General question on forced-air heating/cooling


I've never had a forced air system. I'm looking at an apartment which has forced-air heat and a/c. Do these systems pull in fresh air from the outside or just recirc air in the apartment? I ask because the windows in this unit aren't going to make it simple to stick a fan in and do my own airing out of the apartment.
Thanks...Pat
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Patrick Maloney wrote:

There's usually a fresh air inlet as part of the forced air system, but most of the air is recirculated.
You can always just open a window, no?
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Does the fresh air aspect apply to both heat and a/c?

Well, my point was that, due to the layout, there is no cross ventilation. Also, they're not double-hung windows (slide sidways), so getting a fan into one wouldn't be easy.
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AC doesn't use air for combustion--no need to get outside air. 80% efficient furnaces use inside air for combustion--it (combustion air) gets replaced by sucking air into the house anyway possible--through windows, cracks, outlets etc. 90+% furrnaces use outside air for combustion. All air is recirculated, eliminate drafts. MLD

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MLD wrote:

In most newer (i.e. tightly sealed) buildings, there isn't enough air exchange through walls, doors, and windows. Google for 'sick buildings' of 1970s and 80s. So most newer buildings have a small intake on the return air to provide fresh outside air.
Depending on the age of the building, it could have fresh air intake or not.
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You should ask either the super or possibly the municipal building permits office. In Ontario, Canada, the building code was changed approx. 1980 to require outside air intakes for all forced-air furnaces (and my house was retrofitted, but I do not know if this was mandatory on the (government) landlord.)
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Don Phillipson
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Albany, NY area
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Is the fresh air for general circulation or just for combustion? Many codes call for outside air for combustion in the heat exchanger, but nothing on the house side that is circulated. That may have changed also though.
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not likely that the hvac system can do a fresh air change in most apartments, depending on the climate and sometimes on who pays for the utilities, of course. see also: http://www.buildingscience.com/resources/mechanical/default.htm
Patrick Maloney wrote:

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buffalobill wrote:

I've owned and lived in many homes here in the northeast USA and have never seen one that has an outside air intake for mixing with the conditioned air. Newer homes do tend to have air intakes to bring in air for combustion. They do this to avoid sucking air from the basement, which in turn leads to conditioned air being lost, and cold outside air being drawn in to the rest of the house.
Unless the house is built for high energy efficiency and specifically sealed extra tight, the house will have enough leakage that bringing in air is not necessary. In fact, folks go to great lengths to seal off significant sources of leakage to avoid wasting energy. On the rare occasions when you do need to ventilate, like after burning something on the stove, just open a window or door.
If you do insist on venting, then the best way is to have a heat exchanger that transfers air from the outgoing air to the incoming, recovering most of the energy that would be lost if you just open a window or pump in outside air. Houses that are super sealed for energy efficiency use these.
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