Tag Archives: HEAT PUMPS

TROUBLESHOOTING HEAT PUMP SYSTEMS / RECOGNIZING LOW AIRFLOW

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One of the more common problems found in service work is low airflow across the indoor coil. This situation can be due to coil restriction, inadequate/damaged ductwork or dirty filters to name a few. Much has been written in regards to airflow and it’s critical role for proper heat pump operation and performance. The design value for indoor airflow with A/C’s and heat pumps has, to the best of my knowledge, always been 350-450 CFM per ton. Values less than 350 generally create operational problems like coil frosting in the cool cycle or high head pressure in the heat cycle. Values greater than 450, though the lesser of two evils, would produce situations better described as performance problems…poor moisture removal in the cool cycle, or low discharge temperatures in the heat cycle. But, with typical residential systems, excessive airflow is rarely a problem, simply because residential duct systems are rarely oversized. So, the usual situation becomes one of recognizing and correcting low system airflow.

All residential equipment, be it heat pump air handlers, gas furnaces or packaged units, is limited in its capacity to move air and the limiting factor is system external static pressure. External static pressure is for all practical purposes, a measurement of the resistance encountered by the air as it moves through the duct/air distribution system. The standard for maximum ESP is about 0.5 inWC, and residential duct systems have to be designed and sized, to meet this capability of the blower. Otherwise, the ESP may exceed 0.5 and the airflow per ton will be less than 350 CFM…

With no practical understanding of system operation, determining airflow volume would require taking some kind of measurements and doing some calculations that eventually provide a CFM value. Or, if a specific value of airflow is required, so will be the measurements and calculations. But for me, most of the time, the issue comes down to either having enough airflow, or not. And by “enough” I simply mean, the system will run 24 hours without frosting the indoor coil in the cool cycle, or producing excessive head pressures in the heat cycle. Now, if you’re attempting to evaluate overall system performance and efficiency, which depends to a great extent on airflow, “enough” probably isn’t adequate. But keep in mind, service calls are usually situations where the homeowner was content with the system performance yesterday, but not today. So, I always start with the assumption the airflow has at some point in time, been satisfactory, adequate or “good enough”…and most of the systems I find fall into this category. All I need to do is return the system airflow volume back to or near, whatever it was on Day 1, regardless what that actual value is. And that usually amounts to changing a filter, or cleaning the coil.

Airflow calculations aren’t necessary to decide there isn’t enough. If, in the cool cycle, the suction pressure is low and the superheat is low for fixed orifice systems, or normal for TXV systems, the airflow is low. If, in the heat cycle, the head pressure is high, the cause is either overcharge or low airflow, or both…sometimes, techs will overcharge a fixed orifice system in the cool cycle to correct low suction pressure resulting from a dirty indoor coil. But you can ask a few questions to determine if that’s the case. Common sense dictates indoor coils will restrict over time, due to a variety of possible reasons. So when you see symptoms of low airflow, that’s the first thing to suspect. Occasionally you get lucky and find a dirty filter, but more often than not, the coil is the culprit.

More discussion: HvacR Professional.Com

You can see a more in depth explanation of refrigerant system operation with low evaporator air and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” rental video:

Troubleshooting Heat Pump Refrigerant Systems

(The content of this post is intended for consideration by trained service personnel only)

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TROUBLESHOOTING HEAT PUMP SYSTEMS…RESTRICTED LIQUID LINE FILTERS

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Liquid line filters occasionally restrict for whatever reasons, resulting in a significant pressure drop across the filter. The restriction forces more than the normal amount of liquid refrigerant to remain in the condenser coil, which will increase the subcooling value. Head pressure will usually drop a little or remain near normal, while suction pressure will be low and superheat high, due to the evaporator coil being underfed. Of course, where pressure measurements are taken relative to the filter location will determine “head” pressure readings. If the filter is located upstream of the pressure port, the reading would reflect the pressure drop across the restriction and be low. One of the best methods for recognizing a restricted filter is a temperature drop through the filter. The restriction and pressure drop most often creates “flash gas” which generates some refrigeration effect, and causes the filter to cool. A noticeable difference in temperature between the inlet and outlet of the filter is a definite symptom of some restriction. The following clip from the heat pump refrigerant video provides a good visual.

Troubleshooting Heat Pump Refrigerant Systems

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TROUBLESHOOTING HEAT PUMP SYSTEMS…DEMAND DEFROST VS TIME-TEMPERATURE

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The time-temp type controls are, simply put, an electronic timer with a mechanical coil temperature sensor (defrost thermostat, DT, DFT etc). The timer “runs” all the time, or at least during the compressor “on” cycle. If, after the timer reaches the end of its preset period between calls for defrost (30, 60, 90 min etc) and it sees the DFT “closed”, it initiates the defrost. The timer doesn’t know if it’s summer or winter. It’s the DFT’s that decide what the seasons are, and normally close at some temperature around 30F. So, if the DFT fails closed for some reason, although the coil temperature is actually closer to 100F than 30F, the “board” will initiate the defrost cycle…

Demand type controls are a little more sophisticated. They use two thermistor devices to monitor both the coil temperature and the ambient temperature. The “board” then becomes master of the seasons. When the difference between the coil and ambient temps reaches some design value, the board interprets the difference as a need for defrost. It’s important to keep in mind, as the outdoor coil frosting increases, less heat transfer can take place due to decreased airflow, and subsequent reduction in available heat energy. So, the boiling or evaporation inside the coil slows, resulting in a lower suction pressure and lower saturated coil refrigerant temperature. The temperature differential is an indirect measurement of frost build-up. And most, many, maybe all, demand controls include a design outdoor temperature value, above which, the board won’t initiate a defrost…those values I’m familiar with are around 40F. So if the outdoor temp stays above the “upper limit”, excessive frosting that could result from, say a failed condenser fan motor, won’t produce a defrost cycle.

