Why are HVAC blower motors located at the beginning of the plenum box?

[mikedc]

They are usually placed right after air enters the plenum box, and they push the air through the evap & heater cores.

Is this done for noise reduction? Does it make a big difference?

What about air temps going through the fan? Would that be a factor?

[Lonnies Performance]

Those type of fans typically push air better than they pull it, plus there is more of a moisture/condensation issue downstream of the evaporator, which should be typically avoided for electronics.

[Twentyover]

Centrifugal pumps work on pressure ratios, if the inlet pressure is lower (such as after sucking through a wet evaporator), the flow is reduced. To achieve the required flow would require more blower motor. I will note that early Japanese HVAC's occasionally used sucker fans on HVAC systems, and who knows what Detroit thought up in the 50's. But the point is that you get reduced performance.

When I was in HVAC design, we still used brush type motors, the brushes are cooled by a cooling pipe molded into the motor housing or added as a rubber tube between the scroll and the brush end of the motor. Outside temperature had little effect on brush temp- usually this was a function of motor current demand.

Moisture- If I could emulate a sigh here, I would. OEM's are SUPPOSED to have dry plenums, so rain and snow melt never get to a motor. Yeah. Sure. Condensate that gets airborne off the evap usually stays entrained in the airflow and rarely affects the motor. Most water damage (and the manufacturer I worked for had this problem across the board on his cars) is from water entrained in the airstream in the plenum at entry to the blower.

[mikedc]

I appreciate both responses.


Greg - Are there rules of thumb about the airflow "port size" in the HVAC dash box? Like starting from the inlets, through the blower & cores & internal doors, and out to the various vent openings? Do you maintain a consistent total port area from one end of the dash box to the other?

What happens after that? The usual design has 3 different possible outlet paths (windshield defrost, center AC, and floor). Do they give each path enough "port size" to match the whole HVAC box alone? Or do they compromise them smaller, in consideration that people sometimes want to run air through 2 (or even all 3) paths at once?


I still don't know whether I'm serious about trying to build a custom dashboard HVAC box or not. But I've gotten interested in how these things are designed. I can't think of another part on a classic car that is so large & complex, where there is so little info available about it in general.

[Twentyover]

...
Greg - Are there rules of thumb about the airflow "port size" in the HVAC dash box? Like starting from the inlets, through the blower & cores & internal doors, and out to the various vent openings? Do you maintain a consistent total port area from one end of the dash box to the other?

I like to see 20 sq " on the blower inlet from an unrestricted cowl plenum. I never got it, and the plenum was never unrestricted, but 18-20 sq" is reasonable.

After the cowl pass thru- the OEM supplies an "environment"- this is the space he has alotted to you. You use every cubic mm you are alotted- it includes actual system volume and clearance volumes between your stuff and other people's stuff. You want as much volume as you can get to keep air velocities low and noise at bay. Bigger issue today than it was 15 years ago when cars were alot noisier. Also, greater volume reduces downstream pressure increase, improving blower performance. Is this important? Is it important to you? You can always jack up motor output to get the airflow you need

A stupid little example here. I was working with the OEM platform engineer on a cost reduction to eliminate a shutoff door to the 2nd row outlets. Redesigned the duct the door was located in, smoothing out a wicked steep angle change in the air path. We were able to achieve similar airflow through the duct with 3 volts less feed to an admittedly too large motor (that was always run @ <12v) and- the big news that sold the deal- a 5 dbA reduction in noise. OEMs KILL for that kind of sound pressure reduction.

So the answer is no, we did not try to maintain consistant flow areas. The blower inlet is suction, you need more area than downstream when you are running pressure thru the system.

Another area that has changed in the last 20 years is airflow volumes vs mode. Generally, about 250 cfm will be adequate airflow for cooling an intermediate size interior, assuming good outlets. but in heat and defrost, the HVAC controller will throttle the motor down to maybe 150-160 cfm. While this reduces the total heat transfered from the heat exchangers, the reduced airflow raises the air temperature, increasing passenger comfort. As long as you can cast the air to the correct location, this works.

