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Water Tank Temperature After Shower

JustinSJustinS Member Posts: 200
I am trying to understand something...

Not sure what the starting temperature in the tank was when I took my shower this evening but afterwards, I went to check on the tank, discovering that the tank was already being reheated but that its current temperature was only 120F???

The storage temp is 140F and the boiler is supposed to come on at 134F...

The tank is a TT Smart 80 using a TT Prestige Solo 110...

BTW, the showerhead is max flow of 2.5gpm...

It wasn't a long shower, nor was the water set particularly hot... I'm planning on checking tomorrow what the temperature is before my shower but in the mean time, can someone help me understand why a relatively short and not very hot shower would cause the temperature to drop so low?


  • icesailoricesailor Member Posts: 7,265

    What cloud did you pull that theory from?

    Any water tank, Hot water tank, that is a common shaped and installed tank that has been sitting for some undetermined (but adequately long) time, will be filled with water that is approximately the temperature of the location that the control is sensing. And the sensitivity of the control. If for example, the water in the tank is at 140 degrees because that is what the control has controlled the heat source to create, it might be slightly hotter in the top of the tank than is in the bottom of the tank. Internal circulation will equal the temperature over time. If hot water is drawn out of the tank, as it leaves, cold water replaces it. In order for water storage heaters to work, they must place the new and incoming cold water at the bottom of the tank where it can mix with the coolest water in the tank. If it is a gas or oil fired water heater, the burners will heat the water in the bottom of the tank and the higher temperature mixes as it rises. But it will not get to the 140 degree water temperature until the cold water stops entering and the hottest water at the top stops leaving. It only takes a 7.5 degree differential in temperature to cause circulation in a tank. Once the water in the tank reaches a level where the whole tank is within 7 degrees, circulation stops. When ice forms on the top of a lake, the bottom of the lake is 39 degrees. No matter where you go on earth.  

    If you look at the EPA tag plastered over the side of the tank that gives you what some wag considers what it will cost to run it, it gives you performance numbers. If it is a 50 gallon tank, it will give you a "First Hour Use" which is supposed to make you think that it is how much it will develop continuously. It isn't. It is the volume of the tank, and how fast the water can be heated in one hour. Plus some unknown fudge factor that is like vitamin supplement claims.  50 gallons of hot water plus 18 GPH (Electric WH with a 4500 watt element)added to that (68 gallons) plus that FM number that no one can explain. Its like when you hit a strange combination of keys on your computer and you immediately know that you did something very very bad? Because the screen goes blank and suddenly, the screen is filled with smiling faces and signs of the devil. And it has absolutely no meaning to you. Those things, FM. Magic.

    As far as being able to calculate at any one moment in time what and where the temperature is in a water heater is, is pure speculation on the part of the speculator.

    For anyone not understanding, understand this. There's a big difference between heating water in a closed system to heat radiators than there is to heat water in an open system where you do not know  the varying flow rate and the temperature of the incoming cold water. If the incoming cold is flow restricted and the rate of entry is set, it becomes lust like a heating system with a Delta T  that would turn any Wethead heater upside down.

    We who deal with it, or have dealt with it, wish we could deal with a 20 degree Delta T in a shower.

    Whatever someone comes up with for an answer as to how much hot water is in a water heater tank and at what temperature it is, while trying to figure out if there is enough, there isn't enough. Even twice as much isn't enough in the time you need it. The rest of the time, you don't need any of it.
  • JustinSJustinS Member Posts: 200

    I was thinking about that this morning as far as the position of the temperature sensor...
  • icesailoricesailor Member Posts: 7,265
    Cumulonimbus Clouds:

    If the hot water draw from the tank is lower than the energy input of the water heater, and there are short draws on the tank, the water at the top of the tank can be much higher in temperature than the bottom of the tank. It can be street water cold. It can stratify and have varying layers of hot and cold water from the bottom to the top of the tank. Its called :Stacking. Oil fired water heaters are notorious for this. There is no known way to accurately predict what the temperature of the hot water is in a water heater tank at any given moment of a cycle. .
  • SWEISWEI Member Posts: 7,356
    SMART indirect temperature sensors

    Are located roughly in the middle of the tank, or at least the bottom of the sensor well is.

