The house really is a system

September 12, 2000
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Here’s the problem facing the housing industry: We can’t predict the IAQ performance of a house until after it’s built. However, every problem has a solution, and this is no exception.

The house is a system, and we could predict its IAQ performance if we had the right information. We need information on the components that have a major impact on IAQ, such as combustion equipment and ventilation equipment. But in place of information, we have confusion.

The confusion became dramatically apparent at a recent meeting of the Home Ventilating Institute (HVI). HVI is the trade association for manufacturers of residential ventilation equipment. HVI’s main business is improving indoor air quality. Ventilation is our business, and that’s what ventilation does — it improves IAQ.

HVI members expressed frustration over confusing theories connecting ventilation with house depressurization or flue backdrafting. We’re in business to improve IAQ, but we’re being told we’re actually harming the quality of the air.

There is no reason for this confusion, unless the issues are scrambled. If we examine the issues, we find three parts: combustion products, ventilation, and IAQ. Then the parts can be logically arranged:

IAQ is the objective, so it’s at the top; then both ventilation and combustion are sub-issues of IAQ.

The difference is that they are on opposite sides of the IAQ balance. Combustion has the potential for adding contamination. Ventilation has the potential for decreasing the contamination. Combustion is a negative and ventilation is a positive. HVI members found this all logical, but confusion remains.

Builder's frustration

From a builder’s viewpoint, it is even more frustrating. Contractors produce a house by combining all the components. The house is required to perform within certain limits.

Problems arise because the builder doesn’t have enough component information; he can’t predict the performance of the house. He has to wait until the house is finished.

That’s not right.

In an ideal world, component manufacturers would furnish exactly what the builder needs. The builder would combine that information into a plan that would accurately predict the home’s performance as a system.

Admittedly, IAQ is complex, but this part of it can be simplified by addressing the HVI group’s frustration. The focus is on combustion products, ventilation, and building pressures.

The writers of building codes and guidelines have attempted to prevent flue backdrafting by limiting houses to 5 Pascals of negative pressure. But that doesn’t work for at least two reasons:

1. It is impossible to achieve and maintain a house at less than 5-Pascals negative pressure.

2. Even if 5 Pascals were achieved, there is no assurance backdrafting would always be prevented.

Positive pressure isn’t the answer either; homes in cold climates must never be pressurized positively, lest moisture be driven into the walls. While numerous forces work on a house to produce positive and negative pressures, contractors are expected to build a house that will remain within a range of 0 to -5 Pascals.

Five Pascals is a very small range of pressure. It’s about 0.020 in. wc. Think of how little suction it takes to raise water 0.020 in. in a drinking straw; 5 Pascals is less than 0.001 lb/sq in.

Have you ever tried to prove by measurement that a house is between 0 and -0.020 in. of pressure? It’s very difficult. Building a house guaranteed to stay between 0 and -5 Pascals pressure is impossible. Proving it is another matter.

Should we be concerned about house pressures? Yes, because house pressure may affect our ability to control combustion products. We must be concerned because we don’t want CO poisoning to kill anyone, or even make them sick. The issues of combustion products in the home are valid because of the threat of CO poisoning.

CO poisoning: some statistics

Houses are tighter; we hear more about combustion product spillage than we did 15 years ago; but is there a recent outbreak of CO poisoning?

The U.S. Consumer Products Safety Commission reports CO poisoning caused 214 deaths in the U.S. in 1993 (the most recent year available). The report doesn’t indicate how many were due to equipment problems, unvented devices, and cars.

The Gas Research Institute (GRI) reports less than 30 deaths annually are related to pipeline gas in a recent years.

That’s serious, but another statistic addresses the epidemic question: Between 1983 and 1993, there was a 34% decrease in CO deaths. Houses are tighter; paradoxically, CO poisoning deaths decreased. Is it appropriate to sound the alarm?

I suggest that CO must not be taken lightly, but the trend does not justify panic.

Lack of info about equipment

When we begin to analyze the control of combustion products for CO safety, we soon become stymied by the shortage of useful information. Let’s talk first about equipment.

Some parties warn about spillage from the water heater flue; yet the burner in the gas oven doesn’t even have a flue, and it’s located right in the kitchen, where people frequently congregate.

The gas range is located there, too. Manufacturers of kitchen range hoods know that a vented hood will remove 80% or 90% of gas range combustion products. But that information is not widely disseminated. (Representing a range hood manufacturer, I don’t feel particularly good about that.)

A recent article talked about unvented fireplaces, and I quote: “ . . . (the unvented fireplace) returns combustion products to the home (possible only with clean-burning fuels such as natural gas).”

I doubt any of us would extend that logic to water heaters and say it’s OK to spill combustion products from a water heater because it’s fired by clean-burning natural gas. Factory-built fireplaces are said to have sealed combustion. However, I’m told that UL tests require glass fireplace doors to leak a little because the glass needs cooling air.

Does every atmospheric furnace have sufficient flue draft? On the other hand, in a tall house, what do we do if the flue creates more than 5 Pascals of negative pressure? Can the furnace backdraft the water heater?

There is flow; clothes dryers build modest pressure but move about 100 cfm. And there is pressure; vacuum cleaners build pressures up to 100 in. wc, but at lower flow.

Speaking of equipment, I haven’t even mentioned ventilation; these issues exist even without ventilation. In fact, they’re worse without ventilation.

Natural forces disrupt the balance

Besides equipment, there are always natural forces at work on a house that are beyond anyone’s control; that’s why I doubt the 5 Pascal limit can be maintained, even if established initially.

