Right thanks for coming in everybody um this week. We're going to talk about about compressors specifically a compressor fault. That is a little less common we talk a about shorter compressors grounding compressors and i want to talk specifically about compressors that have poor compression. Which is the term that we like to use because in the past people will say like a slipping compressor or bad valves in a compressor.

They'll use like specific terms. But in the case of modern compressors in a lot of cases poor compression can happen because either it's out on some sort of safety due to high compression ratio or maybe. Those safeties actually get stuck open. So a lot of times.

They'll be like bypass valves or you have that compliance and like copeland score compressors where the where the scroll lifts. But it can be also due to wear and a lot of other things just want to kind of show you on measure quick a couple things that are going to be telltale signs. This information that we have listed on the screen. I just plugged in so it's not like this did not come from a real system.

But some of the things you're going to notice right away. When you look at it our suction pressure. 190. There are circumstances.

Where that could be appropriate. So this is a 410a system. So that's a 66 degree evaporator temperature or what we call suction saturation temperature. And that would be you know in general 35 degrees lower than our indoor return air temperature our temperature going over our evaporator coil is what we would expect our evaporator temperature to be so if we had an 100 degree house well then that might be appropriate.

But if our house is 80 degrees 85 degrees 90 degrees. Then that is definitely high suction pressure. But then we're also going to notice. We've got 305 on our head.

Which equals a 97 degree condensing temperature. So the actual temperature of the refrigerant is condensing and if we look at our outdoor air temperature. We're showing a 94 degree outdoor air temperature which means that we only have a 3 degree difference typically nowadays you're going to see you know 15 between 12 and 15 16 17. In their differential.

So that's also showing us that we have low head pressure. So we think of low compression. The telltale signs is that we don't have that differential between that discharge pressure and that suction pressure so they're they're coming. Closer together than they should be and that's what we're seeing high suction low head is an indication of poor compression um are there other things that can cause these types of symptoms.

There are and the most common would be on a heat pump in cases where you have the reversing valve that's bypassing internally. So the valve is getting stuck halfway the slide on the valve and so when that happens you can actually get bypass from the discharge side to the suction side through the body of the valve and that can cause some similar symptoms. The typical test. We do to kind of confirm for that is just to measure the two suction lines.

So your common suction line temperature going back to the compressor versus the suction line temperature. That's coming from the evaporator and if you see more than typically eight degrees is kind of what we say and that's just a rule that we made up years ago here in the heat pump markets if it's more than that then that's a sign that the valve could be bypassing. But it's also important to recognize that a reversing valve relies on compression in order to shift the valve. So those little pilot tubes that come out of that little solenoid on the reversing valve that's what actually causes the valve to shift.

So it uses uh compression from the compressor to shift. It so. If you have a compressor that's really not pumping a valve can actually get stuck because of that so if there is some compression. But not enough to fully shift the valve that could cause the valve to get stuck.

So you could potentially see a case where the valve is stuck. But the valve is stuck because the compressor is not pumping if that makes sense. What are some of the other indications well you know an easy way to think about this that kind of summarizes all of this is compression. Ratio so here we show a compression ratio of 156.

Which is definitely low typical modern equipment you're generally going to see compression ratios around 23. And that can vary. A little bit. Obviously you get a much hotter summer day.

Compression ratio is going to go up you have somebody who wants to keep it really cold inside their house. Compression ratio is going to go up and higher compression ratio for no reason just because you got a dirty condenser or somebody keeps your ac really cold inside that's not a good thing. We don't want higher compression ratios than we need because if you have a high compression ratio that means high current system is not going to run it sufficiently it's not going to move as much refrigerant because of mass flow rate being reduced and mostly that has to do with lighter suction gas returning to the compressor if that's the case. But we also don't want compression ratios this low.

Because then we have the circumstances that we're seeing here got a 66 degree evaporative coil. The compressor. We're relying on it to move refrigerant that's its job is to move refrigerant. So if it's not pumping that can be a problem another kind of telltale sign of a compressor that's not compressing properly is it's almost always going to have low current.

So when you're looking at your rated load amps you know typically systems that are operating and again this varies. Quite a bit these are just general kind of you know systems that are running under normal. Conditions you're generally going to see them operating at about 06. So about 60 of your rated load amps and you'll notice this when you're measuring current on a compressor and you compare it to the rla listing on the data tag.

It's almost never going to run that rla listing. It's going to be lower than that but when you have a compressor that's not compressing. It's usually going to be significantly lower than that so that's when you're going to see currents that are less than 50 percent of that rla you know sometimes 40 45 in that range sometimes even less than that and that's going to be an indication that that compressor is not moving as much refrigerant and again that's kind of what happens. Because when you have this refrigerant moving through the compressor and there's not as much of it moving so.

It's basically just sloshing it around it's not pumping it through the system that's going to result in lower current because that motor is going to free spin. It's going to sprint spin faster. Which means that you're going to have lower current. So that's typically what you're going to see in those cases.

You're going to see high suction or high evaporator temperature. You're going to see low head low condensing temperature. Those mean. The same thing you're going to see a low compression ratio.

Which again just kind of anchor. What compression ratio is it's just a fraction of your discharge pressure divided by your suction pressure right at the compressor and this is where it also gets tricky because in most cases when we're doing this we're not really going to get a true compression ratio. Because we're not usually measuring discharge pressure. We're usually measuring liquid line pressure so that means that what we're seeing in terms of compression ratio is actually going to be lower than what it really is which is also why when you read in the books.

A lot of times. They'll tell you you know a compression ratio of 3 is going to be normal that's because older equipment also had higher compression ratios. But it's also because we're not really measuring a true compression ratio. We're measuring something on the liquid line.

