(C) Daniel FriedmanVentilation Design Issues for Tight Homes

  • VENTILATION DESIGN PROBLEMS & SOLUTIONS - CONTNTS: What are the design issues & design solutions for ventilation systems for tight or energy efficient homes? Building ventilation supply air & return air problems & solutions. Building indoor air pressure drop during ventilation. Building indoor air ventilation system noise problems & solutions. Ventilation airflow controls. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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How to properly ventilate tight homes:

This article describes common design issues in making a successful building ventilation system, providing solutions for various building venting and moisture problems with supply air & return air problems & solutions, building indoor air pressure drop during ventilation, building indoor air ventilation system noise problems & solutions, and building ventilation airflow controls.

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Ventilation Design Issues - part 2: Supplies & Returns, Pressure Drop, Noise, Airflow, Controls

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. The sketch shows a very basic building ventilation defect - lack of under-roof venting - and its role in causing ice dams. More subtle is the question of indoor air quality and the building design and mechanical systems that affect that concern.

Readers concerned with good design for indoor air ventilation systems should see INDOOR AIR QUALITY & HOUSE TIGHTNESS andINDOOR AIR QUALITY IMPROVEMENT GUIDE as well as VENTILATION in buildings.

This article reviews the design issues that can make or break a residential ventilation system. The text below paraphrases, quotes-from, updates, and comments an original article, "The Importance of Ventilation, Part II", from Solar Age Magazine and written by Steven Bliss.

Building Fresh Air Supplies and Air Returns

According to Gautam Dutt, of Princeton University's Center for Energy and Environmental Studies, natural infiltration probably doesn't ventilate houses very well. The basement may get the freshest air, drawn in by the stack effect (warm air rising in a building and exiting at high points draws outdoor air into the building from openings at lower levels on the structure). Other rooms in the building may not see much fresh air.

A fresh air distribution system can guarantee good air ventilation throughout the building. This article provides basic design principles and installation tips for fresh air ventilation systems.

At VENTILATION in BUILDINGS we provide a detailed guide to building ventilation, and at INDOOR AIR QUALITY IMPROVEMENT GUIDE we provide in-depth technical detail about the identification and cure of indoor air quality problems, including details about the whole array of house ventilation approaches.

While it makes complete sense to identify and remove significant indoor air quality problem reservoirs such as mold contamination or smelly carpets, furniture, or even animal waste that can explain bad indoor air quality, an efficient fresh air distribution system in a building will also remove lower levels or more subtle air pollutants at their source and will deliver fresh air to the prime living areas.

In practice this means putting exhaust air pickups in rooms that produce moisture and odors, and fresh air supplies in bedrooms and living rooms. Rooms with intermittent use, such as a den used occasionally for entertaining, might have an exhaust pickup that can be opened when needed. (For more information see VENTILATION, EXHAUST ONLY).

Keep fresh air supply registers (see VENTILATION, SUPPLY-ONLY) away from return air registers to avoid short-circuiting of air streams. (This makes sense for heating and air conditioning registers as well.) It's also important to locate air supply diffusers where they won't blow cool air on the occupants. In Europe many window-mounted air inlets are taped over by disgruntled tenants. good air supply locations are near ceilings, in hallways, and in closets (provided the closet has a louvered door).

A closet fresh air supply can help assure that clothing stored there will smell fresh, and properly designed a closet supply can also avoid moldy clothing in these sometimes damp, un-heated storage areas, especially when the closet is located on an exterior all.

Open building space plans that promote good flow of heated or cooled air in a building will also improve the flow of fresh air indoors. When air is supplied from centralized locations such as a hallway, undercut the bedroom doors so that air can enter those spaces.

In central fresh air systems, the kitchen will need an exhaust pickup, plus a recirculating-type range hood. Keep the central kitchen exhaust pickup several feet away from the range so that it doesn't get greased-up (a fire hazard).

Building Indoor Air Pressure Drop Due to Ventilating Systems

During the heating season we don't want positive pressures inside the building as that may push moisture into building walls. Some Montana homeowners, for example, have reported frozen door locks due to unbalanced heat exchangers that pressurized indoor air, sending moist air flowing out through door lock openings.

But we don't want large negative pressures in indoor air either. There is evidence that negative indoor air pressures can be dangerous, not only drawing increased levels of radon gas from soils where that contaminant is present.

But also negative indoor air pressure can cause dangerous backdrafting of flue gases from (atmospherically-vented) heating appliances, even causing the production of potentially fatal carbon monoxide.

Yet it is difficult to maintain a neutral indoor air pressure with balanced airflow.

In the original Ventilation II article, Mr. Bliss argues that a slight negative pressure is preferable except where Radon is a problem. This will avoid forcing indoor moisture into problem areas and may actually help dry out building cavities. This also means that small exhaust-only air ventilation systems are probably fine. To keep negative pressures minimal, exhaust-only house venting systems should have intentional openings (a central duct or through-wall diffusers) to supply makeup air (return air).

But with a tight, negatively pressured house, you simply can't have combustion appliances without outside combustion air. (See COMBUSTION AIR for TIGHT BUILDINGS).

