no hot water only in the tub (sink is fine)

My guest bath tub is not getting hot water. We don't use it that often but the last time we used it had hot water. The sink in the same bathroom gets hotwater only the shower doesn't. I read on the google and someone suggested cleaning the "filter under the faucet". Where is it located?
We are expecting family for holidays. We need to get it done asap. Thanks for all your help.
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ted wrote:

No filter on the tub. Disregard.
Post back with the type of faucet. Single Handle? Brand ? How old?
Jim
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scribbled this interesting note:

I recently had an interesting problem come up in one of our houses.
My tenant called me one morning to let me know she had to take a cold shower that morning, but she was able to give the baby a warm bath in the kitchen.
The hot water was no making it to the bath tub.
So I pack up the plumbing box and other items and head on over. Sure enough, there is no water going to the hot side of the tub faucet. I try the lavatory, and no water there either. The kitchen is fine, plenty of hot water there! I look at the water heater and all is fine with it.
Next step? Clear out a closet and open the access hole. Had to clear out all the little girl toys from the closet first!
I peek under the house and what do I see? I see an air conditioning refrigeration line. And it has ice on it. And it runs directly under the hot water line to the bathroom. Both are well insulated, but that made no difference when the a/c froze up!
I look at the filter in the air conditioner and sure enough it looks like it hadn't been changed since these folks rented the house (and it was a new filter when they moved in, I always make sure of this.) The coils inside are completely frozen too.
In this case, when there was no hot water to the bathroom, the cause was a clogged filter in the air conditioner. Not saying this could be the case for the original poster, but odd things like this happen from time to time. I called my a/c guy and he said to just let run for a while and thaw out and all will be fine. It was the first time in thousands of calls that he'd ever heard of that happening!
Sometimes a plumbing problem is just a plumbing problem. Sometimes it is a hvac problem. Dig in and figure it out!:~)
-- John Willis (Remove the Primes before e-mailing me)
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It's a Moen single handle. one year old cartridge. We replaced the cartridge last year due to a leak. Thanks.
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ted wrote:

I wouldn't rule out a failed cartridge, or a remnant of an old O-ring which is blocking a passage. Pulling the cartridge to inspect should tell.
While it's out, flush the passages by briefly turning the water supply back on. Be prepared for the stream of water which will shoot out. Jim
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snipped-for-privacy@yahoo.com (ted) wrote:

If it is an anti-scald valve, then chances are the insert is plugged with minerals. The water needs to be shut off, the faucet dis-assembled, and the cartridge (~$15) is replaced, and everything put back together.
An episode of askTOH had this on a couple of weeks ago.
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