I'm modeling a tankless system to maintain the tank outlet temperature to setpoint (there's no backup heater in the tank model, so the only heat source is the plant loop connected to tank source side), while I didn't find a good approach to achieve a good control either with variable mass flow rate or with variable temperature setpoint, I'm under a strict requirement of running speed for a project so the simulation time step was 1 hour. So please let me know if you guys have any good suggestion of modeling this system or have any insights how the tankless coil is controlled in reality.
I started putting a heat exchanger in the demand loop of boiler and serving heat to the WaterHeater:Mixed source side, in which case the source side mass flow rate and source side inlet temperature were constant all the time, I tried EMS to control the setpoint manager schedule while unfortunately ended up with this question, the results of this approach were not too bad, but it seemed to be something wrong within the node connection and plant simulation.
Then I surprisingly found if I directly connected the tank source side with boiler plant loop as the only component in demand side and set the Pump:VariableSpeed Pump Control Type to Intermittent, the mass flow rate could change trying to meet the loads, which serves my purpose. However, when the boiler is serving both heating coils and tanks, the tank source side mass flow rate changed negligibly. Maybe it's because when the tank was put in parallel with other heating coils, the dhw loads are not significant compared with total heating demand.
And the results are different between E+ released version and develop version (git "develop" branch), in released 9.1.0, the mass flow rate and tank temperature are plotted as following:
While building in "develop", the plot is:
Their trends are similar but the mass flow rates became more constant in latest "develop" branch. The reason why I have to use the develop codes is that I want to implement the code in bug-fixed branch in my local machine which is synchronized with the latest develop branch. But the results were for the develop branch only without my changes so the impact could be eliminated. My conclusion is that neither of these two is working for me because they are not helping the tank temperature to be controlled at my setpoint (52.6667C).
I tried to use my first model where a heat exchanger was put in the boiler loop and supplying heat for tank in another added loop, I removed EMS, and put the same Pump:VariableSpeed with Intermittent mode, I wish in this model the tank source side mass flow rate could be adjusted by dhw loads, but it turned out to be constant.
I am very interested in how E+ models the mass flow rate in tank source side? Why in some cases pumps get higher priority while in some cases (like HX loop) the pump controls are overrode, the loop settings are almost the same in these two cases. Is there any existing function adjusting the tank source side mass flow rate (or source side inlet temperature) to meet the demand (or to say, to maintain the tank temperature)? If not, is it controllable by EMS?
I am so surprised that it's so hard with existing E+ to model tanks without backup heaters. And the situation became more critical for tankless model because the tank volume is very small, recommended here as 0.00379, it is very easy to fluctuate a lot if it's not well controlled. The only approaches I thought about were controlling mass flow rate or temperature, if there's any other idea, very welcome to let me know.
