By joining them you create a shorter return route for the hot coolant
that does not include the radiator (where the liquid is "cooled"). By
blocking the two hoses all coolant now goes to the radiator before
returning to the engine. With the heater core in the circuit it acts
as a mini radiator removing heat from the coolant (and blowing it into
the cabin) before it returns to the engine.

For the most effective cooling of the "coolant" it needs to run
through the radiator or an intact heater core with the fan on.
Cheers
Don

On 24-Aug-09, at 6:35 PM, miguel pacheco wrote:

> Correct me if I'm wrong, but what you describe as a 'short circuit'
> pretty much simulates coolant flowing through the heater core.
> I have read here, on this forum, that, once the core is removed, the
> in and out hoses can either be blocked off, or joined.
> Miguel
>
> On Mon, Aug 24, 2009 at 3:47 PM, Don Spence<dkspence@telus.net> wrote:
>> Not a great idea as this just creates a "short circuit" for the hot
>> coolant to bypass the trip up to the radiator and allows HOT coolant
>> to return to the engine. Better to plug or clamp off both heater
>> hoses thus sending all the hot coolant to the radiator.  (Thanks to
>> Dennis for pointing this out to me a couple of years ago.)
>>
>> On 24-Aug-09, at 3:22 PM, Automatic digest processor wrote:
>>
>>> Date:    Mon, 24 Aug 2009 16:47:01 -0400
>>> From:    Jeff Lincoln <magikvw@GMAIL.COM>
>>> Subject: Re: rear heater blowout - stranded
>>>
>>> Matt,
>>>
>>> I'm not saying your blocking the hoses will hurt anything - because
>>> I really
>>> don't know - it might.
>>>
>>> What I did when this happened on an '85 I used to own was to get a
>>> length of
>>> high temp hose and fasten it between the send and return hoses you
>>> speak of.
>>> Creating a loop so the coolant could still flow. That "repair" is
>>> still in
>>> place because a friend of mine now owns that van.
>>>
>>> At least that way you could keep traveling with no worries and get
>>> a new
>>> heater core at your convenience.
>>>
>>> Good Luck.
>>
>
>
>
> --
> Miguel