Whatever ventilation approach you take, make sure that you look at the whole house as a system, and that the path of least resistance for makeup air is not down a heating flue.

See VENTILATION, WHOLE HOUSE STRATEGIES for a detailed review of alternative house ventilation strategies.

Solving Noise Problems with Building Ventilation Systems & Fans

Low-energy buildings tend to shield out street noise better than standard houses. But in an otherwise quiet house, noisy fans sound still noisier. Many an exhaust ventilation system has had its plug pulled to silence the hum.

Look for quiet fans. Manufacturers rate their fans' noise levels in sones. One sone is roughly equal to the sound of a quiet refrigerator in a quiet kitchen. Unlike the decibel noise scale, sones are additive, so 2 sones is twice as loud as one, and so on. The average (noisy) 100 cfm bathroom fan rates at 3 to 5 sones. Look for a fan in the 1.5 to 3 sone range.

In central fresh air ventilation systems, do not locate the fan directly above or below bedrooms. If possible, create a sound-transmission break between the main fan unit and any metal duct runs (which we recommend because metal ducts can be cleaned), using a short length of flexible duct.

Also, special "muffler" sections can be added to dampen the sound from a ventilation system.

Suspend the fan unit or air handler in such a way that it will not transmit its vibrations to the frame of the building. This is usually done by setting the fan unit on foam-covered slats that are hung from overhead joists or rafters.

Advice on Providing Fresh Air at Heating Furnace or Air Conditioner Duct & Air Handlers

Is it a good idea to provide fresh makeup air to a house ventilation system by connecting outdoor air through the return air side of a furnace or central air conditioner air handler for a building that is also ventilated by an exhaust-only air system?

This seems fine, if the exhaust fan is running constantly, or if the fresh-air damper is power-actuated and wired to open only when the exhaust fan is running. But running a furnace fan with the vent open and the exhaust fan off will over-pressurize the building.

How about plugging your air-to-air heat exchanger into the furnace? This, too, is difficult for similar reasons. Besides, in a building that is tight enough to need an air-to-air heat exchanger, the furnace will be off much of the time. Take a look at Heat Recovery Ventilation if you are considering this approach.

Solving Problems with Fresh Air Flow & Building Fans - How Much Air Can the Fan Move

Not all fans are created equal, but all fans move less air as more ductwork is added. Each fan has a unique performance curve that describes how much air it moves at a given static pressure - the resistance to air flow caused by the ductwork.

Make sure that the fan you select can deliver the cfm (cubic feet per minute) through the ductwork you plan to install. Not just duct length but cross-section, diameter, interior smoothness, and critically, bends and turns have a big impact on the static pressure the fan will encounter. The static pressure of a duct system can be measured by an HVAC contractor and by some home inspectors, or you can learn how to measure static duct pressure yourself using references from a variety of sources including the HRACI of Canada.

When selecting a ventilating system or exhaust fan, look for a relatively flat fan performance curve. That means that the fan doesn't wimp out as soon as you add a few feet of duct. Choose smooth ducts, short runs, and avoid many turns and sharp turns in the ductwork. Many air ventilation system installers don't get the airflows they expected because they installed cheap fans and poorly designed ductwork.

Manual vs. Automatic Control of Fresh Air Ventilation Systems - Building Owner Controls

Both fully automatic ventilation system controls and fully manual controls are available for vent systems. Mr. Bliss suggests providing both types of controls: install a ventilation system that can work fully automatically, but give the building occupants simple manual overrides.

The most common automatic ventilation system controls include clock timers and dehumidistats. The dehumidistat (or humidistat) responds to changing ventilation needs by sensing the indoor moisture level. They are not foolproof however. In very dry climates they may under-ventilate, and in wet climates or swing seasons they may over-ventilate the building. So the occupants may want to be able to make a little seasonal adjustment to the controls.

Timers and fan/light switches work well in bathrooms. For the kitchen exhaust, the good old manual on-off switch may be best.

Some designers recommend running the ventilation system constantly,at a low speed, speeding the system up when needed, usually for kitchen and bath use. This is fine if the system is quiet and was sized correctly, and it can make for more even, comfortable indoor conditions.

Effective, Quiet House Ventilation Systems Mean Homeowner Will Use Them

Virtually everyone who has worked around ventilation systems, starting with simple bathroom vent fans, can tell you that noisy systems tend to be shut off and simply not used by building occupants. We have seen landlords hard-wire the bath fan so that it will always be on when the bathroom light is on, trying to force tenants to blow high-humidity air outside. We have also seen tenants using a lamp and extension cord to provide bathroom light (possibly dangerous) rather than tolerate a loud bath exhaust fan.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article

"The Importance of Ventilation, Part II" - links to the original article in PDF form immediately below are preceded by an expanded/updated online version of this article.

Ventilation Design Articles


Continue reading at VENTILATION, EXHAUST ONLY or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see INDOOR AIR QUALITY & HOUSE TIGHTNESS for readers concerned with good design for indoor air ventilation systems

Or see INDOOR AIR QUALITY IMPROVEMENT GUIDE as well as VENTILATION in buildings.Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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