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Problem: High air conditioning bills! What could be the cause?

Problem: I am looking for advice on a rental apt(900 sq ft) unit I own.  This long time tenant is complaining of very high electric bills  over the last couple years.   While we have had a milder summer, I did notice that this tenants ac would not be running when all the others would.  She said she was trying to keep the electric bill down.   The unit is all electric.  Trying to isolate the problem.  Had an electrician come in and he indicated that the ac and all other appliances are using the correct amount of ampage that they should.  The ac was checked this spring.

Going to call the ac guy back out.  Had a problem with the condenser in june 2014.  He had to pull the fan and rewire the condenser.  I have a print out of their electric charges from 6/13 thru 7/15, and graphed them.  Their electric costs in the summer months have doubled.    Their winter cost have also gone up, but they have not doubled.  Ameren here charges more per kwh in summer than winter.  I am wondering if the rewiring of the condenser could cause this problem, and would it affect the electric furnace in any way to cause an increase in winter months.  I am possibly looking to replace ac as it is over 20 years old.  Not sure if I will have to replace furnace.  Is there anything else that I could be checking – just don’t want to replace the ac and find it is not the problem.  Electrician said to possibly have the meter checked.  We verified with Ameren that it is the correct meter (no switched meter).  Ameren said that they rarely have a meter go bad and when they do the meters usually go slower not faster.
Thank you for any suggestions you can give and I found your blog/website the most helpful in explaining things. Stacey
Answer:  Hi! Thanks so much for your kind words and interest in our website! There are several reasons why a person can get a high cooling bill. I will try to explain some of the reasons below. High head pressure from a dirty coil (not enough air flow across the outdoor coil) or a condenser fan motor/ fan blade that is not sized right for the unit can cause an electric bill to go up because the compressor is working hard to condense the refrigerate. A dirty coil or not enough air flow across the outdoor coil can cause the unit to not cool well because it raises the temperature of the indoor coil as well as the outdoor coil. Rule of thumb is that the air coming out of the top of the outdoor condensing unit should be 30 degrees hotter than your ambient temperature. So on a 90 degree day the air coming out of the top of the unit should be 120 degrees. Low refrigerant charge or a refrigerant over-charge can cause a cooling utility bill to go way up. I would suggest that you make sure that the out door unit is clean. I usually turn the power off to the unit by shutting of the electricity to the unit and use a garden hose to wash the coil out. If possible take the fan top off and wash from the inside out. This should remove a lot of the dirty from the coil. I would suggest that you have an AC tech check the pressures of the low and high side with gauges to determine if the unit is charged properly and if the unit is running at normal pressures in comparison with the outdoor and indoor temperatures. I would suggest that you have the HVAC service tech check the amp flow on the unit with an amprobe and see how many amps of electricity the unit is using while running. The AC has a label on the side of the unit and the unit should not run over or most of the time not close to the FLA rating (Full Load Amp Rating). Many times high cooling bills can be caused by poor duct work design. The duct work might not be sized properly for the air conditioner or the duct work can be gaining heat from holes in the duct… maybe in the attic. Please make sure the duct work is sized properly for the air conditioning size you have installed. A good HVAC company should be able to do a heat gain and heat loss calculation for your rental home and tell you what size air conditioner and duct work should be installed for the home. Yes, there are many reasons that can cause a cooling bill to be high. I hope you can find out the problem without a lot of expense. I also hope that you have a great day! Steve Arnold,