Your questions answered: The future of single-phase variable speed heat pumps

During the September 13 webcast, attendees were walked through an overview of mini VRF heat pump technology.

By Consulting-Specifying Engineer November 22, 2023
Courtesy: CFE Media & Technology

The decarbonization movement has placed single-phase heat pumps at the forefront of both residential and light commercial building design. Higher efficiencies, lower operating ranges and multizone capability of mini variable refrigerant flow (VRF) has long been a viable alternative to conventional small split heat pumps. That said, the native VRF controls options have been limited in the past.

Looking into the future, VRF systems continue to increase as a popular choice providing a flexible, versatile solution that is ideal for a wide range of structures and industries. This will help mechanical engineers identify how VRF technology can help build better air in building design.

During the September 13 webcast, attendees were walked through an overview of mini VRF heat pump technology, explored various heat pump applications, learned about controls integration and review the advantages of the Lennox Real 1.5 ton to 5-ton product line.

Register for the on-demand version of the “The Future of Single-Phase Variable Speed Heat Pumps” webcast here.

Additional questions are answered by Ronald Roos, Manager of Engineered Sales at Lennox Industries.

What is the availability of replacement parts in the central San Joaquin Valley of California?

Answer: The ease of obtaining replacement parts is a complaint I often hear from HVAC contractors and techs.  It’s much easier and quicker to obtain parts from other products.

Are these products considered Made in the US for federal projects?

Answer: They are not however we have an exception letter to address this concern and we have successfully applied Lennox VRF at military and federal installations across the US.

How do ductless mini splits comply with ventilation code? For outdoor temps down to -20degF, do the heat pumps lock up? or do ductless mini splits have emergency resistance heat?

Answer: IMC 402 and 403 instructs means of ventilation through forced air mechanical means or natural ventilation. The Real ODU’s have the ability to meet the needs of spaces but will require a fresh air source introduced into the space which could be one of multiple different options to meet this need. At -20F we cannot confirm operation but we can confirm that we do not have a low ambient heat pump lockout. That said it is recommend that auxiliary heat be included either as a gas furnace for dual fuel or electric resistive heat.

Based on your slide, is the maximum heating operating temperature of 70 degrees F. sufficient for heating loads in cold climates so comfort can be achieved?

Answer: The max heating temp of 70F is the ambient outdoor temp at which the system will continue to produce heat. The indoor temp can we set above 80F if the occupant so desires but the design conditions will ultimately dictate the systems ability to achieve the designed heating indoor team.

What happens to performance when temperatures reach the upper specified limit of 118F?

Answer: This varies by outdoor unit, the capacity tables on have in depth information. As an example the 3 ton single fan model has 36MBH capacity at 113F, at 118F the capacity drops to 31.6MBH noting a 12.3% reduction in top end capacity. There is a derate and the system would be designed to meet peak heating demand at design conditions.

Please let us know about the actual heating capacity at -13°F for a 3-ton unit.

Answer: VPC036H4M-3PD will deliver 24.3MBH at -13F. More info can be found at

Do these units run off of 120 vAC or 220 volts or? How might these units be added to existing, conventional AC and Furnaces? Cost comparison to conventional AC + Furnaces vs Heat Pump Only? What outdoor temps would require supplemental resistance heating?

Answer: 208V 1 Phase. Merely replace the conventional HP or AC unit and confirm power and pipework can be re-used. Cost will depend on SEER of traditional system but as a rule of thumb the Real ODU’s are on par with 18SEER conventional HP’s. I would typically recommend supplemental heating for designs below -10F or based on desired internal design conditions. Note elevation and pipe length will impact performance.

Only 2 refrigerant pipes from ODU? No simultaneous heating & cooling?

Answer: Correct. This is not heat recovery.

Is there an option/accessory for any of these systems provide “heat recovery” style operation rather than heat pump style operation? I.e. indoor units can operate in heating or cooling mode independently of the mode settings of the other indoor units.

Answer: Great question. There is no means of enabling heat recovery simultaneous heating and cooling operation outside of using a different heating source. Example is furnace coil dual fuel system with an additional VRF ductless head. Furnace will take over primary heating and the wall mount if a call for cool exists will receive cooling but not heat recovery based cooling and heating.

Do any of the REAL or other systems feature an “inside-building” compressor so that a dhw de-superheater can be implemented without concerns about freezing of any “dormant” water during winter?

Answer: This is a refrigerant based system and has no means of DHW applications.

