*SOLVED* Extending HVAC Recirculation Times

[SilverPigeon]

Extending HVAC Recirculation Times

Introduction:
Some have been wishing that the HVAC stayed on recirculated air for longer. I'm not one, but I wanted to attempt this in order to learn more about our trucks' HVAC behaviour. Once this exercise is finished, I'll return my truck to its original HVAC settings; you decide how you want your truck to do HVAC. Obviously you do this at your own risk (hypoxia can be real in this scenario).

Rationale:
There are many strategies at play within the HVAC module's software. Some of these have an action of turning off recirculating air. Luckily, many of these strategies are configurable with FORScan which is how it is hoped to achieve longer recirculating air times. After each FORScan configuration change, the HVAC module's will be rebooted in service procedures. The idea being: "dumb down" HVAC until full manual control of recirculation is achieved. FORScan configuration changes were made in an order of both anticipated effectiveness and least harm combined. Changes were made cumulatively; they were not backed out before making the next change.

Testing:
- Testing will be conducted in up to 6 phases, with four tests in each phase. Where expedient, all four tests will be run consecutively within a single drive cycle.
- In order to remove possible variances due to atmospheric conditions, the drive cycles will be performed on days with similar weather conditions.
- Each drive cycle follows the same route as pictured below. It is a mix of suburban and country (some highway) roads. The speed zones are denoted on the picture. If the test required more time to complete, the loop section was repeated.

- Elapsed times are measured on an iPhone 16 Pro using the clock app in stopwatch mode. Due to driver attention requirements, the times taken will be inaccurate by a number of seconds because of recognition and response time.
- All measurements are taken with the following factors common to each test:

• "Auto" NOT used
• Manual fan speed set to 1
• Air distribution set to screen+face+feet (all possible distributions at the same time)
• Recirculation button / soft button pressed which starts the stopwatch.
• Measurement units are metric (°C and km/h)
• Time units are "mm.ss" format
• for consistency, each drive was performed on a separate day.
• all testing done in drive mode "eco".

Phase 1: The "control" phase to establish a baseline for each of the four tests within a phase.
Start of drive conditions: weather: overcast. Ambient temp.: 13. Coolant temp.: 12. ATF temp: 13.

Test Dial Temp. A/C Recirc. Off After (mins.) 1 25 Off 4.39* 2 25 On 5.06 3 15 (LO) Off 5.16 4 15 (LO) On 5.08

Observations: I was surprised by the consistency in turn-off times. It seemed that one primary strategy was in control and 5 minutes was the limit.
* I probably forgot to start the stopwatch on time

Phase 2: The most simple and sweeping changes which cause no harm. In this phase, Cabin Air Refresh (CAR) and Cold Discharge Based Recirc strategies were disabled. Also, HMI Override Partial Recirc (HOPR) was enabled (HMI = Human-Machine Interface).
HVAC 733-01-06 *xxx xxxx xx-- E to 3
Start of drive conditions: weather: mostly sunny / partly cloudy. Ambient temp.: 15. Coolant temp.: 27*. ATF temp: 14.

Test	Dial Temp.	A/C	Recirc. Off After (mins.)
1	25	Off	5.07
2	25	On	5.03
3	15 (LO)	Off	5.06
4	15 (LO)	On	5.06

Observations: Nil effect. Another strategy is still in effect, turning recirculated air off, so all the disabled strategies had not reached their times to turn off recirculated air. Also, HOPR had no effect. This surprised me.
*my bad: I started the truck before noting coolant and AFT temps.

Phase 3/1: Even though HOPR was enabled, I wondered if Smart heating-Partial Recirc (SPR) might be the overriding strategy. In this phase, all I did was disable SPR.
HVAC 733-01-05 x*xx xxxx xx-- 2 to 0
Start of drive conditions: weather: mostly sunny / partly cloudy. Ambient temp.: 16. Coolant temp.: 15. ATF temp: 16.

Test	Dial Temp.	A/C	Recirc. Off After (mins.)
1	25	Off	5.02
2	25	On	not off after 72.52+ (test terminated)
3	Not run
4	Not run

Observations: In the case of heat + A/C, recirculated air could have potentially been on for the duration of any drive. This may have an effect upon compressor life. Compressor duty cycle requires monitoring to determine if this is a significant concern.
Actions:

1. Rerun and complete Phase 3 tests while monitoring compressor duty cycle.
2. Tests 3 and 4 are important to establish behaviour during colder than ambient temps. These require the appropriate atmospheric conditions in order to be run.

