H V A C S y s t e m D e s i g n
Your HVAC system can do some or all of the following: heat, ventilate, cool, humidify, dehumidify, or filter the air in your room. But it may also transmit the noise of rushing air, the creaking of hot water pipes, the rumbling of motors, compressors, and burners. And even when off, it may provide a flanking path to defeat your soundproofing efforts.
In a forced air system, conditioned air should enter the room slowly enough in order to reduce air movement noise. A good design picks a happy medium between the quiet of slow moving air and the required volume of air movement for the temperature and ventilation requirements of the room.
In a low air speed HVAC system, cooling ducts can be placed in the ceiling or high on the walls so the the heavy cool air drifts down over the space naturally. And of course heating ducts on the floor or low on the walls will let warm light air drift up into the room. These duct placements eliminate the need for rushing air to be forced against the natural convection currents in the room.
To slow the air just before it enters the room, we can specify still air boxes in which rapidly entering air is dispersed and distributed more slowly through a number of ducts. A still air box can also help reduce noise transmission both in and out of the room.
If you are custom building a room, you may wish to consider either hydronic or electric radiant floor heating; it's very comfortable and should be quite quiet if designed and built properly. Because the surface area of the floor is so large, a floor temperature as low as 75-80 degrees Fahrenheit can evenly heat the room. In winter, radiant heat does not contribute to low humidity as does forced hot air. Also, because radiant heat eliminates the need for radiators and ducts, the acoustic layout of the room and optimal system setup need not consider these obstacles.
Radiant floor heating may actually reduce energy costs too. Some of the heat radiated by the floor directly warms you without warming the air in the room first, thus reducing your body's heat loss. Therefore you may feel warm in the room even though the actual air temperature may be a bit cooler.
There are a couple of disadvantages to radiant heat. When you return from an extended vacation, if you set the temerature down while you were away then the radiant heating system will need more time to warm the room because of the larger thermal mass involved. Also, care must be taken during construction and any subsequent renovations to protect the heating tubes or wiring in the floor from being punctured. Needless to say, it is very important to have an HVAC contractor with experience and a good track record with radiant heat in order to get a proper installation if you decide to go this route.
You may also wish to consider radiant cooling, particularly if you live in a dry climate. When a conventional air conditioning system cools air that is both hot and moist, the excess water condenses and drips into a drain. While there will be some dehumidifying with radiant cooling, you can hit what is considered the dew point while lowering the temperature and not dehumidify the air. So with radiant cooling, the moisture will remain in the air. This means that in humid environments it is possible that it might leave the room feeling somewhat clammy. In dry climates however this isn't a problem because the air contains no excess of moisture.
If you are considering a radiant system, we strongly suggest that you contact an experienced HVAC contractor and visit one of their installations to see how you like it. One aspect of ventilation that should be taken into consideration is indoor air quality (IAQ). If you utilize either a Counter-Flow or a Cross-Flow air-to-air heat exchanger then you can bring fresh air in without affecting the indoor air temperature that much when either heating or cooling. A Counter-Flow air-to-air heat exchanger typically operates at about an 80% efficiency. Whereas a Cross-Flow air-to-air heat exchanger typically operates at about a 75% efficiency--though the advantage is that these are usually physically smaller units. For instance Zehnder manufactures various units and has a good educational website so that you can learn more with regard to maintaining good IAQ. If you would like to control the humidity and temperature of a room, then you need either an air conditioning unit with dehumidification capabilities or a separate dehumidification system when it is hot outside. Conversely if it is cold outside then you may wish to add a central humidifier in order to maintain the humidity at a recommended level of 35% or so. Given an overall tight building spec where everything has been optimized, a well-engineered HVAC installation may be able to hold temperature within +/- 1.5 degrees and humidity within 2% at a cost of never shutting the unit off. Though with a system design like that, because the compressor is always on, the operating costs may be substantially higher. For a state-of-the-art HVAC system, it is actually possible to hold temperature to within 3/10 of a degree Fahrenheitt and 1% humidity, though a laboratory grade system at that level certainly isn't needed for the average residence! If you are interested in learning more about HVAC here is a book which you might be interested in: A House Needs to Breathe…Or Does It?
