[Sam]

“This is the same dreaded message you receive while
driving in extreme heat and the cruisairs will shut down due to the hot
highway and they usally won’t come back on line until is get cooler.”

I kept getting the same “HI PS” shutdown on our rear cuiseair unit this past weekend. We were in 105 degree heat, sitting in an open field at a music festival. I tried spraying the ground beneath the bus with water, and that didn’t help. I then tried blowing a large fan under the bus, and that seemed to help some. We just had one shutdown of the rear air unit after that. All was well after the temperature dropped below about 95 degrees in the evenings. But, once the “HI PS” shutdown ocurred, the unit would not run until after several hours of cooling passed. I never could figure out why just the rear unit out of four cruiseairs was affected, however.

[Larry & Diane]

On hot days I have found it helpfull to raise the bus as high as possible in order to get as much airflow under the bus as possible,you might also check and make sure that the condensor fan on the rear unit does not have any restrictions.I have replaced the condensor fan with a larger fan on one of my units and it was 104 in Lexington,KY on Saturday and that unit preformed the best of all of the 4,it does have a different sound when running but the flow is much greater.I also piched up 2 degrees of differential after the install.

[Archived]

Like Jack said, raising the coach will help a lot. Liberty’s tend to be a little more sensitive to this when parked. Something else that will help a great deal is running the condenser blower continuously when the A/C is on. This is an easy modification to make, and will help keep the condenser cool. Also some coaches have rubber baffles that extend down below the coach to minimize recirculating hot air. Make sure they are not missing/damaged.

[Archived]

Just wanted to add a Hardy THANK YOU to all of the shared input from those who know,for those who did not know, the best ways to keep us all as cool and comfortable as possible while in this heat spell.Thank you all.

[William]

Now that this thread seems resolved I have another question that is similar but different. If this is the wrong place to post this, please let me know.

When using the heat pump side of my center Cruisair and the outside temperature starts getting down around 45 degrees F, I often get a “LO/PS” code and the unit shuts down. None of other Cruisair units, which are also in operation at this time does this. What am I being told? (I see from this thread that I am experiencing low suction pressure, but that doesn’t mean much to me). Is serious damage about to occur? What should be done?

Thanks for any and all help. As Jon says, “whether it is accurate or not”.

[Michael]

I would have the refrigerant charge checked. When the units are acting like heat pumps the unit is actually operating backwards with the evaporator functioning as the condenser, in essence transferring heat out into the house instead of transferring it to the outside as it does when functioning as an AC.

John Klopp……….any advice?

[Archived]

The low pressure condition is low refrigerant as Jon indicated.

Unfortunately
the charging of one of these units on a bus is a series of compromises. The
original design of the unit intended it to be used as a close coupled system
meaning the evaporator and condenser were close together. The condenser was
water cooled producing a condensing temperature that would seldom exceed 100
degrees F based on a water temperature of around 85 or less.

In
order to put the system on a bus the water cooled condenser was replaced by an
air cooled condenser. This increased the condensing temperature and reduced
system capacity below the published data for water cooled systems. The condensing
temperature could be increased to as high as 140 degrees F. Keep in mind that
the condensing temperature is typically 25 to 30 degrees higher than the ambient
air temperature entering the condenser.

With
R22 refrigerant, having the unit trip on high pressure at 425 PSI represents a
condensing temperature of 161 degrees F.

ALL
compressors and manufactured air cooled units are ARI/ASHRE rated at a
condensing temperature of 130 degrees F. Any time the condensing temperature is
higher than 130 degrees F the published cooling capacity of any unit will be
reduced. Based on an estimated published cooling capacity of 14,000 BTU/hr. I
would expect the actual cooling capacity to be reduced to about 10,000 BTU/hr.
capacity when close to trip off on hi pressure.

The
critical charge of a system like this is caused by a condenser with insufficient
capacity for the conditions it is operating in. The condenser has three
functions. The top rows are used to DE superheat the refrigerant. The
refrigerant will reach the condenser as a super-heated refrigerant vapor. The
temperature will often time be 75 degrees hotter than the condensing
temperature. The superheat is caused by the heat of compression and the cooling
of the electric motor inside the compressor.

The
second stage is where most of the heat transfer takes place due to a change in
phase of the refrigerant from a vapor to a liquid. The bottom of this area
represents the condensing temperature.

The
third section is sub cooling and is located on the bottom rows of the condenser.
The sub cooled liquid refrigerant may be sent to a receiver then to the liquid
line feeding the evaporator. Sub cooling needs to be sufficient to ensure that
the pressure drop of the refrigerant in the liquid line between the condenser
and the expansion valve is less than the sub cooling. Any pressure drop or
increase in refrigerant temperature greater than the sub cooling will cause the
refrigerant to flash to vapor and deliver liquid with bubbles to the expansion
valve reducing capacity.

