Just how warm should air be blowing from fan coil units in heating?

[back2space]

Looking for some advice.

We've had an engineer out many times now to our ground floor office units and all engineers are saying units are operating properly. It's a chilled water cooling system with FCU's that have both cooling and hydronic heating coils. Boiler is set to 75c flow temp. Air onto the coil at 20c and air off the coil at 24-27c depending on fcu. Water in at 45c and water out at 39c which to me seems very poor when the boiler on the roof 3 floors up is set to 75c and has a return temp of near to 65c.

Boiler has even been turned up to 90c and this made little difference. With such a high boiler temp I would have thought the air off would be much warmer.

Thank you

[chemi-cool]

What is the air flow rate in cfm? Can you measure it?
Did you check that the coil and fan blades are clean?

[back2space]

Coil is clean, and fan and motors have just been rewound but in the area where we are getting the most complaints these fan motors are running slower than they did before they were rewound. Even so at this you would expect the air off to be warmer than those that had a higher airflow through them. I'm not sure of the cfm though I'm afraid.

[chemi-cool]

Obviously indoor unit fan is a major part of the problem.
Have you checked coil temp?
What sort of water valve [if any] controlling the hot water flow?

[back2space]

Obviously indoor unit fan is a major part of the problem.
Have you checked coil temp?
What sort of water valve [if any] controlling the hot water flow?

I am working by asssiting the FM but auditing when we have engineer visits. There is an actuator (modulating) that presses down on a pin similar to on a TRV at home. These have all been verified to be working. The water in is approx 45c and water out approx 39c.

Seem to keep getting excuses from the engineers. For example the AHU that supplies fresh air into the space is normally set to bring air in at 22/23c and they have said we need to increase this to 26C to introduce further heat in. This then causes some areas to overheat.

Regarding the fan motors, they tell us that the motors were reconditioned to design specs and that the original motors were too big for the FCU!! so these new ones are slower but are apparently based on the design speed of the FCU? They were rewound as they are in use 24/7 and were getting to about 12 years old so some had burnt out. New genuine ones are no longer available for these Lennox FCU's so the next best thing was a rewind of the coils.

[Brian_UK]

If the heating flow temperature is only 45°C then that is the root cause of your problem.

Until you have design water conditions available at the fan coils you are chasing an ever decreasing circle.

[back2space]

If the heating flow temperature is only 45°C then that is the root cause of your problem.

Until you have design water conditions available at the fan coils you are chasing an ever decreasing circle.

Yes I know but the engineers keep telling us that you are going to get temperature losses in the pipework from roofs pace down to ground floor. I don't quite get that unless the pipes were uninsulated. The pipes by hand feel scalding hot into the unit but the guy who last visited placed a probe on the pipes and got 45C going into the coil! Air blowing from the units is tepid to say the least! I am thinking strainers or coils are blocked but they seem to disregard this fact! 45C isnt that hot and you wouldn't feel them as scalding when you put your hands on, so I think the issue is with the units!

Now they are on about moving all the supply and return grilles about! The problem at introducing 25/26c fresh air into the space at ceiling level is you convince the fan coils that they have heated the room upto temperature and this then means that they go into cooling. Any reccomendation on what the supply air should be set to in winter?

[install monkey]

is there any flexi connections on the pipework?- usually the fur up with crap restricting the flow

[r.bartlett]

I would verify that temp drop and eradicate it. That isn't normal on a properly balanced system. On those figures you are probably only getting 50% of rated duty. Follow the pipes down the building measuring as you go between floors.Check insulation also but they should know if it's insulated or not. At some point you may discover where the issue lies. for example, it's possible there is a valve open somewhere mixing flow and return.

[back2space]

is there any flexi connections on the pipework?- usually the fur up with crap restricting the flow

no flexi pipes, just solid pipes

[install monkey]

strainers before comissioning set?

[back2space]

strainers before comissioning set?

Sorry not sure what you mean fella?

