- Tech Tips
When you work on a heat pump system and you want to test defrost there are many different test procedures to follow to test the board and sensors.
Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.
Lots of pins and jumping involved.
But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.
A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use a (freshwater) ice bath to test it.
If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the outdoor fan off and see when it closes by using an Ohmmeter and testing against a line temperature clamp in the same location.
On a defrost thermostat you can also easily jump it out to test the board since it is just open and closed.
A defrost “sensor” is generally a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.
You CANNOT jump out a thermistor with a typical jumper to test.
P.S. – A podcast about Heat pumps is available HERE
When I first started in the trade as an apprentice we worked on a lot of Trane heat pumps that used crankcase heaters that slid into the compressor sump on the big orange Tyler reciprocating compressors like the one below.
It was very common for these heaters to break off where the wire entered the rod and short against the bottom of the condensing unit. Some of the old timers I worked with would say “This is Florida, we don’t need those things here”, disconnect it and move on.
I later learned that isn’t the correct approach
Systems that have crankcase heaters, have them for a reason and while outdoor ambient temperature is one factor it isn’t the REASON crankcase heaters exist. Refrigerant is attracted to the refrigerant oil in the compressor when the system goes into the off cycle, the amount of refrigerant in the oil and the rate at which it moves into the oil depends on the type of refrigerant and oil and the temperature of the compressor.
When the compressor is off for a while a significant quantity of refrigerant can migrate to the compressor and condense. When the compressor comes on the refrigerant rapidly expands and foams the oil, forcing it out of the compressor and into the system. This is called a “flooded start” and will eventually result in compressor damage due to lack of lubrication, it also decreases system efficiency due to the oil in the system inhibiting the transfer of heat.
Strategies like hard shut off expansion valves, liquid line solenoids help to keep liquid refrigerant out of the compressor and oil separators help to keep the oil in the compressor and out of the systems but the trusty old crankcase heater is still a simple and commonly used strategy to prevent flooded start. If you find one that is failed you would be better off replacing it instead of taking the word of techs who tell you just to cut it out, like I once did.
Take a look at the specs from this Copeland scroll compressor pulled from the Copeland Mobile App (which is an incredible app by the way).
This is a single-phase compressor so the amperages listed are based on an amperage reading from the wire connected to the common terminal.
LRA is locked rotor amperage which is the expected measurable starting amperage and RLA is rated load amps, meaning the amperage it will draw when running normally at its rated load. You may wonder why there are two different RLA ratings here… that’s not what this tech tip is about but if you get the app and click the i with the circle around it you can find out.
The point is we are always taught to measure amperage on common with single-phase motors, but do you know why?
A single-phase motor like the one shown above has three terminals (Common, Start and Run) but only two actual windings (Start and Run). The common terminal is just the “common” point between both of the windings so when we measure the amperage on common we see the total current of both windings.
In tradeschool we learn Ohms law which teaches us
VOLTS = AMPS X OHMS
However, when we try to apply that in the field we realize some things pretty quick that get in the way of applying that neat little formula
So to summarize….
YOU AREN’T GOING TO BE ABLE TO ACCURATELY APPLY OHMS LAW IN THE HVAC/R FIELD
When we measure the ohms of windings from terminal to terminal it is mostly meaningless because the readings are often very low anyway… sometimes so low that your meter becomes inaccurate.
Notice how low the resistances are of this same compressor.
The real resistance of the motor only shows up when it is energized with alternating current and the magnetic fields begin to interact, this total resistance when energized is called impedance.
We do know that the start winding has a higher static ohm value than the run winding and that when we add start to common and run to common together that it will equal run to start (which is a fairly obvious statement since common is just a center point) and that if the thermal overload is open we will measure OL between C-R and C-S but will read the combined value R-S.
These are all true and are reasons to pull out the meter but this still doesn’t tell us anything about the title of this article and you are probably wondering what the heck I’m driving at.
I’m making sure we are all on the same page before I drop a start winding fact bomb on you…
But one more thing we need to come to an agreement on.
The run winding is connected “Across The Line”, in other words with one leg of split phase power connected to Common and the other to run. The current that travels through that run winding is completely a function of the total impedance of that winding which has several factors including the static winding resistance, the inductive reactance of the windings and the back EMF that builds as the motor starts running.
In other words… the amperage starts high because the resistance starts low in the run winding and amperage goes down as the motor gets up to speed because the total impedance increases.
Remember, ohms law teaches us as resistance goes up, amperage goes down if the voltage stays the same.
