undersized unit ??

[moondawn]

Hi There

new to this forum, very impressed with reading through posts, learnt things from reading.

wonderin if anyone can help

the other day went to a site where a hitachi ducting unit outdoor unit ras 6 was faulting on 02 fault. checked pressures on condensor suction was at 28-30 psi and head pressure at 90 psi. decided to decant system. decanted in total 9.3 kg of gas maxmum total charge for one of those systems should be no more than 7.8kg i think. system would not say more than 35 mtre run. put the pre charge in which is 6.1kg of r22 gas.commisioned system and ran with no faults. was told that sysytem should run at 50-55 psi and that head pressure about 200 psi. ran system and got pressures 32psi and 160 psi unit was cooling well.
one question: would systems pressures be runnin lower if indoor unit is undersized . i am thinkin sysytem has come overcharged as its maintained people have seen pressures very low

thanks in advance

[moondawn]

thanks for your reply.

could you suggest how i would go about restricting condensor airflow.

sorry if asking simple questions only learned what i know from engineer who trained me and a basic refrigeration course. my company is only small and does not send on courses very often at all

thanks

[moondawn]

hi marc. could you post how i would go about checking the sub cool and superheat never done this before. can this not be calculated by using the manifold gauges

im eger to learn also do you know of any books i could buy which would help on the refrigeration cycle and subcool and superheat the kind of things i am unsure of

thank you

[Alan B]

Hi Marc,
By measure the SCT and SST do you mean read the pressure gauge and then convert to temperature(or use the relevant temp scale on the gauge)?
Alan B.

[moondawn]

Hi There Mr Cooling magic

Thanks For your input.

Am I Right In Thinkin then the Outdoor condesnsor would of faulted due to Low Pressures within the system.

I Also Had a leak in the room where the ducted unit is serving. it had leaked on to the ciling tiles all around where the ducting was near the evap coil

thanks in advance

[frank]

You can also get low pressure readings if the airflow over the indoor coil is restricted, as in a severly iced coil or blocked filters. Sounds like the last guy tried to bring the pressures up by adding more gas resulting in an overcharge instead of finding the problem.

Check the ducting serving the grilles. Is it restricted? Is it way too long and causing excessive pressure loss and causing the fans to stall? Do you know how to take a volume flow reading and compare it to the fan spec?

Manchester is not far from us but it will cost you to get us to site to sort it

[moondawn]

not had a problem with the unit since. told them to keep the unit runnin at 21-22c as reckon it can not cope due to undersized.

thanks for your reccomendation on books Marc. i have ordered 3 of them hopefully will be able to get my head round them.

[Daddy Cool]

Depending on the exact model used, you may be able to interrogate the subcool & superheat temps from the jard wired controller or if you want to get some sun, the outdoor unit 7 seg digital display.