I have a 1300 sq ft 42 year old house and recently purchased three very efficient window air conditioners with digital temp readouts on them. One is 12000 btus and the other two are 6000 btus each. The reason I bought three is to better distribute the air among the house and I try to keep the house at 76 degrees when I am in need of cooling. I cannot install central air right now because I have a hydronic heating system. Anyhow the larger unit cools the kitchen, living room and dining room but does not run long when it is on, about 5 minutes max. I know air conditioners are more efficient when they run longer. Does anyone have any suggestions to help me with this problem? Thanks!