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Have Hot Air Heat.. Want to add Central Air.

I presently have a 2yr old Forced Hot air Furnace and was told I could not install central air utilizing my present ducts as they are "too small"? If they really are, should the duct work be done by the Central Air installer or my Oil heating service?

  • January 22 2008 - US
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Answers (9)

This sounds like BS to me.  The air handler (fan) is typically part of the furnace unit.  If the ducts are big enough for heat, why not A/C?  I'm not an expert, but I've installed two furnace/central air systems.  Is your furnace gas?  It may be different for oil.  I would get another quote/opinion in any case.

  • March 18 2008
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Existing ducts can be too small, see for example

 

 

  • March 19 2008
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Existing ducts can bee too small, see for example Adding Central Air Conditioning To Existing Homes

  • March 19 2008
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Though it may be possible to move more air through small ducts, the static pressure increases, decreasing the efficiency of the system, and making much noise.  Depending on how much change in temperature you are trying to make from the ambient temperture, you will either need the supplied air to be a larger difference in temperature, or move more air.

 

Also, some heating systems don't have return air ducting (especially if the house was older and originally had gravity heating).  The return air is more critical for cooling; though in either case there should still be a minimal amount of outside air for normal air changes.

 

So, the biggest question is how many BTU of cooling do you need, and how many BTU of heating do you need, and what CFM level are you planning on delivering?

 

I'm not a mechanical engineer so I would recommend you have a mechanical engineer look at your situation for proper sizing.

 

Also consider vent locations; heat naturally rises so venting heat high may not get the heat where it is needed; and cool air natually drops, so placing cool air near the floor may not get the cooling distributed where it is needed unless it is being drawn up in the equipment you are trying to cool.

 

  • March 19 2008
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  • gvw3
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I had central air put in my home about 10 years ago. At that time my home had 4" ducts and only one return. They made a new plenum so the air would go through the A coil. It worked ok.
A few years ago we had a new system installed that has a return in every room and 6" ducts connected to a large run that ran the length of our house. All new furnace, A coil, Condenser. It work ok to. I really don't notice that much of an improvement.

  • March 19 2008
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How many CFM?  Are you sensitive to noise?  Perhaps you just became used to it?

 

With only one return for the whole house, did the house really cool uniformly, especially if doors were closed?

 

  • March 19 2008
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  • gvw3
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You need a 1" gap on bottom of the doors. This allows for air flow. A good freind has been in heating and air for about 40 years told me that was the way they did it years ago. It seemed to work fine.

 

Since that time my nephew his grandson has redone our system to todays standards. Cold air returns in all rooms, 6" ducts and in place of the plenum a whole house run. I really didnt see a big improvment. This is the Chicago area so we dont see to many really hot days.

'

  • March 20 2008
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shortstuf,

 

There are significant issues other than human comfort when central AC is retrofitted to a building without adequate provision for return paths back to the air handler, for example excessive moisture and mold growth, I've seen them at home inspectons here in chicago, and they are even worse in hotter, moister climates, see for example THE PROBLEM OF PRESSURE - TAMARACK TECHNOLOGIES, INC.

  • March 21 2008
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If the ducts are efficient to heat the home they will be effiecient to cool the home. However, you will have to have someone do a load calculation to determine the cooling load of the home. Once you know what your cooling requirements are you can size the cooling coil properly in order to deliver the temp air needed to cool the home. In rare cases the cooling demand might be so great that no matter how low the delivered air temp it may not be able to meet the cooling requirements of the home. So in essence the ducts would be "too small". But again a cooling load calc will be needed to determine this.

For air conditioning usually the supply air is 55F. Now if the ducts arent large enough to deliver enough air at 55F to cool the home you have two choices. 1- You can over size the cooling coil to deliver cooler air (45-50) or 2 - You can adjust the fan speed to deliver more air. To do this you either need to change the shive on the fan pully or change the motor because the fan will need to overcome the increased static that will be created when you try to push more air through the ductwork. This could make the ducts noisey but depending on the rating of the ductwork and the velocities it may not be an issue. The easiest solution is to drop the air temperature. Just make sure the condensing unit is sized correctly to give you the design temp.

Now one thing you really need to think about is whether the ductwork that exists is insulated. If the ducts arent insulated warm air will condense on the outside of the duct and you'll have it raining in the ceiling and walls..............not good.

As far as the return air goes if the home is already set up for hot air heating then cooling shouldnt be a problem. Assuming the air makes it back to the system fine now and it was designed correctly it shouldnt matter whether the air going back to the air handler is warm or cool.

Hope this helps

-LS

  • March 27 2008
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