Heater Core Bypass For A/C performance

[Sean J Blackburn]

Just thought I'd post the results of my heater core shutoff on my truck's ac performance.

It was 95 today and with the valve open (coolant flowing through the heater core) my vent temps while driving were 44.5. With the shutoff valve closed I dropped 5 degrees to 39.5!

5 degrees doesn't sound like much but on a 20 minute drive today it made a real difference in cooling and comfort. I'd definitely encourage anyone who is unhappy with their vent temps to try this out, it really seems to have made a difference in my comfort while driving.

 

[alex6666]

is their any downside to shutting off the flow to the heater core? i'm not a mechanic guy ;( i have really ****** a/c. recharged not much difference, sprayed the a/c rad little difference.

 

[Sean J Blackburn]

Not that I know of! I've done it on an 05 5.9 Cummins and a 10 5.7 hemi with no Ill effects.

 

[Jeepwalker]

You shouldn't need to shut off the coolant temp to get good AC at 95*. I wonder if there's a problem with your AC (overcharged... plugged oriface??).

The heating and cooling in modern vehicles is pretty interesting. They're like a mini high-tech commercial heating and cooling system all in one very small box. Even in the dodge ram. What's amazing is how high-tech they are and people hardly even notice it. The first thing you have to remember in the temperature control world is that temerature & humidity are both linked to comfort. That's just how it is. You can have cool air but it will feel uncomfortable and dank if the relative humidity (RH) is too high.

In the commercial HVAC world (not houses ...but commercial buildings), the cooling coil in HVAC equipment is positioned first in the airstream: Warm, humid air enters the box and flows through the cold cooling coil (AC). As it does so, it drops the temperature below the dewpoint and moisture condenses on the cold AC evaporator coil (like dew forming on a cold beer can). The air leaves the cooling coil much cooler AND DRYER because a certain amount of moisture has condensed out of the incoming air. Then it passes through the heating coil. In the commercial world the heating coil temperature is ususally modulated. But in cars they modulate the mixing of 'some' cold air via dampers through the heating coil (they do that in the commercial world too sometimes). I said "some" air ..not all. By mixing the cold air through the warmer coil you also continue to lower the RH (by raising the temperature) and bringing the discharge air up to a certain comfort level. So the air leaving the hvac box is cooler and a lot lower humidity. Generally AC is going to work better by using the Recirculating Mode (Using AC usually goes into a certain amount of recirc mode anyway) ...which continually feeds air back through the AC which ALREADY was dehumidified previously. By continuously recirculating dehumidified air through the AC coil it really brings down the humidity and feels a lot more comfortable, even if the temperature hasn't lowered a whole lot. Its cooler but it 'feels' better.

In a car, the cold air (it's not REALLY "cold" btw, ...it's w/o as much heat, actually), even if it doesn't pass through the heating coil, it warms up on it's path to the passengers via the hot vent ducts and hot interior temperatures. It's fighting a hot dash and a lot of solar load as you know. But if after a while the cabin temperature becomes too cold you can turn the temperature 'up' which bypasses some (or more) of the cold air through the heater coil.

This is also why the defrost kicks in the AC compressor ...do dehumidify the air so lower humidity air pulls moisture off the windshield faster.

Anyway, most car AC systems should perform very well at hot temperatures by design. It gets a lot hotter in the middle east ...and way more humid in FL and Asia. BTW, Bankok and places in Asia are the most taxing on HVAC than anywhere on earth due to their heat AND humidity. So, 95* isn't much of a stress-test.

 

[Jeepwalker]

Some warm air could be leaking past the damper door (damper motor not great??), and it's possible the seals aren't 100% great. The seals would be tough to get at w/o great expense ...and usually they don't present a problem. They could be at 95% and that would result in some warm air leaking, but probably not enough to affect good cooling. In fact 5* might be within the design of the system. After all, you're talking about a damper door which is literally 1/8" thick plastic ...not thermally insulated steel or fiber. 5* isn't that much. So, of course some heat is going to normally radiate through the plastic vent door. And did you do that test with air blowing through or static?

But anyway, that's how the system works. You might have your system double-checked by an AC pro. Maybe your system is a bit low on refrigerant. There's something called 'superheat' which manufacturers don't always get right. If you can achieve that your system will be at it's optimal cooling efficiency. You need a guy who really knows his AC working on it ...not a generalist. Shaft seals are the weak point in automotive AC, where leaks can occur. But I'm not suggesting your vehicle has a leak. Maybe the cooling was always just outside the window of 'great' when it was new.

 

[Sean J Blackburn]

95 is a cold day in the summer in Phoenix lol. It gets over 115 here regularly. Also, it wasn't bad before the valve, it was running about 45 degrees. After the valve, about 39. When it is as hot as it is in Phoenix you need every advantage you can get! Also, there is nothing wrong with my ac system. I just had it inspected and filled to spec before doing this.

 

[Jeepwalker]

Well if that works for you, like you say, every advantage helps.

There is one potential issue to be aware of worth mentioning ...just something to be aware of. This is something new car engineers probably struggle with. Putting a valve to the heater core is somewhat controversial. It's been reported on some classic car forums I also go on, and even the Jeep XJ forum, that installing a valve at the heater core can actually kill a heater core. It comes down to the valve's placement, is it a byass valve ...or is it even worth using one at all?

