Fan Coil Unit Inlet/Outlet Pipework Temperature Difference?

[elecrobot]

Hi All,

We have recently moved in a building and trying to troubleshoot issues with the Fan Coil Unit (FCU). Please see details below. If someone can please give some advise based upon your experience, it will be greatly appreciated.

With regards to Fan Coil Unit, what we have observed is as below. Can you please discuss with your technical department to advise:

We have a 4-pipe Fan Coil Unit. There are 2 pipes (flow in/flow out) to heating coil and 2 pipes (flow in/flow out) connected to cooling coil (Cooling coil is not in use and Chiller switched off) within same FCU. There is a manual valve on the inlet to Fan Coil Unit (FCU) heating coil and an actuated PICV on the outlet of the FCU heating coil. We have removed the actuator just to keep the testing simple and check the water flow from FCU is good from the heating coil. Both inlet valve to FCU and the PICV on FCU outlet are manually open for this testing. What we have found during testing is when the FCU is switched off via local isolator switch, inlet and outlet pipework from/to FCU are very hot which makes sense as the water is circulating from the FCU as both inlet and outlet valves on FCU are in open position. However when the FCU is switched ON from the local isolation switch, the inlet pipework temperature remains very hot as before but the outlet pipework temperature drops to normal (slightly warm) after few minutes. My question is what is the temperature difference between the inlet and outlet pipework when fan within FCU is running? The air coming out of the FCU is at good flow but it's only slightly warm which I don't think is enough to warm the room.

[Brian_UK]

Depending on the actual design of the building and the FCUs they were generally designed around the boiler flow/return temperatures which would have been - flow 82 degC and return 72 degC.

Good operation depends on the correct airflow rate, correct water flow rates, clean air filters, clean water filters, clean coils and fan scrolls.

Temperatures only mean something when measured with a thermometer, subjective hand felt temperatures are not good enough to prove/disprove the system.

[frank]

The airflow caused by the fans being energised will absorb some of the heat from the heating coil and raise the outlet air temperature above the air inlet temperature . The volume air flow will determine the amount of difference. Low air flow = high difference, high airflow = low difference, all down to the dwell time.
As Brian says, lots of variables to look at.

[elecrobot]

Many thanks both.
“Low air flow = high difference, high airflow = low difference, all down to the dwell time”…. This is very good point. I was thinking other way round i.e. by increasing fan speed i will get more heat faster which is not correct in water based heating systems. I will try reducing the fan speed again and do some temperature measurements.