Heat Pump cycling problem

[Xander08]

I have a Goodman heat pump model SSZ16024. My service tech recently checked the charge and operating of the system. All was made OK.
The system in heat mode was working OK until the condenser fan motor crapped out. The tech replaced it with new.
A few days ago, In heat mode, I noticed the HP compressor and condenser fan come on, run for 5 minutes or so and then shut off. It restarted and ran for about 4 minutes and then shut off, then 3 minutes, then 2 minutes, then 1 minute. I checked the thermostat and it was calling for heat all the time. I was very concerned about destroying the compressor so I shut off the breaker to the HP. When it was running the suction line was getting hot and the compressor seemed louder than normal...maybe my imagination.
It's 18 degrees here in PA now and I've been running on electric heat since this problem arose. I'm waiting for my tech to get back to me. It's not an emergency because I have the electric heat to keep warm but I would like to have an idea of what the problem might be.
Any ideas appreciated.
Xander

[beenthere]

Too many possibilities.

First thing though. Have you checked/changed your air filter recently.

Other then that. You need the installing company out soon, or your going to hate your electric bill.

[Kevin O'Neill]

I have a Goodman heat pump model SSZ16024. My service tech recently checked the charge and operating of the system. All was made OK.
The system in heat mode was working OK until the condenser fan motor crapped out. The tech replaced it with new.
A few days ago, In heat mode, I noticed the HP compressor and condenser fan come on, run for 5 minutes or so and then shut off. It restarted and ran for about 4 minutes and then shut off, then 3 minutes, then 2 minutes, then 1 minute. I checked the thermostat and it was calling for heat all the time. I was very concerned about destroying the compressor so I shut off the breaker to the HP. When it was running the suction line was getting hot and the compressor seemed louder than normal...maybe my imagination.
It's 18 degrees here in PA now and I've been running on electric heat since this problem arose. I'm waiting for my tech to get back to me. It's not an emergency because I have the electric heat to keep warm but I would like to have an idea of what the problem might be.
Any ideas appreciated.
Xander

If you are using a pleated air filter, it will restrict air flow more than a standard filter. That can cause the head pressure switch to trip. The same thing goes for those washable plastic electrostatic filters, sometimes called Paul Harvey specials. A dirty fiberglass filter can do the same thing. Make sure all of your supply registers are open as well.

If you don't get it fixed soon, you could double your electric bill.

Good Luck.

[Xander08]

Filter is 20x25x5 Skuttle NEW. I know that returns are insufficient relative to supply. Additional returns havn't been installed yet. I'm about 300cfm short.
I need some educating on HP operation in heat mode.
I know the evap coil acts as the condenser in heat mode....so the compressor takes the gas, compresses and heats it, it goes to the evap where heat is transferred to the cold indoor air, cooling the gas. Flow is then thru a TXV where pressure is let down, further cooling the flow, then to the outdoor coil where heat is transferred from the outside air to the refrigerant, then back to the compressor.
In heating mode, why is airflow across the evaporator coil an issue...the refrigerant on both sides is a vapor?
In heat mode, what causes the high head pressure that shuts off the compressor?
Thanks

[beenthere]

Low air flow in heat mode, means your not extracting enough heat to condense the vapor to liquid.
Condensing to a liquid is what lowers the pressure.

[Milk man]

Low air flow in heat mode, means your not extracting enough heat to condense the vapor to liquid.
Condensing to a liquid is what lowers the pressure.

No it don't.

The metering device is what causes the pressure change.

[beenthere]

No it don't.

The metering device is what causes the pressure change.
Not on the high side. As far as change of state pressure changes.

No heat rejection, no condensing.
No condensing, the head pressure will be high.

[Milk man]

Not on the high side. As far as change of state pressure changes.

No heat rejection, no condensing.
No condensing, the head pressure will be high.

Yes because the compressor and evaporator (slightly) will add heat to the system.

But form the compressor is superheated vapor that condenses into a subcooled liquid in the condensor. It does not change pressure when it condenses.

Head pressure is high due to not being able to reject heat, not because it's not condensing.

[beenthere]

If its condensing, it rejecting heat.

As it condenses because it has rejected enough heat to condense.

The liquid, takes up less space, so there is a drop inpressure.

[Xander08]

Hot gas at high pressure goes into the evaporator and is cooled by the indoor air across the coils. The outdoor TXV lets down pressure to supercool the gas to a liquid.
If it's not cooled enough in the evaporator, due to restricted airflow, the pressure will rise to the compressor cutoff switch setting.
Sound right?
As a test, I can add return airby uncapping the return duct, upstream of the filter, and see if the HP still cuts off.
John

[beenthere]

Your close enough.

[Milk man]

No.

