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Insulation Heresy

Since virtually all the authorities — utilities, building codes and the Federal government — are agreed that in terms of energy conservation, the more insulation in a building the better, it is worth hearing from one of the only voices around saying that it simply ain't so.  The voice belongs to Jay L. McGrew, civil, mechanical and chemical engineer, former space scientist (he worked on heating and cooling design for the moon shots), and currently president of his own R&D company. Applied Science and Engineering, near Denver. 1

McGrew does not deny that more insulation will cause less heat loss; he simply argues that beyond a certain point, there are other, simpler, cheaper things to do to heating systems that will conserve far more energy than can be achieved by stuffing more insulation into the walls.  His data is not warmly received by the insulation manufacturers ("Johns-Mansville," says McGrew, "has lobbyists in every state in the union."); the gas companies are upset with his alternatives; and McGrew increasingly finds himself telling legislators drafting energy conservation codes that they simply don't know what they are talking about.  All of which tends to make his arguments into something of a one-man crusade, albeit a crusade with hard data.
For the last year or so, McGrew's company has been monitoring the energy use apd heat loss of existing housing in the Denver area. 2   The data gathered from this whole systems approach points to gas furnace design and operation as a prime area for improved energy conservation.  McGrew adds, "In terms of having the greatest impact on national energy conservation, the two simplest things to do are to adjust gas furnaces so they operate more efficiently, and to install room air circulators."

GAS FURNACES

Natural gas is used for more than half of the space heating in the U.S. The most common system is natural gas/forced air, in which the gas burns and warms a cast iron heat exchanger.  Air blown through the heat exchanger is warmed and continues through ducts into the house. These systems are cheap, safe and maintenance free, but their design criteria assumes natural gas to be cheap and plentiful, which has been the case until recently.  But no longer and increasingly less in the future.

McGrew claims a gas/forced air system is "marginal as a heat transfer device because it does not maximize the overall efficiency of the furnace."  For example, the American Gas Association (AGA), the industry's self-regulating body, rates the efficiencies of gas furnaces.  A typical figure of 75% efficiency means 75% of the heat produced goes out of the furnace into the building and the other 25% is lost up the gas vent stack.

The catch is that these ratings are for the furnace alone, not the whole system of furnace, blower, and duct-work throughout the house.  Also, the rating considers a furnace that is on and warmed up, although in actual use of course the furnace cycles on and off repeatedly — when it is off the heat exchanger loses its heat up the stack. Taking the whole system's efficiency into account, as McGrew has done in his Denver study, results in average efficiencies for gas/ forced air systems of 20 - 30%.

This kind of figure is what has the gas utilities up in arms with McGrew, but since they are a measure of BTUs burned versus BTUs delivered to a room as heat, they are much more meaningful than the AGA rating of an isolated furnace in a testing lab. 3   Adds McGrew, "A furnace contractor can tell you the AGA rating, but none of them are able to tell you the actual efficiency when the furnace is hooked up to the air ducts of a house." 4

What helps?  Turning down your thermostat does not affect the size of the flame in the furnace, and neither does closing heat registers in empty rooms.  McGrew's solution is to reduce the orifice size of the gas jet, which reduces the size of the flame. Turning down the gas flow would accomplish the same thing, hopefully getting more heat into the rooms than goes up the stack, but gas companies are — with some justification — horrified at the idea of Mr. or Mrs. Average Customer fiddling with the furnace.

Another way to increase efficiency is to cycle more air through the ducts.  Putting a larger diameter pulley on the fan end of the blower drive belt will help, or even adding a squirrel cage blower to force more air to circulate.

McGrew also thinks the average furnace is way oversized.  "Atypical furnace is designed to have a 1/4 duty cycle on the coldest day of the year," says McGrew.  "This means that on that day the furnace runs only 1/4 of the time. That furnace is 4 times too big — on the coldest day of the year it should be running all the time." The standard contractor's rule of thumb in sizing a furnace for a house is that the furnace should produce 100 BTUs of heat for each square foot of floor space.  McGrew believes 20 - 25 BTUs/sq. ft. of floor is adequate to do the job.

MAXIMUM INSULATION: 3-1/2 INCHES

Since manufacturing fiberglass insulation requires a lot of energy, McGrew is not in favor of squandering the final product.  Especially when a typical home has 60 square inches of holes intentionally poked through its insulated skin, all of them losing heat to the outsid'e.   (Furnace and hot water heater vents, bathroom vents, kitchen range exhaust hood vent, and clothes dryer vent.)

With these kinds of leaks going untended, and with gas furnaces wasting more heat than they deliver, McGrew's data indicates that it is just throwing money away to use more than 3-1/2 inches of insulation in either the walls or ceiling of a house. Three and a half inches in the ceiling doesn't even meet the FHA code minimum standard; and many new houses are now being framed with 2X6, instead of 2 X 4, walls specifically so they can carry thicker insulation in those walls.

Instead of modifying a poor design, or changing a careless construction practice, our consumer economy would always prefer to sell us a new solution, or at least more of something we already have.  Jay McGrew's measurements mean that an insulation sales pitch couched in the rhetoric of energy conservation may not be the whole story.

1.  Applied Science and Engineering, Star Rt. Box 96 A, Littleton, CO   80120.

2.  Applied Science and Engineering is always looking for more houses to monitor.  If you live in or near Denver and would like to know, what the figures are for your house, give them a call (697 - 4493).

3.  Comparing gas furnaces to other heating systems, such as electric, is also instructive.   McGrew's figures indicate that if you assume electric heat to be 100% efficient, and leave out the considerable energy required to make the electricity (the "gross source energy"), then a gas furnace is only 40 - 50% as efficient as electric heat in comparably insulated houses.   A recent Utah survey of Salt Lake City houses, which did include "gross source energy" in a gas/electric comparison study still gave the edge to electricity: 177,000 BTUs were required per square foot of heated space per year with a gas system, versus 154,500 BTUs/sq. ft./yr. for electric heat.   (Amory Lovins, what do you say?)

4.  Gas water heaters dont fare much better.   McGrew's estimates that 32% of the total energy used in a house in Denver goes to heat hot water, yet a recent study of 31 homes by his company concluded the average efficiency of gas hot water heaters in those homes to be only 46%.



1.   Applied Science and Engineering, Star Rt. Box 96 A, Littleton, CO   80120.
2.
Applied Science and Engineering is always looking for more houses to monitor.  If you live in or near Denver and would like to know, what the figures are for your house, give them a call (697 - 4493).
3.
Comparing gas furnaces to other heating systems, such as electric, is also instructive.   McGrew's figures indicate that if you assume electric heat to be 100% efficient, and leave out the considerable energy required to make the electricity (the "gross source energy"), then a gas furnace is only 40 - 50% as efficient as electric heat in comparably insulated houses.   A recent Utah survey of Salt Lake City houses, which did include "gross source energy" in a gas/electric comparison study still gave the edge to electricity: 177,000 BTUs were required per square foot of heated space per year with a gas system, versus 154,500 BTUs/sq. ft./yr. for electric heat.   (Amory Lovins, what do you say?)
Gas water heaters dont fare much better.   McGrew's estimates that 32% of the total energy used in a house in Denver goes to heat hot water, yet a recent study of 31 homes by his company concluded the average efficiency of gas hot water heaters in those homes to be only 46