Suppose you have a forced-air natural gas furnace. At the end of its life, you are considering replacing it with an ENERGY STAR model. In the cold northern states, that means 95 AFUE with an efficient furnace fan motor (blower), often an ECM. How do you model this change to determine cost effectiveness? Or to give your program "credit" for the energy savings? You want to get credit for both natural gas (heating fuel) savings and the electric savings due to the motor. If ENERGY STAR upgrade is not chosen, the fallback would be to install a 90 AFUE furnace with regular PSC motor I guess.
I've heard that some energy modeling programs have trouble giving electric credit for the furnace fan savings. The programs are set up only to give natural gas savings for the furnace measure. But maybe there's a way to tweak that?
There's more to it than that. The blower efficiency gain comes using an ECM for low speed vs. a PSC on low. On high speed the power difference is negligible. ECM is chosen more for comfort than energy savings. ECM vs. PSC payback time simply isn't there unless they run the blower 24/7.
Today's 90%+ furnaces require more airflow than ever to protect the heat exchanger. 50% more airflow for the same BTU as the old natural draft furnaces. Old 100K furnace needed 1,000CFM, new is 1,500CFM or more. Higher airflow can cause draft complains, but also can better mix the air floor to ceiling.
Don't forget DRAFT INDUCER efficiency. My 90% furnace uses a SHADED POLE motor which pulls 150W. A PSC inducer is closer to 80W. ECM is only used for multi stage/modulating furnaces, wattage varies with gas input rate.
You model the blower motor as an "appliance", then the ECM becomes an "improvement". Put some of the furnace cost to the blower improvement (say $1000).
As Bob alluded, there are quite a few "it depends" when it comes to savings. Typically furnaces are not only grossly oversized to the building, they are oversized to the duct. If you are not aggressively downsizing equipment that ECM may actually drive electric cost up.
Ideally you will have ESP between .08 and .4 and you will see good savings on the fan. Smaller equipment will run longer. This is more efficient and more comfortable, but if the duct is leaky you may lose all those efficiency gains through the leaks.
Good furnaces will also have ECM inducer motors. I suspect besides saving electricity, this allows for maximizing heat transfer from flue gases at different stages. For real savings to show up the project needs to take all these interactions into account.
As you know, the house is a system of interconnected systems. Home Performance considers all these interconnections and brings as much into balance as the situation will allow.
+1 on the typical GROSS oversizing of furnaces. A "2 burner" (typically 50k or smaller) will heat over 3/4 of the houses in the South, and many in the North. Yet they are rarely installed as the only heat source in a single family home. Most homes get 4 burner furnaces, although there are some 3's in smaller homes and 5's in larger homes.
Customers have come to associate the "dragon effect" of a hot blast followed by freezing as a characteristic of gas heat since furnaces are so often oversized. Electric heat is oversized also, just not nearly as much. Copper is expensive, so the larger electric heaters have a higher installed cost.
You are right about being able to model details accurately. But then we are challenged with how to model actual useage as well. For your question here's REM's suggested solution:
At this point we suggest the following process:
1.Create two units in the appropriate Library, each one representing a single operational stage of the same device.
2.Include both units in the Mechanical Inputs screen.
3.Based on your expectation of how much of the seasonal load (i.e. heating season or cooling season) will be met by each stage of the device, manually set the "Load Served (%)" for each unit.
·This value will depend on the specific control scheme the device has programmed for switching between stages.
·Keep in mind that you must set the Load Served % separately for space heating and space cooling.
·Remember that if you have multiple systems in the home, you will have to set the Load Served % for each stage of each device, so that the overall house load is 100% in each season.
It's great that you making the effort to also capture electric energy savings from furnace replacement, and taking the opportunity to upgrade the motor efficiency as well. Being out west, I don't have experience with software used up north/back east to model for programs, but appears you've received some good feedback on that.
I did want to comment on the discussion regarding energy performance of ECM motors overall. The operational benefits of ECM furnace motors certainly include their ability to readily ramp down fan speed (to accommodate lower heating levels and/or zoned systems), as well as the ability to maintain a consistent flowrate under varying static pressure conditions. Both aspects can improve system comfort performance. The former can also result in energy savings, while the latter could actually increase electric use if static pressures are too high (as tedkidd mentions).
Note also, however, that ECM furnace motors, as well as those for pumps, also have efficiency benefits compared to PSC versions even if operated at a single speed. Supported not only by manufacturer data but also by CEC (California Energy Commission) and DOE research, ballpark assumptions of 50% increase in efficiency right out of the box are not unreasonable. ECM's cost effectiveness of course depends on how much the fan or pump motor is used, but ECM options are increasingly available and their incremental cost continues to drop. For typical high efficiency equipment including furnaces, ECM motors are already the default.