Heater core flow direction?

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Bonnewagon

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Someone on another site (not us) swears that swapping heater core hoses from one side to the other made a huge difference in temperature output. He reversed the normal "IN" and "OUT" hoses. This was after a good flushout. On his vehicle they were both 5/8" so it was easy. I'm thinking it should not matter which direction the coolant flows but maybe I am wrong?
 

Ugly1

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Someone on another site (not us) swears that swapping heater core hoses from one side to the other made a huge difference in temperature output. He reversed the normal "IN" and "OUT" hoses. This was after a good flushout. On his vehicle they were both 5/8" so it was easy. I'm thinking it should not matter which direction the coolant flows but maybe I am wrong?
It would not matter. The fact he flushed it out made the difference. A little air burst with the water back flushing the flow direction cleans out the build up and old stop leak ( if it was ever used). You have to be careful not to use to much pressure and shred the core.
 

Bonnewagon

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After he flushed he took the temps at the ducts. He claims it was around 90° and after swapping hoses it was 134°. I know that it "should" not matter and agree. But another guy chimed in and verified the results. I am going to try this on my Jeep. It uses 5/8" hoses on both sides.
 
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69hurstolds

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All he did was remove the crud in the flush, likely unclogging some passages allowing more hot fluid to run through the heater core increasing air temps. If you clean out a radiator or heater core every now and again, you tend to get better heat transfer. Imagine that. A better test would have been to put the hoses in the same place after the flush and do the temp measurements to confirm the effectiveness of the flush. I'd challenge him to swap hoses again and see what his temps are. I'd bet they would the same.
 
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Bonnewagon

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I know it sounds hokey. I am going to try it and see. What harm could it do? For that matter-why is my Pontiac using a 3/4" hose going in,and a 5/8" hose coming out? What is the concept?
 

Ugly1

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I know it sounds hokey. I am going to try it and see. What harm could it do? For that matter-why is my Pontiac using a 3/4" hose going in,and a 5/8" hose coming out? What is the concept?
[
It varied from manufacturers engineering as to what they did. Some it’s for heating/water flow others it’s noise reduction ( should hear them whistle!) and have to add a restriction part in the hose. Others it’s just cost.
 

Bonnewagon

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OK. I reversed the cooling lines. The duct temps went from 120° to 145°. Not huge, but a difference just the same.
 

69hurstolds

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OK. I reversed the cooling lines. The duct temps went from 120° to 145°. Not huge, but a difference just the same.
Wow. With that, you can bake a chicken darn near. Since it flows through the same heat exchanger, all things else being equal, you should be seeing the same temps. Unless there's some sort of a flow change by reversing the heater hoses. Some tubes are spiral shaped, others have twisted plastic inserts to induce turbulent flow and minimize flow noise.

With the heat transfer equation, you still should be pumping out the same BTU in theory, regardless of which direction you go through the heater core. Something had to change.

Was this WITH a pre-flush or without? If you didn't flush the core first, there's a possibility you basically did an inline backflush on the heater core unblocking some crud by pushing junk the other way. I don't know. Simply speculating. Regardless, better and hotter heater isn't a bad thing.
 

Bonnewagon

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Yes, it was flushed first. It seems to clog up once a year, right when it gets cold. So I was reading about it and the idea of reversing the hoses was proposed. I rig up a garden hose to attach to the heater core tubes and blast away. I alternate each tube. After a big glob of crud blasts out, they flow just fine either direction. All I can think of is that the inside of the core is designed to flow well in the normal direction. Going the reverse way may cause a slight impedance that slows the flow a bit. But as we all know from radiators, the slower the flow, the more heat can be released. 25 degrees is a nice increase for doing nothing but reverse the hoses.
 
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69hurstolds

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Technically, the higher the mass flow rate, the more heat is transferred, assuming the delta T remains constant. I know it's hard to get your head around that concept, but faster flow in a radiator (to a point) is a GOOD thing. Turbulent flow rules the day.
 
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