Stay tuned for more

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[Steverinofla]

And are you including in your gear an O2 meter? (For YOUR safety)

 

[SilverPigeon]

No, I effectively cut the test off at 60 minutes, plus the 12 minutes to get home. In my case, there's only me in the cab and there is plenty of oxygen for that space and time. I envisage a 7-hour drive with a cab full of people as being a prime candidate for hypoxia.

P.S. Sorry for the continual edits, the site was playing merry hell with edits and saves.

 

[Porpoise Hork]

And are you including in your gear an O2 meter? (For YOUR safety)

Even when on full recirculate the system fresh air blend door is still partially open and supplies a continuous supply of outside air to the cab while the blower is on. The positive pressure is then vented out the external cab vents on the back of the cab. This is across the board and not just a Ford design. While not directly linked to our lovely safety guy Ralph Nader, it was included in a push for various passenger safety regulations that came about in the 60-70's.

 

[S14suspense]

Even when on full recirculate the system fresh air blend door is still partially open and supplies a continuous supply of outside air to the cab while the blower is on. The positive pressure is then vented out the external cab vents on the back of the cab. This is across the board and not just a Ford design. While not directly linked to our lovely safety guy Ralph Nader, it was included in a push for various passenger safety regulations that came about in the 60-70's.

I keep mine on Auto and recirculate as much as possible and it obviously still pulls in fresh air pretty easily. Still smell skunks and other odors while cruising down the highway.

 

[Porpoise Hork]

I tested a bunch of different cabin filters and found that in summertime this PureFlow filter works amazingly well at blocking or significantly reducing nearly all the skunk and other external odors from outside the truck. It does an just an ok job at exhaust fumes so in winter I switch to the HEPA variant as it does a far better job with these fumes but it does restrict airflow a bit more. That's why I only run it in the winter months when using the heater. Honorable mention also goes to the Fram cabin filter for summertime use and it's ability to remove odors like fast food from the cab fairly quickly. They all effectively pull out odors for around 3-4 months, but after that the impregnated carbon and baking soda gets exhausted.

 

[SilverPigeon]

So, today with the limited time I had for the truck was all about finding PIDs to monitor A/C compressor use. The plan is to repeat Phase 3 tests 1 & 2 while logging data for 2 PIDs I found:

• Variable A/C Compressor Valve Duty Cycle - Commanded % and
• Variable A/C Compressor Control Circuit Current - Measured A

The logs will sample every 0.5 seconds for as long as the test runs with recirc. on, or 60 minutes, whichever occurs first. I've trialled this log setup in the driveway to make sure of valid data collection. Log sample shows % for compressor on, and photo % for compressor off. Control current read A for compressor on and A for compressor off.

I cannot run the real tests today as start conditions cannot be met. I will report back with data asap.
I have decided that in order for tests 3 and 4 in all Phases to be valid, ambient temps must be at least 20°C for A/C to properly engage for a cooling-down of cabin temps to the requested 15°C (LO).

PIDs:

A/C off:

A/C on, initial cabin cool-down:

A/C on, normal running:

Sample log:

 

[SilverPigeon]

So, today was frustrating because, when running Phase 3/2:

Test	Dial Temp.	A/C	Recirc. Off After (mins.)
1	25	off	5.04
2	25	on	5.02
3	Not run
4	Notrun

That's right, damn thing turned off under the same test conditions as before. Either:
1. Phase 3/1 Test 2 was anomalous or
2. Phase 3/2 Test 2 was anomalous or
3. I haven't yet found a setting to do the job.

There was some use in today though: A/C log data shows that even at 25°C, and after the evaporator has been sufficiently cooled by compressed refrigerant, the compressor always works to keep the evaporator at around 6-7°C. This means to maintain a warmer than ambient temperature with A/C on, the heater core is compensating for cold refrigerant. Is it a fully loaded compressor at this point? Not sure how much power it's robbing from the engine during this scenario.

Onwards to more configuration and testing next week.

 

[Porpoise Hork]

That's some interesting findings. Always knew that when having the AC on with a higher temp setting meant the heater core had to overcome the cooled air charge but really surprised that they have not changed the run strategy maintaining the evap temps.

Modern auto AC variable speed/stage compressors are surprisingly efficient, only requiring between 2-3 hp to operate under most situations but may peak up to 5hp. Compare that to units from 20 years ago which could require upwards of 10hp to operate. Definitely a significant improvement. When under full throttle or heavy engine loads the truck temporarily shuts the AC compressor off to provide all available power as demanded by throttle input. This happens on ICE only and PB models as well.

