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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Hows it going i am attempting to fault find a tachi ducted

Customer Question

hi hows it going i am attempting to fault find a hitachi ducted ac unit model RUA-30HS
the fault that is occering is i am getting normal high discharge pressure and sunction presure i have just replaced the reversing valve as it was faulty but now i am getting this strange fault i am suspecting it could be a faulty checkvalve so can you plz help me out
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.

Hello, can you tell me if used a dry nitrogen purge while soldering the reversing valve, and a vacuum pump after that work was done and before recharging the system?

Did you replace the refrigerant filter drier, with one that has a two way arrow on it.

We can go from there.

Customer: replied 1 year ago.
I used dry nitrogen faced system out and replaced filter dryer with a two way filter the comppressor has just recently been replaced also
Expert:  Phil replied 1 year ago.


Tell me what the outside and inside air temperatures are the same time I need to know the head and suction pressures exactly... and what refrigerant you have in the system, and if you liquid charged it, or vapor charged it.

Customer: replied 1 year ago.
I charged the system by weight I put 8 kg of refrigerant into the system according 2 the sticker on it it requires 10 kg I liquid charged it into the discharge side the refrigant I am using is r22 the suction pressure I am getting is 120 psi and discharge pressure I am getting is around 380 psi also the system is a capillary system so there is no sight glass on systyem outside temp was like 45 c and it was like 35 inside this ducted ac has 2 stages on it and the other compressor pressures were 60 psi and the same discharge pressures
Expert:  Phil replied 1 year ago.

Hello again,

Please verify that the house is 35C inside (equivalent to 95F) and that the outside air is at 45C or about 112F... if that is the case, you are way out of range for evaluating the refrigerant charge... the system is not designed to run efficiently at all in those temperature ranges... the pressures.

So far however it looks like the refrigerant charge might very well be entirely correct and that the system is not able cool the house down properly in these conditions (net capacity drops by about 25% as you approach 45C outside... a capillary tube system will then run the indoor coil temperature up to 21 or 22c , and that is not enough to do a lot of cooling,

Tell me the voltage supplied to the unit, and how much each leg of of the 3 phase power at each compressor is reading... it should be very close to full load amperage in these conditions. I need the exact numbers on each leg.

The system is rated at 310,000 btu/hr or around 30 tons, it will typically cool 10,000 to 15,000 square feet of space in a modestly warm not overly humid climate.

Tell me about the space, square footage and type, office space, industrial mfgr, restaurant etc... and the nearest city so that I can assess the weather conditions for you.

We can go from there.

Expert:  Phil replied 1 year ago.

ooops I typed 'house' it should be building.

Customer: replied 1 year ago.
the building is a big 200 man mess hall located in mount Isa qld each leg on the compressors is drawing roughly 22 amps each there load limit is 24 each the compressor is also tripping out on internal load which is caused by the high suction pressures what I don't understand though is the other stage on the ducted unit is running all ok drawing the same amps but a normal suction pressure it is like a 30m by 10 m shed 4 metre high and fully insulated the unit has just stared doing this fault would a jammed open check vave located near the capplary line in cause this problem
Expert:  Phil replied 1 year ago.

Hello again, it does indeed sound like the check valve is bypassing liquid and is driving the suction pressure up. If the check valve you are talking about is fit at the indoor coil, it can be removed, and capped off since the system can no longer go into heating mode.

Before you do that however, I would like to see pictures of the capillary tubes and check valve, taken at several different angles and taken from about 5 feet away so that I can see how the entire coil is piped in that area.

Another picture that shows all 3 compressors, taken from about 10 feet away so that i can see all 3 compressors and the suction line piping will be helpful as well.

You can use the paper clip icon at the right end of the tool bar above, or the '+ attach fines' link to the left of your 'send' button to send me the pictures.

With such a hot building inside, 35C, I would expect the suction pressure to be high..

Tell me what the other stage suction pressure is running, and very importantly I need to know if the suction line on that compressor is running cool, cold or sweating.

We can go from there.