Can you post a photo of both the water heater & the safety valve?

The line connected to the safety valve is not a cold water line . It is a releif valve line that cannot be connected to anything except the atmosphere . the fact that it goes up through the ceiling is interesting , they usually discharge right next to the heater within 6 " of the floor , however up will work as long as it terminates in plain sight so one knows there is a problem . DO NOT CONNECT it to any OTHER PIPE !!!!!!

The reason that I asked to see a photo is because there is a "relief valve" that does attach to a water line and I need to confirm what type of valve they have before offering a solution to their question.....

On the other hand, if it is a standard "TPRV" (temperature/pressure releif valve) thye valve MUST BE installed in the upper 6" of the water heater pressure vessel and the discharge line MUST be connected dowwward where it will flow by gravity force to the exterior of the structure or to an approved "Indirect Waste Receptor" IT MAY NOT BE CONNECTED TO A WATER LINE and IT MAY NOT BE RUN VERTICALLY FROM THE VALVE.

Confusing post. It's not clear where OP was from, but in NZ and Australia, for some reason (not much freezing weather there) expansion/pressure relief water tends to be directed up through the roof. The T&P valve is a little different, too.

Must be a British thing, I guess, both countries got their standards mostly from the UK.

Valve was Watts L70A. On Googling it, it is a "Hot Water extender tempering valve"

Hot water heater has one cold water line in On hot water line out connecting to hot water main at ceiling. . Half way up to ceiling this valve acts as a tee with an cold water line coming in from behind and connecting to the cold water line.

What you have there is a very desirable feature, and one that I highly encourage my clients to allow me to install.

Years ago when they still did laundry in Hot water it was common for water heaters to be set at 180 or 190degF, especially if the water heater was supplying a dishwasher because the health codes require 180degF water for the sanitizing rinse.

The Medical profession says infants, children and ppl with sensitive skin can be seriously injured by water temps exceeding 130degF, so the Plumbing codes were ammended limiting the temperature of domestic hot water to not more than 125degF. Now most laundry detergents work fine in cold water and nearly all domestic dishwashers have a built in booster heater to provide the 180DegF sanitizing temp. (When you turn the "power saver" switch on it disconnects the booster heater to save energy).

Within a short time it was realized that trained plumbers could be relied upon to set the water heater thermostat to the code prescibed 125degF, but there was no way to prevent the homeowner from coming behind the installer and turning the temperature up, so in order to protect themselves from future liability most domestic water heater manufacteres installed thermostats that only go to about 140degF, and instead of marking the temperture on the control knob they just put an index at 125degF, indicating the correct temp to operate the heater.

There is only one small problem with that solution. The human body temperature averages 98.6degF and most ppl like their shower slightly warmer than body temp, which means an average shower is 105 to 115degF and some even like a Hot shower which is in the range of 120 to 125degF. The problem here is that back when the water heater was firing at 180 a typical shower was approximately a 60/40 mix of Hot & cold water, but once the source temp at the water heater was turned down we now have approximately a 95/5 mix of hot & cold water. Keep in mind that even with a code required 2.5gpm shower head a 10minute shower consumers 25gal of water, and if that water is coming from a water heater that is only producing 125degF water, it is very easy to run out of hot water before the shower is completed. Now add to that the fact that most new homes have at least two if not more bathrooms and it is quite common for two or more ppl to be taking a shower at the same time so the supply of hot water is critical. The standard solution today is to install bigger & bigger water heaters, in fact, where a three BR house previously had a 40gal water heater it is not uncommon to see an 80gal heater or even two 60gal heaters in homes today.

Now you have to understand that while you are drawing the 125degF water off the top of a typical water heater, that heater is also being constantly filled with 35 to 55degF cold water from the bottom, and as that cold water enters the tank is dilutes the stored water, which results in loosing temperature in your shower quicker.

The WATT's tempering valve is actually a mixing valve, so you can turn the temperature up on the water heater and as the super heated water enters the Watts vavle it is mixed with cold water so the output from that valve is at the code max of 125degF. The end result is that you have 25% to 35% more usable hot water from the same size of tank.

Now in regard to your original question. I cannot see how it is all hooked up at your house so I cannot comment on the second cold water line, but I have attached an illustration of how we typically install a tempering valve and I am certain you can connect it with just one pipe coming down from the ceiling. The only thing you have to remember is, the horizontal line from the cold water supply to the tempering valve must be connected to the coldwater line before the water heater main water shutoff valve.

Q: would you be allowed to tap off that 'hotter than 125*' line and run that to a DW or clothes washer in order to have hotter water at those appliances?

Q: would you be allowed to tap off that 'hotter than 125*' line and run that to a DW or clothes washer in order to have hotter water at those appliances?

The answer to your question is "maybe, depending upon the circumstances".

The code limit of 125degF is to insure that no one in the house would be exposed to water hotter than 125degF. If you were installing a frontloading washing machine you could make the argument that the water fill cycle does not begin until the machine is loaded and the door is closed and locked, therefore there would be no risk of exposure, BUT-it must be understood that the plumbing of the structure is intended to last the life of the structure, whereas any given appliance has an expected life of 12 to 15 years, and we have no way to insure the next machine would be a frontloader. If it was replaced with a top loader the fill cycle will operate whether the lid is open or closed so in theory the future risk exhists.It should also be noted that nearly all residential grade laundry soaps are formulated to work just as well in cold water.

It could be argued that the dishwasher does not fill until after the door is closed, but in this case you don't need the hotter water because all residential dishwashers have a built in water heater element to insure they have the required 180degF sanitizing temp available. The energy saver switch turns the built in water heater element off, so if you desire to have the 180degF sanitizing temp simply leave the energy saver switch in the on position. There is still a slight energy savings here because it would use the 125degF water for the washing cycle, but it superheats the rinse water to 180degF for rinsing.

Now in a commercial setting it is a whole new ballpark. Large commercial dishwashers have a built in water heater except for the small undercounter type that we typically see in a coffee shop. The commercial undercounter dishwashers have no water heater but the health codes demand that they be supplied with a minimum of 180degF for the rinse mode. In fact, the health inspectors carry a small toothpaste type tube of temperature sensing paste. When inspecting a commercial dishwasher he/she will apply two or three stips of the past on a plate or cup and place them in the dishwasher. The paste is typically a light green or gray color but when heated to 180degF the paste will change to a bright pink color. They then apply the paste and run the dishwasher through a cycle then check the color of the paste when it comes out to insure the sanitizing water was 180degF.

Now, while nearly all residential water heaters have a thermostat that limits them to a maximum of 140degF, commercial dishwashers will fire to 190degF.

In this case, when we install a commercial water heater that will supply water to both the dishwasher and the kitchen and restroom sinks we would install a tap on the line under the tempering valve to supply 180degFg to the dishwasher, then we install a tempering valve as shown above to supply water to the sinks.