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This is an air conditioning question:

Must a 4 ton variable speed air handler use a 4 ton heat pump or can a 3.5 ton heat pump or smaller size heat pump be used instead?

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    This sounds like a homework question.
    – Tester101
    Nov 13, 2017 at 3:59
  • Thanks Tester. Just asking the question for my dad who is trying to fix his home following weather damage. Simply looking for some guidance to make sure he isn’t buying a more expensive heat pump than is needed.
    – Andrei
    Nov 13, 2017 at 6:38

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The question you ask about a "4-ton air-handler" is that you can expect the air-handler to produce enough air supply to support a 4 ton A/C unit requiring about 1600 cfm. The actual air flow is dependent upon the total resistance to air flow in the whole system. Since the motor supplied is a variable speed or more probably a 4 speed motor, then you could utilize a lower speed for a reduced air flow. The actual air flow needed to support an A/C unit is not an exact science but is an approximation of the air flow needed. So, to answer your question "yes", you can use that air-handler on an A/C unit smaller than 4 ton. The 4-ton is the maximum size A/C you should use

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  • I agree can't go smaller but larger will work.
    – Ed Beal
    Nov 13, 2017 at 13:58
  • the 4 ton air-handler will work on a maximum 4 ton A/C. It will also work on units somewhat smaller than 4 ton. You said 3.5 ton and I say "no problem".
    – d.george
    Nov 13, 2017 at 18:00
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It depends on the details of the system of interest. Some brands will communicate and limit the air handler to the available capacity, some will assume matched capacity and underperform like a traditional system, some will display an error code signifying “system configuration not supported”, some will just refuse to run and display nothing. It may be helpful to understand what happens in a system with an oversized indoor air handler.
An air handler is just a coil, which is surface area for heat exchange, and a blower which presents a mass flow of air to the heat exchanger. The rate of heat exchange depends on the temperature difference and surface area. Since the temperature difference depends on the mass flow rate changing the blower speed changes the rate of heat flow. A variable speed blower system can do that for itself.
A traditional (dumb, single speed compressor) system with an over-sized indoor unit can be right-sized by changing the blower motor pulley to reduce the blower wheel ROM and with it the air mass-flow rate or by using ductwork balancing dampers to choke the airflow rate to reduce it. If nothing is done to change the air handler’s heat flow rate the condensing unit will fail to deliver completely liquid refrigerant in cooling mode when the outdoor temperature rises enough to reduces the rate of heat rejection at the condenser below the rate of heat absorption at the evaporator. The gas bubbles in the refrigerant will reduce the volume of liquid refrigerant passing through the expansion valve limiting the refrigerant mass flow rate available for evaporation. That is to say, it works fine until it gets hot outside then it starts delivering lukewarm air.
In heating mode, the indoor coil is the condenser and the outdoor coil is the evaporator. The indoor condenser will always deliver a completely condensed liquid to the evaporator, the compressor can’t deliver enough hot refrigerant gas to heat the entire coil at any time and the supply air temperature coming into the indoor rooms may always be lukewarm depending on just how oversized the indoor air handler is. The lukewarm effect might tend to limit the lowest outdoor temperature the system is comfortable to use at, but it will also help it defrost more rapidly in defrost mode.

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