Pipe connecting hot and cold

Does anyone know what the pipe connecting the hot and cold under bathroom sinks is for? I just started seeing these in 1 year old construction. There is also some sort of valve on the line. Maybe backflow or pressure regulator.

1 Like

Hot Water Recirculating System.


Thermostatic valve to control the water temperature to the bathtub so that little kids don’t get scald while playing in the tub.


Thermostatic Mixing Valve


The two Marcels nailed it.


Oui Oui! :grinning:


Thanks guys! Google was getting me nowhere on this one.


This valve is used for bathtubs & showers with 2 handled faucet assemblies.

This temperature limiting device is used on single handled faucets

They should be supplied/set to satisfy IRC Section P2708.4 Shower control valves

Been required around here for much longer than a year.

Edit: Was trying to reply to OP. Pretty sure Marcel already knows this.


@bfeltner This valve is used for bathtubs & showers with 2 handled faucet assemblies. >

I have one on a toilet it keeps it from sweating.

1 Like

Yes and no. All modern single valve tub fixtures MAY have a built in stop to adjust the water temperature to satisfy local codes. Go back a number of years and tub valves were sold without the ability to adjust water output temperature. Look hard enough and I’ll bet you can still buy them somewhere.

I inspect plenty of homes with these valves. Last week I had one measure 130F on a ocean rental. Imagine the lawsuit if a child was scalded using that shower.

Bottom line is don’t assume single handle tub valves can be adjusted.

1 Like

Very true & I routinely find bath/shower temps above 120 at new & old homes. Highest was a month ago. 167F at the shower in a 1 year old home of an elderly couple.

I never comment or verify if there is a limiting device. I simply state that observed water temperature was too hot (include pic) & should be limited to 120 or less at each tub or shower with a device that is independent of the water heater thermostat. …have a licensed plumber install and/or adjust an appropriate device(s).

For any new guys- If I find tub/shower temps to be 120 or less, I simply do not comment. I do not try to verify if it is because of a limiting device or low water heater setting. Yes, the new occupant may increase the water heater & thus faucet temperature but you can’t save people from every “what if” scenario.


High temperatures, 140°F and above, should not be introduced into ordinary residential or commercial pipes that go to end user taps and showers etc.
Install mixing valve to temper or cool water before it enters water lines that go to taps.
Resource: Advantages mixing valve

Even at 60 °C – 140 °F, the setting on most electric water heaters an estimated 25% of water heaters are contaminated by legionella bacteria that tends to grow in the lower temperatures ‘at the bottom’ of water heater. Such bacteria can cause a form of pneumonia.
When the power is initially turned on the upper element is energized which heats the water *in the upper third of the vessel. The lower water remains cooler. It’s only when the upper third of the vessel is heated to the set temperature of the ‘upper thermostat’ the power switches to the lower element.

The average hot water temperature measurement I read on the closest faucet is ><125 °F.

Front page of Robert Young’s Montreal Home Inspection Services Inc Google Web Site.

122.9 °F.

Zurn brochure:

Installation and maintenance:


Working as a young plumber in the early 70’s (before the “oil shortage “) we installEd these on most toilets to stop tank sweating, which was the major cause of rotting floors around a toilet. I have not seen one installed since then but they worked great.

1 Like

How did that work? There’s no hot water to the toilets.

Bob it worked great. The ones we installed were made just for this application. They were rough brass and much smaller then the one pictured. You turned the dial with a screw driver and it had marks 1-10. You set it to just take the chill out of the water.

1 Like

They would have to bring hot water to the toilet for it to work or connect it to the hot water and get a hot seat with multiple flush…

Thanks for the reply. Haven’t seen one on a toilet, yet.

The mixing valve is located in the basement between the floor joists it can be easily missed as it is small and blends in with the rest of the plumbing.

1 Like