Heater Control Valve Requirement

jonn6464

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Hey FABO,

I'm installing an aftermarket A/C & Heater combo in one of my projects. Do I absolutely need a heater control valve? I understand that if it's not there, the engine coolant will just flow through the heater at all times. But, without the heater blower running there won't be any hot air coming out of the vents. Is this an issue? Is it a problem for the heater core?

Thank you for any replies.
 

RustyRatRod

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Lots of older cars didn't come with one, so why not? My 64 Valiant does not have one. It controls the temperature with the HVAC box.
 

72bluNblu

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The heater core will still dissipate heat into the cabin even if you don’t turn on the blower motor. Maybe it won’t blow out of the vents, but it will still raise the temperature inside.

Also, it depends on your AC. If the new AC unit uses the same vents and box as the heater core is in, well, when you turn on the AC it will blow that hot air too and you will significantly reduce the ability of the AC to cool your car.

A manual valve in the heater hose is really easy to add, you just turn it off when it’s “AC season” and you’re good to go.
 

str12-340

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even though the heater control valve in earlier cars had the valve in the heater box, it still effectively stopped water from circulating in the heater core, right? I'm pretty sure they moved the valve to the engine compartment to make servicing easier and to avoid collateral damage like antifreeze soaked carpets.
 

jonn6464

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The heater core will still dissipate heat into the cabin even if you don’t turn on the blower motor. Maybe it won’t blow out of the vents, but it will still raise the temperature inside.

Also, it depends on your AC. If the new AC unit uses the same vents and box as the heater core is in, well, when you turn on the AC it will blow that hot air too and you will significantly reduce the ability of the AC to cool your car.

A manual valve in the heater hose is really easy to add, you just turn it off when it’s “AC season” and you’re good to go.
The A/C is a separate under dash unit, self contained. The heater core is in a different box, with it's own outlet ducts.

I was thinking there would be some radiant heat just from the hot coolant passing through, but I wasn't sure how much that will be felt.

I saw a little electric valve for $30 and thought maybe I could wire that into the heater fan switch. Switch ON=Valve Open
Switch OFF=Valve Closed
 

str12-340

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I'm getting ready to buy a new system for one of my cars, what brand of system are you using?
 

72bluNblu

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The A/C is a separate under dash unit, self contained. The heater core is in a different box, with it's own outlet ducts.

I was thinking there would be some radiant heat just from the hot coolant passing through, but I wasn't sure how much that will be felt.

I saw a little electric valve for $30 and thought maybe I could wire that into the heater fan switch. Switch ON=Valve Open
Switch OFF=Valve Closed

Yes there will still be radiant heat in the car from hot coolant going through the heater core if you don’t have a valve.

I know I definitely notice the heat in the car on longer drives if I forget to close the temp control slider on the dash that closes the heater valve. I don’t have AC in my car, but it’s really obvious if I forget to reset the temp slider in the spring and the fall. I need the heat in the cool/cold mornings but in the hot afternoons leaving the slider on warm heats the cabin up fast even without the blower going.
 

RustyRatRod

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even though the heater control valve in earlier cars had the valve in the heater box, it still effectively stopped water from circulating in the heater core, right? I'm pretty sure they moved the valve to the engine compartment to make servicing easier and to avoid collateral damage like antifreeze soaked carpets.
My 64 Valiant has NO heater control valve as I mentioned in post #2. The temperature is controlled by the doors in the HVAC box. There is NO valve that stops hot water flow.

Also, FWIW, we feel no heat whatsoever in the cabin. I totally rebuilt the heater box shortly after I got the car, so that may have something to do with it. Also, the fresh air doors and vents work very well, so I'm sure they make up for some of it possibly.
 

RustyRatRod

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Yes there will still be radiant heat in the car from hot coolant going through the heater core if you don’t have a valve.

I know I definitely notice the heat in the car on longer drives if I forget to close the temp control slider on the dash that closes the heater valve. I don’t have AC in my car, but it’s really obvious if I forget to reset the temp slider in the spring and the fall. I need the heat in the cool/cold mornings but in the hot afternoons leaving the slider on warm heats the cabin up fast even without the blower going.
Kitty and I sure cannot tell any radiant heat at all on Vixen. But as I said, I rebuilt the box when I got the car and the fresh air vents work very well so I'm sure that helps.
 

Mike69cuda

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On A bodies, the non ac cars circulate coolant all the time.

On factory ac A body cars the under hood control valve turns off the coolant flow.

If you have aftermarket air, you need to install the system the way it was designed for it to work as designed.

In any of the above cases you can always install a $5 heater hose manual shutoff to keep the hot water out of the passenger compartment during the summer if you want.
 

67Dart273

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On several of my vehicles "in the old days" I've installed a manual valve, just "hung it in the hose. I used to use a generic 1/4 turn ball valve, 3/8 NPT and 3/8 NPT nipples. They work pretty good with 5/8 heater hose
 

BillGrissom

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I have an under-dash AC knee-knocker box in my two A's. I put a valve in the heater hose of all, using a small vacuum switch to a vac-operated water valve. I tapped the "on" wire inside the knee-knockers. As a half-measure, you could tap the AC clutch wire since already under the hood, but that would only stop flow when the compressor is running. Wasn't a big deal since the AC has a separate cabin fan, but I figured why not do like Chrysler did in my 1969 Dart w/ factory AC.

In one I used a little vac switch from a Porsche ($10 ebay) and in others a vac switch from a 1980's Mopar (PickNPull). The water valves have 1 vac line w/ spring to reverse. One is for a 2000's Ford ($8 ebay) and other a metal generic one ($12 ebay). I did the same in my 1985 M-B 300D since the solenoid water valve ("monovalve") is notoriously problematic, especially with the poor rebuild kits from MTC (India). Later M-B used the same setup I have, and indeed one company packages that as a retrofit (pricey w/ M-B parts). If I recall, my 1969 Dart factory water valve had 2 vacuum hoses, but most today use a spring return.
 
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