I am in New York on Long Island.
I have a mixing valve on the hot water output of my hot water tank.
I have a natural gas fired Burnham ES2 and a Burnham 50 gallon indirect storage tank.

I keep the setting on the hot water tank to maintain 160 degrees with a 10 degree differential.
The mixing valve is set to output 125 degree water.

Here's my question. In the summer when I'm not running the boiler to heat the house, does it cost more to keep the tank temperature at 160 and use the mixing to lower the temperature, or should I lower the tank temperature back down to 125 and bypass the mixing valve (I have bypass valves to cut it out)?

At first thought, it seems obvious that it would cost more to keep heating the water to 160 rather than 125, because the boiler needs to fire more often to keep the water hotter.
But is that really true, since with the mixing valve inline, I'd be using the hot water up at a slower rate, so that should cause the water to need to be heated less often?

Are there any other reasons to use the mixing valve or not all year round?

Heat loss is a function of the difference in temperature between the tank and the area around it, therefore the hotter the water in the tank the "faster" it will lose its temperature and have to be reheated, regardless of how much, or little, you use the hot water. YOUR usage is almost irrelevant because you use about the same number of therms either way.

So then are you saying that it would indeed cost me more to keep that tank heated to 160 than 125?

yes, just as it would cost you almost nothing to heat it at 100 degrees, and nothing at about 80 degrees, assuming it is in a heated building. However, at 160 degrees the heater may have a shorter lifetime.



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Natural gas cost aside, are there any reasons to keep using the mixing valve all year round (or not)?

I use the mixing valve to get "more hot water" out of my 50 gallon tank, but there's really more of a need in the winter. During the summer, just the 50 gallon tank alone is generally enough.

Some considerations I've heard:
- keeping the hot water tank to 160 kills bacteria in the water that wouldn't happen at 125.
- the water here is ground water with lots of garbage in the water (iron, minerals, who knows what else??) and it has a tendency to leave deposits on anything it touches. Will the mixing valve last longer if I cut it out when I don't need it since the water doesn't flow through it, or will it have the opposite effect since there isn't always water running through it flush it all the time?

you use the mixing valve not to save money but because you run out of hot water.
so there is a reason to use it BUT if you don't run out of hot water then there is no reason..
125-130 deg F will kill legionnaires disease.

125-130 will stop Legionaires from 'growing/spreading'

145+ will kill it

from Cashe-Acme:

Legionella Facts & Prevention
Legionella is an aquatic bacteria that thrives in warm water environments and is the cause of Legionnaires Disease. It is commonly found in hot water heaters, potable water supplies, hot tubs, cooling towers, fountains, swimming pools, etc. and multiplies in conditions especially between 68 – 122 °F. The Legionella bacteria can spread to humans when breathed in.

An estimated 22K cases of Legionnaires disease are reported in the U.S. each year2. It is known to cause fever, chills, and a cough, which can be dry or may produce sputum. Some patients also have muscle aches, headaches, tiredness, loss of appetite, and, occasionally, diarrhea. Legionella may also cause cases of pneumonia that may be difficult to diagnose. Of the approximately 2.4 million cases of pneumonia that are diagnosed each year in the United States, about 18,000 are confirmed as Legionnaires Disease and up to 600,000 cases of Legionnaires Disease are misdiagnosed as pneumonia because the hospitals do not perform tests for Legionella.3

Improved design and maintenance of cooling towers, plumbing systems and hot water heaters to limit the growth and spread of bacteria are the foundations of Legionella prevention.



How the Issue is Being Addressed Around the World
The answer to preventing both Legionnaires disease and hot water scalding is to store distributable hot water at a high temperature and temper it to a safer temperature before distribution to the fixture.

A water temperature of 120°F does not kill the Legionella bacteria; a hot water temperature of 140°F is required at which Legionellae dies in 32 minutes. Hence it is recommended that the water heater be set at a safe hot water temperature of 140°F. The Legionella disinfection range is 158 – 176 °F.

Several countries around the world, including Canada and many in Europe, have adopted regulations requiring water to be stored at a higher temperature and delivered at 120°F or lower. The aggressiveness with which countries have addressed this issue indicates the importance of water storage safety. In fact, according to the Safe Kids Worldwide Campaign, “all code-making bodies at the [U.S.] national and regional level have established plumbing standards for newly constructed homes and residential units requiring anti-scald technology and a maximum water heater temperature of 120 degrees F.”


therefor:

keep the tempering valve in service

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



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Excellent post Bern

thanks bern... as usual, you are a wealth of information..

y'all are welcome

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

You've given me a good reason to keep it on all year round. Thanks for your insight.



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