But even more than that compression ratio is absolute discharge pressure divided by absolute suction pressure the difference between absolute and gauge pressure is just adding in that atmospheric pressure so when we're measuring with a gauge. If i have a gauge open in this room and i'm measuring zero on the gauge. We know that that gauge has been zeroed out to atmospheric pressure in this case because we're in florida. We're pretty much at sea level.

We're at the bottom of an ocean of air that sits above. Us so that's 147. So in order to do the math you take your suction pressure on your. Gauge you add.

147 to that you take your discharge pressure on the. Gauge you add 147. To that you divide your discharge pressure with that amount added in by the suction pressure with that 147. Added.

In that becomes your compression ratio measure. Quick does that for you so you don't have to do that additional math. But just keep in mind that it isn't going to be true. And it's not going to be exact if you're measuring on your liquid line because your liquid line pressure is going to be slightly lower than your discharge pressure again not that that makes a big.

Difference but what you will notice is if it's running properly. 23 to 27. Is going to be sort of typical when you start to drop below. 2.

That's really when you have an indication of poor compression and so that tells us we've got a compression issue. We're also going to measure our current. If our current is abnormally low. That's also going to be an indication of poor compression.

If you have for example a reversing valve that is bypassing that's generally going to result in your compressor. Having the normal or even maybe a little higher current. So that's where you measure that temperature across the reversing valve. If that shows a high number and your compressor is running normal to high current and you're seeing these sort of readings.

That's where you would kill more towards that reversing valve being your issue. Just keep in mind with the reversing valve. It's not that magnetic solenoid that actually shifts the valve that magnetic solenoid shifts. The pilot valve which is kind of like a small reversing valve inside the reversing valve and then those pressures from the compressor those differential.

Pressures are what actually shift the valve and that's why you can't shift reversing. When the system's off the system pressures are equalized you know at the same level that valve's not going to shift. Because it requires that discharging suction to slide the valve so what causes this well there's a lot of things that could potentially cause it old school reciprocating compressors. You know they have pistons.

If the valves broke in the pistons or if there was too much space around the piston. So the cylinders actually had wear in them. That's where you would start to get this most commonly it would be broken valves. So one of the valves would break you have on the upstroke you have your discharge valve on the downstroke you have your suction valve.

If one of those breaks. The refrigerant is just going to move in and out of the cylinder. And it's not going to be efficient at all so that's where in the past. They would call this bad valves.

But you also have rotary compressors. You have scroll compressors in those cases you could have significant wear inside the compression chamber. That could cause it i mean we know with a scroll you've got your stationary scroll on top and then you have your oscillating scroll on the bottom and so if there's a lot of gap between those where they fit that could cause it. But actually.

Most commonly what causes poor compression is actually safeties so in the case of like your your copeland compliant scroll. You actually have this ability for the scroll to kind of move around and that's so that it can deal with liquid or it can deal with you know heavy solid contaminants. Even potentially they come through that compressor without destroying itself and so it's actually a design feature. But if it does lift in many cases you got to shut it off let it equalize before it's going to seat back down another thing you see with scrolls.

Nowadays is that they're really not designed to pump down because when you start to pump down a system that suction pressure is dropping that head pressure's staying the same or even going up a little bit as you're as you're pumping it down. And so that means your compression ratio is skyrocketing once your compression ratio hits a certain point that compressor will often go in to bypass itself. It will go into a safety mode. So that way it doesn't damage itself and commonly.

Nowadays what they'll often do when they go into safety mode is they'll actually bypass discharge gas over top of that thermal limit in order to shut the system off. So basically saying. The system is running in an unsafe condition for the compressor high compression ratio high discharge pressure low suction pressure those are bad for the compressor. So let's discharge.

Some of that gas out of the top run it over to that thermal limit. So that way it's going to shut the system off also we've seen in a lot of cases when you short cycle. A compressor say you you're working cleaning a drain and the system is running and you jiggle the float switch now in some cases and again this shouldn't happen and some people say it doesn't happen. But it does almost every service tech in our market at least has seen it especially with lg scroll compressors.

They the compressor can actually jolt and run start running backwards and when a scroll compressor runs backwards it doesn't pump it's really bad for it too so if you've ever worked on three phase. Scrolls and you have two of the legs reversed that three phase. Scroll is going to run backwards sounds bad. It's really bad for it and definitely something you want to avoid.

But in some cases. We'll run into this so the solution to a lot of these sorts of problems is to shut the system off let. It sit put your amp clamp on the line. Get your gauges hooked up then start it up and so because in many cases.

What's happening is there's something that's causing really high head pressure or really low suction pressure. And that's what caused it to go into this kind of safety mode. So shut. It off give it a little bit have everything connected so when you start it up you can watch it and it may not be the compressor's fault is the point so you wouldn't want to have another problem that caused the compressor to do this you replace the compressor now you have the exact same thing you want to eliminate through process of elimination eliminate the reversing valve eliminate every other possibility before you replace a compressor that's not pumping.

But what you're going to see in your measurements is going to be the same it really comes down to compression ratio and current. But what you're going to notice. First generally is going to be abnormally high suction again keep in mind. If it's in hot pool down.

Because you have a really warm space that would be expected you're going to have low head pressure. So you're going to have a ctoa that's very low in this case. We're showing 97 degrees and outer temperature of 94. So that's very low when you take that ratio that gives us our compression ratio and indicates that issue.

And you're going to have low running current and again just keep in mind just because it says that rla on the data tag that you know being lower than that would be normal. But it would be significantly lower than that based on what you'd normally see that's it nice and quick have a great day thanks for watching our video. If you enjoyed it and got something out of it if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click. The notifications bell to be notified when new videos come out hvac school is far more than a youtube.

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