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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.