Letter to HVAC Contructor

See also: HVAC, Home Appliances & Utilities: Energy Units * hvac(101)*Johnson-Cs*tm9y(Inst.)*tc17(Inst.)*ecobee*Superheat-Subcool*

2021 July 14

Co: My old friend, Pega Hrnjak, Distinguished Research Professor and Director of “The Air Conditioning and Refrigeration Center (ACRC)” at University of Illinois at Urbana- Champaign (https://acrc.mechse.illinois.edu/)

Hi Scott (HVAC Contractor) and Phil (Distributor Technical Advisor),

I want to downsize my AC from 3.5 to 3 ton-refrigeration and have both Energy-Star, 2-stage AC and keep 2-stage furnace as agreed before

This is to discuss another option of HVAC systems and to “demonstrate usefulness of downsizing (with proper control) of AC system” and lowering equipment cost compared to standard sizing, while improving comfort and energy efficiency?

I want to discuss the following:

I am still unsure about the possible options for multi-speed and variable speed blower ECM motors for 2-sage furnace and 2-stage AC:

NOTE that if the evaporator cooling is impeded (choked) the compressor will still have to dissipate its power to the surroundings by increasing its condensing-unit temperature (and refrigerant pressure in the condenser), functioning as an electro-mechanical heater without any refrigeration effect (if evaporator fully impeded), until it its protection shut-off or until its overheating failure due to melting of its components.

NOTE also that proper refrigerant charge in AC system is very important to provide required super-heating in evaporator and sub-cooling in condenser. Low refrigerant charge will increase super-heating and decrease sub-cooling, and high refrigerant charge otherwise, thus reducing performance and efficiency. 

Let us discuss possible control options for 2-stage Furnace and 2-stage AC systems.

Thank you.

Best Regards – Prof. M. Kostic

www.kostic.niu.edu