Wind creates negative pressure on the leeward side of the house. The strength of the pressure is proportional to wind velocity squared. If the wind goes from 10 to 30 knots, the negative pressure on the leeward side increases nine times. That can’t be controlled.

Open a window on the leeward side, and negative pressure quickly develops inside the whole house. Then it takes even less wind to exceed -5 Pascals. How do we deal with that?

Every house has a natural stack effect; the pressure in the basement is always lower than the second floor. Air leaking through second-floor ceiling fixtures lowers the pressure at the furnace in the basement.

Some have tried to provide information, but it isn’t complete. ASHRAE has information. And C.Y. Shaw, Ph.D., of the National Research Council of Canada, has published some. The Ontario Code a few years back tried to mandate a pressure-neutral house, resulting in the famous “Ontario Opening.” Why was that dropped?

There’s a shortage of component information, and there are outside forces that can’t be controlled. Those are the reasons for the problems.

There is an important relationship between combustion products and IAQ.

IAQ is gaining importance

IAQ is gaining in importance because public awareness is growing. People hear about Sick Building Syndrome at work and carry that awareness home. When there is no indoor smoking the odors aren’t masked, and people think the air needs improvement.

Increasing respiratory complaints have sharpened public awareness of IAQ. Documented increases in asthma and allergies may be related to increases in contaminants. Medical people are working on the causes, but we’re not ready with the solutions.

There’s more to IAQ in a house than control of pressure. Let’s examine some other issues, without making the discussion too complex.

There are many contaminants; excess moisture is the most common. Up to this point, I’ve not stressed the need for ventilation, but it is essential for the control of moisture and other contaminants. There must be a balanced approach; if pressure is the overwhelming issue, up to the point of sacrificing good ventilation, it works against IAQ.

Good IAQ depends on both careful management of combustion products and excellent ventilation. With combustion only, we have only the problems, the negative side of the balance. We need the positive effects of ventilation.

IAQ is a very old problem. Let me read from an old law book about how to deal with houses that have been affected by poor IAQ caused by excess moisture:

The priest shall then order the house to be cleared out lest everything in the house become infected. If the priest, on examining it, finds the infection on the walls of the house consists of greenish or reddish depressions which seem to go deeper than the surface of the wall, he shall quarantine the house for seven days.

On the seventh day the priest shall return to examine the house again. If he finds that the infection has spread on the walls, he shall order the infected stones to be pulled out and cast outside the city.

The whole inside of the house shall then be scraped, and the mortar that has been scraped off shall be dumped outside the city.

Then new stones shall be brought and put in the place of the old stones, and new mortar shall be made and plastered on the house.

If the infection breaks out once more after the stones have been pulled out and the house has been scraped and replastered, the priest shall come again; and if he finds that the infection has spread in the house, it shall be pulled down, and all its stones, beams and mortar shall be hauled away outside the city.

It’s the law of Moses, Leviticus chapter 14 (selected verses, but no words changed).

Moisture problems are worse now because the windows have glass, which disturbs natural ventilation. Therefore, we must provide mechanical ventilation.

HVI agreed upon a plan

Earlier I mentioned the HVI ventilation equipment manufacturers discussing their frustrations. You can now sense those frustrations. You can also sense the frustrations of others such as builders and code writers.

The HVI ventilation manufacturers agreed on several points, briefly stated:

  • The house is a system and builders need information to be able to plan.

  • Ventilation makes a positive contribution to IAQ.

  • Most combustion equipment can affect IAQ negatively.

  • Manufacturers of components must work together to communicate.

No single organization has the information needed to plan a house as a system. HVI decided to take the lead and adopted an ambitious, three-part plan:

Part 1. Convene and organize meetings of a joint consortium of the entities involved. There are representatives for combustion equipment, flue equipment, kitchen appliances, ventilation equipment, and others.

Part 2. Establish a primary, overall objective. (We expect something like, “Our objective is to develop and disseminate information so that builders and contractors can plan the house as a system regarding IAQ.”)

Part 3. Develop and disseminate the information defined by the objective.

Anticipated steps for developing the information are described in the following paragraphs.

Some of the information is logically developed in sequence. At present the following steps are visualized.

Step 1. Establish participants’ areas of agreement. Possible items of agreement include the house is a system; combustion products in the house can be a problem; we must agree upon allowable levels.

Step 2. Characterize the sources. Establish information for the combustion equipment found in houses. Included will be furnaces, water heaters, factory fireplaces, site-built fireplaces, wood stoves, gas-fired kitchen ranges, ovens, and any other equipment of that type.

Step 3. Define the transport (flue) factors. Establish information for flue equipment, and how it relates to the combustion equipment. Included will be metal flue products and masonry flues, the equipment which transports combustion products to the outdoors.

Step 4. Define the effects of ventilation. Establish information for ventilation equipment and how it relates to the preceding. Included will be exhaust fans, range hoods, balanced ventilation equipment, ERVs, clothes dryers, built-in vacuum cleaners, and similar equipment.

Step 5. Define the effects of weather. Establish information for the relationship between all of the above and climatic forces, considering all climate zones.

Lively discussions are expected; we hope they will be fruitful. The purpose of the consortium is to bring the people with the information together under one common goal — to make it possible to plan a house as a system.

I’m excited about this plan. Just imagine:

  • Key equipment manufacturers working together, sharing and coordinating simple and essential information;

  • Information being made available to builders and contractors all over the country;

  • Builders planning a house with the information, knowing in advance that the quality of the air in the finished house will be good — all the time; and

  • Contented homeowners living in a house with the air quality they expected.

All of that is possible if those with the information work together . . . and if everyone else encourages them to work together!

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