A common failure with residential equipment condenser and blower motors is not actually the motor, but the run capacitor. Probably the majority of the time, the capacitors fail “open”…the motor can’t develop the necessary torque to actually begin rotation. So, it justs sits in a “stalled” condition, pulling above normal amps…the video gives some thoughts on the subject.
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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 is a critical element of 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 value actually is. And that usually amounts to changing a filter, or cleaning the coil.
You don’t have to calculate airflow to determine 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…quite often, 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.
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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.
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One common mistake beginners make with split system heat pumps is connecting to the service valves during the heat cycle, thinking they will read a high and low system pressure…not so. During the heat cycle, refrigerant flow is reversed between the indoor and outdoor units, so instead of cool, low pressure vapor returning from the the indoor coil, hot high pressure vapor is moving to the indoor coil. Once inside the indoor coil, the vapor condenses and the liquid line is now carrying refrigerant away from the indoor coil at a high pressure. There is likely some pressure drop between the two service valves, but the liquid is still at somewhere near the condensing pressure.
The only tubing in a split system always at low pressure is the section between the reversing valve and compressor. Which is why most heat pump condensers have a third schrader pressure connection tapped into the constant suction pressure tubing. Some of the early heat pump systems didn’t have the third pressure tap, making system analysis difficult to say the least.
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The search terms used in navigating to the blog are still providing subject material for posts. A recent entry was “heat pump high head pressure heat cycle”. Well, I’d guess that same heat pump would have low suction preeeusre and maybe low superheat in the cool cycle.
One of the more common problems found with residential equipment is a dirty / restricted indoor coil. Short return ducting, leaky return ducting, operating without filters, poor return location are some of the things that can lead to an eventual restriction of indoor coils. In the heat cycle, the indoor coil is the condenser. Any restrictions will decrease the airflow through the coil, affecting the heat transfer ability of the coil to expel heat from the hot refrigerant vapor, which results in the head pressure running higher than normal. In many cases, the situation can be compounded if someone has added refrigerant during the cooling cycle to increase suction pressure due to the same condition. Low airflow across the indoor coil during the cooling cycle will oftentimes result in the indoor coil frosting. With fixed orifice systems, adding refrigerant will eventually raise the head pressure, forcing more liquid through the orifice into the indoor coil, raising suction pressure to a level providing a saturated temperature greater than 32F. However, superheat is low or nonexistent, creating a potentially damaging situation for the compressor. Long term effects considered, it’s much better to correct the airflow problem.
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Superheat is the term used to express the increase in temperature of the refrigerant vapor before it exits the evaporator coil. Some amount of superheat is always desirable to ensure no liquid refrigerant reaches the suction inlet of the compressor. Most mechanical refrigeration systems are designed to operate in such a way that all the liquid inside the evaporator coil completely vaporizes before reaching the coil outlet. With the vapor still exposed to the warmer air passing through the coil, it will absorb additional heat and undergo an increase in temperature. The increase above saturated temperature is superheat.
With fixed orifice systems, superheat is affected by the volume of air passing through the coil. If the volume of air is less than design, there is less heat available to evaporate the liquid. So the liquid travels farther through the coil tubing before completely vaporizing, leaving less time for the vapor to gain superheat. So, low airflow through the evaporator coil results in low superheat measurements. Of course the reverse would be true for excess evaporator airflow.
Superheat with TXV systems isn’t generally affected by airflow, since the TXV has the ability to throttle down the rate of liquid feed and maintain the design superheat value.
With air-to-air heat pump systems low airflow is the most common problem, usually due to dirty coils or duct system deficiencies.
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The symptoms of an overcharged system differ with the type of metering device feeding the evaporator coil, fixed or TXV. In either case head pressure and subcooling are higher than normal. With a TXV system, suction pressure and superheat are normal. But a fixed orifice system will have high suction pressure and low superheat. The abnormally high head pressure will force more refrigerant through the orifice. The additional refrigerant in the evaporator coil causes an increase in pressure and, the liquid travels farther along the tubing before completely vaporizing, which results in less superheat.
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TXV sensing bulbs are charged with the same type refrigerant as the system the TXV is used with. This is necessary to keep the P1, P2, P3 stuff balanced. If the bulb charge was different from the system charge, the relationship between suction pressure and bulb pressure would probably create a balance point that would result in an incorrect superheat value.
The refrigerant in the sensing bulb reacts to changes in temperature, by increasing or decreasing in pressure. The pressure is transmitted to the diaphragm whose deflection will change in some proportion to the pressure change. The diaphragm is mechanically connected to the TXV variable orifice, so changes in pressure end up changing flow rate through the valve. Increases in pressure are the result of the bulb warming. If it’s attached to the suction line, the TXV translates the warmer suction line as an increase in superheat, and opens the valve orifice a little, allowing more liquid to feed the evaporator coil, which will eventually lower the superheat. If the suction line and sensing bulb cool down, the TXV see’s a reduction in superheat and closes the orifice a little, reducing liquid flow which eventually raises superheat.
If the bulb loses its charge for whatever reason, the only internal forces remaining are suction and spring pressures, which will close the valve. I see this quite often as a result of the sensing bulb cap tube in contact with some of the coil tubing. Over time, vibration and “rub” can cause the cap tube to wear through, resulting in loss of charge.
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Someone used a search term referring to “normal heat pump suction and head pressures”. Air-to-air heat pump system pressures depend on the type refrigerant, the mode of operation (heat or cool), SEER value and entering air temperature at the indoor and outdoor coils, just to name a few variables. For R-22 systems operating in “cool”, normal suction pressure would be 70+/-, corresponding to a saturated coil temperature of around 40F. Head pressure will vary significantly with SEER and outdoor ambient. On a 90F day, head pressures 225-250 psi would be reasonably normal for 10-12 SEER equipment.
In the heat cycle, the suction pressure will follow the outdoor temperature. In order for heat transfer to take place, the saturated coil temperature has to remain well below the outdoor air temperature. I usually expect to see a 10-20F difference. If the outdoor temperature is 50F, the suction pressure would probably be in the 50-60 psi range. If the outdoor temperature is 25F, the suction pressure could be in the 30 psi range. Head pressures can be expected to run in ranges similar to the cool cycle. Heat pump system analysis in the heat cycle is usually less than absolute. If the unit has a P-T graph comparing system pressures to indoor and outdoor temperatures, that’s you best tool for evaluating the system. There are no superheat or subcooling values provided by mfg’s, to my knowledge.
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On a direct ignition gas furnace service call, you find the furnace goes through the firing cycle as designed until the circulating blower starts and then the flames “blow” out into the area around the in-shot burners. The problem is due to:
a) blocked B-vent
b) failed combustion chamber
c) restricted combustion chamber
d) excess manifold pressure
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