Basic rules of thumb for residential AC

[JCole21961]

I am not an AC expert but I am a mechanical engineer with some theoretical knowledge of the basics. I have a 3,000 ft2 masonary house in Brazil in which I had installed a Hitachi central air system. I have double-pane glass and 4" of insulation on the ceiling slab.

The contractor calculated 8T of cooling and installed 2 x 5T heat-pump systems. These have given nothing but trouble, tripping on high pressure in summer and low pressure in winter. I am also disappointed in their performance when they do run.

I am interested in some basic guidelines as follows:

1. What is the typical temperature drop across the evap. in cooling mode?
2. What is the typical rise in temp across the condenser (indoor unit) in heating mode?
3. On a typical summer day with outside DB at 90F and indoor WB at 70F what should the typical refrigerant suction and discharge pressures be (R22)?
4. What is the typical temperature of the evaporator coil during cooling under the above conditions?

These systems do not have TEV's but capillaries instead. There is a check-valve around the capillaries in the condensing unit (cooling bypass) but no check around the capillaries in the indoor unit.

Any help will be appreciated. JCole

[The Viking]

Sorry, I know this doesn't answer your questions but...

What you are describing, HP cut out in cooling and LP cut out in heating, are classical symptoms of reduced/restricted airflow through the outdoor coil.

Have you checked that the outdoor coils are clean/clear and that the fans are working as intended.

Blocked coils will also reduce efficiency.


BTW, what brand/model are they?

[JCole21961]

The units are Hitachi, fan unit RVT100B8Z, Evap RCT100BP, Comp. 2x RAA050BS. The 100 series (10T) actually has two separate 5T coils in parallel so the system is really 2x 5T units in the same duct system. It is brand new so the outside coils are clean and the fans blowing strongly. However, by my measurements, the indoor coil is getting about 20% more airflow than the specs call for. Could this be causing the problem? The technicians charge the system using superheat and subcooling parameters but by my experience the pressures seem low (45psi suction, 160psi discharge). My Carrier systems in the USA range 70-90psi suction and 250-270 discharge in cooling mode.

[sunilbhat]

I am not an AC expert but I am a mechanical engineer with some theoretical knowledge of the basics. I have a 3,000 ft2 masonary house in Brazil in which I had installed a Hitachi central air system. I have double-pane glass and 4" of insulation on the ceiling slab.

The contractor calculated 8T of cooling and installed 2 x 5T heat-pump systems. These have given nothing but trouble, tripping on high pressure in summer and low pressure in winter. I am also disappointed in their performance when they do run.

I am interested in some basic guidelines as follows:

1. What is the typical temperature drop across the evap. in cooling mode?
2. What is the typical rise in temp across the condenser (indoor unit) in heating mode?
3. On a typical summer day with outside DB at 90F and indoor WB at 70F what should the typical refrigerant suction and discharge pressures be (R22)?
4. What is the typical temperature of the evaporator coil during cooling under the above conditions?

These systems do not have TEV's but capillaries instead. There is a check-valve around the capillaries in the condensing unit (cooling bypass) but no check around the capillaries in the indoor unit.

Any help will be appreciated. JCole


As a thumb rule your heat load will be about 25 Tr minimum. because you have not said about orientation of bldg.

1 typical temp drop should about 6-8 degree in cooling mode

2 Cant say

3 For typical situation you mentioned Discharge would around 260 PSI and suction 65 psi for aircooled condenser

4 Typiclal temp of coil to about 0 degree celcius

good bye Sunil Bhat