Fresh Air

[chrisboy27]

Hi Guys

I carry out maintenace and service calls on a building in Cardiff which has just had a new install of mitsubishi city multi.

The evaporators are ducted fan coil type using the ceiling void as there return also there is a main ahu putting fresh air into the void.

The direction of the ducted fresh air is directly blowing into the return of the fan coil.We have had a few issues with rooms not reaching temp but the return temp showing it has reached temp.

Is this caused by the return air sensor picking up a false reading from the fresh air.

Thanks in advance for you help

[Thermatech]

This is a very common problem with ducted indoor unit when the ceiling void is used for return air.
The temperature of the air in the ceiling void can be very different from the air temperature in the room at ocupant level.
The temperature of the fresh air being supplied into the ceiling void is also critical.

Some cost saving has been made on the installation by not installing ducted return or not fitting remote temperature sensors in the room.
but
the customer ends up with poor temperature control :confused:

One method to resolve this problem is to install remote temp sensors at a suitable location in the room & remove the standard heating mode 4 deg diff.
The next best option is to put the remote sensor in a ceiling return air grill which is only getting return air from the room & no fresh air & leave the standard heat 4 deg diff.

The actual fresh air supply temperature is critical for the overall temperature control in the building so if its much warmer or much cooler than the desired room temp you can get all sorts of cold draft & overshoot problems.

The best fresh air option is the Mitsubishi Lossney energy recovery system because it can be interlocked with the City Multi Mnet controls system & will only work with the city multi units & never fight against them.
It will also automatically provide free cooling when the outside temperature is cool enough & the indoor units are calling for cooling.

Unfortunately to save cost cheaper heat recovery fresh air systems are often installed which have no electronic interlock or communication with the Mitsubishi CM systems & then the fresh air plant may not be doing what is ideal for good temperature control or energy efficiency.

Measure the fresh air temp & the on coil air temp.
If the unit is heating make adjustment for the 4 deg diff & monitor room temperature.Then you can see whats happening.

I find that the best option is to leave temperature data loggers in for a week or two & then you get a much better overall picture of whats happening.
I also monitor the City Multi system with the Mnet lap top computer & then put together some graphs to show the Mitsubishi system data & the data logger temperatures.
Then you can show the customer exactly what is happening with recorded data & make suggestions to improve the temperature control & let them decide based on how much they are prepaired to spend.

I do alot of this type of trouble shooting & fine tuning work & so I regularly deal with this type of issue on VRF ducted systems because consultants & contractors will always try to reduce installation cost. But it makes some fine tuning work for me :D so that ok.

[Brian_UK]

Just an addition to what Thermatech has said about moving the sensors.

Admittedly my problems some years ago was with four pipe fan coils but once again it was a common return air path via the ceiling void.

Some cures were made by fitting a remote sensor just through the ceiling grid but careul positioning was required to avoid the movement of heated/cooled air over the sensor.

The best solution we found, at the time, was to fit a small grille, 100mm vent, into the ceiling with the sensor mounted behind it and a small low voltage axial fan to pull room air through and over the sensor.

[Zigzag]

Hi Guys,
Another quick solution to this problem is to set the wall controller to monitor the room temp if the controllers are in the same room.This will give a better reading than the void

[Thermatech]

But 9 times out of 10 the remote controller is not in the best possition in the room for accurate temperature monitoring
and
Many office blocks do not have any local remote controllers because a centralised controller or BMS control system is used.

[chrisboy27]

Hi Guys,
Another quick solution to this problem is to set the wall controller to monitor the room temp if the controllers are in the same room.This will give a better reading than the void


This system works on a central controller and bms.

I have now managed to talk the installer into putting room sensors in.

Thanks for the advice guys.

p.s Where did you get the mnet from and how much was it.