Best place for return air sensors, in the room or ceiling void?

[back2space]

Hope you guys can offer some advice please.

having issues at work with temperature control. The system is a chilled water circuit with chilled water provided by a chiller on the roof and ducted fan coils in the office spaces. The chiller supplies the cold water at 9c usually however if high outdoor ambient another compressor comes in to bring the water down to 5C I am told.

Air is supplied into the office spaces through the standard square office diffusers and return air is through the same looking diffusers however these return air diffusers dont have ducting on, they are just open to the false ceiling void which is used as the return air plenum. The return air sensors are also laid in the ceiling void. I am Assuming they should be laid above one of the return air grilles not just scattered about in the void.

An air handling unit also on the roof ducts fresh air into the office space that runs through the ceiling void into diffusers down into the office space.

We are having issues with temperature control. The bms system is set at 21C throughout the building however the actual temperatures in the office space can be as much as 25C but the BMS system reporting ceiling void temps of 21/22C.

We believe that the return air sensors are sensing the ceiling void temperature rather than the true room temp. To combat this our a/c contractors poked a couple of the return air sensors through the return air vents in the areas that were reporting poor temp control so they were sensing temperature within the office space. This improved temperature control within that "zone" however we have sinced changed a/c maintenance contractors and since summer has started there has been some office changes, and new rooms created and the same problems have arisen however the new a/c contractors do not seem to think poking the sensors through the vents will make any difference as it will not be sensing properly.

Just wondering what you guys think?

Thank you.

[paul_h]

hot air rises, ceiling void temps are mainly a problem in heating mode (reading higher than it is and shutting of heating too early).
In cooling mode, floor level is normally cooler than ceiling level, so you'll never get the problem of the cooling shutting off too early using ceiling temps.
Your problem may be all the modifications doen to the room, or just old equipment past it's use by date and can't do the job anymore.

[back2space]

There has been a lot of modifications done to the room, it was open plan before now its a secure room with 6 fan coils, we thought the issue was with there not being enough fan coils but the issue is still the same even with only 4 people working in there on a night instead of the usual 60.

So we still think its an issue with perhaps the fresh air supply being circulated at 18C into the ceiling void which mixes with room temp air which then gives false readings to the bms making it think its reached set point when it hasnt.

Another possible cause could be cooler air being blown out the diffusers across the ceiling and being sucked straight back into return air grilles.

Either they are telling us the system is set at 21C in the hope of us physcologically thinking its cooler because they say it is or its set at a much warmer temp.

We reached up to the diffusers and there was cool air blowing out of them so the system is in cooling but perhaps not as much as it should.

The bms states in % what the fan coil is doing it might be in 40 per cent cooling for example.

[Brian_UK]

Age old problem with ceiling void mounted fan coils I'm afraid.

What we have done in the past was make up same fan units, using something like computer fans, fitted them to a box/grille that was fitted into the ceiling tiles themselves.

The air is drawn over the temperature sensor from the space and a better response was obtained.

Just hanging the sensor through the tiles can improve things but it needs to be done with some thought.

If they are in the airflow from the diffusers then false readings will be taken.

Also are the fan coils running at the correct airflow rate/fan speed? The air needs to be 'thrown' from the diffuser; the air with cling to the ceiling before dropping downwards.

If the airflow is low then the air drops too quickly and does not induce an air from the lower regions.

[back2space]

Age old problem with ceiling void mounted fan coils I'm afraid.

What we have done in the past was make up same fan units, using something like computer fans, fitted them to a box/grille that was fitted into the ceiling tiles themselves.

The air is drawn over the temperature sensor from the space and a better response was obtained.

Just hanging the sensor through the tiles can improve things but it needs to be done with some thought.

If they are in the airflow from the diffusers then false readings will be taken.

Also are the fan coils running at the correct airflow rate/fan speed? The air needs to be 'thrown' from the diffuser; the air with cling to the ceiling before dropping downwards.

If the airflow is low then the air drops too quickly and does not induce an air from the lower regions.

THank you Brian for the detailed response, I know that could cause problems if they get caugt in the airflow from the diffusers. From what I can gather the fan speeds are on max as they didnt want to put more coils in so they just maxed the airflow.

They need remote sensors in really but I dont think they want to spend the money.

