Mitsi VRF problem - not all units heating suffeciently

[r0ss]

A workmate of mine phoned me today asking if i had any ideas concerning a problem of his on a call out he visited.

Last week he done a routine maintenance on 19 Mitsubishi slim ducted units (12 on a master BC box and 7 on the slave) and all units were working very well. This was when the weather was a tad warmer >5c.

However he goes to the job today with complaints some units arent managing to heat the rooms they are in. The temperatures outside today have been around the low minus'. The clients want room temperatures of 23 degrees and whilst many of the rooms are getting to around 20/22 degrees, there are some which are struggling to even get to 18/19 celcius. The rooms he says are small with size 20/25's in them.

There are no error messages and the installation was done less than a year ago and has worked faultlessly up until this freezing cold patch.

I think he is clutching a bit at straws so hopefully someone here has a bit of an idea in a direction he can follow.

Hopefully somebody can,

thanks a lot

[thebigcheese]

vrvs will struggle to heat in this weather, was actuly talking to another guy about this yesterday

[VRVIII]

A workmate of mine phoned me today asking if i had any ideas concerning a problem of his on a call out he visited.

Last week he done a routine maintenance on 19 Mitsubishi slim ducted units (12 on a master BC box and 7 on the slave) and all units were working very well. This was when the weather was a tad warmer >5c.

However he goes to the job today with complaints some units arent managing to heat the rooms they are in. The temperatures outside today have been around the low minus'. The clients want room temperatures of 23 degrees and whilst many of the rooms are getting to around 20/22 degrees, there are some which are struggling to even get to 18/19 celcius. The rooms he says are small with size 20/25's in them.

There are no error messages and the installation was done less than a year ago and has worked faultlessly up until this freezing cold patch.

I think he is clutching a bit at straws so hopefully someone here has a bit of an idea in a direction he can follow.

Hopefully somebody can,

thanks a lot

Providing the system is operating correctly, the problem is usually caused by a system being undersized for the capacity required at colder ambient temps.
This is down to poor system design.

IE.

Calculations based on nominal capacities/temps
Pipe run length capacity drop not taken in to account
Consultant spec being reduced to save £££

[back2space]

vrvs will struggle to heat in this weather, was actuly talking to another guy about this yesterday

Why would this be the case if it was sized correctly?

Sorry just interested.

[r0ss]

Thanks a lot for the quick replies chaps


Providing the system is operating correctly, the problem is usually caused by a system being undersized for the capacity required at colder ambient temps.
This is down to poor system design.

IE.

Calculations based on nominal capacities/temps
Pipe run length capacity drop not taken in to account
Consultant spec being reduced to save £££



We were sort of leaning towards this idea but obviously he doesn't want to pin the blame on the contract manager / consultant just in case things flair up and he loses any more work through this firm. But then at the same time spending 12 hours on site trying all sorts with little or no result has to be taken into consideration as he feels he will be regarded as pretty incompetent by the client who he has to deal first hand with.

Its a critical job because its in a doctors surgery. Not good at all

[back2space]

Thanks a lot for the quick replies chaps



We were sort of leaning towards this idea but obviously he doesn't want to pin the blame on the contract manager / consultant just in case things flair up and he loses any more work through this firm. But then at the same time spending 12 hours on site trying all sorts with little or no result has to be taken into consideration as he feels he will be regarded as pretty incompetent by the client who he has to deal first hand with.

Its a critical job because its in a doctors surgery. Not good at all

How about turning a few of the units off and seeing if the other rooms get upto temp, this would then tell you if its under capacity for these sort of outdoor temperatures.

[VRVIII]

How about turning a few of the units off and seeing if the other rooms get upto temp, this would then tell you if its under capacity for these sort of outdoor temperatures.

Did you check the indoor air on / air temps with all indoors operating? You could then try back2space suggestion and turn a couple of indoors off then re check air on/off.

What are the operating pressurs?
Is there any ice build up on outdoor heat exchanger?

I think Theratech would be the best person to give you advice on Miti VRF.

[The Viking]

Quick check;

Compare the capacity of the outdoor unit with the total capacity of all the indoor units.

All the main manufacturers allow for 'diversity' when VRV/VRF systems are designed, I can't remember Mitu's figure of the top of my head but it is something like 125%.

So in theory even a well designed system might have 62.5kW worth of indoor units hooked up to a 50kW outdoor unit.
The only time it will become a problem is when all indoor units are asking for 100% and only getting 80%.
(IF this is the case then it can be overcome by turning one unit in to cooling or OFF, then when the rest of the building heated up put it back to normal)

Another thing to bear in mind, now the system is working at it's hardest so any lack of gas will show up...

