Does the 97-01 Heater Core Box Design Limit Airflow?

Its that time of year again to freeze my butt off driving to work in my Jeep. I've often noticed that the airflow from my vents is much lower when the temp knob is set to HOT compared to COLD. The HVAC unit makes more noise when set to HOT compared to COLD as well. xjguy98 has some good pics of the heater core compartment and it looks pretty restrictive.

http://www.naxja.org/forum/showthread.php?t=963507&highlight=heater

I borrowed an air velocity meter from work and I get ~500 fpm out the vent when set to HIGH & HOT, and ~1000 fpm out the vent when set to HIGH and COLD. I can feel the difference with my hand, just nice to put actual numbers to it.

Does everyone else experience this airflow difference? Just checking to see if thats normal for the 97-01 Jeeps. My heater blows hot, just doesn't seem to be very intense.

I am about a week into owning my first 97+ and I haven't noticed any difference. I did use the cold setting to make sure it worked before I purchased the Jeep.

Why is it the time of year to freeze your butt off? Is it blowing so weak that it doesn't keep the Jeep warm? I'd say you have a problem if so because the heat in our XJs can cook a turkey.

fool4wheelin said:

I borrowed an air velocity meter from work and I get ~500 fpm out the vent when set to HIGH & HOT, and ~1000 fpm out the vent when set to HIGH and COLD. I can feel the difference with my hand, just nice to put actual numbers to it.

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Out of curiosity, would the difference in density between cold and hot air affect your machine's readings? I don't know if this would have anything to do with it or not, but figured I'd mention it.

Does everyone else experience this airflow difference? Just checking to see if thats normal for the 97-01 Jeeps. My heater blows hot, just doesn't seem to be very intense.

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Mine seems to be fine, but if the vents and ducts aren't in alignment I can experience weird airflow restrictions (though that usually affects both heating and cooling). Might want to check that everything behind the vents is square with them, including the one to the rear.

BruceB83 said:

Why is it the time of year to freeze your butt off? Is it blowing so weak that it doesn't keep the Jeep warm? I'd say you have a problem if so because the heat in our XJs can cook a turkey.

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Because the temperature is below 0°...

Right, Its blowing so weak. It changes hot to cold OK, so I'm assuming the blend door is functioning properly, just blows weaker on HOT and there is more noise coming from the HVAC box like there is an airflow restriction.

casm said:

Out of curiosity, would the difference in density between cold and hot air affect your machine's readings? I don't know if this would have anything to do with it or not, but figured I'd mention it.

Mine seems to be fine, but if the vents and ducts aren't in alignment I can experience weird airflow restrictions (though that usually affects both heating and cooling). Might want to check that everything behind the vents is square with them, including the one to the rear.

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Its a Dwyer 471 Digital Thermo Anemometer, and it also measures temp, so I'm assuming it factors that into it.

What do you mean out of alignment? I do have an aftermarket stereo that was in the rig when I bought it, maybe they screwed something up when they isntalled it.

fool4wheelin said:

Because the temperature is below 0°....

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Yikes!

fool4wheelin said:

Right, Its blowing so weak. It changes hot to cold OK, so I'm assuming the blend door is functioning properly, just blows weaker on HOT and there is more noise coming from the HVAC box like there is an airflow restriction.

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Kind of strange. All I can think that moves when you change the temp is the blend door. Now if your also moving it from recirculate to fresh air that could cause it to drop some. Most cars HVAC system blows harder on recirc than it does on fresh air. Could this be in play here?

fool4wheelin said:

Its a Dwyer 471 Digital Thermo Anemometer, and it also measures temp, so I'm assuming it factors that into it.

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Fair enough. I know next to nothing about how these things work, but figured I'd toss it out there

What do you mean out of alignment? I do have an aftermarket stereo that was in the rig when I bought it, maybe they screwed something up when they isntalled it.

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Well, it's been a long time since I seriously tore the dash apart so can't remember exactly how everything routes back there, but there's an air duct that runs through the centre console along the top of the transmission tunnel to the rear vents. I've had that get out of line a couple of times after pulling the console and it has affected airflow out of the front vents - my guess is that the change in backpressure with it out of line weakens the flow in other places; you may be experiencing something similar at the front. However:

It changes hot to cold OK, so I'm assuming the blend door is functioning properly, just blows weaker on HOT and there is more noise coming from the HVAC box like there is an airflow restriction.

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The noise makes me think it's probably not duct placement. When you say the HVAC box, are you referring to the cabin fan or the blend doors? Also, what exact year are we talking about here? I seem to recall that there was at least one revision to the heater core in the '97-'01 timeframe, but could be off on that.

BruceB83 said:

Kind of strange. All I can think that moves when you change the temp is the blend door. Now if your also moving it from recirculate to fresh air that could cause it to drop some. Most cars HVAC system blows harder on recirc than it does on fresh air. Could this be in play here?

