Charging an air source heat pump during cold weather has always been a bit of a problem. The problem is that the amount of refrigerant circulated decreases as the outdoor temperature drops. Why is this? Well, as the outdoor temperature drops, the evaporator temperature has to drop in order to be able to absorb heat from the outdoor air. The lower evaporator temperature produces a lower evaporator pressure. The lower evaporator pressure increases the compression ratio because there is now a greater difference between the suction pressure and the discharge pressure. The higher compression ratio means that the compressor does not circulate as much refrigerant.
At a 45°F outdoor temperature, a typical air source heat pump produces a heating capacity roughly equal to its nominal cooling capacity. At 17°F outdoor ambient, it produces about half as much heat as it does at 45°F. This difference in capacity is directly related to the amount of refrigerant being circulated. The rest of the refrigerant is just sitting somewhere – normally in either the accumulator or the charge compensator. So a system operating at 17°F outside could have perfect pressures even if it only had half of its factory charge. That is why you can be way off checking a heat pump by pressures in the heating mode.
Some manufacturers provide heating performance pressure charts, but refer to them as “check” charts. They are intended to check the system operation at specific conditions, but are NOT intended as guides for adding refrigerant. The problem is that you don’t have a good way to judge how much refrigerant is stored out somewhere in the system. I can hear a bunch of you saying that measuring superheat and/or subcooling solves that problem. While I AM a fan of checking both, they still just measure the refrigerant that is circulating.
There have been some interesting methods used, such as measuring discharge superheat. For discharge superheat, you measure the temperature and pressure of the discharge line right as it leaves the compressor. It should be somewhere around 60°F warmer than the discharge saturation temperature. So if you have a 410A system running at a discharge pressure of 318 psig (saturation temperature 100°F), the discharge line should measure 160°F. A lower temperature reading indicates an overcharge and a higher temperature reading indicates an undercharge. The surest way to charge a heat pump in the winter is to recover the refrigerant, evacuate the system, and weigh in the correct charge. If you have performed a repair on the refrigerant system, then this will save you time and insure a correct charge.
Showing posts with label Heat Pumps. Show all posts
Showing posts with label Heat Pumps. Show all posts
Monday, December 12, 2016
Sunday, February 14, 2016
Mini-Split, Multi-Split, and VRF Systems
Unless you have been living under a rock, you are sure to have noticed the rise of mini-split, multi-split, and variable refrigerant flow systems. This segment of the HVAC market has spawned a whole new vocabulary. The discussions about this equipment and even the technical literature are replete with this new jargon. So I thought I would spend a few moments to discuss some of the more common terms.
Unitary
This is not a new term, but you may often hear traditional systems referred to as unitary. Really, this just means that the system is a manufactured unit, instead of a field built system. However, it is often used to designate traditional HVAC systems: split systems as well as packaged units.
Mini-Split Systems
Mini-split systems are indeed split systems, with an outdoor condensing unit and an indoor blower-coil, which is often referred to as a head. Some common heads are high wall mount, ceiling mount, floor mount, and compact cassette – which tucks away above the finished ceiling. As the name mini-split implies, both the outdoor and indoor units are much smaller than found in traditional unitary systems. For the most part, the indoor units are designed to be mounted in the space they are conditioning, with no ductwork. For this reason, many people also refer to mini-split systems as ductless systems. However, mini-split systems are actually available with both low static and high static ducted indoor heads which are designed to be installed in a concealed area. Static refers to the static air pressure difference the blower is designed operate against while moving the air. Low static units are designed for very short, single runs – not complete duct systems. High static indoor units are designed to be used with complete duct systems. They are used more in commercial applications than residential applications.
Multi-Split Systems
Multi-split units are designed to be used with multiple indoor heads. The idea is to install one outdoor unit to supply from two to four indoor units. On typical multi-split units, the refrigerant line sets for all heads run back to the outdoor unit. The outdoor unit has connections for up to four indoor units. Another type of system which is often referred to as multi-split uses a small refrigerant network for up to nine heads. This type of system has one line set with branches that wye off at each indoor unit. These are really more like a small variable refrigerant flow (VRF) system.
Variable Refrigerant Flow
The term variable refrigerant flow (VRF) has two uses within the industry. The more global definition simply refers to any system which varies the flow or refrigerant to match the load. This is usually accomplished by changing the compressor speed. Truthfully, most mini-split and multi-split systems fit the global definition of variable refrigerant flow. There are now several unitary systems which incorporate variable refrigerant flow. Most of these systems use variable speed compressors to match the compressor capacity and refrigerant flow to the load.
However, variable refrigerant flow (VRF) also has a more specific meaning which refers to a specific type of system. These VRF systems distribute refrigerant via a network of refrigerant piping that supplies multiple indoor heads. Heating and cooling is distributed using refrigerant lines instead of ductwork.