The start winding is connected through a run capacitor and potentially some other start gear and not connected “across the line” like the run winding. This means that the current that moves through the start winding is limited by BOTH the total impedance of the winding AND the capacitance of the run capacitor and any other start gear.
Here is an image from an oscilloscope on this very same compressor referred to above with 197V applied, a proper run capacitor and no hard start kit…
Take a long close look.
Notice the blue line is the RUN WINDING CURRENT and the red line is the START WINDING CURRENT.
Notice that ALL of the true inrush current occurs on the run winding and the start winding current doesn’t go up until the run winding current starts to go down?
That’s because unless the start winding has some form of start capacitor it cannot draw any amperage higher than what the run capacitor will allow. In essence, the run capacitor becomes a ceiling or current limiter that allows only so much stored current per cycle and no more.
Try it sometime.
Measure the running amperage on the start winding with a capacitor slightly larger, slightly smaller and then with none at all. You will see higher amps, lower amps and then (obviously) no amps.
Try taking an inrush reading on the start wire of a compressor with no hard start and see what you get.
Then try it with a hard start.
Notice anything different on the start winding amps? Can you see the moment the back EMF removed the hard start from the circuit? Was the TOTAL amperage actually lower with a hard start or was the time to start decreased and more current shifted to the start winding?
Have you ever wondered why your old refrigerator never needs service, gauges installed, and can run for 30 years that way maybe only needing an occasional cleaning of the condenser? For crying out loud, utilities are buying these old energy hogs through some programs because they never seem to die. Why do they last so long? A good evacuation, a correct refrigerant charge, and maintaining a sealed system. Evacuation is the most important part of an installation followed by charge and airflow to assure the efficiency, reliability, and longevity of equipment. When it comes to evacuation, in an industry plagued with bad information, I do not know why I was surprised to learn that many technicians think it is OK to measure the system vacuum at the vacuum pump. Now I wouldn’t say it’s like driving from the back seat of a car, only because it’s much worse than that. It’s more like driving blind from the trunk. In my opinion, a senseless practice, and simply poor practice when it comes to proper evacuation. If you hate your micron gauge and think that evacuation is impossible or dark magic, I think we have stumbled across the reason. Now I am not going to blame technicians for this, as somewhere along the line, (likely when the marketing department took over-engineering) the 1/4″ test port on a vacuum pump became a service port for evacuation and at times a mounting point for the vacuum gauge and the pump blank off valve became the isolation valve for vacuum. I know this is true because I have talked to many salesmen at tradeshows who have confirmed this to be their understanding. But here is what is important to understand, the 1/4″ test port is nothing more than that, it is a test port. It is designed to provide a port to test the ultimate pulldown vacuum of a vacuum pump. It is not, and was never intended for evacuation or a permanent location for the vacuum gauge, and the blank-off valve is not intended to isolate the vacuum from the system. Let’s start with some basics. Pressure and vacuum are two completely different sciences and cannot be treated the same. Here is a fundamental example. Given a straw, you could easily blow out a candle from more than a foot away. Take that same straw and try to suck the candle out, and even inches away it is not possible. Vacuum is not directional, if you have ever used a cracked straw, you know they are pretty much useless for drinking. The vacuum pulls from wherever it can. A vacuum is simply a reduction in pressure, and it is strongest at the pump inlet and gets weaker and weaker as it moves away from the pump toward the system. This is due to friction and leakage, which means the vacuum is weakest at the furthermost point away from the pump. The pressure differences are extremely small that create the flow back to the vacuum pump. These pressures are typically as low as .002 psi. Remember, a vacuum is limited by physics. The deepest vacuum we can achieve is -14.996 PSIG. That said, the only way to reduce the friction and increase the flow is larger hoses. A 1/4″ hose has such a low conductance speed that it should never be used for evacuation. Using a 1/4″ hose chokes the vacuum pump—no matter how big—down to about 0.5 CFM at 1,000 microns. Hoses that are 3/8″ or 1/2″ inside diameter are the smallest that should be used for evacuation. That said, the vacuum at the hose inlet can be much more different than the vacuum at the pump connection, and again, much more different at the far end of the system. But wait, there’s more! Schrader cores provide additional pressure drops, and we all know there are times when
