Heater Core Direction

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Pierps

Pierps

10 Year
Joined
Mar 14, 2011
Threads
29
Messages
689
Location
Seattle
At the risk of sounding like an idiot, does it matter which direction the coolant flows through the heater core ('76 FJ40, SBC 350)? I searched but couldn't find an obvious answer.

I just re-built my heater using one of Shane's kits. Sand blasted, re-painted, etc. Huge improvement in air flow in all 3 settings (heat, vent, defrost) but when I set the heater to "Heat", all I get is regular temperature air, even at 190-200 degrees.

When I had it out, I flushed the heater core, and when I did so, it didn't seem to matter which direction the water flowed. It flowed nicely regardless. No gunk came out of the core.

When I touch the hose going into the heater core at 190-200 degrees, it's hot... but the other hose is only warm. It makes me wonder whether the heater core is only one way...

If it's two-way, what could explain this? Again, when I flushed the heater core, the water flowed through really well. It didn't have any obstructions.

Thanks in advance!!
 
it doesnt matter. Either way hot water will flow through the core allowing the air to warm as it passes through.
 
That's what I thought... It's weird that it doesn't blow warm air... Maybe the engine temp isn't high enough?
 
Pierps said:
Maybe the engine temp isn't high enough?
Click to expand...


Very possible. Also, the core itself may be clogged with the usual cooling system scale and stuff...also, the valve to the heater core may be likewise clogged and/or simply not opening.
 
x2 Direction dosn't matter. The core will heat up no matter which direction. However, some heater control valves are directional, usually they have an arrow on them to indicate direction, and will function better if installed correctly. They will work installed backwards but may leak by in the off position causing the core to remain hot all the time. Check function of the heater control valve.
 
Thanks guys. When I had the core out, I flushed it with water. The water came out at the same rate it went in, and the water was clean... So I didn't think I needed to use muriatic acid or CLR as many people recommend.

Another bit of background... The PO got rid of the oem heater lines and valve, and put in sweated copper pipe and a typical plumbing valve. I redid the plumbing in the same fashion. So the valve is brand new, and I've confirmed that it's open.

The line with the incoming coolant is very hot just before the heater (after the valve). The outgoing line is only warm. So it seems that either the core is clogged (i don't know how that could be since I flushed it) or the engine temp isn't high enough. To be sure, it hovers at about 191 degrees.
 
Oh, and another thought- when I reconnected the heater and the lines, obviously the heater was empty and I lost a bit of coolant in the process.

So I knew I would need to add coolant and burp the system. I'm wondering if maybe I have some air in the system?

I simply added coolant to the radiator and reservoir.

To burp it, I just removed the cap on the radiator and ran the motor until it reached temp and the thermostat opened. I didn't see any air bubbles, and the coolant level kept rising and rising until it spilled over and I had to shut off the motor.

Sorry, I'm not very mechanically inclined, but I'm trying to learn.

Thanks!!
 
It sounds like it should work. Are the heater hoses connected to the correct spots on the engine?
 
Thanks everyone for your replies. Just wanted to follow up. I've driven the 40 more, and I am getting warm air. It's not HOT air, but it definitely is warm.

I've never had the heater working this well before, so maybe i just expected too much. The climate here in Seattle isn't all that bad, and I get plenty of heat through the floor, so I'll probably be fine this winter.

Thanks again.
 
There should be a temperature differential between the inlet and outlet of the heater core, but it should be highest with the fan on high (extracting heat from the heater core). If you have a hot input and a lukewarm output with the fan 'off', then you have a flow issue with your heater core in some fashion.

If you reach no good conclusion, I'd pull the core and put Muriatic acid in it, until it stops bubbling (indicating that all the calcium/mineral gunk has been dissolved. Flush well with water and reinstall. I did this with a '77 heater core and had a nice hot heater afterwards.
 
Thanks!

You're right- if the fan is off, the coolant shouldn't be losing much heat going through the core. Makes good sense.

Now I feel like a bonehead for not using muriatic acid while I had the heater removed. The weird thing is that when I ran water through it to flush it out, there seemed to be plenty of flow. The water came out just as fast as it went in. It didn't seem restricted in any way.

At this point I think I'm going to just wait until it gets cold enough for me to care. If the heater isn't performing at that point, I guess I'll have to take it and use the muriatic acid. Doh!
 
I have a similar problem. I've pulled my heater core out a couple times to flush with muriatic acid, and everything seems clean. But I still can't get good heat in the winter time. I'm hopefully going to go through my cooling system this winter and see what's going on. I think it may be the valve or it may be the water pump not putting out enough head to flow through both heater cores. I have gotten really good heat out of it in the past, so I know the core is clean. It's gotta be something else that I haven't pinpointed yet. Maybe if I use one of Shane's rebuild kits and make sure it's all sealed up tight that will make a difference.
 
I used one of shane's kits for this project. Huge improvement in air flow, and he provides every little thing you need.
 
FYI

Not trying to be a jerk here but it does matter which hose is the intake and which is the output. The heater core is a "Heat Exchanger". Due to gravity you always fill a heat exchanger from it's lowest point to get maximum efficiency out of the exchanger. If you take this to the extreme you could imagine water just flowing through the exchanger without filling the exchanger to capacity, you won't get maximum surface area covered.

There is one exception to this rule that I can easily think of and that is a radiator. The reason it is backwards to a normal heat exchanger is that it is used for the reservoir for the water pump which requires a "head pressure" for intake.
 
fjgoober said:
FYI

Not trying to be a jerk here but it does matter which hose is the intake and which is the output. The heater core is a "Heat Exchanger". Due to gravity you always fill a heat exchanger from it's lowest point to get maximum efficiency out of the exchanger. If you take this to the extreme you could imagine water just flowing through the exchanger without filling the exchanger to capacity, you won't get maximum surface area covered.

There is one exception to this rule that I can easily think of and that is a radiator. The reason it is backwards to a normal heat exchanger is that it is used for the reservoir for the water pump which requires a "head pressure" for intake.
Click to expand...

You're not being a jerk at all... I appreciate your thoughts.

I didn't consider the gravity element when I asked my question- I was thinking more in terms of whether there was some kind of one way valve inside the core (obviously there isn't since I was able to flush and back-flush easily). But the idea of the direction going against gravity as a means of completely filling the core is one I hadn't considered.

When I re-installed the core, I connected it the same way the previous owner had it connected- the line coming out of the water pump was connected to the higher of the two fittings on the heater core. If I understand what you're saying, it should actually be the other way around.

That said, do you think it matters all that much? The fittings on the core are offset maybe 1-2" on center from one another, and the core sits horizontally. So there isn't much of a height differential.

I'll definitely take your word for it if you believe this could cause the heater to under-perform so much.

Thanks!

Peter
 
I doubt this is your issue, but back in college after I put an SBC 350 in, I inadvertently hooked my heater inlet hose to the thermostat housing and you know the rest. It'd blow nice and hot and then cool down and then hot...Anyway, after about a week of head scratching I finally figured it out. Like I said, probably not what's going on with yours, but thought I throw that out there. Good luck!
 
190-200 should be plenty hot.

Once warmed up, the heat coming out on the heat setting feels hot enough to burn bare skin. Can’t remember which thermostat I’m running, but back when the digital temperature gauge was working it was usually around 200-210.

Are you sure the heater valve is fully open? Slow flow could theoretically limit temperatures... even though I’ve tended to find it’s hot or hotter... warm isn’t a thing.
 
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