Lennox unit tripping on LP please Help!

[lees8110]

Hi all,

Please bare with me as im still new to refrigeration and this site.

The previous engineer to have worked on the unit suspected the unit was undercharged and quoted to reclaim, pressure test and re charge the system.

So I have repaired the leak and re-charged to the factory required 7.71 Kg of new R407c.

When the unit runs in cool my head pressure cycles from 10 bar to 18 bar as the condense fan comes in and out and my suction pressure starts at 4bar and slowly creeps down to 2 bar when the LP switch drops out the compressor,

On heating (Heat pump) My suction pressure stays around 3 bar and the discharge starts around 10 bar and climbs up to 25 bar (with the condense fan running) when it then goes out on HP.

Does it seem the system is undercharged??? I am scratching my head with this one

Thanks in advance for any advice given, and great website too learninig alot!

Thanks again Lee.

[Gary]

Sounds like insufficient indoor air flow to me.

[airefresco]

Yep, first thing that came to my head was dirty filters or dead fan motor.

[Gary]

When the unit runs in cool my head pressure cycles from 10 bar to 18 bar as the condense fan comes in and out...

As you have rightly observed, airflow through the condenser has a major effect upon high side pressure. The same is true of airflow through the evaporator and its effect upon low side pressure.

In point of fact, the temperature and volume of air flowing through both coils has far more influence upon pressures than refrigerant charge does. Whenever pressures are not what you think they should be, the number one suspect is airflow, not refrigerant charge.

Do yourself a favor: Every day on every job, always check for airflow problems before you even think about putting your gauges on the system.

[bebad]

true true, Gary.

[wingman]

Do yourself a favor: Every day on every job, always check for airflow problems before you even think about putting your gauges on the system.

I had to learn that too

Nowadays it spares me a lot of time and work.

[lees8110]

Thank you all very much for your advice,
I did check the filters before anything they had been renewed and the coil visibly looked in good order, Is there any specific method of testing the airflow to prove this?

[Gary]

Check the temperatures of the air entering and leaving the coil and subtract one from the other. The difference between these two temperatures is called the evaporator delta-T (evap dT). If the evap dT is much more than 20F/11K there is an airflow problem.

Check for blocked registers and/or returns, incorrect fan speed, incorrect fan rotation, dirty fan blades, worn pulleys, worn or slipping belts, etc.

[lees8110]

Ok I have checked and tightened the fan belt, I seem to have a reasonable air flow and return, I have been advised off Lennox technical to replace the TEV and the drier? Does this seem necessary? as my next precaution I will measure my evap dT

[icecube51]

let the TEV be,nothing wrong whit that one. maybe some other bloke that was trying to adjust,and instead de-regulate it.its funny you didn't changed the filter-drier when you touk out the refrig.

Ice

[lees8110]

So are these not common to go then I take it(TEV)?

I realise now how stupid of me to have not replaced the drier now, just still new and forgot to order with the parts.

So is this looking like the culpret? blocked drier???

[Gary]

Ok I have checked and tightened the fan belt, I seem to have a reasonable air flow and return, I have been advised off Lennox technical to replace the TEV and the drier? Does this seem necessary? as my next precaution I will measure my evap dT

Measuring the dT is not a precaution, it is a means of figuring out what is wrong with the system. Do you want to know what's wrong or do you want to continue guessing?