"When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the evaporator through a very tiny, narrow hole. On the other side, the liquid's pressure drops. When it does it begins to evaporate into a gas."

http://www.energyquest.ca.gov/how_it_works/air_conditioner.html

[Milk man]

Hot gas at high pressure goes into the evaporator and is cooled by the indoor air across the coils. The outdoor TXV lets down pressure to supercool the gas to a liquid.
If it's not cooled enough in the evaporator, due to restricted airflow, the pressure will rise to the compressor cutoff switch setting.
Sound right?
As a test, I can add return airby uncapping the return duct, upstream of the filter, and see if the HP still cuts off.
John

No.

Starting at the compressor. It compresses low pressure gas into a high pressure gas. The high pressure gas enters the indoor coil and is cooled (condensed) to a high pressure liquid. The high pressure liquid leaves the indoor coil to a metering device. The metering device causes a change in pressure and the high pressure (high temperature) liquid is flashed into a low pressure (low temperature) gas. The low pressure gas leaves the outdoor coil and then enters the compressor to be compressed into a high pressure gas again.

[Milk man]

The only pressure drop in the refrigeration system should be at the metering device.

Condensing a high pressure/high temperature gas to a high pressure/high temperature liquid will not cause a pressure drop.

[Xander08]

So what causes pressure to rise high enough to trip the compressor on the high pressure switch?
Merry Christmas to all and to all a good night.
John

[beenthere]

The only pressure drop in the refrigeration system should be at the metering device.

Condensing a high pressure/high temperature gas to a high pressure/high temperature liquid will not cause a pressure drop.

The liquid is not at the same high pressure as the vapor.



Take a metal container that has a lid that seals.
Fill that container with with steam, put the lid on.
As the steam condenses(changes form vapor to liquid), the pressure in the can decreases, this will be evident by the metal container being crushed by the atmospheric pressure.

The water will still be 212°.

The pressure at the begining of a condenser coil is higher then at the condenser coils outlet.

[Milk man]

So what causes pressure to rise high enough to trip the compressor on the high pressure switch?
Merry Christmas to all and to all a good night.
John

Low air flow across the condensor coil.
Non condensibles in the sealed system.
Discharge line restriction.

[Milk man]

Who told you that their is a pressure drop from the compressor to the metering device? And how much pressure drop is acceptable?

Here is another link explaining the refrigeration cycle and it says nothing about a pressure drop in the condensor.

http://www.europe.honeywell.com/70_refrigeration_control/EN5B-0024UK07%20R0505.pdf

[Airmechanical]

The liquid is not at the same high pressure as the vapor.
that is undisputable, after the hot gas is cooled/condensed the pressure is lowered

that's why your hot gas discharge pressure is higher than your subcooled liquid pressure



.

[pecmsg]

Let’s see if this clears this up.

Page 7

http://sporlan.jandrewschoen.com/10-135.pdf

[beenthere]

I said pressure drop from one end of condenser coil to the next.

The pressure drop of the refrigerant lines is caused by piping restrictions, and vertical heigth difference of the evap and condenser.

You ever work on a Trane RTU with the access fitting that is between the condenser coil and the compressor. And then add a access fitting just after the coil.
You see the pressure drop.

Check the piping charts of your brand for long line sets. They should have a max length listed. Or, tell you what their acceptable max pressure drop is for the line set.

York has listings for pressure drops of 32 PSIG on the liquid line.
Those of course are for friction and static losses.

If I look hard enough, I think I can find one that allows up to 50PSIG pressure loss.

[Milk man]

The high side changes state, not pressure.

If it changes pressure, then how much?

[Airmechanical]

The high side changes state, not pressure.

If it changes pressure, then how much?


when i check a system with a hot gas access and a liquid access it's usually around 10 psi difference (liquid pressure being lower)

why there is a difference could be an all day discussion:(



.

[Milk man]

I said pressure drop from one end of condenser coil to the next.

The pressure drop of the refrigerant lines is caused by piping restrictions, and vertical heigth difference of the evap and condenser.

You ever work on a Trane RTU with the access fitting that is between the condenser coil and the compressor. And then add a access fitting just after the coil.
You see the pressure drop.

Check the piping charts of your brand for long line sets. They should have a max length listed. Or, tell you what their acceptable max pressure drop is for the line set.

York has listings for pressure drops of 32 PSIG on the liquid line.
Those of course are for friction and static losses.

If I look hard enough, I think I can find one that allows up to 50PSIG pressure loss.

Are you saying the pressure drop is from the friction of the coil/piping or from when the refrigerant changes state?

[Milk man]

Let’s see if this clears this up.

