Scale build up on coils, causing heating problems?

[weldon]

I service the units at a printing company. The indoor coils on several of the smaller units have scale and corrosion build up on them that I have been unable to remove through cleaning. I have saturated the coils with cleaner, brushed the fins, cleaned them with a high amount of water back flushing them the opposite way the air flows and used industrial compressed air. Non of which seems to work. The cleaner I am using is mild and safe for aluminum. Corrosion was present before I started servicing units. I was hoping scale could be cleaned out without removing the coils. The two units with the most scale are 3-ton heat pumps. One is a Trane the other an old Amana. The Trane keeps running in emergency heat mode when ambient temp drops into the low 30's. The Amana frosts completely over (not frozen) if the ambient temp drops into the low 30's. I have made leak repairs to 5 of the 22 units at the plant. These are two of the units, leaks were repaired 6 months ago systems were pulled into a deep vacuum using slight nitrogen charge to pull multiple vacuum while also using heat gun to release any refrigerant in compressor oil as well as low areas in the lines. New refrigerant was weighed back in; pressures and superheat were well within specifications. With units switched in cooling mode and 90 degree air temperature around units was simulated system pressures and superheat were still within specifications. Could the scale build up be causing the emergency heat to run too much on the Trane? On the Amana could the scale build up be cause the unit to work harder and create the frost so quickly that the defrost controls cannot keep the unit clear? What needs to be changed to decrease the time span between defrost cycles and increase the time of the defrost? The Amana has an unadjustable defrost timer that appears to have been rewired although with no wiring diagram I am unsure how to wire it to original specifications. Any help would be greatly appreciated.

 

[spector]

Be sure to check with the guys in the hvac-talk.com forum. They can best help with your questions.

 

[Yeldud]

Having a heat pump switch into backup heat mode in the high 30's does not sound unusual, especially if it is an older unit, which can't operate at as low an outdoor ambient as some of the newer heat pumps. I assume switchover occurs due to a low outdoor air temp lockout thermostat, so that device may also deserve some scrutiny, if the units are trending towards switching to backup heat mode at higher temperatures than previously observed. If the lockout thermostat is adjustable maybe it's not set correctly or out of calibration. Sounds like the Amana is trying to operate in heat pump mode at too low of an outdoor temp, and is beating itself up. I would also check air flow rate on this unit, to make sure it's within spec. Fouled fins would cause loss of performance in both a heating and cooling mode, and because performance is normal in cooling mode, I doubt this is a significant contributor to abnormal heating mode operation. Just two cents worth from a consultant but certainly not a heat pump expert.
regards, Yeldud