Each HVAC project is different. Lifestyle considerations, performance issues, budget, and personal preferences mandate a customized approach to each individual job.
In a forced air system, conditioned air should enter the room slowly enough in order to reduce air movement noise. A good design picks a happy medium between the quiet of slow moving air and the required volume of air movement for the temperature and ventilation requirements of the room.
In a low air speed HVAC system, cooling ducts can be placed in the ceiling or high on the walls so the the heavy cool air drifts down over the space naturally. And of course heating ducts on the floor or low on the walls will let warm light air drift up into the room. These duct placements eliminate the need for rushing air to be forced against the natural convection currents in the room.
To slow the air just before it enters the room, we can specify still air boxes in which rapidly entering air is dispersed and distributed more slowly through a number of ducts. A still air box can also help reduce noise transmission both in and out of the room.
If you are custom building a room, you may wish to consider either hydronic or electric radiant floor heating; it's very comfortable and should be quite quiet if designed and built properly. Because the surface area of the floor is so large, a floor temperature as low as 75-80 degrees Fahrenheit can evenly heat the room. In winter, radiant heat does not contribute to low humidity as does forced hot air. Also, because radiant heat eliminates the need for radiators and ducts, the acoustic layout of the room and optimal system setup need not consider these obstacles.
Radiant floor heating may actually reduce energy costs too. Some of the heat radiated by the floor directly warms you without warming the air in the room first, thus reducing your body's heat loss. Therefore you may feel warm in the room even though the actual air temperature may be a bit cooler.
There are a couple of disadvantages to radiant heat. When you return from an extended vacation, if you set the temerature down while you were away then the radiant heating system will need more time to warm the room because of the larger thermal mass involved. Also, care must be taken during construction and any subsequent renovations to protect the heating tubes or wiring in the floor from being punctured. Needless to say, it is very important to have an HVAC contractor with experience and a good track record with radiant heat in order to get a proper installation if you decide to go this route.
You may also wish to consider radiant cooling, particularly if you live in a dry climate. When a conventional air conditioning system cools air that is both hot and moist, the excess water condenses and drips into a drain. While there will be some dehumidifying with radiant cooling, you can hit what is considered the dew point while lowering the temperature and not dehumidify the air. So with radiant cooling, the moisture will remain in the air. This means that in humid environments it is possible that it might leave the room feeling somewhat clammy. In dry climates however this isn't a problem because the air contains no excess of moisture.
If you are considering a radiant system, we strongly suggest that you contact an experienced HVAC contractor and visit one of their installations to see how you like it. One aspect of ventilation that should be taken into consideration is indoor air quality (IAQ). If you utilize either a Counter-Flow or a Cross-Flow air-to-air heat exchanger then you can bring fresh air in without affecting the indoor air temperature that much when either heating or cooling. A Counter-Flow air-to-air heat exchanger typically operates at about an 80% efficiency. Whereas a Cross-Flow air-to-air heat exchanger typically operates at about a 75% efficiency--though the advantage is that these are usually physically smaller units. For instance Zehnder manufactures various units and has a good educational website so that you can learn more with regard to maintaining good IAQ. If you would like to control the humidity and temperature of a room, then you need either an air conditioning unit with dehumidification capabilities or a separate dehumidification system when it is hot outside. Conversely if it is cold outside then you may wish to add a central humidifier in order to maintain the humidity at a recommended level of 35% or so. Given an overall tight building spec where everything has been optimized, a well-engineered HVAC installation may be able to hold temperature within +/- 1.5 degrees and humidity within 2% at a cost of never shutting the unit off. Though with a system design like that, because the compressor is always on, the operating costs may be substantially higher. For a state-of-the-art HVAC system, it is actually possible to hold temperature to within 3/10 of a degree Fahrenheitt and 1% humidity, though a laboratory grade system at that level certainly isn't needed for the average residence! If you are interested in learning more about HVAC here is a book which you might be interested in: A House Needs to Breathe…Or Does It?
Each HVAC project is different. Lifestyle considerations, performance issues, budget, and personal preferences mandate a customized approach to each individual job.