Letter to HVAC Contructor

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:

For modulated (small-step, multi-stage variable speed) furnace and modulated AC (small-step, multi-stage variable speed compressor), the “true” VARIABLE-SPEED ECM air-blower motor is needed to accommodate variable flow rates (speeds) and variable torques (static pressure and variable powers) based on pre-programed and sensors’ feed-back parameters.
For 2-stage furnace and 2-stage AC, is it possible to pre-program or set different blower speeds for 1st stage (for say, 66% of heating or 66% cooling ) power and higher blower speed for 2nd stage (100% power, would be similar to variable modulated settings for 66% and 100% modulation), or the flow rate is the same for 1st and or 2nd stage, or it is optional setting by installer?
If the size of condenser and evaporator coils (heat exchangers, HX) are chosen to match 2nd-stage, maximum 100% heating/cooling capacity, then they will be oversized for 1st-stage (say 66% cooling/heating capacity) or even more oversized for minimum modulated power (33-40%). Lower than maximum thermal-power (1-stage heating or 1-stage cooling) should be balanced by lower air-blower speeds and/or by smaller-above refrigerant “saturation temperature” above outside over-condenser-air temperature, and/or smaller-difference below refrigerant “saturation temperature,” below inside over-evaporator-air temperature (thus much higher 1-stage efficiencies for "oversized" HXs, about, 1-stage 23.5 EER, 2-stage 13.5 EER for 17 SEER TC17 unit), since thermal heating or cooling correlation is Q=A(HX)* [T(R)-T(Air)]*H(air-speed). Heat-eXchaner coil area, A(HX) is sized constant for 2-stage (thus oversized for 1-stage), and indoor air temperature is desired to be constant (about 75F), so over-all heat transfer coefficient, H, may be varied by blower air flow (i.e., air-speed), or temperature-difference of conditioned air-to-refrigerant in evaporator (e), Ae-Re (usually 30F=[Ae(75+55)/2-Re(20+50)/2]=65-35=30F), and temperature-difference of refrigerant-to-air outside in condenser (c), Rc-Ac (usually 30F=[Rc(150+100)/2-Ac(105+85)/2]=125-95=30F). The T(R) is average refrigerant temperature inside HX coil (inside evaporator or condenser respectively).
If TXV (Thermostatic Expansion Valve) modulate refrigerant flow (to maintain desired refrigerant-superheating) and evaporator pressure (refrigerant temperature inside coil) it should be beneficial for dehumidification? Otherwise 1st-stage lower cooling power (all else being the same) will increase evaporator temperature and reduce dehumidification at constant air flow (oversized coil for 1st stage, say 66% cooling), unless air flow is reduced accordingly (lower set speed for 1st-stage and programed variable speed for modulated variable cooling) to prevent saturation-temperature increase in evaporator. Therefore for improved dehumidification a lower air flow is better (350 CHM/ton) in humid climate as compared to 400 CFM/ton standard (good in dry climate) and even lower air flow for 1-stage?

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