Would a HGB work?

[marc5180]

Hi guys,
One of our other engineers has been working on a Lennox packaged unit this week that keeps tripping every few hours on LP when on heating.
The discharge is running at 11bar and the suction at 2 bar.

As the outdoor coil starts to freeze up this drops the suction pressure even furthur and overnight the unit trips on LP.

The refrigerant charge has been checked and is correct.

If he blocks part of the indoor coil on heating then this raises the discharge to 16bar and the suction pressure increases to 4bar and the unit doesn't trip on LP.

The only problem he has is how to keep the discharge pressure up?

He can't block part of the coil as this will probably act as lack of condensor capacity.
The fan is a fixed speed fan and not fan speed controlled but even if it was then i'm not sure if that would work as the air volume into the room would decrease.

The only thing i can think of is a hot gas bypass after the TEV to keep the suction pressure up but i don't know much about them and in what type of application they are used.

Can anyone shed any light on it?

[lana]

Hi there,

This is normal and the system must have some kind of defrost (outdoor Unit). This happens in split heat pump units on heating mode. When ambient temp. drops then you will have LP problem.
HGB will reduce your heating capacity and it will open the valve most of the time which on the other hand is a waste of energy.

Cheers

[chemi-cool]

You can control the situation by adding a thermostate that sense the outdoor coil temp and activates the 4 way valve just before the LP switch it off.
it should sense the temp at the low part of the coil as the discharge enter the coil from the top.

[marc5180]

Hi, the system does have a dynamic defrost ( not sure what this is) that does seem to work but maybe not often enough.

You could watch it all day and it would be low but wouldn't trip but as soon as you return in the morning it has tripped on LP.

Putting a defrost stat on the coil linked to the 4way rv would work but could I use this as well as having a dynamic defrost?

[NoNickName]

The outdoor coil surface is insufficient for the ambient conditions.
What was the ambient conditions at which the heat pump was designed for?
HGB will fix the problem, but will cause the system to spend energy for keep itself running, and prevent it from releasing any heat in the heated space.
HGB is not the way to go. I'm afraid it can't be fixed. There's a limit of ambient temperature, under which the heat pumps are consuming more energy than what they are releasing. The result is an inevitable low pressure alarm.

[Brian_UK]

Which model is the unit, some Lennox packaged HPs have data tables down to -28°C air on to the outdoor coil. Mind you the duty has dropped by 84%.

[marc5180]

The model number is a Lennox BHK 020 NS.

[Brian_UK]

Have you checked the control settings?

From service book...
Q:
LP cut out due to not enough defrost on heat
pumps

Ans:
Adjust the CLIMATIC settings to extend the
defrost cycles or shorten the time between
defrosts

[mad fridgie]

Install you HGV soley for the point of keeping the machine running (set just above LP trip point), but be careful that the hot gas does not directly impact on the defrost coil sensor.
I do not know your unit sorry, but if the indoor fan is single phase you could use a phase cutting pressure control, to reduce fan speed when required.

[NoNickName]

Which model is the unit, some Lennox packaged HPs have data tables down to -28°C air on to the outdoor coil.

thanks for giving me a good laugh.
Mind checking the corresponding evap pressure of let's say -33°C?

Rereading the thread... why is the discharge at 11 bar? What refrigerant is it?

[mad fridgie]

thanks for giving me a good laugh.
Mind checking the corresponding evap pressure of let's say -33°C?

Rereading the thread... why is the discharge at 11 bar? What refrigerant is it?

Ever worked on a freezer, what evap pressure do they work on? There is your answer.
Fixed speed comp, fixed heat transfer area and air flow on evap and cond, low SST, little mass flow, little heat of rejection, effect low discharge pressure.
COP above 1.

[NoNickName]

I did, and have you ever seen a heat pump freezer?

An evaporating temp. of -33°C is 1.4 barg for R410A, let alone other lower pressure refrigerants. A common LP switch is 2 barg. 3.5 barg for R410A.
Anyway, with a condensing pressure of let's say 40°C (air side heating) or 50°C (water side heating), the final pressure would be 23.4 or 29.8 barg.

I'm afraid that lift would not be allowed on most compressors. The COP would be VVEEEERRRYYY far from 1. Considering the defrosts cycle, it would approach zero.

