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Indirect Controls

mattmia2mattmia2 Posts: 817Member
I have an HTP SSU-45 heated by a UFT-100. I can run it out of hot water during 1 shower in the winter if I am not careful. I realize a low flow shower head could probably fix it, but I am looking for a way to fix the hot water production problem. It is controlled by the thermistor connected to the control in the UFT-100.

I took a number of videos of the supply water temp and the indirect tank sensor temp and the supply water vs the firing rate.

I found that the stagnation in the tank means it takes several minutes of large demand for the cold water to reach the temp well, then the boiler takes another couple minutes to pre-purge, fire, ramp up, and start producing full output. The supply temp is only about 160 until the demand ends and the firing rate is 100%, so it is dumping between 85,000 and 95,000 btu/hr in to the water once it starts, but because of the lag in the sensing I think it is just so far behind it can't catch up. By the time the boiler is firing the tank temp has fallen to between 70 and 80 degrees and doesn't rise above about 90 until the demand ends.

The zone controls aren't set up to make a DHW call a priority but the only thing that might be calling at that time is about a 5,000 btu/hr radiator. I have the DHW supply water temp set to 180 and my incoming water is about 35 degrees. The swt doesn't get to 180 until after the demand has ended. The tank temp is set to 125 with a 5 degree differential.

Is there a common strategy or standard control to solve this problem? i have a couple ideas:
1. Measure flow and begin firing the boiler and circulating the DHW zone when a large average flow has been detected for a few minutes. This would require a flow sensor that can measure instantaneous rate and a control that could integrate a sliding window of the past couple minutes.
2. Add a buffer tank.
3. Add a button in the bathroom to my control system that would fire the boiler at the tank setpoint and start the DHW zone circulating(on about a 5-10 minute timer) so the boiler would already be hot and firing when the tank cooled enough at the level of the tank sensor to produce a DHW call through the boiler.
4. Run the tank at 140 and temper it to 120. I don't like this idea because of the high chlorides in my water and the tank only being warranted for a 140 degree setpoint and tempering valves not functioning so well with low flow rates.

It produces plenty of hot water in the summer. I had a 40,000 btu/hr 50 gallon direct fired gas tank and that would also run out if I wasn't careful about usage, but in theory this should be putting about 3x as much heat in to the water. The old gas tank would fire every time you washed your hands so it was firing earlier in the demand. It had a very loud power vent so you knew when it was on.


  • EBEBRATT-EdEBEBRATT-Ed Posts: 6,509Member

    The only thing I can think of is pipe a loop between the hot and cold DW pipes above the tank. Put a circulator and an aqustat in this loop also a check valve. The aquastat will run the pump. Set it for 125 deg. The circ will circulate the tank and keep the whole tank hot.

    35 degree inlet water is really cold
  • mattmia2mattmia2 Posts: 817Member
    I had thought about a dhw re circulation system with a demand button that ran the pump on a timer just to not have to run water down the drain to get hot water, but all my fixtures are pretty close to the water heater so it would never save its cost in water, but it could solve this problem.

    The water comes partially from the Huron river which is relatively wide and shallow and is stored in some relatively small tanks and the system is not that big so it doesn't have a lot of time in mains in the ground to pick up heat. It looks like it is about 45 now but it has been relatively warm for winter the past few weeks. I have measured it at around 33 or 34 some winters.
  • HVACNUTHVACNUT Posts: 2,827Member
    The water heater is not on priority.
    Is the water heater piped as a separate loop, Tee'd off the primary loop and wired into the boiler, or is the water heater piped on the same manifold with all the space heat zones?
    Does the boiler know to max fire on a DHW demand?
  • mikeg2015mikeg2015 Posts: 1,118Member
    What is the return water temperature? Should be 25-30f delta T during a call. Of higher, the flow rate is too low for maximum transfer. That being said, if the burner is at 100% then you are good.

    If it’s at full fire rate, you should get about 2gpm at a 80f temp rise.

    So you are not getting all the heat.
  • BillyOBillyO Posts: 67Member
    are the supply and return piped to thank sized properly?
  • SuperTechSuperTech Posts: 1,126Member
    I'd start with priority zone control for the indirect tank. I have my boilermate piped into the secondary manifold on an injection pumping setup and I have no problem with running out of hot water.
    Ideally you would want the indirect piped off the primary loop and operating with its own circulator. A delta T pump works nicely for this application.
  • hot_rodhot_rod Posts: 12,540Member
    edited February 8
    A RIB relay could turn it into a priority demand. Also, can you lower the sensor on the tank to detect cold faster?
    Higher operating temperature with a mix valve?