In point of fact, most OEMS have a foot heat model, and requirement for airflow at those locations. A typical driver side will have 3 feet- RH foot on the throttle, LH foot to the left of the brake pedal, and your other LH foot kit of in the middle close to the base of the seat cushion in a relaxed position. You need to hit them all with airflow, so you have a truly tortured outlet port shapes. You may also need to downsize them to get the velocity to reach these locations.

I should note most of this development work is done on CFD models after the initial layout work is done

What happens after that? The usual design has 3 different possible outlet paths (windshield defrost, center AC, and floor). Do they give each path enough "port size" to match the whole HVAC box alone? Or do they compromise them smaller, in consideration that people sometimes want to run air through 2 (or even all 3) paths at once?

usually only two paths at a time
Defrost- Windshield and side window demist
Blended Defrost and Heat
Heat- Foot heat , with bleeds to defrost and side window
Bi-level- Cooler face vent and warmer heat outlet
A/C- Face outlet

And the outlets are sized to meet the requirements for each of the three major functions as stand alones- we would size defrost, size heat, and size A/C- and use mode door position to control outlet location airflow volumes

I still don't know whether I'm serious about trying to build a custom dashboard HVAC box or not. But I've gotten interested in how these things are designed. I can't think of another part on a classic car that is so large & complex, where there is so little info available about it in general.

Well, 40 years in HVAC design turned me from a fresh faced kid to a grumpy old grey haired fat man. Is that really what you want?

The only real handbook, and I hesitate to call it that, was a deal written maybe 30 years ago by a guy named (and I'm sure I'll mis-spell it) Alex Khargelis. Forget the formal name of the book, and it wasn't even really HVAC specific, it was about interactions between the engine coolant temp and flow, refrigeration system, and airflow systems. I took a class he offered 20 odd years ago, and the handbook was an assembly of photocopied stuff from all over the place- different font types and sizes, hand drawn pictures, etc. The handbook came with the class. Every HVAC engineer I've known (except for the kids who are just coming into the workforce now) had a copy, or at least know of the book and it's author. I still have my copy in a box of crap I brought home when I retired.

[mikedc]

I like to see 20 sq " on the blower inlet from an unrestricted cowl plenum. I never got it, and the plenum was never unrestricted, but 18-20 sq" is reasonable.

After the cowl pass thru- the OEM supplies an "environment"- this is the space he has alotted to you. You use every cubic mm you are alotted- it includes actual system volume and clearance volumes between your stuff and other people's stuff. You want as much volume as you can get to keep air velocities low and noise at bay. Bigger issue today than it was 15 years ago when cars were alot noisier. Also, greater volume reduces downstream pressure increase, improving blower performance. Is this important? Is it important to you? You can always jack up motor output to get the airflow you need

That matches my uneducated gut feeling about what is a good design. A physically bigger system would push the same given amount of air more slowly, which means quieter operation.

Is that the biggest reason for the noise reduction in recent years? The modern systems look physically bigger and I see a lot of airflow smoothing efforts. Molded rigid plastic vent passages. The aftermarket AC conversions (and older OEM) use round corrugated flex-hose between the plenum box and the dashboard vent outlets. That strikes me as a source of noise, or at least turbulent airflow. But it would partly depend on how well the installer laid it out.

A stupid little example here. I was working with the OEM platform engineer on a cost reduction to eliminate a shutoff door to the 2nd row outlets. Redesigned the duct the door was located in, smoothing out a wicked steep angle change in the air path. We were able to achieve similar airflow through the duct with 3 volts less feed to an admittedly too large motor (that was always run @ <12v) and- the big news that sold the deal- a 5 dbA reduction in noise. OEMs KILL for that kind of sound pressure reduction.

A 5-decibel reduction from a vent design tweak? Dang. I take it that was a pretty kinked-up design to start with.

That obstruction area also sounds very close to an outlet, which is very close to the driver's face. It would be easier to hear that noise than an obstruction buried way down inside the plenum box.

