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VSD recirculation pump with DHW loop in eQUEST

asked 2016-10-18 14:33:51 -0500

dpud12's avatar

updated 2017-05-08 16:10:46 -0500

Hi All,

I am trying to model a recirculation pump with a variable speed drive for a domestic hot water loop, however, have not been able to control the pump as expected. From my understanding of the technology, the point of having a VSD on a recirculation pump is to be able to modulate the flow of the recirculating water so that a desired temperature is maintained at the end of the supply portion of the loop (say 105 F since that is what most people want for showering). This way you can save both water heater consumption and pump energy. However, I do not see a way to specify this type of control in eQUEST.

Whenever I set the drive type to Variable Speed, I get a consumption reduction as anticipated, but the part load profile shows that it is simply at 50% the entire time (see image attached) rather than varying with the system/environmental conditions as expected. I have the control type set to Load Reset and the Min Alarm Temp set to 105, which matches the Loop Delta T specified of 25 F (supply temp 130 F). Changing the loop delta T and the Min Alarm Temp has no impact on the operation of the pump. Thank you in advance for any insight you may be able to offer!


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answered 2016-10-18 19:12:59 -0500

pflaumingo's avatar

updated 2016-10-19 09:31:47 -0500

If you set the 'Loop Recirc Flow' (LOOP-RECIRC-FLOW) you are setting a constant recirculation flow and not a variable speed one. If you want it to be variable speed then you need to specify the 'Loop Minimum Flow' (LOOP-MIN-FLOW). This minimum flow is the minimum fraction of the supply design that the loop will run before the recirculation pump turns on. If the design is process flow is 15GPM and the minimum flow is set to 0.5 then anytime the fractional value times the design flow drops below 7.5GPM then the recirculation pump will kick on as needed at a variable flow. It does seem to be a little buggy in that you have to set the Loop Recirc Flow to a value greater than 0 such as 0.01.

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I have adjusted my circulation loop by setting the minimum flow ratio, but it won't let me leave the input for Loop Recirc Flow (LOOP-Recirc-FLOW) blank. Will entering zero ensure that eQUEST will look towards the minimum flow ratio, or is there anything else that I need to do? Thank you very much for your help!

dpud12's avatar dpud12  ( 2016-10-19 07:59:52 -0500 )edit

Also, does this mean that there is no way to have the loop controlled by the return leg temperature? So rather than having to set a Min Flow Ratio, I would like to just set the return leg temp I want to maintain.

dpud12's avatar dpud12  ( 2016-10-19 08:04:49 -0500 )edit

Edited answer regarding the blank flow. I'm not sure if you can directly set a return leg temp.

pflaumingo's avatar pflaumingo  ( 2016-10-19 09:37:37 -0500 )edit

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Asked: 2016-10-18 14:33:51 -0500

Seen: 970 times

Last updated: Oct 19 '16