The situation faced by the tempering valve here is that neither stream is warmer than setpoint. So setpoint cannot be met by any action that the valve could take. Since it is modeling an anti-scald device, not a more complex 3-way valve, it chooses to do nothing to intervene. It is only setup to redirect flow around a tank that is too hot. So the tank gets the draw.
With a stratified tank the use outlet will be warmer and you get into a situation where the controller would do what you expect.
Something else is cooling off the tank -- typically it is solar panels running at night.
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Thursday, August 28, 2014 9:48 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] Tempering Valve in DHW loop with a solar tank [1 Attachment]
Dear Mr. Griffith,
thanks for the prompt response.
It is a WaterHeater:Mixed. In my opinion it is not the solar tank which causes the high DHW energy demand. It is rather the tempering valve which does not regulate the mass flow rate properly.
As you can see in the attached diagram, there is always a mass flow rate on the use side of the solar tank regardless of the inlet and outlet temperature. There should only be a mass flow rate when the inlet temperature is higher than the outlet temperature.
Another problem is the setpoint node you have to specify in the TemperingValve. Obviously is must be the same node of the plant loop (setpoint temperature 60°C). So if the the solar tank temperature is higher than 60 °C the tempering valve tries to decrease this mass flow by mixing it up with the return flow of the DHW demand loop.
Regards,
Christian
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Posted by: "Brent Griffith" <Brent.Griffith@xxxxxxxxxxxxxxxxxx>
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