A/C icing back on suction

[carlfoster]

Hi all, I'm having a problem with a close control a/c unit that we've been trying to fix all week

The a/c uses 407C, has a TEV and a long pipe run. From what I've been told, 4 a/c condensing units were moved temporarily while roofing repairs were done. The engineer who moved them had problems with the one a/c, and wasn't sure of the reason.

I went to look at the unit and was told by the building manager that first thing in the morning the whole suction line onto the compressor was a solid block of ice. So he switched it off for an hour to let it defrost. When we looked at it, it wasn't icing back but quite cold.

I discovered that the engineer who moved the units put reclaimed gas into this unit, so decided that this could be the problem. So we recovered the gas, evacuated the system overnight and recharged with fresh gas.

It appeared to be working fine during the day, but this morning the manager called again saying it was iced up again.

I had a look and found the pressures a bit low so adjusted the condenser fan speed control to bring the head pressure up a bit hoping that the higher pressures would prevent another frost up.

Do you guys have any other ideas what the problem could be?

[tony--1]

hi
i know its silly but have you checked fan/s on indoor unit

[NoNickName]

I think you've got one of these
1) liquid solenoid not fully open or clogged
2) liquid filter clogged
3) txv clogged

This is especially true since they moved the units, so debris is probably been ingressing.

[goodguy]

Is this a R-22 retrofit to 407c or new installation?

[carlfoster]

Hi, thanks for the replys.

hi
i know its silly but have you checked fan/s on indoor unit

Yes, the indoor fans are fine.

Is this a R-22 retrofit to 407c or new installation?

This was a 407C new installation 5 years ago. The units were moved a few weeks ago.

I think you've got one of these
1) liquid solenoid not fully open or clogged
2) liquid filter clogged
3) txv clogged

This is especially true since they moved the units, so debris is probably been ingressing.

1) There is no solenoid
2) We changed the filter drier when we recovered the gas.

3) Possible. This is something I suspected but the fact that it iced all the way back to the compressor threw me on that one. Especially when you consider that the units are on the roof, 5th floor, and the evaporators are on the ground floor.

I checked the temperature of the liquid line and it was 35, and then 40 after I slowed the condenser fan. Do you think that maybe it doesn't have enough gas? I mean, I charged it to subcool, but maybe the head pressure isn't enough?

[lana]

Hi Carlfoster,

First of all R407C is a zeotropic blend and must be charged as liquid. If the system is charged as gas then there is no R407C in the system, the refrigerant is something else Secondly, the oil is very sensitive to moisture and must be worked with very carefully.
When there is a frost on the suction line means the evaporating pressure is low and the corresponding temperature is below 0°C. Low LP results from one of the 4 reasons:
1- Lack of expansion valve capacity.
2- Lack of refrigerant.
3- Pre-expansion on liquid line.
4- Lack of evaporator capacity.

First three reasons make the superheat (SH) to increase and the no 4 makes the superheat to decrease. So by measuring the superheat you can find out which one of the reasons to eliminate.
By measuring the sub-cooling (SC) you can pin point the problem. If there is a lack of refrigerant then sub-cooling decreases. Here are the tips:
1- SH up, SC normal, LP down, no temp dif. in liquid line.
2- SH up, SC down, LP down.
3- SH up, SC normal, LP down, temp. dif. in liquid line.
4- SH down, SC normal, LP down, possible flood back.

Also be careful if you have sub-cooling then maybe there is a pre-expansion but there will be no temp. dif. in the liquid line

I hope this helps.
Cheers
Lana

[NoNickName]

I checked the temperature of the liquid line and it was 35, and then 40 after I slowed the condenser fan. Do you think that maybe it doesn't have enough gas? I mean, I charged it to subcool, but maybe the head pressure isn't enough?

I don't think so, it looks like liquid is warm, so there shouldn't be excessive subcool.
Also it doesn't look like refrigerant is missing either.

[goodguy]

hey, just a quick thought. Maybe the equilizer line on the txv is metering liquid into suction?
good luck

[carlfoster]

Hi Carlfoster,

First of all R407C is a zeotropic blend and must be charged as liquid. If the system is charged as gas then there is no R407C in the system, the refrigerant is something else Secondly, the oil is very sensitive to moisture and must be worked with very carefully.
When there is a frost on the suction line means the evaporating pressure is low and the corresponding temperature is below 0°C. Low LP results from one of the 4 reasons:
1- Lack of expansion valve capacity.
2- Lack of refrigerant.
3- Pre-expansion on liquid line.
4- Lack of evaporator capacity.

