HVAC advice

[hovrens]

I added 365 sq ft to my already 1335 sq ft house. I have a 2.5 ton heat/AC system. I bought a new one about 1 year ago (before I new I was adding). I do not want to buy a new one until I see how inefficient my current 2.5 ton undersized unit will be.

Until then can I just continue the duct work another 16 ft and branch out to two bedrooms and a small bathroom?

Would a booster (inline) fan do me anything?

Is installing ducting like any other plumbing as far as sealing joints, adding tees/elbows?

[Rich]

Most ductwork will install end to end (sleeve type) and then screw the joints and put duct tape (not the grey stuff - metal duct tape). Some types don't even require screws just the tape.
I had mine sized for an 1800sf house - 4 tons. Although I don't know what your heat loss and gain is - so 2.5 tons might be just fine. Your biggest problem will probably be getting return air back from your new space.

[roger g]

Lack of return air is one of the biggest problems with forced air systems. Most houses have one or two return air openings with one maybe in the hallway then you close all the bedroom doors and wonder why they are so cold. A friend of mine who still doesa it the old way, he put s a return air grill in EACH room.


roger

[Rich]

Right on roger. If you can't get return air grilles inside the rooms of existing homes you can put a transfer air grille above doors (I've even seen them in doors). You will not get enough air from beneath and around doors to compensate.

[roger g]

Another problem is where the same ductwork for heating is used for airconditioning. Actually 2 problems or at least makes a person think. In heating mode (also a/c but we'll talk heating for now) the home owner is always told to clean the filter for proper airflow. Alos as an installer we are told to check to make sure of the proper temperature differnce across the heat exchnager. I forget what the temp is but not enough airfl0w and the exchanger could overheat and crack. Too much and you get air that is too cool and you might affect the condensation requirements of the design. Air flow, air flow and more airflow is what they talk about.
So now the homeowner wants an a/c coil installed inside the plenum. Take a look at the bottom of any "A" coil and see how small the opening is. We have to put in pieces of sheetmetal inside the plenum to make sure all the airflow goes through the A coil. Most time we cut off more than one half the airflow. In Canada we have big plenums with small tonnage a/c's which means small A coils.
I asked a furnace rep about all this airflow that wasn't happening not even including the restriction just going through the A coil. He just shrugged and said that it was all factored in. Yeh right! I might look like a cabbage but I don't think like one The other problem with the ductwork is that when figuring the heating load you must consider where the cold is coming from. Usually the north side. So in fact most of the heating should be on the north side. Nobody does this, they just make all the runs equal but it shouldn't be that way. Now you use the same ducting for a/c where the most heat comes from the south and that is where you want the most cooling. You can see the problems. Go into most houses in the summer and the winter and you can FEEL the problems.


roger