Page 7

http://sporlan.jandrewschoen.com/10-135.pdf

Good link

[beenthere]

Are you saying the pressure drop is from the friction of the coil/piping or from when the refrigerant changes state?
Those were for pressure losses due to line length friction loss and static losses(height differences).

The pressure drop from the inlet of a condenser coil and the outlet varies.

As air siad, usually around 10 PSIG.

Some lower, some higher.

Some of it from state change, some of it from the coils design creating pressure drop due to friction.

[Milk man]

Would you say it was a greater pressure drop for commercial equipment than residential splits?

[beenthere]

Never thought about it.

[Milk man]

I was thinking residential equipment. Where you thinking commercial equipment? That might be where the misunderstanding came from.

I don't think there is much of a pressure drop through a residential condenser, but I could be wrong.

[beenthere]

I think you'll find weather its residential or commercial.
The amount of PD, will vary with the amount of subcooling.

[Milk man]

When I went through tech school we used Refrigeration and Air-conditioning text book. Second edition.

Chapter R24, page 274, talks about troubleshooting. Page 276 and 277 both show refrigeration and air conditioning systems showing the same pressure through out the high side.

On page 275 it states the pressure remains the same through the condensor coil.

So this is what I was remembering and why I disagreed. The only reason I remember is it was one of the questions I got wrong on my final for this class when I was in tech school. (That was a while back)

I can believe in the real world there could and is a pressure drop, but not enough to make a difference in most cases. And the next time I get my hands on a machine with a discharge port I'm going to see for myself how much.

[Milk man]

So you're saying because the refrigerant does not condense there is less room in the condenser for the vapor. The vapor is compressed tighter together and that is what raises the head pressure?

That's interesting.

[beenthere]

You ever put a head control on a condenser, and stop or lower the condenser fan speed to raise the head pressure by not cooling the compressor dischare gas as much. So you don't condense all of the refrigerant.

[Milk man]

You ever put a head control on a condenser, and stop or lower the condenser fan speed to raise the head pressure by not cooling the compressor dischare gas as much. So you don't condense all of the refrigerant.

I thought you where adding heat to the refrigerant via heat of compression from the compressors and not rejecting what is picked up by the evaporator.

[beenthere]

When you slow or stop the fan.
Your lowering the amount of heat your rejecting.
HOC will increase some. But basically, your simply lowering the amount of heat your rejecting(lowering system capacity).

With the fan stopped or slowed, the condenser coil is mostly vapor. Very little liquid.
When the fan restarts, or speeds up, you condense most of the vapor into liquid, and can tell its being condensed by the drop in pressure.

[Milk man]

What you have said makes a lot of sense. Thank you for taking the time to explain it.

[beenthere]

Your welcome.

[Airmechanical]

I don't think there is much of a pressure drop through a residential condenser, but I could be wrong.


you will see the same type of pressure drop whether its a 1-1/2 ton split residential, or a 30 ton split commercial



.

[heaterman]

Always nice to read a thread were theory and practical application come together.

[Xander08]

Gentlemen,
I didn't mean to start an argument among Professionals.
Given all that has been said....I conclude that If return air is insufficient, hot refrigerant is not cooled and condensed to liquid in the evaporator; thus raising gas temperature/head pressure and tripping the compressor on high pressure.
Sound right now?
Happy Holidays
John

[heaterman]

They are playing nice. It's an exchange of knowledge, experience and theory. It would be easier to give you a definative answer if we could "lay hands" on your unit and check temps and pressures but I think you have the jest of it.

[Airmechanical]

Gentlemen,
I didn't mean to start an argument among Professionals.
Given all that has been said....I conclude that If return air is insufficient, hot refrigerant is not cooled and condensed to liquid in the evaporator; thus raising gas temperature/head pressure and tripping the compressor on high pressure.Sound right now?Happy HolidaysJohn

always remember, with a heat pump;

while in the cooling mode, the indoor coil is the evaporator

while in the heating mode, that same indoor coil is now the condenser;)



.

[beenthere]

Gentlemen,
I didn't mean to start an argument among Professionals.



I didn't see no arguments.

[Milk man]

Gentlemen,
I didn't mean to start an argument among Professionals.
Given all that has been said....I conclude that If return air is insufficient, hot refrigerant is not cooled and condensed to liquid in the evaporator; thus raising gas temperature/head pressure and tripping the compressor on high pressure.
Sound right now?
Happy Holidays
John

Turned out to a very good thread. Just needed to get the subject brought to the front. It will help all that read it.

[captube]

Turned out to a very good thread. Just needed to get the subject brought to the front. It will help all that read it.

Yes, it was a refresher course for me :)

[juststartin]

That's why I love this site. I learned as much in this thread as I learned in an entire day of class!!:)