New Coil

Jeff Lawrence_24

Professor,

I have a customer that needs a new furnace. We want to replace the furnace and the evaporator coil and wait until spring (or later) to change the outside unit. The condensing unit is about 17 years old and I want to put in the highest efficiency evaporator coil that I can.

The existing coil is a regular, run of the mill builders grade coil, just like the condenser is. I don't think it will hurt anything to install the new coil on the old condenser, but I'd like your opinion.

I even think his power bills may go down a little because of the higher efficieny coil even with the old (7 SEER?) condenser because the increased surface area and the TXV.

Thanks.



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Eugene Silberstein 3

Great Idea But be Very Careful

The idea behind replacing the coil at the time of furnace replacement is awesome and I definitely give the nod to that. The system will operate more efficiently with the larger evaporator coil and the TXV. But, be very careful when charging the system.

The older system was a low efficiency system with a fixed bore metering device. This means that all of the refrigerant is circulating through the system all the time when the compressor is operating. When you install the new evaporator coil and TXV, the refrigerant flow will be modulated by the evaporator superaheat. In times of low evaporator load, the TXV will close, causing the refrigerant to back up in the condenser coil.

Since the condenser coil is not rated at higher efficiencies, the effective surface area of the coil will be greatly reduced as more and more of the coil is being used as a receiver.

Since there still a fair amount of nice weather down in Georgia, I would go ahead and do the replacement, but be sure to keep the condenser subcooling as close to the middle of the range (18 degrees) as possible.

It is very easy to overcharge systems with TXVs if the condenser subcooling is not closely monitored. WHen you get close to the proper charge, be sure to add refrigerant in 4 ounce increments and allow the system to operate for a few minutes between additions. I know this is difficult in the trenches and very easy to say in the classroom (clock ticking, customers waiting and all), but do the best you can.

Also, make certain that the customer is aware of what you are doing in the event, however small it may be, that you are not the one doing the condensing unit replacement in the spring.

Regards to the family.