• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Removing heater control valve.

UltimateG

NAXJA Forum User
Location
Tahoe
Hi,

I am preparing to permanently remove my heater control valve and just run the hoses straight into the heater core.

I’ve read that these hoses are not all the same size. I’ve believe both heater core hoses are 5/8”, The water pump outlet is also 5/8”, and the hose into the thermostat cover is 11/16” Is this correct?


So, is it possible to stretch the 5/8” onto the larger pipe at the thermostat cover?
Or, Is there some other simple way to connect them together?

Thanks,
G




94 sport, 4.0. A/C
 
Some wonder just why there are two sizes. The physics behind the decision is as follows:

By utilizung a larger return line that supply line, low pressure on the return side is automatically created for the volume of coolant being pumped. This assists the flow of the coolant up hill to the core by a very large margin.

On my 98, the hoses are 5/8" dupply side and 11/16" return side. So, the cross-sectionaal area of the 5/8" line is 1.2265625 square inches and the 11/16" line is 1.484140625 square inches which is an increase of 21%. In terms of fluid dynamics, 21% is a huge change in volume. If this increase was applied to the engine displacement, the new displacement would be 4796.44cc's I think everyone will agree that is huge.

If the OP decides to hard line the heater core, the two sizes must be used for the system to operate correctly as designed.

This ends todays lesson on fluid dynamics, class dismissed. Hope the theory helps someone...
 
Some wonder just why there are two sizes. The physics behind the decision is as follows:

By utilizung a larger return line that supply line, low pressure on the return side is automatically created for the volume of coolant being pumped. This assists the flow of the coolant up hill to the core by a very large margin.

On my 98, the hoses are 5/8" dupply side and 11/16" return side. So, the cross-sectionaal area of the 5/8" line is 1.2265625 square inches and the 11/16" line is 1.484140625 square inches which is an increase of 21%. In terms of fluid dynamics, 21% is a huge change in volume. If this increase was applied to the engine displacement, the new displacement would be 4796.44cc's I think everyone will agree that is huge.

If the OP decides to hard line the heater core, the two sizes must be used for the system to operate correctly as designed.

This ends todays lesson on fluid dynamics, class dismissed. Hope the theory helps someone...

Are you saying the flow is from the water pump, through the heater core, into the thermostat housing?
 
Deja vu...

At least on a 97+, the supply line (top of thermostat housing to the top of core) is the larger diameter hose. The return line (bottom of core to water pump) is the smaller diameter hose.

I know there are a few different cores listed for various generations of the XJ, but I believe that some of the earlier years use the same core as the 97+. Anyway, what I'm getting at is that the inlet on the heater core is larger than the outlet, and I think that's true for most, if not all, XJs.
 
For the 97+ setup.... Volume per second remains constant regardless of inlet or outlet diameter (if it didn't, you would eventually create a black hole on the supply side as the volume on the outlet side kept going up). Fluid in the smaller pipe travels faster but that would be true if their order was reversed too, so it doesn't really affect the overall pressure. It just doesn't matter on this kind of setup which side is big or small. The only reason one way would be more or less efficient is that some more of the work from the initial pressure source would be lost over the longer distance, which is possible but not significant factor.

Now then... On my Cadillac, the heater core pipe is larger than the bypass return, and the valve just blocks off the core. So when you open the valve, the resulting difference in pressure (as determined by ~6 feet of tubing) causes the fluid to divert that way. There is less restriction on the heater core-return than the regular return and the fluid dynamics performs as described

The older XJ had a more complicated design, but was probably similar to the Cadillac (I converted mine a long time ago so don't have anything to look at). I suspect the newer style just kept the old-style setup for parts compatibility, since there is no significant impact on performance.
 
Last edited:
Are you saying the flow is from the water pump, through the heater core, into the thermostat housing?

The flow is from the T-Stat housing to the core back to the inlet side of the pump.

The volume of coolant pumped is a constant. The pressure drops on the return side to enhance flow. If there were not a pressure differential, the coolant would stall in the return line.

This is the very reason I return my core to the upper hose instead of to the pump. As I run an electric water pump, the impellor has been removed from the mechanical pump. This created a situation where by the pressures on both sides of the core were the same and as a result, no flow through the core. By allowing the coolant to bypass into the upper hose, the pressure differential was reestablished and the core started working as it should.

Physics... Just physics... Look up "Bernoulli's Principle" for the math.
 
Okay, I did the job this afternoon. So to answer the original question:


The outlet on the thermostat cover and the inlet on the top of the heater core are 3/4". All it took was one standard 3/4" heater hose to connect the supply side of the heater core.

The outlet on the heater core and the inlet to the water pump fit a 5/8" heater hose.
 
And that makes me wonder. That stupid leaky control valve I bought from Napa has 5/8" fittings on all four corners. That means the supply side has 3/4 hose slipped onto a smaller 5/8 fitting.


Could this be the reason why it's always dripping? What a stupid design.
 
At least according to RockAuto, the thermostat to valve, and valve to heater hoses are 5/8" on one end, and flare out to 3/4 on the other. The thermostat housing is 23/32, so I would presume the 5/8" end connects to the valve.

It would seem like the 97+ hoses would easily replace the valve and separate hoses. Curious if that's actually the case.
 
Last edited:
Yeah the older style has tapered hoses with different sizes at each end. So you have to buy fitted hoses if you keep the valve.

The newer style hoses will work. I replaced mine with regular cut-to-length hosing but I used the newer style hose clip on the valve cover stud to hold the hoses in place, and that has been working fine. The newer style have 90-degree bends and that opens up the engine bay a little better, but the cut-to-length has worked fine.
 
Well, I just did a little more research, and learned some more. I installed the heater hoses wrong, I crossed the in with the out. The upper pipe on the heater core is an outlet, and the inlet is below. So, to correct what I wrote above:


The outlet on the thermostat cover is 3/4". It connects to the inlet on the heater core, which is 5/8".

The outlet on the heater core is 3/4, and it connects back to the water pump inlet, which is 5/8.
 
Last edited:
Back
Top