Another area that has changed in the last 20 years is airflow volumes vs mode. Generally, about 250 cfm will be adequate airflow for cooling an intermediate size interior, assuming good outlets. but in heat and defrost, the HVAC controller will throttle the motor down to maybe 150-160 cfm. While this reduces the total heat transfered from the heat exchangers, the reduced airflow raises the air temperature, increasing passenger comfort. As long as you can cast the air to the correct location, this works.

Interesting. So basically, the system decides to blow slower-moving hot air directly at the passengers rather than move more total hot air into the cabin.

In point of fact, most OEMS have a foot heat model, and requirement for airflow at those locations. A typical driver side will have 3 feet- RH foot on the throttle, LH foot to the left of the brake pedal, and your other LH foot kit of in the middle close to the base of the seat cushion in a relaxed position. You need to hit them all with airflow, so you have a truly tortured outlet port shapes. You may also need to downsize them to get the velocity to reach these locations.

I should note most of this development work is done on CFD models after the initial layout work is done

Foot heating is a funny thing. If there's not enough then the driving experience is miserable. Too much, and I find it makes me sleepy.

usually only two paths at a time
Defrost- Windshield and side window demist
Blended Defrost and Heat
Heat- Foot heat , with bleeds to defrost and side window
Bi-level- Cooler face vent and warmer heat outlet
A/C- Face outlet

And the outlets are sized to meet the requirements for each of the three major functions as stand alones- we would size defrost, size heat, and size A/C- and use mode door position to control outlet location airflow volumes

How did the old bi-level modes ever work to deliver two different temps at the same time? I think of temp control being done with either a blend door or a heater core flow valve.

Well, 40 years in HVAC design turned me from a fresh faced kid to a grumpy old grey haired fat man. Is that really what you want?

The only real handbook, and I hesitate to call it that, was a deal written maybe 30 years ago by a guy named (and I'm sure I'll mis-spell it) Alex Khargelis. Forget the formal name of the book, and it wasn't even really HVAC specific, it was about interactions between the engine coolant temp and flow, refrigeration system, and airflow systems. I took a class he offered 20 odd years ago, and the handbook was an assembly of photocopied stuff from all over the place- different font types and sizes, hand drawn pictures, etc. The handbook came with the class. Every HVAC engineer I've known (except for the kids who are just coming into the workforce now) had a copy, or at least know of the book and it's author. I still have my copy in a box of crap I brought home when I retired.

That's surprising at first but not when I think about it longer. The education system always lags an industry's demand by decades.

[Twentyover]

That matches my uneducated gut feeling about what is a good design. A physically bigger system would push the same given amount of air more slowly, which means quieter operation.

Is that the biggest reason for the noise reduction in recent years? The modern systems look physically bigger and I see a lot of airflow smoothing efforts. Molded rigid plastic vent passages. The aftermarket AC conversions (and older OEM) use round corrugated flex-hose between the plenum box and the dashboard vent outlets. That strikes me as a source of noise, or at least turbulent airflow. But it would partly depend on how well the installer laid it out.

Improvements in overall sound levels drives the need for reduced sound levels from HVAC. You don't want to be the system the end user hears operating while tooling down the road

Corrugated tubing in general presents a greater restriction that a smoothwall duct, reducing airflow or requiring more blower output. It also makes it more difficult to aim the air at the outlet to better get to your target location.

A 5-decibel reduction from a vent design tweak? Dang. I take it that was a pretty kinked-up design to start with.

That obstruction area also sounds very close to an outlet, which is very close to the driver's face. It would be easier to hear that noise than an obstruction buried way down inside the plenum box.

It was a little more than a design tweak- the duct was redesigned to smooth flow and eliminate the all aspects of the door. And the door was in a booster duct going under the console to the 2nd row passengers.

It really was acked up- the original design was going to source air from one of two locations, and the duct tooled for such. Then the decision was made to eliminate one location, so the porting was flashed over closing the opening. but all the crap obstruction was left. I assume that decision was made just prior to production launch, I was with another place when the car was launched so have no detail on why the decision was made.

