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Question DetailsAsked on 6/6/2017

I am looking to replace my Carrier Condenser unit with the next size up, is this advisable? Advantages/Dis-advant..

It is a 2000 yr model Carrier unit that I think uses R-410A. Our house is approximately 1450 square foot with a vaulted ceiling in the living area, and insulated well with not so good Vinyl double pained windows. I wasn't planning on replacing the inside coil unless necessary, just looking for more efficient cooling and to save some money by not having to replacing the whole system.

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Messing with the configuration of an HVAC unit is almost certain result in lower operational efficiency, because you would have mismatched components - and generally your operating performance (heat removal) will be limited by the lowest capacity componetn -0 the evaporator coil in your scenario.

For other readers - in the Midwest and some of the deep South the "outside" unit is called the "condensor" while the inside coil is called the "evaporator", even though technically the "condensor" is only the outside condensor coil which the fan pulls air through to cool the refrigerant. Of course, to make it even more confusing, the fan and compressor motor starting capacitors are also called condensors by many people - who ever said English is not an interesting or challenging (and commonly very nonsensical) langauge ?

Anyway, putting in a larger capacity outdoor unit (compressor and condensor coil and control valve) would likely be a waste because the evaporator coil would most likely not be rated for that heat exchange capacity, so it could not remove that much heat from the air flowing past it. Also, because it would be undersized, the low side gas pressure would run too high so the unit would not run right. Therefore, the TXV valve would have to be adjusted (or might self-adjust in some models) to restrict the gas flow to achieve the specified pressure differential - essentially throttling the compressor down to the rating of the evaporator coil. Alternatively, depending on system specifics, the new unit might overdrive the evaporator, causing excessive cooling at the coil as too much gas is evaporated in it, causing coil freezeups and rapid compressor fallure due to freezing of moisture in the lines or the coil or the filter/drier. This is a very quick way to trash an A/C system.

Also - doing this mismatch (unless your coil happens to be rated both for your current unit and the next larger size - sometimes is, using a throttling/expansion orifice or "coil plug" at the inlet to restrict refrigerant flow for the unit rating) is illegal so technically no HVAC tech should do it, risking a $10,000 EPA fine as I recall for the mismatching parts on an energy efficiency rated unit, and also risking losing their EPA Section 608 certification - which risks a $37,500/day fine if they then continue to operate without that license. Penalties are similar to defeating the pollution controls on your car - and look how that went for pretty much all the major car manufacturers with their diesel emissions control defeat devices.

Doing this mismatch will almost certainly also void the warranty on the new outside unit - partly defeating your purpose of saving money because future repairs would cost more if you have any component failures in the warranty period.

Also - the inside coil now almost always comes with the new outside unit as part of the installation package anyway when you buy a new unit (which is commonly cheaper as a complete system than if bought just as a replacement outdoor unit anyway) - with the the evaporator coil comprising only a few hundred $ of the system cost - yet that is the component which very commonly goes out first and costs generally 1/3 to 1/2 the cost of a new system to replace, so getting and installing the new evaporator coil with the new system is probably the best bang for your buck of any component of the system, especially as modern coils have a disturbingly shorter life compared to the 15-20 years which used to be common before manufacturers went with the cheaper aluminum (which some are now moving back away from due to class action suits).

Bottom line - if your outdoor unit is shot on a 7 year old unit, either replace only the defective component with an identical rated one, or replace your entire system from end to end. (Though sometimes the refrigerant tubing is left in place and reused, though if only a short run through the wall and readily changed out I recommend changing it - or at a minimum at least have it throughly solvent flushed to clean the old refrigerant, oil, and metaillic compressor wear debris removed from it. Some new unit warranties REQUIRE new tubing, so check warranty before reusing the old.)

You can find a lot of previous questions with answers about repairing versus replacing A/C units, and the life cycle and economic factors affecting the decision, in the Home > HVAC link under Browse Projects, at lower left.

Answered 3 years ago by LCD

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