I would like to build a hot water loop with condensing boiler.
Is there any curve available in OpenStudio?
How should I change the curve?
I also tried to change the loop leave water T, delta T, but simulation results were totally unreasonable which showed no heating after May. Is there anything else I need to set up?
Take a close look at the flow rates for each loop and how the pumps are being controlled. What kind of pumps are you using? Assuming that your're using constant speed primary pump(s) and a VFD on the secondary pump(s) it is important to stage your primary pumps based on the secondary flow. You always want the primary flow to exceed the secondary flow for primary/secondary systems. Otherwise your plant delta T will degrade and the expensive condensing boiler stops condensing. See the attached image. 
Thanks! That does solve my problem perfectly. I found I used variable pumps for both primary and secondary loops. After I changed primary pump to constant speed pump, the results looked reasonable. BTW, how to set up the stage of the pump in OS?
The primary pumping is straight forward. There should be one primary pump for each boiler and the primary pump should only run when the associated boiler is running. You may need to define the PlantEquipmentOperation:HeatingLoad object for the primary loop in order to stage boilers. I know the OS team has been working on adding this object to OS but I'm not sure on the status. I would ask another dedicated question in regards to this.
If modeling multiple secondary pumps I recommend using the HeaderedPumps:VariableSpeed object which allows you to define how the pumps should be staged.
@Yan @Lincoln FYI @Kyle Benne has added the PlantEquipmentOperation:HeatingLoad (and the other operation schemes to OS). I'm actually testing them at this very moment. They'll be available through Measures in 1.9.0 at the end of this month.
Not as far as I'm aware, and there are no relevant measures in BCL that I could find.
To change the curve you can just set the relevant values in the OS:Curve:Biquadratic object by clicking on the boiler in your plant loop and scrolling down to the curve parameters (see image). The default is a flat curve with Coefficient1 Constant set to 1 and everything else set to 0. If you change Coefficient1 Constant to 0.9667 and Coefficient2 x to -0.1667 then that is the same as the condensing boiler in the HospitalLowEnergy.idf example file.
See the other answer.
@Jamie Bull, I input 135F for design loop exit T and 30F for T difference.
That seems reasonable, so not sure what the issue might be.
@Jamie Bull, I did several tests and found the problem actually happened with primary/secondary loop. In my case, the hot water loop was primary/secondary loop with condensing boiler. When I used primary only loop with condensing boiler, the results were reasonable. Is there anything that I should take care of when I set up the primary/secondary loop with condensing boiler such as the setpoint for each loop?
@Yan I'm not sure. It might be worth opening up a new question for that part of the problem as it might take a bit of troubleshooting.
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