The Hotline Newsletter

Summer 2002

"Our Heater is Just Like Cambridge"

We hear this statement from competitors who try to quote a "draw-thru" make-up air heater as an equal substitute for a Cambridge "Blow-Thru®" space heater. The Space Heating Case Study shown above proves there is a big difference in operating costs and performance. Don't be fooled. This equipment may look similar but the technology inside is quite different. Only Cambridge offers a Blow-Thru® design certified for 160°F.

Max Temperature Rise and Discharge Temperature
The heater's max temperature rating is crucial. Higher ratings translate into smaller units, lower horsepower motors and reduced energy costs. Why? Because the goal for space heating is to provide just enough tempered outside air to meet building air infiltration requirements with extra heating capacity (Net MBH for conduction) left over for comfort heating or freeze protection. This part of the heater specification is so important that our competitors have come up with some creative, but very misleading ways of disguising their lower temperature rating so they can tell a contractor, "our heater is just like Cambridge" when in fact it is not!

For example, the Cambridge 160°F rating is for both temperature rise and discharge temperature based on 0°F inlet air. Beware, some manufacturers artificially raised their ratings by "pulling a fast-one" and basing their max temperature rise rating on -40°F inlet air. Actually, their draw-thru design hasn't changed at all and still has a 120°F max outlet temperature with 0°F inlet air.

Draw-Thru vs. Blow-Thru®
Why is the gas burner and blower arrangement so important? The simple answer is lower operating cost, better performance and longer service life. The technical explanation starts with the fact that all blowers produce a fixed volume of air, but the density (mass) of that air changes as it is heated.

Draw-Thru (Make-Up Air)

• Blower is downstream of burner
• Discharge CFM is constant
• Components in the hot air stream

A draw-thru heater, with the blower downstream of the burner, is best suited for make-up air applications because the discharge CFM is constant despite fluctuations of outdoor air temperatures. This is important for matching large exhaust fan requirements. However, heating inlet air before it goes to the blower has disadvantages for space heating. First, it limits the max temperature rating. Most draw-thru heaters can only be certified with a temperature rating between 90°F - 120°F. Second, the pre-heated air is less dense before it goes to the blower so it must heat more air to satisfy the building's total heat load.

Blow-Thru® (Space Heating)

• Burner is downstream of blower
• Inlet CFM is constant
• Components in the cold air stream

The Cambridge S-Series Blow-Thru® heater provides more heat output (BTU/CFM) than any draw-thru design. It is the best way to apply direct gas-fired technology to a space heating application where the primary concern is heating the building, not supplying a large volume of make-up air. Locating the burner downstream of the blower has two advantages. The first advantage is obvious. Motor, belts, bearings and other components are located upstream of the burner so they always stay cool. This increases service life and reliability. It also allows the Cambridge heater, with its patented burner, to be certified for 160°F. Second, when the colder (denser) inlet air is heated to 160°F, more usable heat (Net MBH) is available for space heating.