The obvious difference between time-temp and demand controls is the frequency of defrost cycles actually taking place. The time-temp controls can initiate every 30 minutes under the right conditions, with maybe every other defrost being unnecessary. The demand control, on the other hand, will initiate only when the coil actually needs a defrost…which of course, improves the heating efficiency of the heat pump system.

You can get a full explanation and illustrations of all the common electrical failures in the “Troubleshooting Heat Pump Electrical Systems” videos for rent:

Troubleshooting Heat Pump Electrical Systems

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TROUBLESHOOTING HEAT PUMP SYSTEMS…DIAGNOSING TXV’S

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The illustration below is a fairly typical cross section of a thermostatic expansion valve.

..The valves open or close, as a result of the pressures P1, P2 and P3. P1 is the sensing bulb pressure, P2 the evaporator (suction) pressure and P3 the spring pressure. The sensing bulb pressure varies with sensing bulb temperature, which is, more or less, the suction line temperature. If the suction line warms, the sensing bulb warms, the refrigerant inside the bulb warms and expands, so the pressure increases…vice versa for when the suction line cools.

The spring and suction pressures have a closing effect on the valve. The bulb pressure has an opening effect on the valve. When a system is at steady state operation, the refrigerant flow is constant, so P1=P2+P3. When the heat load on the evaporator coil changes, the rate of refrigerant evaporation or boiling inside the coil, will change. This affects both the evaporator (suction) pressure and suction line (sensing bulb) temperature, so P1 isn’t equal to P2+P3. At that point the valve will open or close a little, trying to get the pressure formula back to an equality…

Whether or not we can fully comprehend all that is pretty much academic, relative to diagnostics. You just need to remember the purpose of expansion valves in the first place: to control or maintain a relatively constant system superheat…

So, if you’re trying to troubleshoot a valve, all you really have to do is decide whether or not it’s doing what it’s supposed to do…to do that, you need to measure the system vital signs: low pressure, high pressure, superheat and subcooling. With those four values, you can diagnose a problem with the TXV or something else.

But first of all, if you measure the superheat and get, say 10-15 degrees, regardless the other numbers, the TXV isn’t faulty, because it’s doing what it was designed to do…

If you measure 30 degrees superheat or 2 degrees superheat, there’s a problem somewhere that could be due to a faulty valve. You have to analyze the other system vital signs before deciding the valve is suspect.

You can see a more in depth explanation of TXV operation and illustrated failures in the “Troubleshooting Heat Pump Refrigerant Systems” and “Troubleshooting TXV’s” rental videos:

Troubleshooting Heat Pump Refrigerant Systems

Troubleshooting TXV’s

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TROUBLESHOOTING HEAT PUMP SYSTEMS…REVERSING VALVES

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The reversing valve is the component of a heat pump system that determines whether the system runs in heat or cool. They are in fact, an assembly of two valves: the main valve which actually directs the refrigerant flow in the system, and a pilot valve which controls the main valve. The pilot valve applies system pressures to the ends of the main valve, suction pressure to one end, discharge pressure to the other, creating a pressure differential which will force the main valve slide piece to shift in one direction or the other. This design allows the heat pump system pressures to actually switch the reversing valve position. A solenoid capable of switching the main valve directly would, no doubt, be very large. I’m oversimplifying the design and operation a little, but my intent is simply to summarize the function of the valve, because if it fails mechanically, your only option is replacement…an intimate knowledge of the inner workings turns out to be academic.

Valve failures will generally be 1) solenoid coil failure, 2) “stuck in heat or cool” position or 3) stuck somewhere between heat and cool positions. Coil failure is usually fixable. You only need to verify the absence or presence of coil voltage in the appropriate cycle, to eliminate wiring problems. Coils can short out or go open and in most cases, a new coil can be substituted.

Stuck valves could be the result of a pilot or main valve problem. In either case, I’ve had no luck in making a “repair”…I’ve “unstuck” a few, but the fix was only temporary. They most often stick again.

The last failure situation is the valve stuck somewhere between cycles, which is usually a difficult situation to diagnose. The valve slide position is such that the net result is a significant amount of leakage between low and high side pressures, producing symptoms of a faulty compressor…high suction and low head pressures.

There are several methods used by service people to confirm or eliminate the valve, involving tubing temperature measurements. More involved procedures amount to isolating the compressor, then eliminating it as the problem.

You can see a more in depth explanation of reversing valve operation and troubleshooting techniques in the “Diagnosing Reversing Valves” rental videos:

Diagnosing Reversing Valves

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