Interesting. So basically, the system decides to blow slower-moving hot air directly at the passengers rather than move more total hot air into the cabin.

By reducing airflow, the outlet temp rises. If you run full airflow, the core is 'overblown', that is, the outlet air temp is lower, and you're blowing more of that heat out into the atmosphere through the exhausters. Target locations are unchanged

...
How did the old bi-level modes ever work to deliver two different temps at the same time? I think of temp control being done with either a blend door or a heater core flow valve.

Magic. And testing. Lots and lots of testing.

HVAC's are typically series reheat systems (air goes through evap then a fraction is directed to the heater core and the remaining fraction bypasses the heater core.) so in the blend chamber, you direct cooler air that by passes the heater to the vent outlets, and warmer air that went through the heater blended with unheated air toward the floor. There are a couple ways to do this- most are proprietary to the manufacturer. The big struggle is the temp linearity curve and temp stratification between outlets

One startup electric we were working with had a single evaporator front and rear, and multiple electricheater cores, one for each seat. They aren't around anymore

That's surprising at first but not when I think about it longer. The education system always lags an industry's demand by decades.

[mikedc]

Improvements in overall sound levels drives the need for reduced sound levels from HVAC. You don't want to be the system the end user hears operating while tooling down the road

Corrugated tubing in general presents a greater restriction that a smoothwall duct, reducing airflow or requiring more blower output. It also makes it more difficult to aim the air at the outlet to better get to your target location.

Is the corrugated round ducting (vs molded smooth passages) a major difference in noise? Or is it more like 'every little bit helps'? Even if I don't make a custom-done HVAC plenum box I'm still dealing with the matter of ducting. 50yo factory HVAC setup, aftermarket setup, custom job . . . they all require ducting & vent outlets.

When I look at the aftermarket (and even some older OEM) vent outlet items, they don't impress me. Many of them basically just run a corrugated round tube into the back side of a little rectangular box, and the front side of the box is the slatted opening.

By reducing airflow, the outlet temp rises. If you run full airflow, the core is 'overblown', that is, the outlet air temp is lower, and you're blowing more of that heat out into the atmosphere through the exhausters. Target locations are unchanged.

That sounds sort of like trying to run an engine without any thermostat. The coolant blows through the system too fast. It doesn't spend enough time inside the radiator getting cooled off.

Magic. And testing. Lots and lots of testing.

HVAC's are typically series reheat systems (air goes through evap then a fraction is directed to the heater core and the remaining fraction bypasses the heater core.) so in the blend chamber, you direct cooler air that by passes the heater to the vent outlets, and warmer air that went through the heater blended with unheated air toward the floor. There are a couple ways to do this- most are proprietary to the manufacturer. The big struggle is the temp linearity curve and temp stratification between outlets

Sounds pretty complex. I guess that's why "bi-lev" isn't that common nowadays.

The modern systems that I've taken apart (well, 1990s-2000s systems) . . . I recall them just having a blending chamber for the heated & cooled air, and that's it. I can't remember seeing one with a real provision for delivering two different temps of air to the cabin at once. But, that said, I was just taking them apart to change cores & blowers. I wasn't exactly studying them from a design POV at the time.
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Is there anything important about the dash box's cabin air inlet (for recirculating cabin air) besides making it large enough and decent total airflow? Does it matter whether that air inlet is horizontal or vertical? I've seen a lot of OEM ones with horizontal openings there.


Do cabin air filters make a big dent in airflow? Is there anything to know about sizing them or placing them?

I assume the earlier in the system the better, just to keep everything clean. I saw an OEM dashboard setup on something (can't remember what it was) where they had squeezed a cabin air filter in between the blower motor & evaporator core (so the filter was the very first thing to receive pressurized air from the blower). The filter was about exactly the same size as the evaporator core itself. I remember thinking that was a nice packaging job. But I wondered if having a filter the same size as the evaporator core (square face area) would be big enough.

