# Bypassing the heater core



## histoy (May 22, 2019)

I have a factory AC car and I want to stop the hot water from flowing through the heater in the summer. I installed a 3/4" shut-off valve in the hose leading from the heater core to the water pump. I understand that this hose is the return, but the valve does stop the hot water flow.

I've read where a couple of guys stated that they installed a water bypass valve for the heater. Could someone please provide some details on how this was done. There doesn't seem to be much room behind the engine to install 3/4" valves for blocking & bypassing the water flow from the engine to the heater.


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## BLK69JUDGE (Jun 10, 2010)

whats your car ?


coolant flow direction - The 1947 - Present Chevrolet & GMC Truck Message Board Network

is your valve on the correct side of flow ??


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## GTOJUNIOR (Aug 7, 2011)

Most common bypass seen under-hoods of modded cars and/or cars with heater core issues is to totally take it out of the loop.
No hose(s) connected to core nipples this is a short term way to deal with issues and subjects the core to further issues without fluid running through it.

Look for a late model vacuum or electric actuated Bypass valve assembly. These are set up to truly Bypass the core while keeping it in the loop if needed. 
They will have duel inlet and outlets with the switch that opens and closes the lines/hoses, something like shown below. 
They are compact enough to be tucked/hidden if desired.


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## histoy (May 22, 2019)

My car is a '66 GTO. Technically the shutoff valve should be on the input side of the heater core, not on the return, to keep hot water from entering the heater core.


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## PontiacJim (Dec 29, 2012)

histoy said:


> My car is a '66 GTO. Technically the shutoff valve should be on the input side of the heater core, not on the return, to keep hot water from entering the heater core.



Use a 3-port diverter valve and matching hose nipples: https://www.amazon.com/L-Port-Forged-Valve-Handle-Female/dp/B0741417WN


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## histoy (May 22, 2019)

Yes, that would do it PontiacJim, but it would take 1 in each hose to make it work, and the return would have to be 3/4". Thanks for the suggestion. And thanks to GTOJunior for your suggestion.


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## PontiacJim (Dec 29, 2012)

histoy said:


> Yes, that would do it PontiacJim, but it would take 1 in each hose to make it work, and the return would have to be 3/4". Thanks for the suggestion. And thanks to GTOJunior for your suggestion.


Yep, it would and that's why that valve has threaded ends to insert whatever pipe fitting/adapter needed to match your hose requirements. Pretty simple stuff if you have a plumbing store or Home Depot or Lowe's near by.


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## Lemans guy (Oct 14, 2014)

here are some photos of mine, just put a shut off valve in main line and one in bypass as well. this is summer running, in the fall I reverse the valves. close one and open one. it lays pretty flat, has worked great for years,...just outside air blows thru all the vents when heater core cut off. but you must put your heater lever all the way to the right, that is what allows the air to blow thru the heater core.

works great


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## histoy (May 22, 2019)

Thank you Lemans guy. I appreciate the pictures.


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## the65gto (Oct 9, 2008)

On my 65, I just been able to connect the hose that normally goes to the heater core (input side) directly to the passenger side cylinder head connection.


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## John Schutt (Aug 27, 2018)

Many Gm Vehicles have had the vacuum valve that stops the supply flow,
https://www.carid.com/four-seasons/...MIu6mbhoHB4wIVEtvACh0xFA3LEAQYBSABEgI_7_D_BwE
These don't always stop the hot water from migrating to the heater core through the return.
Why not use the below that bypasses the core until its needed?
https://www.amazon.com/ACDelco-15-5...ocphy=9006397&hvtargid=pla-569618123922&psc=1


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## histoy (May 22, 2019)

Thank you for your suggestions. I have a manually operated shutoff valve in the 3/4" return line that does basically the same thing as the first one you mentioned, but I'm not sure how that impacts engine cooling and water circulation in the block. I looked at your second suggestion, but it has only 3 water connections. I found an AC Delco #15-5533 that has 2 5/8" & 2 3/4" connections. That looks like it would do the trick. Thanks for leading me to an easy solution.


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## John Schutt (Aug 27, 2018)

Look at:
Four seasons 74800
AC Delco 15-5302
GM 14044414
Its a metal valve 5/8" and 3/4" bypass, meaning the coolant will still flow as designed.
Non are available local to me.


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## histoy (May 22, 2019)

Thanks


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## John Schutt (Aug 27, 2018)

I Ordered mine yesterday, I should have done this earlier as we're approaching record setting temps.


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