Heater Hose Question....

[Larry S.]

Tech question: On mid-years (and probably other years) the heater hose is two sizes, 3/4" and 5/8" I.D. Why did they use two different sizes to and from the heater?

[Bill I.]

Re: Heater Hose Question....

Larry, my guess is so the guys installing them didn't get them mixed up. Bill

[Dan D.]

Re: Heater Hose Question....

I'm not sure either, but I have thought about this before and I believe it may be that the 3/4" lets more water fllow into the heater core, the 5/8" being smaller will impede the flow out of the core. This increases water pressure in the core and thus increases heat transfer. Someone chime in if I am wrong with this analogy. -Dan-

[John H.]

Re: Heater Hose Question....

I'm not sure either, but I have thought about this before and I believe it may be that the 3/4" lets more water fllow into the heater core, the 5/8" being smaller will impede the flow out of the core. This increases water pressure in the core and thus increases heat transfer. Someone chime in if I am wrong with this analogy. -Dan-

Dan -

Coolant flows from the intake manifold fitting to the heater core inlet (bottom nipple) through the 5/8" hose, and from the heater core outlet (top nipple) to the water pump fitting through the 3/4" hose.

The heater core inlet fitting is at the bottom to minimize any "gurgling" sounds inside the car from air bubbles or flow surges through the core. Having the inlet at 5/8" diameter and the outlet at 3/4" diameter promotes more even flow through the core, as the unit pressure is always lower at the outlet than at the inlet.

[Joe L.]

Re: Heater Hose Question....

Dan -

Coolant flows from the intake manifold fitting to the heater core inlet (bottom nipple) through the 5/8" hose, and from the heater core outlet (top nipple) to the water pump fitting through the 3/4" hose.

The heater core inlet fitting is at the bottom to minimize any "gurgling" sounds inside the car from air bubbles or flow surges through the core. Having the inlet at 5/8" diameter and the outlet at 3/4" diameter promotes more even flow through the core, as the unit pressure is always lower at the outlet than at the inlet.

John-----


Yes, many later GM cars use the same size hose for inlet and outlet to the heater core. However, most of these use a special "valve" in the system which is really nothing more than a connector or tee with a reduced size orifice in it to achieve the same thing as the different size hoses. A picture of a typical tee with an integral reducing "valve" is seen below:

[Dan D.]

Re: Heater Hose Question....

Sounds like I was wrong. But appreciate the education. -Dan-

[Larry S.]

Re: Heater Hose Question....

Thanks to all of you for your replies and now I understand the logic of the two different sizes of hoses.