The pipe work connecting each FCU is scalding hot so I think that it must be strainers or blocked coil?

[Brian_UK]

Some fun isolating valves contain strainers, these thing need checking.

I would suggest that a single for is selected and the pipework disconnected and components examined.

Having 'red hot' flow pipes but cold return demonstrates that there is insufficient flow the question is why?

[back2space]

Sorry maybe I didn't clarify properly, both the return and flow are red hot. But air off is poor. I'm trying to do this whilst currently suffering from the yearly Man flu lol

[r.bartlett]

Sorry maybe I didn't clarify properly, both the return and flow are red hot. But air off is poor. I'm trying to do this whilst currently suffering from the yearly Man flu lol

Water in at 45c and water out at 39c which to me seems very poor when the boiler on the roof 3 floors up is set to 75c and has a return temp of near to 65c.

What exactly are you saying??

[al]

Is there a primary and secondary loop, as said above water leaving at 65 should be at 65 at entry to fcu, strainers would want to be blocked to extent you would feel it, either get your thermometer out or leave it to FM company, very hard to diagnose issues based on "feelings", temps needed etc

[back2space]

Yes I think I will end up doing this as I am struggling to get anywhere with the fm at present who are clueless and send electrician out most the time to heating faults.

just a question what would you recommend fresh air supply temp be at in winter or summer?

[hyperion]

As has been mentioned earlier, is there a secondary circuit via either a calorifier or secondary TMV which is limiting the flow temperature to 45degC? The only sure way of establishing the flow temperature on the main pipework and the fan coils is to accurately measure it. This might help to pinpoint where the restriction is, however it is unusual for all of the FCU's to have a clogged up water circuit. There has to be a common denominator, such as a three port bypass valve incorrectly set causing the lower temperature or a pump that is not circulating at the correct flow rate.

[install monkey]

supplying red hot water to fan coils can cause the plastic around the heating actuator to go brittle and break off

[vijay_study]

Hi, design delta t for the hot water system is 10degc. However we are getting delta t as 6degc as mentioned in above post. We may have set their boiler temperature at 75 degc. However measurement at outlet of boiler may have not been taken. Probably we should use the same thermometer to measure boiler water leaving temperature and water entry temperature to coil. This is because temperature drop of 30degc is impractical. Hope above information is of help. Vijay

[back2space]

supplying red hot water to fan coils can cause the plastic around the heating actuator to go brittle and break off

yes had this issue already, but I've been told not to touch the boiler and leave it at 85c

[paddyaircon]

I had this issue once. The fault lay with the chw pipework being operational and actuators opening to flood coils with chilled water. This came to my attention whilst maintaining a rooftop vrf at same site and noticed the chillers running up in mid winter. BMS fault in the end. Tempory fixed by switching off chiller and pumps.Chilled water vs LPHW-!!

[frank]

Has the heating circuit been properly dosed?

I would hazard a guess that the ground floor circuit is full of crap leading to poor flow rates. Temperatures can be OK but if flow is compromised then you have symptoms such as you describe. Lots of heat on the roof but as it is flowing so slow, it loses significant heat by the time it is at ground floor level.

As previously stated, you need to isolate and remove one of the FCU's at ground floor level and inspect the internals of both the coil and pipework.

[back2space]

Has the heating circuit been properly dosed?

I would hazard a guess that the ground floor circuit is full of crap leading to poor flow rates. Temperatures can be OK but if flow is compromised then you have symptoms such as you describe. Lots of heat on the roof but as it is flowing so slow, it loses significant heat by the time it is at ground floor level.

As previously stated, you need to isolate and remove one of the FCU's at ground floor level and inspect the internals of both the coil and pipework.

Hi Frank,

I am not sure, over the years we have gone from several different suppliers who maintained the building. I am trying to develop into FM from currently working on the FM help desk but I am trying to get into the FM Management side of things but HVAC is one area I have a big interest in. Lets put it this way the engineers that have attended I have sometimes been the one educating them on things and telling them things they didn't know. Engineers often say " are you an engineer" as they are surprised at my knowledge and experience.