[Thermatech]

The index is critical.
Add up total capacity of all indoor units & see what the capacity of the outdoor unit is.
If the capacity of the indoor units is more than the capacity of the outdoor unit then the heating performance at the indoor units will be reduced as the outdoor unit can only ramp up to 100%.
When you factor in the correction factors for reduction in performance due to over index / long pipe lenghts & low ambient / defrosting then you might find the system cannot produce the heat needed.
Ok the rooms are small but at best a size 20 is only going to do about 2kw. But do the overall system correction factors & you might find that with all indoor units requesting max heating the size 20 might only be doing 1 to 1.5kw. Alot of design engineers forget to think about this as I have found out many times when attending site meetings with consulting engineers.

In this extreem weather condition which the design engineer may not have designed the sytem to cope with then the best option is to allow 24hr operation instead of allowing the building to cool down overnight.

If the system is not SOG & the compressor is flat out but not overheat on the discharge temp & the system is dofrosting properly then whatever heating the system can provide is the best you will get.

The other problem with ducted indoor units can be the type of supply air grill. Alot of design engineers consentrate on good performance in cooling mode as the priority & use supply air grill which blow horizontally across the ceiling.
This is hopeless when max heating is required because the hot supply air stays at ceiling level & never gets down to occupant level but returns at high level to the indoor unit which then thinks that it does not need to heat so much as room temp close to set point but at low level in the room it is still much cooler.
So some times remote room temp sensors can help.

Another classic is using ceiling void return air but the ceing void is large pitched roof loft space & so the units are trying to warm up the huge loft area also.

Another classic is the fresh air not being preheated & eveybody assumes the City Multi has a problem but infact it is very cold air being pumped into the building which is the problem.

Remember all City Multi units have standard 'energy save' control stratagy which mean that when to return air sensor sees the temperature at 2 deg C from set point the indoor unit starts to reduce heating capacity but aiming at a higher sub cool target which results in the LEV valve closing down a bit so with reduced refrigerant flow through the coil the unit does a bit less heating.
So if indoor units get to 2 deg C near to set point then they are working ok & are in the 'comfort zone' far as the unit control stratagy is programmed.

Every winter I trouble shoot lots of sites with poor heating performance problems with City Multi systems & 9/10 times its down to design.
I tend to monitor the system with the Mnet monitor laptop & if there is some system fault then it can be quickly identified or otherwise confirm that the system is working to max capacity & the enduser just need to operate 24hrs during the very cold weather.

[back2space]

Quick check;

Compare the capacity of the outdoor unit with the total capacity of all the indoor units.

All the main manufacturers allow for 'diversity' when VRV/VRF systems are designed, I can't remember Mitu's figure of the top of my head but it is something like 125%.

So in theory even a well designed system might have 62.5kW worth of indoor units hooked up to a 50kW outdoor unit.
The only time it will become a problem is when all indoor units are asking for 100% and only getting 80%.
(IF this is the case then it can be overcome by turning one unit in to cooling or OFF, then when the rest of the building heated up put it back to normal)

Another thing to bear in mind, now the system is working at it's hardest so any lack of gas will show up...

My outdoor unit on my multisplit system is over capacity if I have all three indoor units on heating and they are all asking for 100% heat, so I have to turn one of them off and then 100% can be given to each indoor unit, otherwise with all 3 on they are not getting full capacity required to raise the room temp. Once the other 2 rooms reach set point I can then turn the 3rd on which quickly comes upto set point and the system happily runs all day like this then.

[back2space]

The index is critical.
Add up total capacity of all indoor units & see what the capacity of the outdoor unit is.
If the capacity of the indoor units is more than the capacity of the outdoor unit then the heating performance at the indoor units will be reduced as the outdoor unit can only ramp up to 100%.
When you factor in the correction factors for reduction in performance due to over index / long pipe lenghts & low ambient / defrosting then you might find the system cannot produce the heat needed.
Ok the rooms are small but at best a size 20 is only going to do about 2kw. But do the overall system correction factors & you might find that with all indoor units requesting max heating the size 20 might only be doing 1 to 1.5kw. Alot of design engineers forget to think about this as I have found out many times when attending site meetings with consulting engineers.

In this extreem weather condition which the design engineer may not have designed the sytem to cope with then the best option is to allow 24hr operation instead of allowing the building to cool down overnight.

If the system is not SOG & the compressor is flat out but not overheat on the discharge temp & the system is dofrosting properly then whatever heating the system can provide is the best you will get.

The other problem with ducted indoor units can be the type of supply air grill. Alot of design engineers consentrate on good performance in cooling mode as the priority & use supply air grill which blow horizontally across the ceiling.
This is hopeless when max heating is required because the hot supply air stays at ceiling level & never gets down to occupant level but returns at high level to the indoor unit which then thinks that it does not need to heat so much as room temp close to set point but at low level in the room it is still much cooler.
So some times remote room temp sensors can help.

Another classic is using ceiling void return air but the ceing void is large pitched roof loft space & so the units are trying to warm up the huge loft area also.

Another classic is the fresh air not being preheated & eveybody assumes the City Multi has a problem but infact it is very cold air being pumped into the building which is the problem.

