I have been doing a lot of general research to prepare for this winter with a comprehensive winter weather strategy to see the least decrease in fuel economy.
Part 1: Grille blocking
Last year acdii made a grille cover, see this thread, which he then kindly sent me as a gift after he got rid of his first FFH, the Blue Devil.
I ran out of zip ties so I bought more on Amazon planning to put this cover on soon as temps are now consistently below 50oF. Since the temps dropped last week I've seen a marked drop in fuel efficiency. Trips that used to consistently get 60-65 MPG now are only seeing 50-55. My current tank average has dropped from about 57.5 MPG over the first 200 miles to now 56 MPG over 275 miles. These last 75 miles have taken a big toll on my fuel economy averaging only around 52 MPG. Unfortunately, the zip ties I bought are being shipped from China. I'm am quite annoyed at Amazon that nothing prior to purchase indicated that they would be shipped from China and that they would take 3 weeks to arrive. I expected them to come like most Amazon items in about 1 week or less. So while I'm stuck waiting I decided to do more research.
Grille blocking is useful to help the ICE warm up faster and to insulate it to keep it warm longer. The cold winter air flowing over the radiator and engine compartment while you drive quickly sucks the thermal energy out of the components that you want to stay warm for maximum efficiency. Since the FFH is designed to spend a large percentage of its miles with the ICE off this is a problem in winter.
However, you do not want to block the flow of cold air to the electric components. While the ICE is less efficient in the cold, the hybrid computer and electric components are not adversely affected by the cold. In fact, you want to keep those components as cool as possible to keep them from wearing out. Thus it is important to understand where the radiator is for the hybrid system and not block its airflow.
In the FFH you'll see that there are two radiators. A large one with a fan, for the ICE, and a small one that is in front (closer to the grille) than the large one. The small one is for the hybrid components. In the FFH it is located down low and receives its airflow from the lower grille opening. The ICE radiator spans the entire height and gets airflow from both the large upper grille and the lower grille. In my past experiences with using acdii's grille cover on the upper grille (see this thread) I found that even in the summer the ICE didn't get too hot with the upper grille blocked. However, I didn't leave it on because I was concerned that the electronics would get too hot from the limited airflow through the engine compartment even though the inverter radiator being still exposed to air.
Sadly, there is no way currently to monitor the temp of the inverter coolant. Hopefully someone brilliant will figure out the XGauge coding for that soon.
Thus, my plan for this winter is to use acdii's grille cover on the top grille while temperatures are consistently below 50oF. Once the temperatures drop down below freezing I may block one row of the lower grille. I do not want to block any more of that lower grille to keep the electric components as cool as possible.
I discovered that as soon as the car is turned on the coolant is flowing through the inverter radiator. Even though the ICE was not very warm and the inverter coolant was barely warmer than room temperature the coolant was constantly flowing while the car was in park sitting in the garage in my tests this evening. I want to further test this when the car has been sitting for hours and the ICE/inverter components are completely cold but it seems likely that this coolant will circulate non-stop when the car is on regardless of temperature. For this reason I don't want to interfere with that by blocking the lower grille except partially in extreme cold. The reason to consider partially blocking the lower grille in extreme cold is because the ICE is still cooled by that grille and in extreme cold blocking the upper grille only might not be enough to keep the ICE warm.
The common Prius grille blocking links talk about only blocking the lower grille on the current gen Prius since its inverter radiator is located up high. The Prius also appears to constantly circulate the inverter coolant anytime the car is turned on regardless of temperature.
Part 2: Use the heated seats
Instead of turning on the HVAC right away when the ICE is cold I plan to use the heated seats for initial warmth. Turning on the heated seats will not make the ICE come on like turning on the HVAC will. On low each heated seat draws about .13 amps from the HVB or 0.0364 kW. This is a minimal power draw, less than the headlamps.
Part 3: Intelligent HVAC use
I use "intelligent" here to mean that I'm applying my knowledge of the FFH's inner workings to how I control the HVAC settings.
Since below a certain coolant temp threshold the ICE will run constantly to make heat I don't want to turn the HVAC on until the coolant temp is warm enough not to interfere with normal operation. Since the PCM update modified these settings I don't know yet what that temperature is. I also plan to not set the HVAC temp any higher than absolutely necessary.
Parts 2 & 3 will improve efficiency by not causing the ICE to run only to make heat which is not efficient. Part 1 improves efficiency by getting the ICE warm faster and keeping it warm longer.
I don't sacrifice much personal comfort to do this since in the winter I'm already dressed for being outside and don't need much heat anyway. When leaving from home our car is also parked in a heated garage so we always get into a warm car at the start. In any car you wouldn't have heat immediately so turning on the HVAC on from the start does no good. With the grille blocking strategy I hope to dramatically lessen the ICE warm up time so that the delay compared to a gas only car is minimal.
I wanted to share this strategy so that others can consider implementing any of the above steps or adding their own suggestions. Once I know the minimum coolant temp to turn the ICE off and still have heat I will add it.