How to model condensing gas-fired DHW tank in EnergyPlus?

asked 2015-09-18 05:17:23 -0600

Julien Marrec

29706 ●94 ●76 http://www.effibem.com/

updated 2017-05-08 12:16:16 -0600

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

I want to model a tank-style condensing domestic water heater.

It seems that the EnergyPlus WaterHeater:Mixed only accepts a quadratic curve, function of Part Load Ratio (PLR) only, whereas a biquadratic curve function of PLR and return water temp is warranted.

So I'd tend to go ahead and use a Boiler:HotWater and a WaterHeater:Mixed for which I'd at least put the Heater Maximum Capacity to zero to simulate a simple storage tank.

Should I put the boiler and the tank in series on one supply branch?
What parameters for the WaterHeater:Mixed should I use, especially Deadband temperature, control type?
What kind of setpoint managers and availability managers and where?

Any example of a working setup would be much appreciated.

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@Julien Marrec I retagged in favor of SHW over DHW, which may need further debate. My preference is for SHW after ASHRAE, 90.1, etc.

MatthewSteen ( 2015-09-18 20:42:07 -0600 )edit

Sure, SHW is more general and should likely be preferred.

Julien Marrec ( 2015-09-20 05:53:03 -0600 )edit

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answered 2015-09-18 12:34:09 -0600

MJWitte

9740 ●6 ●10 http://www.gard.com/

The water heater "source side" is placed on the demand side of the boiler loop. See example file 5ZoneWaterSystems.idf

There are some special control fields at the end of the object that may be used to coordinate the water heater and boiler operation. The Source Side Flow Control Mode specified when the water heater will place a demand on the boiler loop (or whatever the source is). And there is an optional Indirect Alternate Temperature Setpoint Schedule Name that allows the boiler to meet a different tank setpoint than the primary tank heating element setpoint.

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I've got it setup with a primary boiler loop and a secondary boiler loop. On secondary supply side I have the tank and a TemperingValve in //. But I'm getting problems getting the boiler to be sized correctly and to ensure that the service hot water is at the right temperature at the point of use (it drops below the scheduled temperature, and below the DHW loop setpoint... yet E+ doesn't throw any warning or error about demand not met... I was surprised by this). It's definitely a controls problem.

Would anyone be willing to work on this with me? We could start a dedicated slack channel...

Julien Marrec ( 2015-09-20 05:51:50 -0600 )edit

There is an open issue related to this to add some output variables regarding water use load not met, #4954. It refers specifically to water use equipment, but I'm going to add direct use from the water heater as well.

MJWitte ( 2015-10-06 15:21:35 -0600 )edit

@MJWitte Is this still the best implementation of a gas condensing water heater in Energyplus?

mldichter ( 2019-01-31 16:39:27 -0600 )edit

@mldichter Yes, no change to this. @Julien Marrec Seems it would be fairly straightforward to allow WaterHeater:Mixed to accept a bivariate curve or table. A little more work would be required for WaterHeater:Stratified since it does not have a Part Load Factor curve input (not sure why not).

MJWitte ( 2019-02-04 15:44:22 -0600 )edit

@Julien Marrec I was modeling a tankless coil with a mixed tank and a boiler source side plant loop, for this system the unmet water heating loads are even more obvious due to the smaller volume and more fluctuating temperature. I think the unmet load situation is something with control of source side, the flow rate is turned on/off only once observing the previous tank temperature in each timestep. There's no temp recover for cases below setpoint if source side plant loop is the only heating source in tanks. The logic of heat recover only works for internal heaters.