What can happen with a valve ( especially if installed on the inlet side of the heater core line) is, crud and sediment can build up in front of the valve over the months of non-use. It can be rust sediment, mineral deposits, pieces of gasket material, hose or whatever, which became dislodged. Whatever crud is floating around a cooling system -- the older the vehicle the more there'll probably be. Guys have showed the crud on like the XJ forums. Then, when an owner turns on the valve, say in the fall, they my 'slug' any built-up crud which happened to collect and get trapped in the no-flow area of the valve or hoses. That crud can go right into the heater core. That can lead to plug-ups which may or may not be able to be dislodged. The passages inside the heater core are quite small. If the valve is on the output side it's may not be as bad because it has time to go through the system and hopefully get broken up before it goes through the h/c. But even that is controversial b/c guys have indicated sediment can still build up to a degree in the inlet side (of the coolant line) because the coolant is basically not flowing and some debris can collect in the hose. I dunno. It comes up on the XJ forums because heater core failure has been rampant on those vehicles and they came stock with shut-off valves. At least the early ones did. And so did GM cars of the 70's-80's.

So there's controversy on weather the risk of having to remove the dash to replace the heater core makes it even worth using a valve at all, or if so, where to put it, or use a valve with a built-in bypass (preferred) ....or remove the hoses and them out in the fall prior to using the heat. Esp if there is a good-working heater damper which closes off the heat during normal use.

Not trying to scare ya, but pointing out something to be aware of, so you don't inadvertently run into a situation of unintended consequences which lead to more expensive downstream issues. Removing a dash to replace a heater core (if that's what it takes on a Ram, is a LOT of work). I've done it on other vehicles. If the valve is on the downstream (discharge) side of the coil's coolant flow, that would probably be a safer spot for it. A bypass valve would be even better.

Here's what a bypass valve looks like. There are different styles (this is just one). There are also solenoid actuated bypass valves too (do a search).

https://www.amazon.com/Four-Seasons-74809-Heater-Valve/dp/B000C2QSVQ/ref=sr_1_14?crid=23NK3S8D1VKZ2&keywords=coolant+bypass+valve&qid=1661535938&s=automotive&sprefix=coolant+bypass+valv%2Cautomotive%2C211&sr=1-14

 

[John Jensen]

I experienced the same, the coldest vent went from 58 down to 42.

Jeepwalker, FYI, there is a TSB 24-001-20 for 2019-20 trucks reporting poor AC performance. Several have elected to have the TSB completed and several have installed a cutoff valve to employ when conditions warrant it. There have been no reports of heater core damage. Maybe in classic cars but not in a Ram 6.7.

 

[Sean J Blackburn]

Thanks for the heads up jeep walker, but I've been running the same setup on my diesel for 8 years with no issues. If the cooling system has sludge in it then someone isn't doing their maintenance

 

[Jeepwalker]

Thanks for the heads up jeep walker, but I've been running the same setup on my diesel for 8 years with no issues. If the cooling system has sludge in it then someone isn't doing their maintenance

No problem. I added a link to the bottom my last post (above) what a bypass valve looks like. There are a few different styles, and some electrically or manually/vacuum actuated. Just for reference. They would be better and less risky.

 

[Jeepwalker]

....There have been no reports of heater core damage. Maybe in classic cars but not in a Ram 6.7.

Sounds good, but who's keeping track of old Ram heater core plug-ups? LOL.

 

[alex6666]

thanks for all the info above, I did take it to an ac guy to have the ref recharged, presumably he did a pressure check at the same time but nowadays who knows. I suspect its the damper door and my understanding is that it will trigger a trouble code sometimes if not working. Soon i'll have the truck in for an inspection including trouble codes as its about to go off power train warranty and we'll see then. I live in a pretty hot area of british columbia (kamloops) so mid thirty to fourty celcius is common so a crappy a/c on a relatively low mileage truck is disappointing I suspect i will install the valve if I can find someone I trust to do it.

 

[juancache]

A slight;y related question. I going to replace my heater core, it is leaking really bad. I need to take it to a shop to have the refrigerant removed from the AC system. I've temporarily bypassed the heater core using a 3/4 pex 90 degree elbow to connect the two hoses so I can move in the driveway as needed without leaking more coolant into the cab. I'm about a mile from a shop that can remove the refrigerant from the vehicle. Can I have this in place long enough to drive to the shop and back to have my refrigerant removed, without damaging the engine?

 

[John Jensen]

A slight;y related question. I going to replace my heater core, it is leaking really bad. I need to take it to a shop to have the refrigerant removed from the AC system. I've temporarily bypassed the heater core using a 3/4 pex 90 degree elbow to connect the two hoses so I can move in the driveway as needed without leaking more coolant into the cab. I'm about a mile from a shop that can remove the refrigerant from the vehicle. Can I have this in place long enough to drive to the shop and back to have my refrigerant removed, without damaging the engine?

Sure. The 90 degree elbow is taking the place of the heater core. You could drive it forever that way with no harm to anything

 

[GTyankee]

This looks like it is a cable controlled bypass
It looks like when the water is cut off from the heater core, it reroutes the flow of coolant right into the return hose

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Personally, I close the Recirculation Door & i have a custom cut rug like dash cover laying on the dash board.
It keeps the glare of the windshield & the heat off of the dash

You can see the dash cover in this image, i had just laid it on the dash, after about a week of sun, it flattened out nicely.
It looks like this pix was taken in the winter, when i was driving over to Yuma Arizona from San Diego

 

[Hemi395]

The only downside I can see to bypassing the heater core is could potentially cause some sediment to fall out of the coolant thats locked in there. Since these heater cores are somewhat prone to being blocked this could be a concern...