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Question DetailsAsked on 12/12/2016

i replaced the thermostatic shower valve and still only warm not hot

new tankless water heater . Only in one of three showers has the problem . Can't return the $240 valve and have the same problem . It's a expensive Graff fixture with the one temperature mixing handle. o rings look ok on hot side of valve . Could it be hard water build up in the hot water feed line ?

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2 Answers


Obviously since other showers are OK, is not the hot water supply. Assuming you have full hot water to the sink in that bathroom, that would rule out supply pipe problem except right at the shower.

My first guess - here is link to their drawings and installation instructions BTW - pick out the model you have and look at the exploded parts diagram and parts list and installation instructions -

Their system is designed for 149 degree hot water and 59 degree (Fahrenheit) cold water input and to produce 100 degree water coming out, it appears. Except most US houses run about 120-130 hot side supply (or less if long pipe run to usage point or if water heater is turned down for child/elderly safety), and 40-55 cold side except in very warm climate areas, so it would run luke warm in most normal cases - and you need to follow the instructions about how to recalibrate the valve for your particular temperature. Plus many people consider 100 degree water on the warm, not hot side - 105-110 is more normally considered the proper shower temperature by most people without sensitive skin.

So - you would most likely have to adjust the temperature limiting ring (item 9 in the drawing) to the hotter direction (hopefully it has a warmer/cooler arrow on it, or you might have to experiment) to compensate. Not clear from the diagrams or instructions how to access this - sometimes there is an almost flush trim ring with a very small recess or tab you turn, or sometimes a slot in the cover plate yuou can adjust it with using a small pointed tool like a small screwdriver, sometimes a small adjust screw that turns the adjustment ring, sometimes the handle and cover plate have to be removed to adjust it, but having put the valve in (if this was a DIY job) you should be able to figure that out. If done by a plumber then he might come out for less than full rate to adjust it, or tell you how over the phone. he should have tested it when there and adjusted to suit at that time.


If that does not solve it -

The link also provides a help line to the manufacturer - they should be able to help you diagnose it if the temperature setting ring is not the answer, as it likely is.

As to hard water in the pipe to the shower - maybe, but if other pipes are not showing restriction I would not suspect that first. You could check it by testing the hot water flow without the thermostat control unit in - obviously easier to do with a temporary piping or hose connection before the valve houseing is installed, but can be done with some difficulty afterwards. I am not familiar with that brand, but some like Delta the outlet from the pipes into the valve unit is sealed to the outside and open just to the front, in which case after removing springs and O-rings (so they don't get lost) you can judiciously duct tape around the valve to keep any leakage out of the wall, then with one person in the shower area watching and another to turn the water supply valve on for a brief shot, you can run a few seconds of water through the open valve assembly to ensure both hot and cold are running full force. Want to do this with shower doors mostly closed and a large towel handy to suppress the spray if it goes wild, and be sure the valve person can hear you (voice or cellphones) to shut the water off quickly if needed. This will show if you are getting low flow from the hot side - also can blow out any loose debris in the system. In your case - from the diagrams (not knowing what model you have or the couple of thermostatic ones they make) this may not be possible.

If getting full flow and housing, then try pouring some water through the cartridge (if a cartridge type, which yours does not appear to be) to be sure the openings through it are clear - might have had some debris or a piece of rotted O-ring plugged up in it.

Running a fuzzy (not the almost bare wire type) pipe cleaner carefully into the inlet and outlet holes in the valve (after disassembling it with water turned off) to be sure they are not blocked might help too.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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