thermostatic mixing valve (Honeywell AM-1 1070) seized after <3 years?

[Danspot]

My wife has been complaining for a couple years about the tub only being lukewarm (100F) yet sink, shower, etc. gets to 120F, and I finally got to this item on the list.

After doing some research, I looked for a mixing valve under the tub and found none, so went off in other directions until I noticed a mixing valve under the adjacent sink that was flowing to the tub.

It's a Honeywell AM-1 1070 (incidentally the only place they decided to install a mixing valve).

I unscrewed the cap and tried to turn it, no luck. Removed the cap and used increasingly larger pliers and a monkey wrench to try to turn the valve, but all I got were some nice brass shavings.

I'm guessing the valve has seized up. I'd love some perspective and suggestions on what to do, including:

1 - Is it reasonable for the valve to seize up after <3 years and very little use? We have very good water (Culligan-tested). Months ago we had some crud flowing through the hot water line that periodically clogged up the shower head, but that has disappeared.

2 - Am I trying to open the valve the wrong way? I'm trying to turn this brass cylinder that's about 3/16" diameter and 1/2" tall, with vertical grooves down the sides and a screw hole in the top. It won't turn either direction. I was able to rotate a black washer-type thing that is attached to it, which maybe has something do with a preset turning limit; it had no impact on the water temp.

3 - As a rank amateur, are there other options than calling in a plumber? I could theoretically turn off the water to the house, flush the lines, unscrew the hex nut(s) for the valve, take it apart, soak it in vinegar, brush it with an old toothbrush, put it back in, turn it (hopefully), and then put the water back on, but that's very hypothetical. Like, I might lose a part, put it back together wrong, brush so hard a break a spring or screw or something, in which case I'd have a busted valve and I believe an inoperable plumbing system until I can replace it (can't turn the house water back on if the valve is missing).

4 - I guesstimated to the wife $400 for the plumber. Is that reasonable? I'm assuming they could replace the valve, or just the entire setup.

Thanks for reading this and any advice you might have.

 

[CountryBumkin]

Here's the instructions for that valve https://forwardthinking.honeywell.com/related_links/water/mixing_valve/install/62_3098.pdf
You need to loosen the screw on top, lift the knob, then turn to adjust.

If it still doesn't turn, it is likely that it is frozen. Maybe some WD40 or PB Blaster would help. You may be able to take it apart and clean it (but if it do, have a rebuilding kit on hand, as it may not go back together if the seals have swelled (if it has rubber-type seals) then you'll have no water in the house until you get this valve back together.

Is the problem with the Tub water that it is coming out too cold or that when the Tub is full it's too cold - because it could be that your running out of hot water before the Tub is full. Not too uncommon with large Garden/Spa Tubs (i.e. 60+ gallon Tubs) with small water heaters (i.e. 50 gallon WH).

Maybe you could turn up the temp on the water heater (say to 130* assuming it's at 120 or 125 now)) then the setting on the mixer valve would allow more hot through it. But then you would need to have mixing valve already on the shower or its water will be too hot.