Did you not have a heater control valve on your RENIX system (even though if you'd asked me, I'd have told you you were letting yourself in for more work than you needed?) You should be able to just use that one, since it serves the same function either way. It would appear that was how it was done on the 89 that I've got that's converted (only because I bought it that way. I'd not have done it myself - the closed system is just ducky.)
Failing that, there's no real reason at all that the heater valve can't just be deleted. I'm also toying around with the idea of putting in a cable-operated valve as part on the next improvement to my copper heater lines - just because I like having manual control of things. However, I have deleted the heater control valve on my 88 - it's been gone for five years or so - and I'm not having any trouble as a result.
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