When you say -13 F Outdoor:  can I get INDOOR Space to +78 F WITHOUT supplemental electric heating? If not – what?

Answer: Yes you can if designed to meet your space load condition by selecting the Real ODU to meet your load at that condition.

Does your control algorithm for the condenser unit utilize some predetermined ECM set fan speeds on the AHU, or would an infinitely variable AHU fan provide advantages?

Answer: The condenser algorithm operates independently of the IDU therefore the internal thermostat will vary the ECM fan speed and in turn will provide advantages as it relates to temp control and efficiency.

Will these systems change the typical square footage a given unit size will satisfy?

Answer: Example: I have a 3,200 sqft spray foamed home that is fully zoned, the customer is currently happy and comfortable, however the customer wants to add on an additional 600 sqft.

With the variability of these systems, would it be possible to replace the condenser with a real condenser and add an additional air handler for the added 600 sqft, in this scenario the 5 ton system would be operating in a 3,800 sqft  spray foamed home?

Answer: The impact would be minimal between a conventional outdoor unit and the Real’s performance in this instance. Please reach out to us at to verify the impact for you and confirm that for you.

What happens below the guaranteed ambient? does it trip off? or just decline in capacity?

Answer: Declines in capacity but continues to run. It would only trip out on head pressure once the outdoor conditions drop well below the rated -13F temp.

What size circuit breakers are needed?

Answer: Varies by ODU.

Are there mixed air options for when a DOAS would be larger than the OA requirements for the room?

Answer: Yes, we have the ability to adjusted the mixed air accordingly and size the VOSB in that manner.

How much is the heating capacity of the exterior units de-rated at an ambient temperature of -25 Deg Fahrenheit?

Answer: Testing was only done down to -13F.

How will humidity and comfort be controlled in a traditional, existing Furnace/Coil application with the real, since blower speed cannot be controlled?

Answer: Blower speed is controlled by the thermostat which in this instance would be a conventional thermostat.

What is the average efficiency or COP that we see in these units under normal cold climate operating conditions (say ~40 F)?

Answer: Small tonnage single phase heating performance is measured in HSPF. AHRI is the best source for this info however on average the HSPF2 is around 9.0. High of 9.5 and low of 8.8.

Are these condensing units designed for the future refrigerant usage? If not, should we expect these units to work with CO2?

Answer: They are designed for R410A and the A2L variant is in development and will be available prior to the A2L switchover in January 2025.

How does the installed cost of VRF compare to traditional equipment, and is there a gross budgetary estimating number, e.g. $ per ton?

Answer: Cost will depend on SEER of traditional system but as a rule of thumb the Real ODU’s are on par with 18SEER conventional HP’s.

How many low voltage wires will be required at the condenser in a retrofit application?

Answer: 4 wire. C, B/O, Y, W

What differentiates Lennox from its competitors with these type of applications?

Answer: The ability to utilize a traditional indoor unit with a variable speed side discharge VRF ODU is unique to Lennox. Furthermore that ability to direct connect any thermostat and use both one to one or multi zone applications is an industry first.

What is the metering device?

Answer: Standard R410A Thermal Expansion Valve TXV in the indoor unit. Additionally the ODU has a built in EXV.

For a historical building with steam radiators, is it feasible to remove the radiators and replace them with the floor-mount indoor units?

Answer: Yes, it is, additionally there are multiple options available for outdoor units.

Will the real provide a furnace heat enable signal for implementation with an existing split system furnace?

Answer: The control is done at the thermostat. The Lennox M30 and E30 thermostats both have dual fuel HP operation control sequences built in.

What is the maximum number of conventional style ducted air handlers can be connected to the larger capacity outdoor units within the REAL product offering? Are there limitations on higher capacity ducted indoor AHUs connection capabilities?

Answer: It is directly proportional to capacity. 5 ton using 1 ton air handlers can connect up to 5. Merely that the capacity connected matches the total capacity of the Real unit. That said capacity connect can be less than the ODU ie below 100% connected capacity ratio down to as low as 50% of the ODU capacity.

Is nay warranty to get replacement parts on the future, i got experience with another VRF manufacturers that don’t supply the parts after 10 years.

Answer: We strive to continue to supply parts for our products beyond the 10 year mark. We have had our VRF for 9 years and continue to stock abundant inventory of even the first generation sold.

Can you replace a traditional residential Air Conditioner with a split system? What would be some of the implications if any?

Answer: Yes, you can. The installing contractor will need to confirm breaker size, piping condition and ensure all other components are in working order. Beyond this there is no further implications.