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Reversing flow to heater core

The heater on my 2001 ford F150 is not heating. The blower is working, the inlet hose is hot, the outlet hose is cold. Removal of hoses confirms that there is very limited flow through the heater core. I flushed it but got only minimal improvement. However when I back flush (reversing the flow through the core) the flow is great. Here’s the question: Can I reverse the flow through the heater core on a permanent basis? It seems this would give the needed flow of hot fluid through the core to allow heating. Is there a problem with doing this?

I assume the hoses are the same size, I can’t think of a problem with reversing flow. It does seem odd the flow would be better…

Seems there may be a restrictor on the inlet to slow the flow through the core, but I don’t see how reversing would hurt anything. The hoses are the same size and the “crossover” looks easy. I think I’ll try it and see what happens.

Seeing how a new heater core can be had for less than $50. I’d just replace it.

@FoDaddy - how big a job is it?

@texases , I’m not sure, but the OP seems to have been able to get to it in order to test out the original one, so I’m assuming he could swap it out fairly easily.

The hose hookups are often pretty easy to get to, but getting the core out could be a lot of work, if dashboard disassembly is required…


Dashboard has to come out. More than $1200 art the dealer and it would take me a week or more!. $5 for the hose connectors to they this. I’ll know in 15 minutes if it’s going to blow up.

Let’s hope hot antifreeze doesn’t start spraying on you legs. ;=( Or, you may have found the secret cure to all clogged heater cores.

OK, it seems to be working, I have heat and no water or steam in the cab. The heat seems to be +/- 50% of what it should be but I can live with that and some say it may get better as the hot fluid circulates for a while.


What did you use to flush it?

A regular garden hose by itself isn’t powerful enough

You need to use a water nozzle on the “jet” setting. Blast it out from both sides. I’ve done this several times, on various vehicles, with good success.

I have replaced heater cores on your body style F150. It’s a major PITA . . . and there are no shortcuts. The dash has to be removed completely, or at least moved far enough back that you can “sneak out” the heater core

BTW . . . have you been neglecting your coolant? It should generally be serviced every 5 years. Most plugged heater cores, in my experience, are due to neglected coolant

Looks like you’ve got your heat working again, I doubt you’ll have any trouble running it that way, at least for the near future.

BTW @FoDaddy, heater core replacement for this truck bills out at 7 hours, plus servicing A/C system if needed. Not something fun to do.

I was under the impression that the OP had already removed the heater core and was bench testing to when he mentioned that he found that it functions fine when the flow is reversed. I apologize for the misunderstanding.

Makes me long for the 40s and 50s when they put the whole heater assembly under the hood and just ducted the hot air into the car or the Valiants and Darts where you only had to remove one j-boly to drop the heater assembly under the glove box.

The only thing I can think of is. What happens if you flush the trash clogging the heater core back into the block?

How has this worked out for you, I just reversed the heater hoses on my daughter’s Ford focus with same issue and she now has heat again. Will be watching closely

By reversing the hoses, the coolant will flow in reverse direction through the core, but it’ll still be flowing in the same direction as far as the engine is concerned… but that will not prevent it from carrying crud from the core into the engine cooling system.

I’m inclined to agree with PvtPublic. I wouldn’t want to be flushing core-clogging crud into my cooling system either. It has clogged the heater core, it may now plug the radiator… or coat the inside of the engine.

I’d recommend bypassing the heater core and dressing warmly until it can be replaced.

The correct way to flush the heater core is to disconnect both hoses. Then take a 5 gallon bucket with a gallon of water in it and half a gallon of CLR and use a small utility pump such as you can get at Harbor Freight to pump the CLR mixture from the bucket through the core and back into the bucket. Let this run for at least 2 hours. This will dissolve any of the deposits inside the core that are dissolvable. Then disconnect the pumping setup and run at least 30 gallons of clean water, using a garden hose, through the core to flush out the CLR. Then hook up your normal engine heater hoses, refill your radiator, run it enough to get the air out and top off the radiator again

I suspect that after 4 years, OP either fixed the problem, or has frozen in place at the wheel. :wink:

Jeeze. I never looked back beyond Joseph Wagner, who resurrected it.

I still say that something needs to be done with this format. Instead of adding new “features”, perhaps the webmasters can work on that problem.