Cartridge ciculator sticking

I have 2 zone forced hot water heating system with 2 TACO 007-05 Cartridge ciculators. One zone works without a hitch. The second ciculator seems to hum a little and not deliver hot water. (The pump is hot but the lines stay cold). If I tap on the circulator it frees up and runs fine as long as I am calling for heat. Once the system cools I have to force it on again.
I am hesitant to replace the ciculator because it works when running. Is there another system level issue that could cause this or do I have a bad part?
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"ForgottenOne" wrote I am hesitant to replace the ciculator because it works when running. Is there another system level issue that could cause this or do I have a bad part?
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Replace it now and you won't have to when it won't start again in January when it is 20 below zero.
Check out sources as I've seen the from $60 to $200. I got one at the plumbing supply for $20 less than Home Depot.
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responding to http://www.homeownershub.com/maintenance/Cartridge-ciculator-sticking-596384-.htm forgottenone wrote: Would you replace the entire unit or just the "cartridge"?
Seems like the "cartridge" Costs more than the entire unit at most places.
Ed Pawlowski wrote:

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