I tend to agree with you and this is what I am trying to push for one problem unit to be checked properly. We also have one unit that is forever stuck in heating even though when you take the actuator head off this should then mean the pin pops back up (like on a radiator) to the closed/open position. Cant remember exactly which is closed or open, pin up or down? Either way that suggests to me that theres some sort of crud in there reducing flow rates through.

Its frustrating for me as I am not a qualified engineer so its my word against theres but I have a very strong interest and passion for HVAC and a lot of what I know is self taught from my own time. That can sometimes be better than what any course can teach you.

[nike123]

Looking for some advice.

We've had an engineer out many times now to our ground floor office units and all engineers are saying units are operating properly. It's a chilled water cooling system with FCU's that have both cooling and hydronic heating coils.

Boiler is set to 75c flow temp.

Good. Lower will make high condensation of fumes. Higher mean low eficiency!

Air onto the coil at 20c and air off the coil at 24-27c depending on fcu.

You should lower fan speed to lowest to get higher TD! Low air deltaT mean high fan speed or air accumulation in fan coil. Make good air purge on all fan coils.
Fan coil is designed to have 10K air deltaT at medium fan speed! Therefore, at room temperature of 20°C and water entering temperature of 45°C (which is perfectly OK for fan coil heating (low temperature heating = high eficiency) these numbers look OK to me!

Water in at 45c and water out at 39c which to me seems very poor when the boiler on the roof 3 floors up is set to 75c and has a return temp of near to 65c.

That mean that 4 way mixing valve is set properly! 6K temperature difference on fan coil water side is perfect!

Boiler has even been turned up to 90c and this made little difference. With such a high boiler temp I would have thought the air off would be much warmer.

As I said, mixing valve is at action. Boiler should never see return temperature below 62°C because of fume condensation!

[nike123]

Sorry maybe I didn't clarify properly, both the return and flow are red hot. But air off is poor. I'm trying to do this whilst currently suffering from the yearly Man flu lol

"Red hot" doesn't mean anything. What is temperature measured mean a lot!

I repeat! Make good air purge on all fan coils.


Secondly, in order to ensure that fan-coil proper fiunction, balancing of fan coils, which are regulated with 3way valves is totaly diferent than balancing fan coils regulated only with fan speed and fan on-off.



Therefore, second condition for fan coil to work properly is proper water flow.

First case ( 3way valves) require dinamic balancing and second ( fan speed control) could be staticaly balnced!

From your quoted statemant, I would dare to say, system is not properly balanced!

Here, you can find lot of literature:
http://www.imi-hydronic.com/en/produ...g-and-control/

[nike123]

Hi, design delta t for the hot water system is 10degc.

Maybe for pasive radiators!. Fan coils are designed with delta t at water side of 5K!
And, since fan coils are "low temperature systems" flow temperature of 45°C is perfectly within design conditions!
Nevertheless, flow temperature could be higher, but at expense on efficiency, especially with condensation gas boilers!

[nike123]

If the heating flow temperature is only 45°C then that is the root cause of your problem.

Until you have design water conditions available at the fan coils you are chasing an ever decreasing circle.

I bet that 45°C is design flow temperature AT FAN COIL and 40°C is design return temperature FROM FAN COIL.
That doesn't necessary mean that these temperatures are design temperatures for whole circuit!

