HI I may be replacing my 20+ year old Bryant split a/c ...I'm in a 50+ year old house and the original design was 2 seperate 2 ton zone systems ....wouldnt it be more efficiant if I can tie the duckwork together into one Big single zone system ? Is there any tricks used in the industry to tie the return vent and supply vents into one single zone ? If so are there things I need to look out for in terms of balancing the branches both verticaly and horizontaly ? Any tips you could offer or if you know of any web links where someone has written about doing that with their old house would be appreciated. The reason I'm thnking about replacing is because my upstairs system blows hot air ...ive checked the outside condensor,compressor,relays,capacitors as well as the indoor coil,fans and electirical...everything seems to run fine except its just blowing room temperture air. The last thing I intend to do is clean out the indoor coil with a strong vacumn and some of that coil cleaning foam. Does anyone have any thoughts about what the problem might be and if they think that coil foam stuff works ?