Tigerdunes brought up demand defrost versus timed defrost. For the past 18 years, I have dealt with my Lennox defrost scheme. The only times I heard it go into defrost mode were when the unit was first running in heat mode. The unit would switch over from heat to defrost, the defrost cycle would complete and everything would turn off. I never heard it run a defrost cycle when it was not first heating. I rarely heard it go into defrost. I never heard it defrost on a fixed cycle such as every 90 minutes. Am I to understand that a Carrier will just go into defrost mode every X minutes whether it needs it or not and is not in a heating cycle; just turn on and defrost itself? I found this information in the Carrier 25HNB6 heat pump installation instructions: AUTO defrost adjusts the defrost interval time based on the last defrost time as follows: When defrost time When defrost time 3-5 minutes, the next defrost interval=90 minutes. When defrost time 5-7 minutes, the next defrost interval=60 minutes. When defrost time >7 minutes, the next defrost interval=30 minutes. The control board accumulates compressor run time. As the accumulated run time approaches the selected defrost interval time, the control board monitors the coil temperature sensor for a defrost demand. If a defrost demand exists, a defrost cycle will be initiated at the end of the selected time interval. A defrost demand exists when the coil temperature is at or below 32_F (0_C) for 4 minutes during the interval. From that description, if the time interval elapses and the coil is not at 32 degress, then no defrost will take place. That, to me, sounds like a form of demand defrost. Is Baldloonie saying that the coil could be at 32 degrees or less without frost and, therefore, a defrost cycle would be unnecessary? What kind of ambient sensor detects presence or absence of frost? Neohioheatpump asked, "When your old A/C would run, was it producing water?" It was producing water and I saw it dripping from the condensate drain. One weekend I went out of town and came back to find water on the floor around the return box that the fan coil sits on. I opened up the access and found the interior insulation of that box soaked. Turns out the condensate line was plugged. Took awhile for that to dry out. On the other hand, this past summer I did not see water dripping from the condensate drain. The drain line was not plugged and the drip pan seemed dry. I thought it was because I had dried out the air so much with the two dehumidifiers that there wasn't enough remaining for the A/C to remove. Cooling was normal. Running the fan on manual started with the installation of the Lennox and an associated Honeywell Electronic Air Cleaner. The house suddenly became dusty and I blamed the EAC for not doing its job. The Honeywell zone representative came to my house, sized up the situation, and told me to run the fan on manual so that the dust would remain suspended in the air and available to be captured by the EAC. I did that and it was of no help for the dust situation but it did help reduce the temperature differential between the first and second floors during cooling so I kept doing it as a matter of routine. Thus, I cannot say that not running the fan reduced the humidity. Though I complained about humidity, the heat pump contractor never told me that I was humidifying the house by running the fan (on orders from the Honeywell rep). I will continue to run at least one dehumidifier in the completely below grade basement during the summer because I want that area to be warm and dry to avoid mold and mildew. I've been in the basements of other houses in the area that don't use dehumidifiers and they stink. Mike Home said, "You stated in an earlier post that you run the fan in the summer because the upper floor is 2-3 degrees hotter than the lower floor. I think adding the attic insulation will help this." I agree that adding attic insulation could help. But, having a Dutch Colonial house with a barn-like roof that is covered in dark shingles on the east and west sides causes a lot of heat gain through the second floor walls. I'm not keen on opening the walls to determine the amount of insulation or add to it. Changing to a light-colored roof would make the house look out of character with the neighborhood. There are newer shingles that reject heat gain that I will investigate. Mike Home also said, "You could also zone the first and second floors, but this is expensive and probably not feasible with your current duct work." There is but one main feed duct and it runs through the basement. There is no way to segregate the room feeds in this scheme without doing each one individually. I'll leave that expense to the next owner.

Thank you for the replies. I’ve been off on a cub scout campout so have been out of pocket. I will try and answer the questions. The unit that is failing is a 4 ton R22 unit – model SSX160591AA It was installed in 2012, with no indication that a new coil was installed at the same time. There was also installed at the same time a separate 3 ton unit R22 unit – model GSC130301EB. Together these units cool the first floor of the house. So it seems like in 2012 the prior owner chose to replace the condensers only, and had to do it with R22 units to avoid replacing the coils? Which is a little strange because one year earlier, in 2011 a new unit was installed for the upstairs. It is a 5 ton R410a unit – model ssx160591aa . The receipt shows a new coil was installed at the same time. All these units support individual air handlers (three of them). For heat there is a boiler that circulates hot water to the air handlers. Last year I had a lot of ductwork work performed – the HVAC company I worked with says the system does not have enough air intake capabilities, so they vastly expanded all that. It made a noticeable difference. They also reconstructed the upstairs ducting so that there is a return path for the heated/cooled air (it is a dual zone system and did not have a return path). I liked the work they did, but I also worry that it was their work that led to the premature looming death of the compressor. They had the coils and air handlers out to redo the ducting. Thank you for providing answers and perspective. I just ordered a big book from Amazon on HVAC to try and educate myself a bit. Any recommendations of books or websites is appreciated.

I have purchased a number of older homes, and this sounds like par for the course for me. All this "money pit" and "walk away" stuff is perplexing. If you want a new home, then get one. If you want one from 1960, it is going to have systems near the end of their lives and some items to repair. Anything already disclosed or already visually obvious is off the table. No Seller is going to rebuild the deck for you, or replace a roof that is not leaking. You are buying what you see. The only reason for inspection is to reveal unknown issues you will have to deal with in the future. I'd ask for repair of the electrical issues because those can be safety issues. Then evaluate the cost of a new roof and a new deck, compare that with the cost of a similar house in perfect new repair, and ask for some reasonable dollar amount or amount off the purchase price. If you cannot afford to repair these items, think twice about an older home.

What causes excessive condensation in your air conditioning? https://www.heating-air-conditioning.org/ So what exactly is causing this excessive condensation in your air conditioning system in the first place? There are a number of factors that could help explain this phenomenon. For starters, this excess condensation could be due to bacteria found in your air conditioner that is creating a clog inside the drain. This clog causes the condensation pan to overfill, and leak water. Another contributor to this excess condensation could be the condensation pan itself, which perhaps is rusted or damaged and is causing water leaks in your homes. Another cause of excessive condensation in your air conditioning system could be a dirty air filter. This is why regular AC maintenance and air duct cleaning is so important. Your AC's dirty air filters could be blocking the airflow over your system's evaporator coil. This causes the coil to freeze, and when it melts, it could be dripping water into the evaporation pan, leading to water leaks.