    How old is the tank, could it need a delime? A small scale coating on the coils drops heat exchange by 10- 15%

    I have a bunch of flow sensors, designed for 1/2 gpm, if you want to try that route I'll send one.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • mattmia2mattmia2 Posts: 817Member
    edited February 8
    The flow sensor is an interesting idea. I had thought about a flow meter, but not a sensor that triggers at a certain rate. I would want to size it(or have it adjustable) so the shower would trigger it but a faucet wouldn't. Connect it to a digital in of the control processor, trigger a dhw call at about 125 degrees so it heats the new water but doesn't overheat the tank the the thermistor in the tank takes over and finishes the call at the higher temp.
    EDIT: Trigger the dhw call if that flow continues for say a minute and a half or something like that, ideally longer than a washer or dishwasher fill or filling something in the laundry tub.

    I could get a well in through the cross at the cold water input or one higher up in through the indirect coil. Either of these would require a dual control that starts the call based on the lower temp sensor and ends the call on the upper temp sensor since one the boiler started heating and the cold water had been flowing for a while the temp at the bottom or in the coil would no longer represent the tank temp.
  • mattmia2mattmia2 Posts: 817Member
    My thought is to put a well in through here by replacing the 1x3/4 ell with a tee or cross:

    I figure when the zone is not calling the water in the coil will get to the temp of that level of the tank very quickly. I could use that to start the boiler and the dhw zone and heat it to the tank temp on maybe a 5 minute timer until the cold water reaches the higher temp probe and starts a dhw call. It would take some experimentation to figure out where the right setpoint and time is for that. Even during a slow call the regular well gets to about 110 before it starts heating with a 125 degree setpoint with a 5 degree differential.

    Is there some sort of a 1" m-npt to 1/2" sweat that I could use to make a well of a custom length out 1/2" copper pipe and a cap so i don't have to figure out the length and it won't be inexplicably expensive like a pre made well?

    I am not sure if the 0-10v of the UFT overrides the dhw call or only the tt dh call. I have read the manual a couple times and it seems it could be interpreted to mean whichever you want it to.
  • HVACNUTHVACNUT Posts: 2,827Member
    In the manual. Page 52. "Indirect storage tank differential setpoint" 5° to 30°.
    Have you checked that?
    Don't try to reinvent the wheel.
  • BillyOBillyO Posts: 67Member
    like I said in an earlier post, check supply and return piping size, should be 1". Theres a reason for 1" connections
  • BillyOBillyO Posts: 67Member
    sometimes check to see if door is unlocked before climbing thru the window
  • mattmia2mattmia2 Posts: 817Member
    mattmia2 said:

    The supply temp is only about 160 until the demand ends and the firing rate is 100%, so it is dumping between 85,000 and 95,000 btu/hr in to the water once it starts, but because of the lag in the sensing I think it is just so far behind it can't catch up. By the time the boiler is firing the tank temp has fallen to between 70 and 80 degrees and doesn't rise above about 90 until the demand ends.

    The tank temp is set to 125 with a 5 degree differential.

    I could make some more measurements of flow and temps, but i'm pretty sure the return end of the coil being in 45 degree water is pushing the delta t up to whatever it needs to be to transfer the full output of the boiler.

    You figure that in the time it takes to move the cold layer up to the aquastat, pre-purge, fire, and heat the boiler and water above the tank temp there has been about 10 gallons of 45 degree water added to the tank.

    8.3 lb/gal*10gal*1 (btu/(lb *f degree))*(125-45 f degrees)
    =6640 btu
    100,000 bth/hr / (60 min/hr) = 1,666 btu/min
    ~4 minutes of no demand to recover from that start up

    Assuming a 90 degree shower temp and 3 gpm of the blended flow, you would need to put 67,500 btu/hr in to the water to keep up

  • mattmia2mattmia2 Posts: 817Member
    Would whoever disagreed like to explain where ~90,000 btu/hr is going if it isn't going in to the water?
  • mattmia2mattmia2 Posts: 817Member
    I looked at the manual again as some suggested and figured out that this unlabeled table is first draw. With an input of around 90,000 btu/hr instead of the 141,000 btu/hr that the table is written for, we are in the red area somewhere which matches the performance I am seeing:
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