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I appreciate all the time you've taken to answer my questions. If I don't custom-build my own dashboard setup, I might just try modifying an OEM or aftermarket setup to work better. At any rate, I'm trying to figure out where the low-hanging fruit is for improving stuff.

[Twentyover]

Is the corrugated round ducting (vs molded smooth passages) a major difference in noise? Or is it more like 'every little bit helps'? Even if I don't make a custom-done HVAC plenum box I'm still dealing with the matter of ducting. 50yo factory HVAC setup, aftermarket setup, custom job . . . they all require ducting & vent outlets.

When I look at the aftermarket (and even some older OEM) vent outlet items, they don't impress me. Many of them basically just run a corrugated round tube into the back side of a little rectangular box, and the front side of the box is the slatted opening.

The biggest advantage to using smoothwal tubing is the reduction in pressure drop, allowing lower blower speed and noise for similiar comfor levels. Combined with the ability to maximize it's size inside and envelope, results in reduced air rush noise (a real term). Actual sound produced by convolutions are not a real problem until you get into higher airflows

That sounds sort of like trying to run an engine without any thermostat. The coolant blows through the system too fast. It doesn't spend enough time inside the radiator getting cooled off.




Sounds pretty complex. I guess that's why "bi-lev" isn't that common nowadays.

The modern systems that I've taken apart (well, 1990s-2000s systems) . . . I recall them just having a blending chamber for the heated & cooled air, and that's it. I can't remember seeing one with a real provision for delivering two different temps of air to the cabin at once. But, that said, I was just taking them apart to change cores & blowers. I wasn't exactly studying them from a design POV at the time.
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Bi-level remains common, even if the term may have limited use. It's the mode position where the air arrows point to your face and heat.

Good visible examples of internal machinations are most obvious on Valeo HVAC systems (pretty common on Chrysler products in the 2000-15 range.) They used a molded 'crossflow vent' in the housings to manipulate air temp distribution

Is there anything important about the dash box's cabin air inlet (for recirculating cabin air) besides making it large enough and decent total airflow? Does it matter whether that air inlet is horizontal or vertical? I've seen a lot of OEM ones with horizontal openings there.

No magic on recirc inlets- usually positioned to get access the an airspace above the gglove box. The grille is to keep someone's kleenex from ingesting into the wheel, getting spit out and plugging the evap air side. Horizontal or verticle, up or dowm, just looking for smooth entry into the bowl of the blower wheel

Do cabin air filters make a big dent in airflow? Is there anything to know about sizing them or placing them?

I assume the earlier in the system the better, just to keep everything clean. I saw an OEM dashboard setup on something (can't remember what it was) where they had squeezed a cabin air filter in between the blower motor & evaporator core (so the filter was the very first thing to receive pressurized air from the blower). The filter was about exactly the same size as the evaporator core itself. I remember thinking that was a nice packaging job. But I wondered if having a filter the same size as the evaporator core (square face area) would be big enough.

Absolutely impact airflow. Bigger is better.

Placement- a big PITA. The best placement (downstream from the blower) is the most difficut to service. One I like the best is Nissan D Platform (Pathfinder, Murano, Altima, & Maxima. I also happened to be assigned to it after it was released and in production.) Big filter just upstream from the evap. The designers did a great job cclaiming enogh real estate to let this get serviced.

Some filters are located prior to the blower wheel. These will more severely impact airflow, but the blower motors are supposed to be sized to take this into account. Part of the statement of work from the OEM, usually in the form of a performance and durability standard.

If the system is designed for a filter, and there is no lower spec variant that runs sans filter, run the filter. There is a potential, however remote, that at high plenum pressure (high speed, stiff headwind, etc) the absence of a filter can contribute to an overcurrent condition, resulting in a thermal event. A thermal event is not a good event, like a trip to Disneyland or a rock concert.

Anyway, enough for now. I started driving yesterday morning at 6am in Ft Dodge IA coming back to Detroit. It's 1:30 tomorrow morning here, so i will go to sleep now

[mikedc]

Whoa.



Thanks again. I'm gonna paste & save these answers you've been giving.