air heating equipment is classified as being either "recirculating" or "non-recirculating." It's important to understand the difference when specifying or buying a direct gas-fired heating system for space heating or make-up air applications. Beginning this year, each type is now tested and certified to separate safety standards reflecting the fundamental differences between these two concepts for direct gas-fired heating.

Non-Recirculating Heaters heat 100% outside air. Cambridge manufactures only non-recirculating heating equipment. Our M-Series design is used for make-up air applications and our S-Series design is used for space heating applications.

Recirculating Heaters can heat up to 80% inside air that is recirculated through the heater and mixed with at least 20% outside air. This is also often called a "pressurized" heating system because its control system will bring in enough outside air to maintain a fixed positive static pressure in the building.

Two common misconceptions are that recirculating heaters are more energy efficient because they heat less outside air and non-recirculating heaters are just make-up air heaters that help improve indoor air quality. So after 40 years in the business, why does Cambridge only manufacture non-recirculating heating equipment? The answer is simple. We believe they are safer, better and more energy efficient.

Cambridge S-Series "Blow-Thru® Space Heaters

Intermittent operation.
Designed to neutralize infiltration without excessive pressurization. Amount of outside air being introduced is typically less than or equal to air leakage of the building.
Improved IAQ by introducing 100% fresh air.
More energy efficient heating system.

80/20 Recirculation Heater Pressurization System

Continuous operation.
When doors open, heaters try to maintain building pressure by bringing outside air. The additional air is forced out thru the open doors, wasting energy.
Potential IAQ problems from re-heating contaminated indoor air.
Required more MBH and larger horsepower motors.

Here are the facts:

Energy Efficiency

Ok; so how can a Cambridge, high temperature rise, S-Series Space heater that heats 100% outside air be more energy efficient than a comparable recirculating space heater that only heats 20% outside air? It just doesn't seem logical. The answer lies in a number of "logical" assumptions that are just not true.

100% is more than 20%: Not always! 20% of a really big number is more than 100% of a small number. Also, the total amount of heated outside air is a function of how often the system operates. So when it takes a big, 120,000 cfm 80/20 recirculating system to deliver enough BTUs to heat a building, at least 24,000 cfm of outside air is always being heated. When that same building is heated with much smaller, Cambridge S-Series space heaters that only require 28,000 cfm, the non-recirculating Cambridge system actually heats significantly less outside air. That's because it uses intermittent thermostat operation of individual heaters for zone temperature control. Cambridge heaters are typically on only 30% of the time compared to a big recirculating system that is continuously running.

Recirculating heaters never heat 100% outside air: Not true! Just one open dock door or a leaky building will drive an 80/20 system with pressure modulation controls to heat 100% outside air in order to maintain building pressurization. For example, a 10' x 10' door opening at .01% static pressure produces about 400 ft./min of positive air pressure, or 40,000 cfm for just one door opening!

Recirculating heaters use same amount of electricity as non-recirculating space heaters: Definitely not true! With all the emphasis on high gas prices, it is easy to overlook the electric operating costs of these big heaters. Recirculating systems typically must turn the air in a building 1-1/2 to 2-1/2 times per hour to reduce stratification, requiring its large horsepower blower motors to run continuously. This significantly increases electrical operating costs compared to Cambridge S-Series heaters that use much smaller horsepower motors that run intermittently to meet the building's true space heating needs.

Recirculating heaters have high temperature rise capabilities: Again not true! Only the non-recirculating, Cambridge S-Series Space Heater design is certified to the new standards for a maximum 160°F temperature rise/discharge temperature rating. In comparison, 48.9°F is the maximum equivalent temperature rise now permitted for an 80/20 recirculating heating system. The heater's max temperature rating is crucial for space heating applications. Higher temperature ratings translate into a better Btu/cfm ratio, smaller units, lower horsepower motors and the need to heat less outside air. The dramatically lower temperature rise capability of recirculating heaters means they must use a larger volume of warm air to heat the building. In other words, lower temperature ratings result in more energy used for space heating applications.

New ANSI Safety Standards

The American National Standards Institute (ANSI) recently revised their Z83 standards that apply to industrial direct gas-fired air heating equipment to reflect the differences between these two heating concepts. ETL, CSA and others test and certify equipment to these safety standards. Cambridge was the first manufacturer of direct gas-fired heating equipment to be certified to the industry's new, harmonized US/Canadian Standard ANSI Z83.4/CSA 3.7 that now applies to non-recirculating equipment. Our energy efficient S-Series Space Heaters and M-Series Make-Up Air Heaters use 100% fresh air and do not recirculate or re-heat potentially contaminated indoor air.

A new, more stringent ANSI Standard Z83.18 for recirculating direct gas-fired air heaters just went into effect January 1, 2003. It now requires manufacturers of this equipment to use new, more complicated control systems to assure adequate indoor air quality (IAQ). The new control systems reduce heater discharge temperature as the percentage of re-circulated room air is increased. That's right, due to safety concerns, the allowable temperature rise (heat output) is a function of how much re-circulated air is used regardless of how much heat is required in the building.

Manufacturers of direct gas-fired recirculating heaters promote their technology as IAQ friendly. Many code officials and engineers do not agree. In fact the ANSI Standard Z83.18 for recirculating heaters was not adopted in Canada due to concerns about safety and IAQ. It is also important to note that the nature and use of a building may change from its original intended use. Recirculation may be acceptable for the original application but not at all acceptable when the building's use changes. The days of 80/20 systems may be numbered, as many engineers in the U.S. now limit allowable recirculation to only 60%, making it a 60/40 system.

Why does Cambridge only offer Non-Recirculating Heaters

The recirculating air-heating concept tries to take the original direct gas-fired make-up air heater design and modify it to be more energy efficient for both make-up air and space heating applications. The result is that it can be used to do both tasks but does neither of them the best. We know this from our own experience. Cambridge was the first to offer a recirculation design for space heating applications during the 1960's. However, the introduction of our more advanced, high temperature rise technology for non-recirculating space heaters meant there was no longer a reason to offer the less energy efficient recirculating design with its inherent safety and IAQ concerns.