Heating and air conditioning systems

In a sealed system if you blow air out, fresh air will be sucked in to compensate (or else you end up with a vacuum and the house implodes)
A 'simple' heat exchanger works by letting the out going and incomming air flow past eachother separated by a thin metal finned wall so the warm outgoing air heats up the cold incomming air (or vice versa).
There are more sophisticated systems but hopefully you get the basic idea.
Del

In a sealed system if you blow air out, fresh air will be sucked in to compensate (or else you end up with a vacuum and the house implodes

Yes, but imagine how clean the rooms woud be!

The Conditioned air from the heating/cooling system having its Temperature, Humidity and Air Cleaniness modified displaces the air cirulating in the room. Due to change in density therefore, the air in the room go back through the return duct of the cooling system and so.

Hi mac5777,

I have a little confusion on your question "how does heating and cooling system bring in fresh air?"

If I get you right, you want to put air-conditioning unit in your attic that is fine split type or window type, however if you install split type unit and you want to have at least 5 % - 15% of fresh air to come inside you need to install and intake fan whether ducted or from the wall then of course you need also to have ventilating exhaust to exit the amount of air from the intake fan.If you put window type unit you will not be needing to install intake fan because most of window type unit has fitted with fresh air intake and exit damper. (hope you have no plan to put chilled water type).

when somebody told you by "exchanger" maybe he is referring to the evaporator unit as he did not elaborate that it is only for the purposed of making your room cooled by means of air passing to it output will become cool air depending of course on thermostat set point, but mustn't be fresh air that is coming from the outside as it will not passing through evaporator coil.

Kind regards

Roman

Doesn't the exhaust from the furnace (heating portion) have to be exhausted from the house? How does replacement air get in if it is a sealed house. My house is very old and very leaky so it is not a problem for me, but in a sealed house how does that work?

Doesn't the exhaust from the furnace .

These days they usually have a 'balanced flue'. That is two concentric tubes..the inner lets out the combustion products (fan assisted) and the inner draws in the air supply for the burner.

Del

Well of course I knew that Del. And in fact the warm combustion air pre-warms the cool intake air for greater efficiency.

I guess I misread the question, not realizing until told that it was new construction, I thought somebody had gone and sealed up a house where an older unit without exterior intake was an existing situation and the guy was wondering where the air was coming from. I incorrectly deduced that there was no separate intake or exhaust or he would have seen it and not bothered to ask a self-evident question.

the answer to the question in short is that it does not bring in fresh air. The same air is circulated through the furnace and the a/c cooling coil that is in the house in a closed loop. To bring in fresh air a duct can be induced into the heat a/c duct work or directly into the house. the heat a/c system will bring in some unconditioned air from the outside due to the creation of a vacuum in the system when it is running. there are systems that exchanged the air and condition it before it enters the system. these are underused because of their expense and lack of understanding. a fresh air intake of some kind should be used, since it will bring in fresh air from the outside when kitchen or bath exhaust fans are being used. without this opening the house becomes a vacuum and the air that must come from somewhere will come in form the attic or crawl space where it is not as fresh. there are systems available to bring in fresh a some more expensive than others.

Thanks for the help. I'll focus on the details. This is for new type of construction on the US west coast. Plans are to create the high R value by the use of foam insulation in stud-less, hollow walls and a non-vented attic. Specs say the attic will not exceed but 1 to 5 degrees from the living space below. And the equipment can be down sized to save some money. Now to get moisture and oxygen depleted air out and fresh in from the units located in the attic is required. Can an exchanger handle both the heating and air conditioning side? I will be getting info from professionals but I thought this site has always come up with good advice and things to look out for. Thanks again!

The most efficient heating equipment as Del has said will have both an intake from and exhaust to the outside. It will not use interior air at all. Older and less efficient models but still available will require a certain room size to provide adequate air to the furnace.

Here's a typical powered (has a blower motor) residential system.

http://www.popularmechanics.com/home_journal/how_your_house_works/1275121.html

http://www.residential.carrier.com/products/airquality/ventilators/index.shtml

Hi, mac5777!

Confusing as it seems, the object of the sealing game is NOT to prevent the flow of fresh air through the building. It is to prevent undue heat loss. Building codes require a certain air exchange rate for every room of the house, and they mean fresh air! There are as many ways of getting that air in and out as your imagination will allow.

In some cases the air is conditioned through limited access points to prevent heat loss. In other cases, just leave a window open a crack. If the building is carefully sealed, the heat loss (or gain) will be limited. And when the window is closed, it will be even further limited.

Sealing prevents unintentional points of heat loss like inadequate insulation, condensation, air translation points through walls, around windows & doors, etc. from stealing BTUS and your money to keep you comfortable at home.

Mark

Hvac in a home does not bring in any fresh air, unless you duct it in from the outside. The absolute negative pressure generated by the intake equals the absolute positive pressure. Note: I wrote ABSOLUTE. You can do that or to be more energy efficient, investigate an ERV or ERU by such manufacturers as Honeywell, CanFab or Lennox.

In residential and commercial HVAC systems, generally there is a small intake with it's own filtration system that is ducted to the outside. The intake(s) often has a damper in it to control the amount of outside air sucked into the system. For commercial systems that damper is often under servo control and in emergencies (such as a fire, or if there is a chemical release outside) the dampers can be set so that 100% outside air or 100% inside air is used, depending on the circumstance. The HVAC system becomes part of the fire protection system. After 9/11, many have been upgraded to contemplate biowar and chemwar scenarios as well.