[sinewave]

Can you not fit dedicated wall mounted sensors which connect to the return air sensor point on the ducted PCB board?


One of our clients has suffered from this very problem because the standard return air sensor in the ducted unit was either getting false readings from the roof space or from the duct feeding ambient fresh air in to the unit.


Wall mounted sensors (wired correctly! ) have shown a vast improvement in staff comfort!

[back2space]

Can you not fit dedicated wall mounted sensors which connect to the return air sensor point on the ducted PCB board?


One of our clients has suffered from this very problem because the standard return air sensor in the ducted unit was either getting false readings from the roof space or from the duct feeding ambient fresh air in to the unit.


Wall mounted sensors (wired correctly! ) have shown a vast improvement in staff comfort!

This is an option but facilities dont seem to believe there is a problem.

There is though and its temperature control.

REmote sensors would work fine and I have suggested this but they took to putting a couple of the sensors poking through the vents in other parts of the office but not in our part.

[frank]

You've nailed the problem in your last couple of posts.

Members here have given you the reasons why you have control issues, (and deep down, you seem to know this already) but the powers that be do not have the budget to install corrective measures.

Using the ceiling void as a return air plenum with return air control sensors in the ceiling void, will not give good results. You need averaging sensors, 1.6m from floor level in the space out of direct sunlight and draughts.

This discussion could go on and on but nothing will change.

Good luck with it anyhow.

[The Viking]

In my humble opinion...

No sensors should just lay on the ceiling, they should be fixed in a return air airstream or poked through the ceiling but preferable wall mounted.

But, what you are describing with the situation worsening after partitioning were added, leads me to think it might be the very common fault of FTATACG (Failure To Ask The A/C Guy).

More often than not when people want to rearrange their offices, they don't want to pay architects or consultants. After all, it's only a couple of walls and they do know where they want them to go.
As a result they get a couple of quotes from different builders to put the walls up and from sparkies to add some sockets. Then they will choose the cheapest quote.

What nobody had considered is that those new walls upset the air flows for the A/C and heating system.

Is the unit(s) feeding the air to this new room situated within the ceiling void above the room itself?
Is the unit's sensor within the ceiling void above the new room?
Has proper heatload calculations been carried out for this new room?
Is the new walls upsetting the airflows from the grilles?


Questions, questions, questions...

[Abby Normal]

bottom line, if you want to control the temperature in the space, you need the sensor in the space

[Brian_UK]

The awkward state that we have in the UK is that there no maximum temperature standard; a minimum yes but no max.

HSE go as far as saying it should be 'comfortable' but don't clarify it further, of course.

[back2space]

You've nailed the problem in your last couple of posts.

Members here have given you the reasons why you have control issues, (and deep down, you seem to know this already) but the powers that be do not have the budget to install corrective measures.

Using the ceiling void as a return air plenum with return air control sensors in the ceiling void, will not give good results. You need averaging sensors, 1.6m from floor level in the space out of direct sunlight and draughts.

This discussion could go on and on but nothing will change.

Good luck with it anyhow.

Yes I do know what the issue is but in order to get anything to change I need evidence which is why I am putting this together to show them.

They dont take my word for it as the facilities manager is supposed to do that!

[rude]

In my humble opinion...

No sensors should just lay on the ceiling, they should be fixed in a return air airstream or poked through the ceiling but preferable wall mounted.

But, what you are describing with the situation worsening after partitioning were added, leads me to think it might be the very common fault of FTATACG (Failure To Ask The A/C Guy).

More often than not when people want to rearrange their offices, they don't want to pay architects or consultants. After all, it's only a couple of walls and they do know where they want them to go.
As a result they get a couple of quotes from different builders to put the walls up and from sparkies to add some sockets. Then they will choose the cheapest quote.

What nobody had considered is that those new walls upset the air flows for the A/C and heating system.

Is the unit(s) feeding the air to this new room situated within the ceiling void above the room itself?
Is the unit's sensor within the ceiling void above the new room?
Has proper heatload calculations been carried out for this new room?
Is the new walls upsetting the airflows from the grilles?


Questions, questions, questions...

100% accurate.. Also another thing to consider is how old is the building and ac system are, because over time i find that new offices are built to accomodate more ppl working. Which then these ppl need more lighting, computers etc etc and the heat load calculations go up and the poor old system struggles to maintain temps.