    It sounds as though your contractor did replace the tank aquastat with the 12k thermistor (that's how the boiler sees the tank temp you presumably read from the boiler display.)
  • JustinSJustinS Member Posts: 200

    This morning before a shower, I disabled the DHW system - I wanted to see how it would affect the tank temperature without any re-heating affecting it...

    Just prior to the shower, the tank temperature was 150F and immediately after, it was 106F... assuming a street water temp of 50F (I think it's closer to 70F), in order for this 106F to be uniform throughout the tank, we would have had to displace nearly 50% of tank capacity (35 gal)...

    Since there's a mixing valve to ~120F and the ~15-min shower @ 2.5 gpm was cooler than that (maybe 100-105F?), it seems to me that the hot water displaced/used would be more like 20.5 gal...

    Also, when I initiated the re-heating, it only took 18 minutes to go from "106F" to 140F while it normally takes 11 minutes to go from 134F to 140F...

    Based on this, it seems likely that the temperature is not uniform...
  • hot rodhot rod Member Posts: 7,080
    tank performance

    Here is a good article Dave Yates wrote for Contractor magazine, from their online archive. . That 85% number that Dave refers to seem to be close to what I have measured also.

    Keep in mind the performance numbers and standby heat loss can be somewhat misleading. Knowing the test conditions is important.

    One indirect tank manufacturer tested around 30 of their competitors tanks to see how the number related to reality. Then they changed the conditions until they got the tanks to perform to the advertized numbers :)

    In one case to get the "less than 1 degree per hour standby loss" the tank needed to be in a 70° room with 120° tank temperature. So if you do not have the test conditions it's hard to trust, or believe tank performance numbers.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • JustinSJustinS Member Posts: 200

    TT told me that ithe 1F/hr is based on 120F storage in a 70F room

    Where does the 85% come from as far as hot water from tank? Am I missing something but wouldn't the calculation be (115 - 40) / (120 - 40) = 0.9375?

    Also, should I be trying to heat my tank with lower temperature water to get into condensing mode?
  • icesailoricesailor Member Posts: 7,265
    Mixing Hot Water:

    All I am going to say is this.

    If you are maintaining 120 degree water in a storage tank, some tanks don't circulate as well as others, and some pressure balance shower valves don't work as well as others.

    If you want a hot shower, or plenty of hot water for a shower, you need not water, not cooler water. If you lower the boiler inlet hot water temperature so that you can get more efficient by condensing, you're not going to get hot water.

    I stopped at a stop light yesterday on a main drag with 4 lanes going my way. (8 lanes total). I have a 2001 BMW 325XI wagon with 150,000+ miles that will blow the doors off most cars on the road from a standing stop.  Plus out accelerate most cars from 50 to 80 MPH. There were newer cars on either side of me. When they light turned green, the two cars on either side took off like they were at a drag strip to get ahead of me. I laughed. I just drove my normal way to get the computer MPG/Vacuum gauge off the peg as quick as I could. I could only think about how much it cost me to take off and how much more it did them. I caught up with one of them two lights later at a major intersection.

    Just because you can do something, doesn't mean it is always a good idea to do it.

    If you had a thermostatic shower mixer, it wouldn't be mixing at 120 degrees. It takes 130 degrees or more. The heat loss in a house and the energy to compensate for it isn't close to what the energy requirements to heat domestic hot water in a water heater for a shower.
  • SWEISWEI Member Posts: 7,356
    Indirect calls

    on all the mod/cons I have used force the boiler to a preset water temp, then shut off once the DHW hits the preset temp.  The smarter ones use a thermistor in the tank to increase response to a droop and reduce overshoot.  With external controls we can take them to the next level -- firing the boiler to the lowest temp which will achieve the needed recovery.  Combine that with some intelligent setpoint management and we see far fewer ignitions.
  • ZmanZman Member Posts: 3,517
    Smart tank

    The smart tank has a tendency to run uneven temps. I think the high mass of water in the jacket and the heating from the outside has a lot to do with it. I have noticed big fluctuations in may own tank.