 

[SilverPigeon]

So, today was Phase 4. The idea today was to disable the PCM Recirc. strategy:
RAM Air Recirc Strategy (RAMARS) is disabled • Power Off Recirc Delay (PORD) is disabled •
Compressor Overheat Protection (COP) is enabled • PCMR PCM Recirc strategy is enabled
HVAC:

733-01-06

x*xx

xxxx

xx--

My default was 3, changed to 2. Of course, I didn't touch COP; it's one of the things I refuse to, along with durability earlier in the testing. I ran test 1 (25°C, A/C off) and 5 minutes was again the limit.

Right now there is only one more thing I can think of to try: enabling RAMARS above. I'll change 2 to A. It could be that, if this setting is valid in out trucks, this strategy may keep recirculating going.

I've decided that it's no use in doing A/C on tests because I've proven already that the compressor is always running with A/C on but when ambient temps are 5-10 above dial temps, I'll run test 1 again. Stay tuned.

 

[SilverPigeon]

So yesterday a couple of 1 ¼ hour drives to go clay target shooting and back. The night before, I enabled RAMARS in the hope of changing the outcome. I think it's partly relevant; here's why:

In the morning drive, ambient was 2 ° C and recirc again only stayed on for 5 minutes. By the time I arrived, ambient was still low, about 11 ° C. In the afternoon things changed; ambient started at 20 ° C and with the dial set at 21 ° C, recirc stayed on for about 20 minutes. Same when I hit recirc again and dialled up to 25 ° C and same again when I dialect down to 15 ° C. A/C on and off during the drive made no difference.

So far, all testing has been carried out with ambient only as high as 16 ° C. Back a ways when I had recirc on for over an hour, I didn't take notice of the ambient temperature during that time. I currently believe that RAMARS being enabled cut the recirc time down from over an hour to around 20 minutes AT THIS HIGHER AMBIENT TEMPERATURE of 20 ° C. That's the only relevance it seems to have had, so I will disable it again.

Here's what I think is going on: there is another HVAC strategy which is enabled by default: Enhanced Windshield Anti Fogging Strategy (EWAFS). I'm thinking that at low ambient temps, it refuses to allow recirc for more than 5 minutes to stop fogging up the windows (yes, I know A/C can dry the air quickly but I don't think the strategy is as smart as that because A/C doesn't automatically turn on).

So, the next step is to disable RAMARS again, but also to disable EWAFS. I will need to run tests like this:
1. Low ambient temps around 10 ° C or lower, A/C off, fan 1, distribution screen/face/feet
2. High ambient temps around 20 ° C or higher, A/C off, fan 1, distribution screen/face/feet

If those tests both produce recirc times > 1 hour, I can then play around with fan speed and distribution to make sure it sticks. There is no point in using A/C during the tests because we don't want it to be constantly using the compressor (wear & tear).

I'll report back once testing is done with the above ambient conditions met.

 

[SilverPigeon]

So after all that testing, ambient temps have nothing to do with it; it's all down to air distribution. The way it seems to work out is that you can only extend recirculation times when air distribution is set to FACE ONLY, not feet or screen.

That should still satisfy the recirc fiends though, because I'm back to a consistent recirculation on even between ignition cycles. The following pic shows the HVAC asbuilt configuration required; whether or not all are needed is something I leave to others to work out:

'These HVAC settings equate to the easy mode choices of (in red):

If you do it by HVAC easy mode, you should verify those changes have worked by visiting the HVAC asbuilt configuration.

In addition, I have verified that recirculation still leaves a fresh air bleed into the cabin as @Porpoise Hork stated earlier; the PID is in percent closed for the air inlet door position:

As I'm a fresh air acolyte () I will now change my HVAC back to normal; this is all the testing I'll do on the topic. Have fun recirculating guys .......

 

[digitaltrucker]

WOW incredible, I bet you could have fixed our earlier models like my 13, where we used a resistor hack! LOL!

The thing I have always disliked about my 13 and now my 21 is, when I have vent only on, I never get the true outside cool air, it always seems the system is "warming" the air a few degrees.

 

[sbi]

I will just say, as I've said before: My truck never, ever goes to circulation mode when I use AUTO (and I use AUTO most of the time). In 3 years and 9 months I have not seen the circulation button turns on once, even when it is 100 degrees out with 90% humidity.

 

[digitaltrucker]

I will just say, as I've said before: My truck never, ever goes to circulation mode when I use AUTO (and I use AUTO most of the time). In 3 years and 9 months I have not seen the circulation button turns on once, even when it is 100 degrees out with 90% humidity.

I was understanding that while in "auto mode" your system could go to recirc mode, but the light not come on. At least that is what I recall folks saying about my 13.

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