[back2space]

100% accurate.. Also another thing to consider is how old is the building and ac system are, because over time i find that new offices are built to accomodate more ppl working. Which then these ppl need more lighting, computers etc etc and the heat load calculations go up and the poor old system struggles to maintain temps.

The building is about 5 yrs old, and here is a screen print from our bms, the room now converted into a helpdesk secure room has a red line which is the wall of the helpdesk. Has seating for about 60 people and computers.

Problem is that even at night time with 4 people sitting in here, we experience drafts from the a/c so we ended up getting 2 of the fan coils switched off at midnight so that they wernt blowing over heads and onto necks however even before these were switched off the temps remained the same as during the day at 24C even though they have no load apart from 4 pcs and 4 people.

The same fan coils serve the meeting room and the server room, however a wall mounted split is being approved for the server room as currently only 1 air diffuser in there and not upto the job temp is 25C in there constantly.

Theres also a ducted split in here that is completly separate to the main system this is used in case of breakdown of the main system however it is useless and was pointless putting it in as it does not do anything as is undersized. its a 5kw capacity. its been used the last few days to assist the main system during the day.

[Yuri B.]

? Which controlers' sensors are

laid in the ceiling void

If there are

ducted fan coils in the office spaces

how are they controlled (to maintain office spaces' temp.) ? Have the fan coil(s) dedicated to each space its/their own room controller. room sensor, and (a) 3-way valve(s) with solenoid ?

Has the AHU a cooling coill also? If yes, there should be only one return air temp. sensor installed in the extract air duct, not many

scattered about in the void

Are the air flows balanced?

Sorry if I'm not understanding smth properly.

[Yuri B.]

Abby Normal:

if you want to control the temperature in the space, you need the sensor in the space

Thus, elsewhere there are used simple, cheap room thermostats. Installed at some 1.6m height at every controlled place (open, and away from heat sources) they control solenoid valves of their corresponding fan coils.

[back2space]

? Which controlers' sensors are

If there are

how are they controlled (to maintain office spaces' temp.) ? Have the fan coil(s) dedicated to each space its/their own room controller. room sensor, and (a) 3-way valve(s) with solenoid ?

Has the AHU a cooling coill also? If yes, there should be only one return air temp. sensor installed in the extract air duct, not many


Are the air flows balanced?

Sorry if I'm not understanding smth properly.

Each fan coil in the ceiling void has a return air sensor, this can be independantly controlled from the bms so set points can be set on each coil. THey will operate in heating/cooling depending on whatever they need to be doing.

The extract duct is the same duct that branches out to the whole building, 21C air supplied to the whole building at the same time. The AHU for the fresh air therefore has its own cooling/heating coil. But this supplies are at 21C whether the office is requiring heating or cooling.

ON really hot days they sometimes turn the AHU temp down to 18C.

[Yuri B.]

return air diffusers dont have ducting on, they are just open to the false ceiling void which is used as the return air plenum. The return air sensors are ... laid in the ceiling void.

An idiosy to have done so.

new a/c contractors do not seem to think poking the sensors through the vents will make any difference as it will not be sensing properly.

They are making pretexts for not doing anything.

[back2space]

An idiosy to have done so.

They are making pretexts for not doing anything.

YES I think they were cutting corners with the air con.

But I think the extract ducts take the air from the ceiling plenum also so this is supposed to pull the air through the return air grills with no ducting on.

[Yuri B.]

And, probably the "zones" each sensor controls are also too wide? (how many fancoils serves one sensor?). Hopefully, they at least did not mixed up inputs, aggreavating the whole pitifull picture.

[back2space]

theres one return air sensor per fan coil so therefore each zone has its own fan coil.

The helpdesk area is divided into 4 zones.

[Magoo]

What about fitting those variable volume diffusers that are temp controlled for local zones with minimal close off, add a coulple of dummy stats for staff to play with, all phycological [spell check ], old people versus young people, as in whose cold and whose hot at the same time. what did Abe Lincoln say, you can keep some the people happy some of the time, you can keep all the people happy some of the time, but you cannot keep all the people happy all the time.
magoo
magoo

[back2space]

the problem is the system is controlled from BMS so facilties do sometimes say they have turned it up or down to fool you into thinkning its warmer/cooler.