    As stated moving the sensor may help.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JustinSJustinS Member Posts: 200

    How would you move the sensor? Isn't there a fixed thermowelll?
  • ZmanZman Member Posts: 3,517

    The well is in the center of the tank. You can raise and lower the sensor a good distance within the well. Just pop the cover and carefully pull the wire.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailoricesailor Member Posts: 7,265
    edited June 2014
    Tank Control:

    So, you're controlling the temperature in the middle of the tank? Does that really make sense?

    The cold water is injected in some way to the bottom 1/3 of the tank. So you put the sensor 1/2 way up , the middle of the tank? So, the tank has to give up 1/2 +/- its hot water before the controller makes something start to heat the water?

    Gas tank type storage  water heaters have the control in the bottom 1/3 or less. Electric water heaters have two elements/thermostats. The top one works first, then the second one comes on and runs the bottom element. Sometimes, it is the only element that ever runs. They are in the bottom 1/3 to 1/4.

    A lot of TT types I saw had problems with quickly responding to temperature changes. Some had problems sensing at all.

    Put a quality DHW thermostatic mixer and mix the hot and cold. You will never have another problem. Unless you are trying to fill a 100 gallon jetted tub with a 30 gallon water tank in 5 minutes.
  • icesailoricesailor Member Posts: 7,265

    What you describe, a Watts 70A hot water extender is just that. A device to extend hot water from a tankless heater on an oil boiler. If it only lasted 2 years, it was probably installed improperly without the proper heat trap.

    A proper thermostatic mixer on a water heater is a totally different thermostatic animal.
  • hot rodhot rod Member Posts: 7,080
    quality mixers

    the life expectancy of a mix valve, on DHW systems is directly related to the water quality. Water with a lot of minerals, lime, calcium, grit, etc shortens their life. 95% of the mixers we get back are locked up from hard water deposits.

    The hardness, but also the temperature of the "hot" water. The hotter the more the minerals preciptate out of solution. Installing a thermostatic mixer on a tank of 160- 180F makes a huge difference in the service life, before cleaning is required. Also the amount of water that flows thru the valve determines how long before cleaning.

    Catch 'em before they get too much build up and vinegar will clean them out. If they sieze tightly you may rip an o-ring taking them apart.

    Very simple inside a 3 way valve, o-rings, a plastic spool, and a wax cartridge.

    To make an accurate valve that meets the strict ASSE standard takes tight tolerances. Tighter tolerances = more maintenance.

    That being said, look for a brand with a large body, this allows larger components inside and better flow rates.

    Not all thermostatic valves get along well with tankless heaters, the pair tends to fight one another as the tankless ramps up the valve tries to close down and response times between the two get them at odds. There are special thermostatic valves built for tankless heaters, used in Europe where tankless are the main DHW source. They are low Cv and not really adaptable to US markets, however.

    I'd also suggest a tankless with a small tank built in, this minimizes the "cold slug" and behaves better with thermostatic valves, as the valve doesn't see wide and quick temperature changes.

    The other big problem is improper piping when mixers and recirc systems are combined. Either the valve will drift hot, or lock cold if it does not see a constant supply to the hot port to allow it to respond. ideally a 25-27 delta T between hot in and mixed temperature out, to be 100% accurate, that is part of the ASSE test standard.

    As far as I know those "extender" type valves do not have the appropriate ASSE 1070 or 1017 listing and could get you in a bind should someone get burned.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • icesailoricesailor Member Posts: 7,265
    Hot Water Extenders:

    I don't think that Hot Water Extender Valves like a Watts 70A meets the requirements of ASSE 1017 or 1070 specifications.

    I also think that the new 2015 water heater energy savings will throw a large old lag screw into the working gears of water heater tempering/mixing valves.
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