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WaterHeater:Mixed being heated significantly by something other than the water heater?

asked 2018-02-01 17:18:04 -0500

mldichter gravatar image

updated 2018-02-23 17:22:25 -0500

I am running a DOE stripmall model (google drive link) created from the openstudio measure, and the water heaters seem to be heating up way past the plantloop setpoints. To try and figure out what's going on, I changed the value targeted by the WaterHeater:Mixed and WaterUse:Equipment temperature values to a cool 20C from their former values of 48C and 60C.

Schedule:Constant,
Mixed Water At Faucet Temp - 120F, !- Name
Temperature, !- Schedule Type Limits Name
20; !- Hourly Value
Schedule:Constant,
Service Water Loop Temp - 140F, !- Name
Temperature, !- Schedule Type Limits Name
20; !- Hourly Value

Then looking at any WaterHeater:Mixed temperature hourly starts correctly but then grows all the way to the water heater's max temperature.

20.3941488758886
21.181041550316
21.9645761208971
22.7450248886258
23.5221482556748
24.2960830378693
.
.
.
80.3548709036
80.9048281271821
81.4759689809748
82.0455031349025
82.2222222222223
82.2222222222223

I cannot figure out what's going on. The only suspect is this warning (repeated for all the pump in each plantloop)
* Warning * Check input. Pump nominal power or motor efficiency is set to 0, for pump=SERVICE WATER LOOP PUMP

My best guess is this is adding heat into the water being pumped, but there doesn't seem to be anything wrong with the pumps.

Pump:ConstantSpeed,
Service Water Loop Pump, !- Name
Node 131, !- Inlet Node Name
Node 137, !- Outlet Node Name
Autosize, !- Design Flow Rate {m3/s}
0.001, !- Design Pump Head {Pa}
Autosize, !- Design Power Consumption {W}
0.7, !- Motor Efficiency
0, !- Fraction of Motor Inefficiencies to Fluid Stream
Intermittent, !- Pump Control Type
, !- Pump Flow Rate Schedule Name
, !- Pump Curve Name
, !- Impeller Diameter {m}
, !- Rotational Speed {rev/min}
, !- Zone Name
, !- Skin Loss Radiative Fraction
PowerPerFlowPerPressure, !- Design Power Sizing Method
348701.1, !- Design Electric Power per Unit Flow Rate {W/(m3/s)}
1.282051282; !- Design Shaft Power per Unit Flow Rate per Unit Head {W/((m3/s)-Pa)}

I tried changing the 0 to 0.1 in Fraction of Motor Inefficiencies to Fluid Stream, which shouldn't and didn't do anything because that 0 means a perfectly efficient pump.

Any ideas on what the problem might be?

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answered 2018-02-23 17:29:59 -0500

mldichter gravatar image

I found a few problems with the model and got it working.

The * Warning * Check input. Pump nominal power or motor efficiency is set to 0, for pump=SERVICE WATER LOOP PUMP wasn't the problem, but that occurred because the pressure of the pumps was set to 0.001 and the typical value is 173,000.

The water heaters were being heated by the Off Cycle Parasitic Fuel Consumption Rate and on Cycle Parasitic Fuel Consumption Rate being set to about 173W for this electric water heater. First off, this water heater is electric and has no pilot light, and secondly this was like the regular heat source is always on. The water got WAY too hot because it had a heater running all the time.

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Asked: 2018-02-01 17:18:04 -0500

Seen: 68 times

Last updated: Feb 23 '18