[nike123]

3 laws of hydronic systems:

1.Air purging
2.Proper water flow (balancing)
3.Pressure maintenance

[back2space]

Hi Nike123

Completly agree wtih you on all your comments and thank you for your response. Its hard as until we check all these thigns properly it could be any one or a number of things at fault. The fan speeds are manually set on a switch 1,2,3 they design speed says they should be on 2 but that was when the building was commisioned for sectional office space. This floor was originally divided up into two office spaces. It is now one large office space and there are twice the number of desks originally (call centre with smaller desks than previously). Fan speeds have been upped to increase the cooling which works well to be fair. In heating though lower fan speeds means the heat just stays up at ceiling level. All return air vents have the return air sensor poking through the vents as they were not picking up accurate temps being placed just on the underside of the grills and this was sometimes picking up an average temperature of the void. The false ceiling void is used as the return air plenum.

A quote has gone in to move the sensors further down at occupation height (light switch level) but I know by fact that this will not resolve the issues as those units are already calling for 100% heating as the set points have been turned upto 25C to compensate. They are already calling for 100% heating and report return temps of 20/21C.

This is an extract of the email ive sent to the FM this afternoon:

"I know the engineer has given his feedback back to you and you've relayed this back to help desk management but there are points I need to raise with you where I feel the engineer could have done more investigation and the whole visit felt very rushed. This is a big issue and is affecting a lot of people and really needs to be a priority to resolve. A quote has been submitted and whilst I feel this will resolve temperature control in the bigger picture this will not help the units that are already running in 100% heating output already and not achieving the set temperatures. Further investigation is needed as to why there is a 40c temperature drop from the boiler to the flow side of the FCU's as this is not normal on insulated pipework. The air on (air intake temp) is 21c and the air off (air outlet/blown out) temp is only 27c at best. That's s 6c rise and is very poor if the flow temps were 80c which the boiler is set to then we would be getting a lot more heat output from these units. I think further investigation is required OOH when the units can be easily accessed and perhaps one problem unit checked (F013) and diagnosed fully to find out the cause. It is also apparent that 2 supply vents near the lockers are supplied from a unit above the innovation room/office (F114) are turned off. This will be because this area used to be one big open space, when the office was reconfigured and partition put up into the innovation room and office we would not want the temperature in these offices affecting the call centre area. However these units should have been reconnected to the nearest FCU within the area by the lockers which would be (F013) this unit is capable of connecting 4 ducts to it but only 2 are connected to it. Having these connected would therefore improve temperature in this area, which I believe the quote gives brief details into moving of duct work and diffusers etc.

During the visit 2nd floor staff were approaching us when we were in reception and advised of the heating issues on the 2nd floor - there are quite a few complaints of being too hot/cold on the 2nd floor. We both agreed (Ash & I) that this was due to the recent re-position of the air sensors on the back of the fcu's which means they are sensing the average temp of all the air in the void and not the room air temp. I discussed my concerns in one of our conference calls long before the NHS moved in and I advised that these issues would occur but F&G did not wish to take the advice given but now were having the issue of a new tenant complaining it's too hot on the 2nd floor. Imagine what this will be like when it starts to get warmer outside.

We also inspected the reception FCU outside of the lifts. This was recalled 3 times and each time closed down by the engineer who said that no issue could be found as he had asked reception staff and they were reporting no issue. Not sure how reception staff would know if there is an issue or not unless they are stood directly under the unit it either works or it doesnt for office staff etc. It took us 2 mins of looking to discover that the hot & cold water supply valves to the unit had not be turned back on after the actuator had been replaced in December. That's if the actuator was actually at fault and it might have been this issue in the very first place. This unit is now working fully in heating/cooling. What concerns me is that if the engineer can miss a simple thing like this on 3 recalls, on other visits that are affecting a whole area etc are they able to diagnose and investigate properly. It seems that unless concerns are raised the maintenance of the plant is seriously going to fall behind.

As a comparison we don't appear to have any complaints on the 1st floor, this was an area that was fully audited in 2012 and many issues were found and rectified. All strainers were checked which are part of the PPM task anyway and should be done without a need for a job raising. This audit was not carried out around the whole building so the issues experienced on the 1st floor are probably the same issues that are being experienced on other floors"

[back2space]

Maybe for pasive radiators!. Fan coils are designed with delta t at water side of 5K!
And, since fan coils are "low temperature systems" flow temperature of 45°C is perfectly within design conditions!
Nevertheless, flow temperature could be higher, but at expense on efficiency, especially with condensation gas boilers!