First three reasons make the superheat (SH) to increase and the no 4 makes the superheat to decrease. So by measuring the superheat you can find out which one of the reasons to eliminate.
By measuring the sub-cooling (SC) you can pin point the problem. If there is a lack of refrigerant then sub-cooling decreases. Here are the tips:
1- SH up, SC normal, LP down, no temp dif. in liquid line.
2- SH up, SC down, LP down.
3- SH up, SC normal, LP down, temp. dif. in liquid line.
4- SH down, SC normal, LP down, possible flood back.

Also be careful if you have sub-cooling then maybe there is a pre-expansion but there will be no temp. dif. in the liquid line

I hope this helps.
Cheers
Lana

Thank you very much for this info. It does confirm a lot of what I was thinking, and adds a few things to think about.

The thing that confuses me though is that the variable fan seems to change the subcool readings and its tricky to know what the speed should be set to at the current ambient.

The SH seems ok, so the valve seems to be keeping the right setting. It does, however, seem to get cold on the first part of the evaporator and not the end except for the suction leaving. This makes me think that the evaporator is being starved of refrigerant, so either lack of refrigerant in the system or liquid starving at the valve or liquid line.

[lana]

The thing that confuses me though is that the variable fan seems to change the subcool readings and its tricky to know what the speed should be set to at the current ambient.

Hi there,
Glad to help.
What I always do is : run the fans at full speed, then measure and compare temp. and pressures.
The two points you mentioned are easy to diagnose.
If there is a lack of refrigerant then SC goes down.
If there is pre-expansion then SC would be normal.

Remember : If there are more than one fault in the system then it would be very very difficult to diagnose specially when one increases something and the other fault decreases the same parameter, then you will measure the average of that parameter and the decision .... Cheers

[johnl45]

Possible dirty air filters?

[lana]

Possible dirty air filters?

Hi,
If the air filter is dirty then the fault lies in the "Lack of evaporator capacity" family and this would make the SH to decrease.
Cheers

[mr cool]

if its flooding back that hard i would check air flow, tev and controls. if it is running all night the room temp will be low and there for evap is not absorbing the usual amount of heat. also check tev bulb and make sure its insulated. whats the t/d and superheat value?

[TRASH101]

Just to add to your difficulties

with it being a close control system and I take it that this is an intermittent fault. You might try checking that the unit isn't sticking in dehum as most close control units tend to drop fan speed and then reheat.
So if the reheat aint working then the hum will rocket as the temp drops and compounds your evap loading.

[SteveRob]

Look at what differences there has been to the system before it was moved, if it was ok then try and simulate all conditons that were before the move.

Seems like you have done a good thing by putting new R407c into the sytem as now you know you have a true blend.

But the main thing I would concentrate on now is the Pipe run and Ref charge as these are the two items that have been changed, or need to change.

If you have moved the condensers to the ground, five floors, it is a must for at least one liquid trap! also the abient temps could be a whole lot different down there, so yes playing with the cond unit and cond temps, as well as total ref charge will affect this.

Look at the simple things before getting to complicated!

[Freecool]

Hi everone just registered enjoy reading all your comments on various topics but can anyone tell me how you send a new posting having difficulty.
Regarding problem: Check Air on Temp,Air Flow Across evaporator, Filters, Pipe Sizes. Liquid Line Temps. Superheat. Has anything be left in the pipe work when moving? All else remove refrigerant replace TRV, Drier Vac out And recharge with new.

[coztemur]

It seem you have dirth in eev or
if you have a Hot gas bpv it is not working properly

Coskun

[noushad123]

you means suction pressure low and discharge pressure high please change TXV
please you can tell me type of unit, model. TXV is variable type you need to change only the power assembly

[lana]

you means suction pressure low and discharge pressure high please change TXV
please you can tell me type of unit, model. TXV is variable type you need to change only the power assembly

Hi there,

If the problem were from the TEV then the discharge pressure would not be high, it would be the opposite.
Cheers

[lana]

Dear Carlfoster,

If you are reading and following this then tell us how it goes?
Have you found the problem?

Regards

[mianrizwan]

my be the gas shortage.

[simon@parker]

just a thought do u have an in line suction strainer at the compressor had a similar sort of prob on a split changed gas deep vac all that and could not solve it eventually (we did that 3 times) we cut ut the inline strainer and found the oil blocking it must have been moisture or the oil acting funny but that solved it from what you have said evap not fully cold sounds like a blockage or short of gas at evap but then a cold suction 2 blockages ? or restrictions creating an expansion effect ? hope it helps

[goshen]

does anybody look at the dates????????this post is almost 3 years old!!!

[simon@parker]

oh yeah learning still only new to this lmao