Now, some (possibly many) may object to targeting this residential forced-air product, which could be argued as hardly representative of the entire spectrum of commercial-residential heating products (although residential forced-air gas heating represents the largest portion of the heating market in North America).

However, it is this author’s opinion that the Pulse furnace certainly had a hand in changing the heating products of our world.

The Pulse furnace. (Courtesy of Lennox.)

Setting The Stage

But the world at this period in time was in the middle of an unprecedented energy crisis. Everyone, from leaders of countries to individual home and business owners, was looking for ways to cut the United States’ reliance on foreign powers for sources of fuel.

Many industry observers believe the Pulse was a unique residential heating product on the market. Better yet, it offered efficiencies in the 90-plus range at a time when most residential gas furnaces averaged about 60% efficiency.

The manufacturer did not actually invent the high-efficiency furnace. Lennox did, however, come up with a unique design, which is based on the same principle that Germany used to propel the buzz-bombs of WWII.

More than a half-century earlier, Carrier Corp. came up with one of the world’s first 90-plus-efficiency gas furnaces. Unfortunately, it was introduced during the wrong time in history to realize possible success. At the time, natural gas prices were too low.

When Lennox introduced the Pulse, Carrier had enough experience with high-efficiency furnaces to foresee the problems that would eventually come as other, less-experienced companies tried to invent their own high-efficiency designs. In fact, in the mid-1980s, Carrier produced a slide program that showed how higher efficiencies would be achieved and the problems that manufacturers would face. This presentation was shown to hvacr contractors all over North America by Carrier trainers, who foresaw the product recalls that were to follow.

Efficiency-Condensate Link

Add nitrous oxide (a little of the sulfur-based trace elements from combustion) and a dash of hydrofluorocarbons in the air from aerosol cans, and you wind up with an acidic soup that can quickly destroy furnace heat exchangers. Companies had to come up with ways of dealing with the condensation, either by skirting the edge of the dewpoint (staying around 80% efficiency) or providing a drain for the water at higher efficiencies (90%-plus), and designing heat exchangers that were acid resistant.

Even with the above elements in place, it would be difficult for such a furnace to achieve the efficiencies as high as the Pulse. The reason — and history now proves this — is due to the mini-explosions inside this furnace, which were extremely effective in transferring heat because they blew away the natural insulating layer of air that stays next to the heat exchanger surfaces.

Inshot Burners, Venting

Until legislation at the beginning of the 1990s forced all furnace manufacturers to meet a minimum of 80 AFUE, most furnaces used a quiet burner that produced the type of flame that you could see, similar to the surface of a domestic gas cooking range. The inshot burner, by comparison, is a single, loud, noisy flame blown directly into a heat exchanger. It is very effective at achieving higher furnace efficiency.

Now, other challenges had to be met. For example, there was the problem of directing the flame through the heat exchanger so it didn’t impinge on the metal surfaces and cut efficiency by extinguishing the flame at those cooler points. To combat this, special induced-draft fans were installed to suck the flame through the center of the heat exchanger. Of course, all kinds of safety devices had to be installed to make sure that this complicated device worked as it should.

Next came the problem of proper venting. Until the late 1980s, most furnaces were vented into brick chimneys and the wasted heat from the furnaces caused these chimneys to draw. However, when efficiencies were improved and the waste heat was reduced, there was no draw. In addition, cold chimneys became a place for moisture and acids to condense and weaken the structure. New venting techniques had to be devised.

Controls, Hydronics

Soon copper heat exchangers started to replace the old cast iron ones because the heat transfer was better. Everything was getting more efficient, lighter, and more complicated. And, of course, the hydronic people also had to address condensation, corrosion, and venting.

In the 90s, another top-notch technological breakthrough emerged: electronics and digital building controls. These eventually became more common, especially in the commercial boiler field. Energy-saving strategies could be devised and boilers could become integral parts of the new and more successful building control industry. It also meant that new means of staging combustion were being designed so that no more hot water would be generated than times and loads required. And in large commercial applications, this staging often resulted in efficiencies that were equal to the improved means of combustion.

All of this sophistication led to a new look in boilers and water heaters. Gone were the roaring old 5-ton clunkers with their pipes, huge vents, and gauges. In the 1990s, they started to take on a more sophisticated look, so much so that they became presentable enough to be located in some front offices rather than in dusty boiler rooms.