Imagine
a condenser 14 rows high, the top 2 are DE superheating, the next 10 are
condensing and the last 2 are sub cooling. The bottom 2 should be full of
liquid refrigerant. If the system is undercharged, the bottom 2 will not be
full of refrigerant and the sub cooling will be reduced causing the liquid to
flash to a gas prior to reaching the expansion valve. If the system is
overcharged, the liquid refrigerant will back up into the condenser reducing
the condensing section to maybe 9 or even 8 rows. The reduction is heat
transfer will reduce capacity due to a high condensing temperature.

System
charging is easy when you are operating within the intended design standards.
When operating outside of the standard as indicated above, the charge becomes
critical and compromised. The goal of a properly charged system is to provide
the maximum amount of cooling without damaging the compressor. As Jon pointed
out this is generally when the suction line from the evaporator is as cold as
possible. There must be enough refrigerant in the system to reach the expansion
valve in a liquid state. The refrigerant is then metered into the evaporator
were it expands, a phase change takes place and the coils are cooled. If insufficient
refrigerant is in the system, only a portion of the evaporator will be flooded
and capacity will be lost. A similar condition will take place if the superheat
on the expansion valve is set too high. The expansion valve should be set for
8-10 degrees of superheat. In this case superheat is the temperature rise based
on the inlet refrigerant temperature and the outlet refrigerant pressure
measured at the compressor and calculated to be a temperature. The proper
superheat ensures that all refrigerant has made a phase change and is now a
vapor at the compressor inlet. A superheat that is too low will allow liquid
refrigerant back into the suction side of the compressor and destroy the
compressor in a short period of time.

And
now that you understand a basic system, add a reversing valve so you have a
heat pump. The reversing valve will swap the evaporator and condenser sections
to provide heat instead of cooling. Anyone see a problem?

Charging
to get maximum cooling is a compromise in a bus installation. If you want it to
work perfectly in a bus installation with heating capability you will have to
decide if you want to charge for maximum cooling or maximum heat.

My
recommendation is to charge for maximum cooling and never use it as a heat pump
for heat.

You
already have several ways to heat the bus without using the heat pump.

[Ace]

Big thanks John. You are absolutely correct to note since we have multiple heating options in our coaches it makes sense to optimize the AC units for cooling.

[Ed]

Great explanation John, thanks.

[Archived]

John,Do you think that it is a good idea to by-pass (eliminate)the hot gas by-pass valve,and if so should it help performance?

[Archived]

Jack,

In general I
would not remove the hot gas bypass valve. The function of the valve is to monitor
the suction pressure/temperature of the evaporator. If the pressure is too low
the valve opens and hot gas is diverted into the evaporator to keep it from
freezing or frosting.

The reason the
hot gas valve is used is in a low ambient condition the suction pressure will
drop causing frosting. The evaporator pressure drops in a low ambient condition
due to an increase in refrigerant mass flow. The mass flow increases due to a
reduction in condensing pressure allowing the compressor to pump more
refrigerant.

If the valve
is operating correctly and the refrigerant charge is reasonably close, there
should be no way it will be open during a high ambient maximum cooling capacity
day.

If the valve
has been removed, on a day when the coach interior was hot and the outside air
was cool, frosting of the evaporator would be likely to accrue.

[Archived]

I understand what you are stating,AAP and another cruise air service center tell me that they have had so many failures with the plastic hot gas valve that it is not if but when they will fail so they have begun to eliminate them,I guess that the logic is that it is better not to have it than the problems that could happen without it,I was also told that performance should increase but I have had 2 removed and I did not see any increase in performance.What do you think about increasing the condenser fan capacity and is there any negative to doing it?

[Archived]

Thanks for the knowledge and advice. I am convinced. I will have the refrigerant level checked and all of the units maximized for best cooling performance; no more using them for heating.

Cheers.

[Archived]

Jack,

I would go along with removal if you dont try to run the system below an outside air temperature of 60-65 degrees. No increase in performance will be achived unles the charge was incorrect or the valve had failed.

You cant have too much air flow or condenser surface. But, if you manage to get a condenser way bigger than the one you have now, you will need to have hot gas bypass.

[jack]

Thanks for the response,I have 1 cruise air unit that I have replaced the condensing fan and 2 weeks ago it was 104 in Kentucky and that unit is the only one of 4 that did not have high pressure or thermal shutdown sometime during that heat wave,I have 2 more fans that I am going to install soon.