Remember all City Multi units have standard 'energy save' control stratagy which mean that when to return air sensor sees the temperature at 2 deg C from set point the indoor unit starts to reduce heating capacity but aiming at a higher sub cool target which results in the LEV valve closing down a bit so with reduced refrigerant flow through the coil the unit does a bit less heating.
So if indoor units get to 2 deg C near to set point then they are working ok & are in the 'comfort zone' far as the unit control stratagy is programmed.

Every winter I trouble shoot lots of sites with poor heating performance problems with City Multi systems & 9/10 times its down to design.
I tend to monitor the system with the Mnet monitor laptop & if there is some system fault then it can be quickly identified or otherwise confirm that the system is working to max capacity & the enduser just need to operate 24hrs during the very cold weather.

Fantastic post Thermatech, very informative and concise. I do enjoy reading your posts.

Another thing to mention is fan speeds. Fan coils are only going to give their rated duty when the fan is set on high. This is something that I have been told by a few engineers, and whilst low fan speed is better for quieter operation and probably works fine most of the year in extreme conditions the system struggles.

At home today I have had to bump the fan speed up on one of my fan coils as it was struggling in such a cold room.

I also leave it 24/7 operation at present, and at night turn them down to 18C and then the next day they warm up much quicker than if heating up from say 12/13C where the furniture and walls have allowed to cool also.

[Thermatech]

The indoor fan speed issue in max heating situation for an over index VRF system is an interesting problem to get your brain arround.

For example total indoor unit capacity 125% outdoor unit cappacity 100%.

Think what the system does in cooling mode when the outdoor coil (100%) is the condenser. Well the outdoor unit uses outdoor fan speed control to keep ideal discharge pressure target.

Now think about the system in heat mode & all the indoor unit (125%) are now the combined condeser coil which is 25% oversized & there is no fan speed control so the system cant maintain any target discharge pressure because all indoor units 125% are running in high fan speed.

Result poor off coil temps as the discharge pressure / temp is low as the outdoor unit can only provide max 100%.

Some VRF systems cope with this by switching off some indoor units untill the minimum targets are reached & the indoor units which have been allowed to run have good coil temps & then the system will allow some more indoor units to heat ,,, & so on untill all indoor units are allowed to heat.
Any units not allowed to heat by the system control stratagy will show 'standby' on the remote controller.

This tends to go unnoticed as it all happens between 6am & 9am during the morning warm up period.
But if the system is under sized for the very cold ambient conditions we have at the moment then this situation with some indoor units at standby can continue into normal working hours & the end user complains.

Its not a problem is just the way the system tries to cope with condensing target control with very cold indoor unit on coil temp.

[back2space]

The indoor fan speed issue in max heating situation for an over index VRF system is an interesting problem to get your brain arround.

For example total indoor unit capacity 125% outdoor unit cappacity 100%.

Think what the system does in cooling mode when the outdoor coil (100%) is the condenser. Well the outdoor unit uses outdoor fan speed control to keep ideal discharge pressure target.

Now think about the system in heat mode & all the indoor unit (125%) are now the combined condeser coil which is 25% oversized & there is no fan speed control so the system cant maintain any target discharge pressure because all indoor units 125% are running in high fan speed.

Result poor off coil temps as the discharge pressure / temp is low as the outdoor unit can only provide max 100%.

.

Not sure I understand.

Your saying to leave them on low because the indoor units are now the condenser and the outdoor coil is the evaporator that is sized to 100% and cant give any more than this, but the indoor units are bigger than the evaporator should be so running them in high fan speed would mean not enough heat can be produced by the condenser as its already running at 100% and the high fan speeds remove this heat very quickly?

In summer my outdoor unit the fan runs really quite slow really compared to in heating?

Can we move to PM so as not to confuse the post.

[r0ss]

Just want to say thanks a lot guys. Ill be forwarding this thread onto him.

I also will be heeding a lot of the great information given.

Thanks again.

[Matt1982]

2nd that iv learnt something there. Cheers guys.

[airconadam]

yer great post to have a read,I have been to 2 sites recently all with ducted fancoils above the ceiling but above the ceiling there was cold fresh air supplying into the ceiling void thus either making the return air temp sensors flash 8 deg c and knocking the indoor unit off or by lowering the air on so much it lowers the air off ,upon futher inspection found an electric heater battery on an ahu not working. thanks alot

adam

[dannycool]

Excellent posts Thermatech!!!

I have seen the problem of a badly specced Mitsi VRF system before and it resulted in us having to insulate a suspended ceiling just to keep heat in.
The problem was as you say, not consulting the data books to accurately de-rate the original selections.

Mitsubishi do a free (i think) 1 day course on VRF design and selection, on which it shows you how to use the data book properly.

Danny

[TRASH101]

Well put thermatech

All too often unsympathetic endusers (and sometimes engineers) blame the equipment or the design without fully appreciating the extent of the system.

Fresh air make up at -5 celsius is very fresh indeed.

Stratification in heating due to poor air distribution because "its drafty in the summer so we had someone block it off a bit"