The smaller boiler manufacturers weren’t about to be left out of the commercial boiler market fray. They came up with new designs that could be parallel piped and staged on one at a time. As a result, many huge, old boilers were cut up and removed from boiler rooms, then replaced by groups of smaller boilers that could be carted in through standard doorways. The added advantage is not only the realization of higher efficiencies through staged operation, but also that state inspections and 24-hr maintenance often be can avoided because each unit is below the operating range of such requirements.

Japanese heating technology also had a strong influence in the 1990s. Of special interest is their “tankless” water heater. Available for both natural gas and electric energy sources, it features a large burner section that heats water to its desired temperature in its first pass through. Storage tank heat losses are eliminated; water is only heated when it is needed. These devices take up little space and can have a stylish design.

While radiators and other in-room heat transfer devices have certainly undergone startling improvements over the old cast iron monsters, some of the greatest improvements over the past 10 years have come in the area of in-floor hydronic heating systems.

Of course, piping hot water to warm floors is not new. However, what is new is the reliable and inexpensive plastic-style tubing, which was popularized in the 1990s. The means of connecting heat transfer tubes to headers is also vastly improved and more reliable. There is far less chance of tube or connector leakage over the entire life of the building.

Electric Resistance Heating

During the 1990s, many ideas were tried to make electric heaters more comfortable and efficient. In-ceiling and in-floor heat strips often resulted in more energy losses. However, as one electric company pointed out several years ago, zoned electric heating (heating just one room at a time) can be less expensive than heating an entire house with even an efficient natural-gas furnace.

Of course, the great bastion of electric whole-house heating is in the backup role for heat pumps. And while dual-fuel heating (a heat pump connected to a gas furnace, for example) is truly more energy efficient, the higher equipment cost (and the fact that many people prefer the comfort of a gas furnace) keeps this idea on the back burners.

There is still one great market in North America for standard electric-resistance heaters and furnaces. It’s in the tropical portions of this continent, where heating is only required a few days per year.

Radiant Heating

Of course, radiant heaters have been around ever since there has been a need for heating. A bonfire, wood burning in a 50-gal drum on cold days, and most old-fashioned fireplaces were nothing more than radiant heaters. They warm surfaces through radiated infrared rays.

Many outdoor jobs or positions in large warehouse-type buildings would be very uncomfortable without radiant heaters. And the good news here is that they are now available in gas (natural and propane) and electric configurations, another 90s development.

Direct-Fired Makeup Air

Some people ask, “What about all the carbon dioxide and carbon monoxide?” Answer: The main product of combustion from an open natural gas flame is water vapor (which can be considered a good thing on a cold day). If there is proper combustion, no carbon monoxide should be present, and the small amount of carbon dioxide in the large flow of air is negligible.

On the positive side, think of the efficiency! There is no heat exchanger or need for a vent, so nothing is lost. Whenever large amounts of air must be vented from buildings on cold days, or where heat will be lost naturally (as in greenhouse applications), direct-fired makeup air heaters are an ideal choice.

Over the last few years, electronics and improved burner designs have made modern versions of this old and efficient heating concept even more efficient.

Forced-Air Heat Pumps

One of the first big innovation in heat pump compressors came in the late 80s, when domestic scroll compressors became available. The simple design, lack of losses, and ability to handle significant quantities of liquid refrigerant caused many contractors to change their thinking about the viability of heat pumps. Having said that, there’s no need to turn up one’s nose at reciprocating compressors. They, too, have become more efficient and more reliable. With some of the new variable-capacity designs, heat pumps are more efficient and offer better comfort than ever.

When discussing heat pump comfort, the usual knock against this system is that it blows “cold, drafty air.” In the 1990s, electrically commutated indoor fan motors (ECMs) have helped improve this situation. With bonnet temperature sensors and delayed fan-starting circuits, variable-speed motors have helped improve modern heat pumps’ comfort. The bad news is the price to replace one (or its electronic control board).

Water-Source Heat Pumps

Noncorrosive metal construction has, in some cases, greatly reduced the amount of traditional failures of these parts. Then again, improved understanding of how to properly install jobs has also had a positive impact.

Wheeler is a former contractor and instructor who has spent more that 25 years in various parts of the hvacr industry. He can be reached at Cortez Heating and Air Conditioning, Bradenton, FL, 877-755-5211, ext. 15; jimwheeler@ij.net (e-mail).

Publication date: 11/12/2001