I just read this article you posted. I am a fan of what you guys are doing....keep it up. Id like to offer an additional point to the ductwork question. Im a retired HVAC engineer and now providing energy coaching services to residents in Needham. One thing I run into a LOT are houses that are 2 story, less than 3,000 SF and have a 2-4 ton single zone air conditioning system which serves the whole house (both floors), with heat provided by a separate HW boiler with multiple zones throughout the house. The air conditioning supply air quantity to each room on both floors is determined based on the COOLING load in those spaces (heating comes from the HW system). People often ask me if they can use their existing AC ductwork to also serve as the heat source. I explain all the issues you mention in your blog. But in the case described above, I tell them they are not likely to be happy with the heat pump system applied to their existing duct system and used for heating. The problem is that the air supply quantities (CFM) to both floors will need to be different in heating compared to the CFM values in cooling. In summer, more CFM/SF of air is typically supplied to the second floor compared to the first (ignoring internal load issues). In the winter, the opposite is true. Lots of heat (CFM) needed on the first floor compared to the second floor. When you apply the heat pump to this single zone system, you will find that the floor with the thermostat will be happy, while the other floor will either be too warm or too cold. The only way to solve this problem involves modifying the branch ducts (typically in the attic) so that all the 2nd floor branches are on one common supply and the 1st floor on another common supply. Each of these common supply ducts have a motorized zone damper connected to a thermostat on the respective floor to provide 2 zone heating (and cooling) control. In theory, the homeowner could achieve the same objective by seasonally adjusting the manual volume dampers in the branch ducts, but this is not something they will do properly up in the attic. Throttling airflow at the diffusers just generates noise.
People with gas heat who don't want to pay the higher energy costs associated with heat pumps in very cold weather often opt for a hybrid system (heat pump in mild weather, gas boiler in cold weather). In these situations, the problems described above may be more tolerable.
In newer homes, its more common that the contractor has installed separate systems for each floor, in which case you are good to go.
Something to think about when applying these heat pump systems to existing duct AC systems
Ed! This is a super detailed response, thank you so much for taking the time to write it all up. And that's a great point about different sizing needs due to heat rising/cold air sinking - we've run into cases where the AC for a colonial house supplies only to the 2nd floor, presumably because it's easier to just duct from the attic & the cold air sinks anyway.
I've seen a couple of panels in home that allows for 2-zone heating & cooling (for example, Honeywell's HZ311, which works for the gas/AC combo but apparently isn't compatible with heat pumps - they have a different model for that) but I'm generally under the impression that Mitsubishi's ducted equipment & thermostat system generally isn't wonderful when it comes to 2-zone controls.
Any chance you have opinions on thermostats or specific brands of heat pumps that have better support in this department? I know you're retired & out of the field (and cold climate heat pumps are evolving so quickly), but if you could let me know what I should be looking for in a proper controls system, I can run with that.
I just read this article you posted. I am a fan of what you guys are doing....keep it up. Id like to offer an additional point to the ductwork question. Im a retired HVAC engineer and now providing energy coaching services to residents in Needham. One thing I run into a LOT are houses that are 2 story, less than 3,000 SF and have a 2-4 ton single zone air conditioning system which serves the whole house (both floors), with heat provided by a separate HW boiler with multiple zones throughout the house. The air conditioning supply air quantity to each room on both floors is determined based on the COOLING load in those spaces (heating comes from the HW system). People often ask me if they can use their existing AC ductwork to also serve as the heat source. I explain all the issues you mention in your blog. But in the case described above, I tell them they are not likely to be happy with the heat pump system applied to their existing duct system and used for heating. The problem is that the air supply quantities (CFM) to both floors will need to be different in heating compared to the CFM values in cooling. In summer, more CFM/SF of air is typically supplied to the second floor compared to the first (ignoring internal load issues). In the winter, the opposite is true. Lots of heat (CFM) needed on the first floor compared to the second floor. When you apply the heat pump to this single zone system, you will find that the floor with the thermostat will be happy, while the other floor will either be too warm or too cold. The only way to solve this problem involves modifying the branch ducts (typically in the attic) so that all the 2nd floor branches are on one common supply and the 1st floor on another common supply. Each of these common supply ducts have a motorized zone damper connected to a thermostat on the respective floor to provide 2 zone heating (and cooling) control. In theory, the homeowner could achieve the same objective by seasonally adjusting the manual volume dampers in the branch ducts, but this is not something they will do properly up in the attic. Throttling airflow at the diffusers just generates noise.
People with gas heat who don't want to pay the higher energy costs associated with heat pumps in very cold weather often opt for a hybrid system (heat pump in mild weather, gas boiler in cold weather). In these situations, the problems described above may be more tolerable.
In newer homes, its more common that the contractor has installed separate systems for each floor, in which case you are good to go.
Something to think about when applying these heat pump systems to existing duct AC systems
Ed! This is a super detailed response, thank you so much for taking the time to write it all up. And that's a great point about different sizing needs due to heat rising/cold air sinking - we've run into cases where the AC for a colonial house supplies only to the 2nd floor, presumably because it's easier to just duct from the attic & the cold air sinks anyway.
I've seen a couple of panels in home that allows for 2-zone heating & cooling (for example, Honeywell's HZ311, which works for the gas/AC combo but apparently isn't compatible with heat pumps - they have a different model for that) but I'm generally under the impression that Mitsubishi's ducted equipment & thermostat system generally isn't wonderful when it comes to 2-zone controls.
Any chance you have opinions on thermostats or specific brands of heat pumps that have better support in this department? I know you're retired & out of the field (and cold climate heat pumps are evolving so quickly), but if you could let me know what I should be looking for in a proper controls system, I can run with that.