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Question DetailsAsked on 8/14/2015

Can a HVAC system be installed in a second floor crawl space?

I have a 1978 2 story slab home with electric baseboard hear and window A/C. The 2nd floor has 15' cathedral ceilings and a crawl space running the length of the front and rear of the home. Figuring if I can install a HVAC system into the crawl space I can run duct work (if significant cost difference) since I have access to the trusses from the crawl space. I am having windows replaced and my window A/C units will void my warranty. Is this possible and does it make sense with the cost factor?

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Sounds like you have an "attic truss" system or similar - full height second floor over maybe 2/3 the floor width, but at the sides where the roofline is low towards the eaves, unconditioned attic space - like this -

Assuming you have adequate working space there, say about 4+ feet high, it is possible and commonly done. However, it is NOT a "good" location for an air conditioner for several reasons:

1) if the ducting has to run up over the top of the attic room between the top chords of the trusses, right under the roof, the air in them will heat substantially, potentially greatly reducing the cooling effectiveness by the time the air gets to your registers.

2) the attic being a hot zone during cooling season, it greatly increases the operating cost of the air conditioning system, which is proportional to the temperature differential between ambient air temperature and the temperature you are cooling the refirgerant to. In a "normal" outdoor-mounted unit environment, you might typically be in the 90 degrees range outdoors inthe heating system and evaporating the refrigerant in the evaporator coil at maybe 45 degrees - a 45 degree differential say. In an attic it might be 120-135 degrees in the summer, for a temperature differential of 75-90 degrees - so your energy to produce the desired cooling can be increased by a very substantial amount.

3) Also, because the ambient temperature is hotter, it is a lot harder on the air conditioner - the compressor and fan are running a lot hotter, so wear out sooner.

4) Another effect that bothers many people is the noise of having the air conditioner in the attic.

Those effects can be reduced by enclosing the unit in an insulated and sound-proofed box, fan ventilated with outside air to avoid the hot attic air and with thermostatic shutoff if the enclosure overheats, and with the condensor fan vented directly to the outside at a different location than the incoming air vent, being careful that the hot exhaust air does not jsut go right up to the eaves and back into the attic space.

Bottom line - can be and is commonly done, but not the recommended solution.

One other possibility to consider - an outdoors package as usual for the compressor and condensor, but instead either running ducting from an air handler on ground floor or outside and ducting from there (eiother through walls or in an exgternal insulated chase mounted to the outside of the wall), or more commonly running the refrigerant tubing to the cooling demand locations with individual wall-mounted fan-driven evaporators (usually high on the wall) in those rooms. Commonly called a mini-split unit - google that phrase to read about them. Because you are only running tubing, also tend to be a lot less invasive then running ducting, and also generally have individual thermostats so you have individual room control of the temperature.

Another alternative, though rarely done on second floors unless you have a balcony or deck they can be serviced from, is direct through the wall units - basically window air conditioners but installed in holes through the walls.

Once you have read up a bit on air handler and duct units (which commonly use smaller round ducts running at a higher airflow velocity so less invasive to install) and mini-split units and wall-mounted units, then you will be ready to talk to a few Heating and A/C contractors (your Search the List catgeory) about their recommendations and costs.

Answered 4 years ago by LCD

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