We use Moen valves and faucets. A professional, licensed plumber, experienced in performing a tub/shower valve replacements can cut the tile with a grinder or multi-tool, install a new Moen valve and install a Moen repair plate to cover the holes from the previous two handle valve. If performed correctly, there will be no evidence that there was a two handle valve except for the repair plate. It is recommended that you have a licensed plumber perform this type of work, as it will require soldering in the wall in a confined area. This is not something the average DIYer should attempt. Typical cost $800-$1200. Trust me, it's worth it. We have to bail DIYer's out all the time who try projects like this and get in way over their heads. Do not go with the cheapest tub and shower faucet. It is not an easy job to replace one, once it is installed. Purchase the best that you can afford. You only want to do this once. Ask your plumber for recommendations. Purchase it through your plumber, if possible, he will usually stand behind it if there is a problem. He will not warranty one that you provide. If you are going to remove the tile and tub, then the valve replacement is much simpler, however, you are turning this into a major project. Pulling the tub usually results in drain issues and flooring issues. Most of the time, this requires floor repairs or replacement and drain replacements. In most areas of the country, there are also ordinances requiring this work to be performed by a licensed plumbing contractor with a permit. If you hire a contractor, please make sure they have a plumbing license and will procure all necessary permits and inspections that your local municipality requires. If they will not do this, look elsewhere, no matter how cheap they are. Good Luck with your project. Here is a link that might be useful: Lemay Plumbing, Inc., a St. Louis Plumbing Contractor

First use a flashlight and check the seat carefully. Some are square drive and some are hex drive. If it is square drive, make sure your square drive tool is long enough to reach all the way through the seat. If its only making a partway contact it could be causing the seat to bind. I am assuming your using a square drive socket on a ratchet. If so, insert the square drive fully, then hold the ratchet by the head with the ratchet handle pointing up. Turn the ratchet until you have full firm contact with no slack in the drive, then rap the handle firmly with another tool. (I use a 12" crescent wrench to rap the handle). Generally it only takes about two or three good raps and the seat will free up so you can turn it. DO NOT USE ANY PENETRATING OILS, remember that is potable water flowing through that valve and you do not want to risk getting chemicals into your potable water. Once you have the seat out, take it to a True Value or Ace Hardware store and they will have replacements. When selecting a replacement you will note that they have Brass, Monel Metal & Stainless Steel seats. If your seat has what looks like a chip on the bottom side of the face, that is generally caused by water quality. I would strongly advise you to pay a bit more and get the Stainless Steel seat. Also, after you install the seat, Make sure the stem is turned to the full open position before you screw it back into the faucet. If the stem happened to be in the closed position, as you make the final turns to tighten the stem body into the faucet it would push your new washer into the face of the washer and instantly distort and ruin your new washer.

I appreciate the response. We have a full clear view of the entire pipe work - full size basement unfinished, so it is not as bad as most people who have to deal with croll space. The lines all run through joists but none are insulated - the lines are just below the insulation. The main lines are 3/4 PEX that drop down to 1/2 PEX for each fixture. They are all on the same level/height. The kitchen is very close to heater - utility room is next to kitchen. The MB is in the furthest opposite end of the rancher. Just take MB in consideration -- it has three different fixtures - two sinks, one bath tub, and a separate shower stall. The tub gets hot water the fastest - almost in 20-30 seconds. I don't mind the wait for shower so much - it is to be expected with on-demand heater. The most irritating problem is that it does not hold temperature through a short shower. I wonder if the sensors in kitchen and shower fixture are worn out -- they get the most usage. All fixtures are original to the house - which was brand new built - 5 year old.

"Why don't you guy's - roof35 and Mongo come into the 21st century?" geoffrey, seriously? No need to throw bricks. I addressed transitional applications in this paragraph: "Is a new thermostatic shower valve right the easiest way to control temp? Probably. But it's not the only way. Especially in remodels in older buildings. Opening up walls to change a valve can result in changes to plumbing, which can lead to all sorts of issues. When remodeling in older buildings or houses, there certainly are different strokes." Not everyone wants a gut remodel.Not everyone wants to open up a tiled wall to install a new valve. Not everyone want to disturb original plumbing inside of a wall. Not everyone can, for myriad reasons. But in old construction, you can go from having no scald protection to having scald protection by installing an in-line thermostatic valve. And they work just fine. Down the road when a true remodel occurs, then tile can be popped, or complete walls can be opened up, and a shower valve with anti-scald protection can be installed. If you want different water temps in different locations, you can do so. It's not that hard to do. It might hurt to think about it, but with "modern construction" what scald protection do you have at a kitchen sink? None. A bathroom sink? None. The only place you have it is in a shower/tub. If you install an "ancient decrepit" whole-house valve, precious children and Alzheimer-suffering old folk across the globe will have scald protection wherever hot water can flow.