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Good point - and if the defrost/floor setting is being used, that is not a recirculating mode; it will cycle the A/C compressor but also draws in fresh air, so may contribute to the pressure drop depending on where the mode selector is being moved to or from.

Out of curiosity, what happens when you go from full hot to full cold with the HVAC mode selector set to max. A/C? I'd be curious to see if the pressure drop remains constant or not with it on a full-recirculate mode.

When I'm running my heat, I seem to get more airflow when I set the temp control to 3/4 or 3/5 rather than all the way to full hot. The air is still plenty hot, but the flow seems to be higher and I get warm faster. Might work for you? Just a thought.

I'm north of the border, but I am pretty sure your ND temps are a lot lower than mine in the winter. You know you have a cold climate when you have to go to Canada to warm up.

I'm gonna lean toward the blend door as well, or some other aspect of the pneumatic HVAC controls (one of the few doors in there...). My 2001 XJ's heat works too well (extremely hot, with a huge volume of air blown)... I've honestly contemplated using it for heat shrinking in lieu of a heat gun.

BruceB83 said:

Kind of strange. All I can think that moves when you change the temp is the blend door. Now if your also moving it from recirculate to fresh air that could cause it to drop some. Most cars HVAC system blows harder on recirc than it does on fresh air. Could this be in play here?

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Not doing recirc, just using the normal vent and floor selections. FYI, my blend door is cable operated.

casm said:

When you say the HVAC box, are you referring to the cabin fan or the blend doors? Also, what exact year are we talking about here? I seem to recall that there was at least one revision to the heater core in the '97-'01 timeframe, but could be off on that.

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I'm referring to whatever enclosure is around the heater core. I have a '98 XJ. Interesting you ask which year because the guys with 00-01 (you and anthrax323) seem to have awesome heaters.

casm said:

Out of curiosity, what happens when you go from full hot to full cold with the HVAC mode selector set to max. A/C? I'd be curious to see if the pressure drop remains constant or not with it on a full-recirculate mode.

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I'll have to try that out.

BrokenSockets said:

When I'm running my heat, I seem to get more airflow when I set the temp control to 3/4 or 3/5 rather than all the way to full hot. The air is still plenty hot, but the flow seems to be higher and I get warm faster. Might work for you? Just a thought.

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Yes, this is true for me as well. Anything less than FULL HOT has better airflow, but is (obviously) somewhat colder. Which year is yours?

In case no one clicked on the link at the top, here are the pics I was referring to, compliments of xjguy98:





To me it looks like the airflow going through the heater core is way more restricted than if you just wanted cold air. Makes me wish I had the infamous Heat Valve. Funny thing is that in the summer when I have the vent on COLD, it feels warmer than the outside air. Different issue for another thread...

Anyone else with a '98 have the same issues as me? I'll borrow you my Dwyer Digital Thermo Anemometer if you want to check numbers

Why not check the flow of another XJ. You may have blend door problems or somthing in there blocking the heater core if flow is a lot lower.
Bur yes you will have SOME less air flow when all of the air is forced into the core. For sure it's easyer for air to go around the core then throu it.

fool4wheelin said:

Funny thing is that in the summer when I have the vent on COLD, it feels warmer than the outside air.

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Just as an aside: I get the same thing and have noticed it in other XJs of varying years as well; my best guess is that it's down to engine bay temperatures vs. the positioning of the fresh air vents at the base of the windscreen. Their location is perfect to be warmed constantly by heat in the engine bay as well as to draw in heat coming off of the hood.

Anyway, I'm stumped on what's happening with the heat output. Badron's suggestion to check against another XJ if possible isn't a bad one, though.

badron said:

Why not check the flow of another XJ.

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Thats why I'm posting this. No-one else has a Digital-Thermo Anemometer Laying around?

badron said:

Bur yes you will have SOME less air flow when all of the air is forced into the core. For sure it's easyer for air to go around the core then throu it.

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Yes, I agree, but my point is that the core asside, Chrysler put a bunch of plastic shrouding around the heater core (to keep the cold air cold in the summer I'm assuming). If you look at those pics, the air leaving the core has to squeeze through that little 1/2" wide opening before it goes back into the main plenum.

If everyone else with a '98 XJ has the same issue as me, I'll write it off as bad design and live with it, but if everyone else has good flow I'll probably start digging into it further, blend door or the like...

I'll have to take a video with the flowmeter readings and post it.

Well, mine is a '99 but you got me wondering when I read this thread yesterday. While waiting at the drive through tonight I decided to check mine out. Engine was up to normal operating temperature and I had my extended idle activated so I was maintaining a constant 1100rpms. I had the selector knob set to the upper vents (no A/C, no recirc, no defrost) and I alternated between full cool and full heat on the temperature knob. I was just checking it by holding the back of my hand up to the vent, but I couldn't notice any perceptible difference in air flow between hot and cold. Again, it's a '99 (not a '98) but I don't imagine there's any difference between the two. I'm thinking your problem can't be blamed on poor design.