The issue I have with this is why do we need the boiler set to 80c if we are only getting 45C at the fan coil unit? There is a separate supply and return pipework back to the boiler? You would have thought 65C would be the flow temp we would require. I also thought fan coils were more efficient at removing heat from the water than radiators?

[Brian_UK]

With regard to your return air sensor 'hanging down' .

Many years ago we had a similar problem with ceiling void FCUs getting a decent room temperature reading.

What we did, and it proved effective, was to fit a small, 100mm square, grille in the ceiling with the air sensor mounted in it. On top of this we fitted an axial fan such as is used in computers etc to draw air over the sensor.

[install monkey]

you say 80deg at boilers but 45 deg at fan coil- is there a bms controlling a VT (variable temp) circuit? typically if outdoor air is 20deg htg flow would be 20 deg and at 0 deg outside htg flow would be 80? might need to adjust the curve slightly.
but franks logic regarding sludge sounds good, easy way to prove this is oven the vent plug so water drips into drip tray or open a drain **** and see what comes out

[back2space]

With regard to your return air sensor 'hanging down' .

Many years ago we had a similar problem with ceiling void FCUs getting a decent room temperature reading.

What we did, and it proved effective, was to fit a small, 100mm square, grille in the ceiling with the air sensor mounted in it. On top of this we fitted an axial fan such as is used in computers etc to draw air over the sensor.

Good point, weve since poked them through the grills they work better like this in cooling but heating they still dont sense very well as warm air rises they think they have reached temp quite quickly. We have since changed fan speeds to high so that they force the warm air down which has helped a bit. The sensors were previously mounted just above the return air grill which looks like a dummy supply air grill but they were cable tied on, you then rely on the FCU's pulling air through the grills and over the sensor. This wasnt too bad but not accurate enough so they were changed so the sensor was poking just through the vent into the room.

Attached is a pic, this grill just opens up into the false ceiling void and you can see the little copper coloured sensor. These are dotted all about to allow air to return back to the FCU's there is no return air ducting/plenum, the ceiling void is that.FullSizeRender.jpg

[back2space]

you say 80deg at boilers but 45 deg at fan coil- is there a bms controlling a VT (variable temp) circuit? typically if outdoor air is 20deg htg flow would be 20 deg and at 0 deg outside htg flow would be 80? might need to adjust the curve slightly.
but franks logic regarding sludge sounds good, easy way to prove this is oven the vent plug so water drips into drip tray or open a drain **** and see what comes out

No nothing like that on this old Johnsons BMS system. All the heating controls are set, boiler flow temps are 80C and the boiler sensors are apparently achieving these set points.

[nike123]

The issue I have with this is why do we need the boiler set to 80c if we are only getting 45C at the fan coil unit?

As I already said, that has to do something ( a lot) with prevention of boiler condensing high amount of tar and acids in it. If the water in boiler is any lower than 62°C, condensation of fumes occurs!
That is not issue with condensation boilers (High efficiency) because they are designed to harness heat from fumes, which your is apparently not!

80°C/75°C is high flow temperature for fan coil systems, therefore it is lowered wit 4-way mixing valve somewhere between boiler and fan-coil system supply circuit!


Also, if that high temperature comes accidentally to cooling system (chiller circuit), it will eventually pop the safety valves and release all refrigerant in air!

Therefore, 50°C is max flow temperature for fan coil systems also used for cooling purpose.

Simmilary, floor radiant heating flow temperature should not exceed 35°C

[redroge]

Forget the fm company and get a decent h&v company in to recommision the system or you will go on forever messing about with it. There could be various things wrong as the guys have said I,ve even had all the valves in the wrong way round so get somebody who knows what they are doing.