Room temperature flucation problems -PART 2

[remp]

Hi

The conference room air handling unit is not holding the temperature in the space steady and worse still the supply air temp fluciates between 15 and 30 Deg C in any 30 minute spell, this causes drafts and people notice the fluciations.

I have found what I think the problem is: The air handling unit has a water heating coil at 80/70 Deg C.
The coil is only working at 10-30% of its duty most of the time. When the control valve opens say from 10 to 20% open the off coil tempetatrure takes about 15-20 minutes to settle down, it keeps rising before it stops. In the mean time if the BMS did not see a responce in the room it sends another signal to open up more so by the time the first signals responce has kicked in, the control valve is opend even further to compound the probelm.. The temp overshoots the mark in the room. When in reverse it takes ages for the heat to leave the coil and there is the same problem in reverse as the valve closes to release less heat.

Looks like the coil is just too big and a small fluciation in water flow rate takes ages for a responce. Is there stratafacation going on...ever seen probllems like this with a coil before????Any tips?

Thanks

 

[SAK9]

In order to get some meaningful advice,I would suggest you to post some additional info such as design cooling and heating loads,design air on/off, how is ventilation air supplied,if the room has a large component of fabric load etc

If it is an internal conference room,heating is only for the ventilation air(can be high for a conference room).It appears the heating coil is oversized and you may not be able to get it to work.The possible solutions are replacment with either a smaller coil or an electrical heater.If the heating load is only a fraction of the cooling load(possible in a temperate climate),getting a right sized coil will be difficult and the electical heater option may be the way to go.

Anyway going back to the drawing board and looking at the whole problem in a different perspective will be sensible.

 

[Zesti]

Changing the coil in case it is oversized might not be necessary.

It might be a lot cheaper to lower the water supply temperature to the coil (separate from the control valve at the coil).

Do you have a pump at the coil in order to have 100% flow through the coil at all times ? (with the control valve regulating the temperature of the flow)

(It might help to post a schematic of the whole setup, including all sensors and controls.)

 

[remp]

Hi Zesti

I am thinking of putting a pump at the coil as you said to keep the flow at 100%. I think it is the low flow rate that is effecting the perormance of the coil. do you think the pump will help so in effect i have a constant volume variable temperature coil?

Remp

 

[Zesti]

Have you consulted the supplier of the AHU/coil?

Do you have the specifications for the coil and do they match with what you think is needed? (airflow, entering T, exit T, water temperatures)

If the coil turns out to be too large, have them calculate at which water temperatures the coil will deliver the desired temperatures airside at a good waterflow.

You mentioned stratification: if this is indeed happening then the the step above and adding a pump should get rid of that.

- Have you checked the authority of the control valve?
- Is the flow at 100% control valve set properly?
- Might there be other systems on the same watersupply which could influence pressures and flows?