The problem is that its warm enough but not cool enough, it doesnt get below 23C even though its set at 21C so we shouldnt be shutting off diffusers I dont think,

[Gary]

To combat this our a/c contractors poked a couple of the return air sensors through the return air vents in the areas that were reporting poor temp control so they were sensing temperature within the office space.

This seems to say that the temperature of the air entering the return is warmer than the temperature in the void. How does that air get colder in the void?

Do you have supply air ducts leaking cold air into the void?... or perhaps cold air from the fresh air system leaking into the void?... or maybe cold air is being sucked up around the ceiling tiles into the void?

[back2space]

This seems to say that the temperature of the air entering the return is warmer than the temperature in the void. How does that air get colder in the void?

Do you have supply air ducts leaking cold air into the void?... or perhaps cold air from the fresh air system leaking into the void?... or maybe cold air is being sucked up around the ceiling tiles into the void?

Gary unfortunatly all of them are possible, they cut corners when they installed the system, I think cold air is being pulled in through the tiles or leaking ducts.

I will speak to the engineer soon on a conference call.

[Thermatech]

In this case you need an AC enginner to do a full survey of the units.
This should include a health check of the indoor ducted fan coils cooling performance & confirmation of correct setup.

Data log the following for a few days.
1/room temp at a number of locations/ occupant level & ceiling level db & wb.
2/ return air grill / return air sensor temp for each unit db & wb
3/ fresh air supply temp at each outlet in the ceiling db & wb

Also
a/ The fresh air volume needs to be measured at each outlet to confirm correct supply air volume.
b/ a/c ducted units air volume needs to be measured to confirm correct supply air volume.
c/ supply air circulation needs to be monitored to see exactly where in the room the air is going.

Then based on accurate recorded data the engineer can provide a report which will show what the problems are & suggestions for improvements & fine tune the system operation.
Facilities department will then have evidence in the form of data log graphs of temperatures & can make informed decision.

Notes
Fresh air temp always has a big impact on temperature control in this type of application.
Room temp sensor location is always critical.
The ac units in the room were probably designed for average open plan ocupancy not 60 staff & 60 computers.
The circulation of air in the room may not be ideal for the current high density occupation layout.

[back2space]

In this case you need an AC enginner to do a full survey of the units.
This should include a health check of the indoor ducted fan coils cooling performance & confirmation of correct setup.

Data log the following for a few days.
1/room temp at a number of locations/ occupant level & ceiling level db & wb.
2/ return air grill / return air sensor temp for each unit db & wb
3/ fresh air supply temp at each outlet in the ceiling db & wb

Also
a/ The fresh air volume needs to be measured at each outlet to confirm correct supply air volume.
b/ a/c ducted units air volume needs to be measured to confirm correct supply air volume.
c/ supply air circulation needs to be monitored to see exactly where in the room the air is going.

Then based on accurate recorded data the engineer can provide a report which will show what the problems are & suggestions for improvements & fine tune the system operation.
Facilities department will then have evidence in the form of data log graphs of temperatures & can make informed decision.

Notes
Fresh air temp always has a big impact on temperature control in this type of application.
Room temp sensor location is always critical.
The ac units in the room were probably designed for average open plan ocupancy not 60 staff & 60 computers.
The circulation of air in the room may not be ideal for the current high density occupation layout.

The circulation issue is causing problems also, there is a draught that some people complain about.

We have had data loggers in several times temperatures in the winter remained constant at 23.1 - 23.3C pretty accurate however they state the fan coils are at 21C.

From what I have gathered from the engineers, there is plenty of capacity and the problem is somewhere else. As even at night time when only 2 pcs and 2 people in the room, the temperature remains the same.

I am going to ask to have a look at the BMS to see if its actually set at 21C as if it is it could be the actual BMS system that is not controlling properly. We have had issues before where the BMS has thought the chiller was working but it was not, and was only found out when they checked the chiller by hand and it had a fault.

[back2space]

UPDATE:

Ended up working on my own tonight, so thought would be an opportunity while it was quiet to check things out.

Out of the 5 fan coil units that are in here only 2 had their return air sensors in place above the return air grille. The others were just loose in the ceiling void which feels cooler than the office space, I have since repositioned these into the correct places and will see how it goes.

There was also one fan coil unit that was on low fan speed (I) whilst the others are all on high (III).

These are lennox fan coils.

Th