Do you need to use the mixing valve for Indirect Water Heaters

IndependentEddie

I posted this as a response to another discussion but thought it might merit a new discussion.

My question relates to Mixing Valves. I just had a 60 Gal Superstore Ultra installed with the optional mixing valve. The documentation mentioned it as optional. I don't believe that either Gas or Electric fired water heaters need them. My question is optimizing efficiency. We got the bigger tank because with the in-line furnace heater we used to have, you could never fill a tub with hot water. Although only two of us now live in the house it is a 4-5 bedroom with 3 full baths and I thought for resale value having a big tank would be desirable for resale. To me it doesn't seem to make sense to heat the tank to 140 degrees, then use the mixing valve to mix in cold to drop down to 115-120 and then again at the tub or sink mix cold again to get to "consumption temperature" of less than 100. This makes sense to me if you need massive amounts of water to supply so that 60 gallons of 140 degree water would probably supply 100 gallons or more of less than 100 degree water. For us, I would think that the most energy efficient settings would be to set the water temperature for the tank to 120 degrees or so and set the mixing valve to maximum (no cold water). This would supply 60 gallons of 120 degree water which when diluted with cold at the sink or tub would be more than we would ever need. Lowering it even more might make sense. We never wash clothes with hot water and the dishwasher has a heating coil to increase the temperature. Anybody have experiences with this?

kcopp

"Need"... Depends. Will it work w/o one yes. As a measure of scaling protection, Yes.
If the Boiler malfunctions it will protect you.
Some codes require you to install a listed mixing valve on any water heater. Gas, electric or a tankless coil in a Boiler.

Jamie Hall

One advantage to using the mixing valve is that you can keep the storage tank at a high enough temperature to prevent bacterial growth in the hot water, such as Legionaire's. That means a temperature of 140 as a minimum.

Ironman

The IMC requires it to prevent scalding in case the aquastat fails.

As Jamie has said: keeping the tank at 140*+ kills legionella.

Gordy

Another thing to add to @Jamie Hall's post. An added plus of higher storage temps is extended usage . However you must protect the end user from scalding. So mixing valve is required.

hot_rod

If it is a ASSE 1017 listed valve at the tank, you need to keep the tank at 140- 145F. The mixing valve needs a 25-27° difference between hot inlet, and mixed outlet to operate accurately.

So 145 at the tank mixed to 120F supplied to the building. It good to have that 120 to the washer and dishwasher so the soaps dissolve and work well, for showering you probably mix down to 100° + or -.

If you have sections of the hot water piping that are not used often, that is where bacteria can harbor and grow.

With the tank maintained at 140, that should minimize the potential for bacteria to get a start.

Tom_133

Hot Rod,

What mixing valve can we use to pass code for needing it but with the most minor temp difference for it still to work?

Most of my customers hate that the state forces the mixing valve and they have caused quite a few headaches, especially when installed on on demands.

JohnNY

Here in NYC, any time a space heating boiler heats domestic water, a temperature limiting device is required.

j a_2

Limiting devices are very comforting to installers, no one wants issues with occupants,especially young and old getting scalded....It's hard if not impossible to discuss all the different codes in all the different areas this site covers...So I won't, my best advice is to follow the code....

hot_rod

Tom said:

Hot Rod,

What mixing valve can we use to pass code for needing it but with the most minor temp difference for it still to work?

Most of my customers hate that the state forces the mixing valve and they have caused quite a few headaches, especially when installed on on demands.

For point of use it must be a an ASSE 1070, It is required to have a tamperproof cap, 120F max.setting.

At the tank it could be either a 1017 or 1070. The 1070 does have a tighter temperature regulation, but lower Cv.

j a_2

Hot Rod....wondering if that 120 is a national code...last time I went to a code class they said 120 was the minimum and 130 was the max.

mattmia2

This bring up another question, what is the temp swing in a gas or indirect tank? The t-stat has some differential between the in and out points, the tank has cold water entering and mixing near the bottom, the tank has a heat source somewhere in it, and the heat source has some residual heat after the t-stat ends its call.

hot_rod

The manufacturer of the control would have some spec on the differential and maybe accuracy of temperature control devices. Indirect aquastats are usually around 5-7 °, some are adjustable.

If the indirect is controlled by a boiler sensor, then the boiler control would have adjustability for on/ off and differential.

The control regardless of the type only knows the temperature at it's sensor location. Tanks will stack (stratify) hotter on top, I have seen 30° or more on tall skinny tanks.
.
The top could be 130 or whatever and as 55° water enters the sensor location see the differential change and fires the heat source.

The amount of stacking depends on the tank height to some degree.

On solar tanks a second upper sensor is often included to prevent over-heating and popping the T&P valve. Solar control often enable a collector protection function allowing the tank to over-shoot stepping to prevent collector stagnation, the upper sensor puts the brakes on at 190 or so.

A recirc pump changes the dynamics inside the tank also as some blending is going on depending on where that return enters the tank.

Some über design solar storage tanks have sensors every 12" on the tank to layer heat energy in and out, call on various back up, or shuffle loads from different layers in the tank.

lchmb

Is code requiring them on instantaneous water heaters? I was told by a manufacturer they are not wanted on the units.. and install manual say's not to include it..

hot_rod

I think at least 3 states require them on all DHW sources, tanks, indirects, tankless, HX, etc.

On tankless, instantaneous we suggest several feet of piping up stream of the H port for best performance.

Be aware of recirc piping, or use the Caleffi 520 valve, 100% shutoff on H.

Wellness

https://www.treehugger.com/green-food/is-it-safe-to-turn-down-your-water-heater-temperature.html

Legionnaires Disease, or Legeionellosis, is caused by "Legionella pneumophila, a ubiquitous aquatic organism that thrives in warm environments." It was identified after 34 veterans died after attending an American Legion Convention in the Bellevue-Stratford Hotel in Philadelphia in 1976. It is temperature sensitive:

* 70 to 80 °C (158 to 176 °F): Disinfection range
* At 66 °C (151 °F): Legionellae die within 2 minutes
* At 60 °C (140 °F): Legionellae die within 32 minutes
* At 55 °C (131 °F): Legionellae die within 5 to 6 hours
* Above 50 °C (122 °F): They can survive but do not multiply
* 35 to 46 °C (95 to 115 °F): Ideal growth range
* 20 to 50 °C (68 to 122 °F): Legionellae growth range
* Below 20 °C (68 °F): Legionellae can survive but are dormant

That said, no electric water heater that I have ever installed has ever consistently kept the H20 temp above 150, when used normally (showering, washing dishes, etc.), at least as reflected by my temp sensors.

hot_rod

One common accepted practice is to elevate the tank and entire piping system every 24 hours. Time required depends on the temperature you maintain. At 140F for one hour. Less time required at higher temperatures.

No need to keep the tank at the high temperature 24/7.

The key is to circulate that high temperature throughout the entire DHW system. Easier said then done with dead end risers and branches. And POU valves must be installed at every fixture.