BMS Control flow temps for fan coils units

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Hi there,

Can someone explain the attached screen shot from our BMS system. Now its heating season the fm doesn't appear to be setting the boilers correctly. Contractors who attend do not appear to know what values go where. These boilers supply hot water to our 4 pipe fan coils which heat the office building.

IMG_5338.jpg

The values I am querying are Boiler Flow S/P & Flow Hold Off S/P. Usually flow s/p is set to 60 and hold off is set to 50c. However we are not sure if they should be reversed as we are only getting a 2c differential between the return and send temps. Is the flow s/p what we want the flow to be and the hold off is what temperature the boilers reach before the burners stop firing?

No one else seems to know here at all and it seems they are setting silly levels on the boilers such as 80c for both.

Thank you.

[CraigF]

With only 2c diff you don't seem to be exchanging any heat? What are these boilers running?? Calorifiers? Wet coil AHU?

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Wet coil FCU... The reason we have had 2c differential is because we have had a pressurisation unit fault so the system kept tripping off. We have cooling circuit off the chiller and separate heating circuit.

We've now had this replaced and we want to get the best settings we can but we don't really understand these two. Many contractors have been out and don't understand it along with BMS engineers.

[CraigF]

Just out of interest what controls are they??

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Just out of interest what controls are they??

Johnson controls with Lennox chillers/fan coils

[redroge]

Surely flow hold off set point is water temp to be reached before pumps start, there for should be lower than flow set point. Flow set point 80c ,hold off 50c.

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Surely flow hold off set point is water temp to be reached before pumps start, there for should be lower than flow set point. Flow set point 80c ,hold off 50c.

Doesn't seem to make any difference. The flow is still there even if the boilers are at 40c or 80c

[The Viking]

First of all,
Check the spec for your boilers, as most manufacturers insist on a flow temp of at least 78 degrees C...

The "Flow Hold Off" tag has been labled so by the BMS (Building Management System = Control) engineer who set this system up, as it generates confusion the best option is to either look in the O&Ms to see what it actually is looking at or, if for example the O&Ms are missing, get a BMS engineer in to interrogate the system to see what the function is (and maybe re-label the function)

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Yes I think at the next bms maintenance we will ask that question. For now we will put it back to 80c

[bigspee]

75- 80degrees is what you want for a boiler flow temp. You shouldn't run a heating boiler without pumps running, they should be interlocked so that the pump runs and makes a flow switch / diff press switch and then the boilers are enabled. In the picture you have uploaded it shows the boiler set point at 60 degrees and the hold off at 60 degrees... You ideally want set point at 75-80 The hold off set point might be a hold off for the FCU,S ie the coils need to reach the set temp ( hold off) temp before the fans on the FCU's start to prevent the air been blown feeling cold.