heating issue with vrf

[philjd26]

hi
vrf unit giving me grief....the system is installed over two floors, 10 on one floor and three on the floor below which are the last units on the run...on the last three i am having problems with the coil getting up to a reasonable temparture...the hottest they reached is 35c, and as time goes by switches too defrost(outdoor i would imagine)...the pipework to the last three units is reduced in size,could this be a pipework spec problem or a refigerant shortage issue?

rgrds phil

[martinw58]

sounds like short of gas have you run the system in cooling to see if units donot cool

[Brian_UK]

What is the system diversity ?

or

What is the capacity of the indoor units compared with the size of the outdoor?

[philjd26]

capacity is ok and units do seem to get around 4c on the coil, i am starting to lean towards charge problem due to defrost being carried frequwntly..

[SteinarN]

Is it icing evenly on the whole of the outdoor coil?

[philjd26]

to be honest i didnt check if the icing was even on the coil, it displays a warm up symbol on controller..
what will this tell me???

[SteinarN]

If it's ice on the whole coil it's probably not lack of refrigerant causing icing and defrost.

[brunstar]

i would try and turn off the first 5 units and see if the last few units start to heat up and then this will tell you if you have a refrigerant shortage.
as brian said you have to know the capacity index of the connectivity of indoors to capacity of the outdoor, if it is sized at 130% don't expect them all to perform at low ambients when there is a load on all systems.
try turning a few off up the line.

if this doesn't work it could be a pipework size error.
also make sure that your refnets are flat, horizontal as this will effect the flow to the indoor units.
what brand is it?

[Thermatech]

With long pipe runs
the units furtherst away from the outdoor unit will tend to have the loweat hot gas temp.
If the system has any built in design problem like 'over index'
then
expect poor heating performance at units with longest pipe lenght.
In cooling mode the outdoor unit can control head pressure with outdoor fan speed control.
What can the system do to control head pressure in heating mode when the indoor units are the condenser coil & the enduser dictates the fan speed & air volume at indoor / condenser coil ?
then
consider the overcondensing problem when the indoor capacity is 130% & the outdoor unit can only ramp the compressor up to 100%
and
the indoor on coil temp is low

result

poor heating performance

Remember
In the uk most VRF systems are selected for cooling performance with little or no consideration for heating performance.

The system might be SOG
or
might be over index
or
might have some other operational problem.

If you only run these 3 units then you will see if they have the capacity to provide normal heating performance
but
that does not prove SOG or any other possible fault.

[brunstar]

head pressure is controlled at the indoor unit, this is controlled by the EXV, even if the unit is in thermostat off it will allow hot gas to pass throuogh the fan coil, this is why they have a minimum capacity index to ensure that there is adequate refrigerant storage in the indoor coils otherwise the units would have head pressure control. indoor coil temp is also controlled by slowing down the outdoor fan, reducing the coil temp.

[Thermatech]

Minimum opening of LEV valve at indoor unit when at OFF is stratagy to prevent liquid refrigerant & oil being trapped at indoor unit at OFF.
Otherwise system would run SOG & compressor run out of oil.

At thermostat heat mode ON the indoor unit is looking to maintain SC at indoor coil although outdoor unit has comp at 100%.
If outdoor unit cannot maintain discharge pressure / temperature at target condensing pressure / temp
then
indoor units get poor hot gas temp & this makes poor performance at indoor units.
Indoor units are unable to maintain target SC & continually open up LEV valve
but outdoor unit cannot respond with increased comp speed as already at max.
then what ?

[philjd26]

Thanks for replys lads
i learnt something there, this is a sanyo system...i have checked the pulses on the exv to see if its looking for full heat on all 3 units...it opens to 480 pulses which is max with a coil temp of no more than 35...then after time coil temp drops to ambient due to a defrost being carried out..

i was wondering about the high pressure control,is the network of indoor pipework maintained at a constant pressure in heating,as my thinking is that different units have different fan speeds causing various reduction in pressures..ie more movement of refrigerant with indoor on low fan and less on high..

rgds phil

[brunstar]

i also forgot to add some units have a main and sub coil on the outdoor unit, if there is high pressure it can also use the sub coil which is half of the outdoor coil as head pressure control, this half is usually used if all filters are blocked or a severe increase in head pressure.