the core depressor either does not open or barely opens the Schrader valve. This will also play havoc in the evacuation process. These pressure drops, again, will make the vacuum at the pump much deeper than it is in the system. The reality is your pump could be at 250 microns while the system is still well over 2,000 microns. This could lead to catastrophic failure of the refrigeration system over time. Vacuum pump blank off valves leak! Vacuum pump blank-off valves are designed for nothing more than keeping the oil in the pump when the pump is turned off with the hoses connected, in the case of a tip-over, and to keep the oil from absorbing large amounts of moisture while the pump is stored. They are not vacuum rated, and if tested with a micron gauge, most–if not all–will creep toward atmospheric pressure in a matter of minutes. This is the primary reason for vacuum trees with vacuum rated ball valves. Those are intended for that purpose. They are not there only to provide a connection point for multiple hoses. The blank-off valve on the evacuation tree, or evacuation manifold, are less than ideal for system decay testing as it also does not isolate the core tools and hoses that are also a significant source of potential leakage, if for no other reason than simply the huge amount of connections. The reason the blank-off should be closed during isolation is to keep the oil from sucking out of the pump when the vacuum rig is under a vacuum and the pump is off. If you are testing the ultimate pulldown of the pump with a micron gauge, closing the blank off prior to shutting down the pump is critical as you will easily pull oil into the vacuum gauge if you shut the pump off before doing so. So what is a technician to do? After all, there is no other convenient place to install the vacuum gauge on a typical system, there are only two ports on the entire residential system. The answer is vacuum rated core tools. Using core tools allows two important things to happen. First, removal of the core which is a significant restriction, and second, it provides a place for the vacuum gauge that allows isolation of the vacuum pump and hoses. While a core tool is not 100% leak-free, down to about 20 microns, a good core tool will not be a significant source of leakage when the valve is closed and the system is isolated. Core tools should be cycled several times during evacuation to release any trapped air around the valve, but aside from that, the only source of leakage during isolation at that point is between the ball valve and the service port on the system. If a non-permeable connector is used for the vacuum gauge, the source of leakage not has been significantly reduced if not, for all intents and purposes, completely eliminated. Just recently I was reading a post online that “vacuum rated” is nothing more than an industry buzz term. Nothing is further from the truth. Everything leaks, even solid copper lines! It is the leak rate that we are concerned with. Vacuum rated defines the performance under a vacuum and clarifies the leak rate of the core tools or the hoses tested. It tells the user the ultimate vacuum that the tool will perform to without the leakage rate overcoming the ability of the pump at the rated micron level. Vacuum rated means it was tested for the process of evacuation and can be expected to perform adequately down to the rated level. This does not mean that standard core tools cannot be used, but it does mean that the user should test them for the intended purpose to assure that they are tight enough to perform without being a significant source of a leak in a vacuum. Because of the hoses, vacuum pump blank-off valve, manifolds, and core tools leak, we need to either remove or isolate as many of these components as we can from the system during the decay test. Just to be clear, evacuating through a manifold is not a great idea either, as the small porting and hoses also are a significant restriction. Core tools and a proper evacuation rig allow a good evacuation to happen. Making a few small changes in your approach to evacuation will make a huge difference in your frustration level, and make the system last years longer. — Jim Bergmann / MeasureQuickPS. Open the gas ballast if equipped before starting the pump, close it when you hit 1500 microns, then open it again after you have isolated the pump/rig for a few seconds prior to the pump shutdown. This will help keep your oil dry, and assure that you achieve maximum vacuum during the evacuation. It will also help your pump last longer.
If you are used to simple, straight cool split systems you know that the low voltage to the outdoor unit is usually VERY simple with just a Y (contactor power) and a C (common) connected to the outdoor unit in many cases. When the condensing unit controls are strictly two-wire low voltage there is no continuous low voltage power so there are also no timers or other logic in the condensing unit. Usually, in these cases, the LV wires connect directly to the contactor coil.
A heat pump needs to be able to switch between heat and cool and defrost which brings in the necessity for more control conductors and complexity.
A heat pump defrost board like most modern controls contain both loads and switches to control different functions. because it has timers and some basic “logic” the board requires a power supply and for most residential split system boards this power comes from the C (common) and R (hot) terminals from the indoor 24v transformer.
The defrost board also utilizes the constant power on the defrost board R terminal to back feed voltage through the W2 wire back to the secondary heat inside whether it be heat strips, furnace or hydronic secondary heat.
This helps to counteract the cooling effect that occurs when the heat pump when it shifts from heat to cool mode for defrosting. This function is an important thing to test on heat pumps to reduce cold draft complaints during the winter.
Simply force the board into a defrost and check for 24v between w2 and c at the outside board to confirm proper operation or check the secondary heat via ammeter or visual confirmation during the defrost cycle.