[mad fridgie]

I did, and have you ever seen a heat pump freezer?

An evaporating temp. of -33°C is 1.4 barg for R410A, let alone other lower pressure refrigerants. A common LP switch is 2 barg. 3.5 barg for R410A.
Anyway, with a condensing pressure of let's say 40°C (air side heating) or 50°C (water side heating), the final pressure would be 23.4 or 29.8 barg.

I'm afraid that lift would not be allowed on most compressors. The COP would be VVEEEERRRYYY far from 1. Considering the defrosts cycle, it would approach zero.

Yes to your first question, many freezer plants use the waste heat air for drying overalls, as part of the design "a freezer heat pump"
If your heat pump was designed at 7Cdb, 6Cwb (universal standard) with fixed swept volume and fixed transfer area and fan speed, then at -28C ambient, your condensing would not be 40C, it would be very close (within a few degrees) to the air on temperature to the indoor coil (cond), air on would be 20C give or take, and so SCT and SST could be be within the envelope of some compressors with some refrigerants and at those conditions, the COP would be over one at these conditions. (heat output would be pretty low and more than likely have little or no effect but thats not the same as not having a COP lower than 1)

[NoNickName]

You are confusing heat pumps with heat recovery.

Anyway, let's take a compressor like SH120 from Danfoss. The minimum t in the envelope is -25°C for any condensing temperature. And at any given t, there is a limit to the SSH.

[mad fridgie]

Not confused at all,
Your question "have you ever seen a heat pump freezer?" Your are indicating that the chill side is used as a freezer, and a heat pump is where the heat is used.
A copeland DKM-75, on R22, can have a SST as low as -45C and upto +12.5C,
pretty good range?

[mad fridgie]

a heat pump, air cond, freezer, chiller,
They all have the same 4 main components, so are the same, You just pick the working conditions that meet the application, then you call it what ever you want!

[NoNickName]

Cars and trains have the same components too (wheel, engine, brakes, etc). Would you call you car a train?

[mad fridgie]

Cars and trains have the same components too (wheel, engine, brakes, etc). Would you call you car a train?

I have called cars many worse names than a train!

[marc5180]

Have you checked the control settings?

From service book...
Q:
LP cut out due to not enough defrost on heat
pumps

Ans:
Adjust the CLIMATIC settings to extend the
defrost cycles or shorten the time between
defrosts

Hi Brian,

We haven't checked the settings as of yet since there was no service controller on site but as soon as the new one arrives then we will check this, thanks.

It also faults on "no filters installed" every once in a while so we will also check what the pascal setting and readings are.

[marc5180]

thanks for giving me a good laugh.
Mind checking the corresponding evap pressure of let's say -33°C?

Rereading the thread... why is the discharge at 11 bar? What refrigerant is it?

The refrigerant is R407c and the discharge is around 11 bar because the suction is so low.

[install monkey]

the indoor blower fan motor pulley is variable so u can adjust the airflow-be carefull due to the pulley is close to the fan casing.also slowing the fan speed will reduce the jolt on start up when the discharge fractures at the bottom of the condenser.

[caseco]

using an auto reset pressure switch you could break the feed to the four way valve and the indoor fan when the pressure drops to 1 bar feed to four way valve and fan motor would break set lp to six 6 bar and diff of 5 this means once valve drops out it wont come back on till it reachs 11 bar outside heat should clear outdoor coil of ice and unit will run on heat indoor and fan wont blow cold while outdoor is been defrosted

[Gary]

At what pressure does the LP trip? Why isn't the system going into defrost before this pressure is reached?

[Gary]

I think the first thing I would want to check is airflow through the outdoor coil, as any reduction will cause both high side and low side pressures to be lower.

[Mr-Frosty]

i have had this problem on one of our sites but only during adverse cold weather the room was set at 24 degrees at the customers request and was on a 7 day timer it was on at 5am off at 6pm mon to friday and with temps dropping on a night the unit will trip on lp even though the charge etc is correct the way we got around it was to set the timer during winter to run on 24/7 but change the zones so zone a would be 24c during the day and instead of going unoccupied we lower the temp to 19 degrees so it stood more of a chance of reaching set point during working hours and the unit still performs its defrost as and when required. i hope this helps