Fluctuating Back Pressures???

[Daz man]

Folks, I am commissioning a fairly new Airedale condensing unit which feeds a dx coil in an AHU.

Whilst commissioning this morning I was having problems with low back pressures.

I was getting correct head pressures approx 250/270psi but
the back pressure kept fluctuating.

I know I have approx the correct charge of R407c as I have weighed it in myself - according to manufactures instructions.

I have checked the sensing bulb location of the External equalising expansion valve to find correct.

I was thinking of changing the Expansion valve and liquid line drier for new replacements? Also could I still be a couple of hundred grams short of refrigerant?

Any Ideas?? Many Thanks in advance.

[Gary]

Generally, hunting is caused by the orifice being too large for the heat load. Keep in mind that this involves two factors, i.e. orifice size and heat load. If this system has been working properly in the past, then the orifice size is probably correct... so that brings us to heat load.

Very likely there is an airflow problem (dirty coil, dirty fan blades, fan motor running in reverse direction, fan blade flipped over and thus backhanding the air, etc.)

[Daz man]

Thanks for that Gary, appreciate your response.

[El Padre]

I would'nt say that you are short of refrigerant as it sounds like some liquid is returning to the compressor, have you monitored subcooling and superheat?

As Gary has suggested, if its in a comms room airflow is even more critical due to the high sensible load!

Good luck

Cheers

[desA]

Has the evaporator been repaired, or replaced at any time during this machine's lifetime?

[Tesla]

Hi Daz man
I think you should be measuring your sheat, hp and current here and add a little more refrigerant, also check that your coil has not been fouled by construction dust etc. Do this before changing costly components.