One of the difficulties in studying this segment of the industry is the lack of uniform terms. For example, Daikin refers to their VRF system as VRV for variable refrigerant volume. Each manufacturer tends to have their own name for the different network components. The components used to branch off from the main refrigerant line to feed an individual head have several names, depending on the manufacturer. They are called refnet joints (Daikin), branch joints (Mitsubishi), separation tubes (Fujitsu), or Y-branch fittings (LG). Most manufacturers have boxes that take a single refrigerant flow and divide it up among several heads. Common names include branch selectors (Daikin), branch controllers (Toshiba), branch boxes (Mitsubishi), flow selectors (Toshiba), or heat recovery units (LG).
Heat Recovery Systems
Again, the term heat recovery has several meanings within the field. The global definition simply refers to any system which finds a productive use for the condenser heat, such as heating hot water, or dehumidification reheat. In the VRF world, heat recovery systems refer specifically to a VRF system which can heat and cool simultaneously. This is accomplished by moving heat from rooms which need to be cooled to rooms which need to be heated. This way the heat is recovered and reused, rather than being discarded.
Hopefully this discussion will help you make sense of the mini-multi jargon. For more details, refer to Unit 46 Mini-Split, Multi-Split, and VRF Systems in Fundamentals of HVACR, 3rd edition.
Unitary
This is not a new term, but you may often hear traditional systems referred to as unitary. Really, this just means that the system is a manufactured unit, instead of a field built system. However, it is often used to designate traditional HVAC systems: split systems as well as packaged units.
Mini-Split Systems
Mini-split systems are indeed split systems, with an outdoor condensing unit and an indoor blower-coil, which is often referred to as a head. Some common heads are high wall mount, ceiling mount, floor mount, and compact cassette – which tucks away above the finished ceiling. As the name mini-split implies, both the outdoor and indoor units are much smaller than found in traditional unitary systems. For the most part, the indoor units are designed to be mounted in the space they are conditioning, with no ductwork. For this reason, many people also refer to mini-split systems as ductless systems. However, mini-split systems are actually available with both low static and high static ducted indoor heads which are designed to be installed in a concealed area. Static refers to the static air pressure difference the blower is designed operate against while moving the air. Low static units are designed for very short, single runs – not complete duct systems. High static indoor units are designed to be used with complete duct systems. They are used more in commercial applications than residential applications.
Multi-Split Systems
Multi-split units are designed to be used with multiple indoor heads. The idea is to install one outdoor unit to supply from two to four indoor units. On typical multi-split units, the refrigerant line sets for all heads run back to the outdoor unit. The outdoor unit has connections for up to four indoor units. Another type of system which is often referred to as multi-split uses a small refrigerant network for up to nine heads. This type of system has one line set with branches that wye off at each indoor unit. These are really more like a small variable refrigerant flow (VRF) system.
Variable Refrigerant Flow
The term variable refrigerant flow (VRF) has two uses within the industry. The more global definition simply refers to any system which varies the flow or refrigerant to match the load. This is usually accomplished by changing the compressor speed. Truthfully, most mini-split and multi-split systems fit the global definition of variable refrigerant flow. There are now several unitary systems which incorporate variable refrigerant flow. Most of these systems use variable speed compressors to match the compressor capacity and refrigerant flow to the load.
However, variable refrigerant flow (VRF) also has a more specific meaning which refers to a specific type of system. These VRF systems distribute refrigerant via a network of refrigerant piping that supplies multiple indoor heads. Heating and cooling is distributed using refrigerant lines instead of ductwork.
One of the difficulties in studying this segment of the industry is the lack of uniform terms. For example, Daikin refers to their VRF system as VRV for variable refrigerant volume. Each manufacturer tends to have their own name for the different network components. The components used to branch off from the main refrigerant line to feed an individual head have several names, depending on the manufacturer. They are called refnet joints (Daikin), branch joints (Mitsubishi), separation tubes (Fujitsu), or Y-branch fittings (LG). Most manufacturers have boxes that take a single refrigerant flow and divide it up among several heads. Common names include branch selectors (Daikin), branch controllers (Toshiba), branch boxes (Mitsubishi), flow selectors (Toshiba), or heat recovery units (LG).
Heat Recovery Systems
Again, the term heat recovery has several meanings within the field. The global definition simply refers to any system which finds a productive use for the condenser heat, such as heating hot water, or dehumidification reheat. In the VRF world, heat recovery systems refer specifically to a VRF system which can heat and cool simultaneously. This is accomplished by moving heat from rooms which need to be cooled to rooms which need to be heated. This way the heat is recovered and reused, rather than being discarded.
Hopefully this discussion will help you make sense of the mini-multi jargon. For more details, refer to Unit 46 Mini-Split, Multi-Split, and VRF Systems in Fundamentals of HVACR, 3rd edition.
Saturday, February 18, 2012
Heat Pump Temperature Rise
One of the best ways to check the refrigerant charge of a heat pump during the heating season is to measure the temperature rise across the indoor coil and compare it to data published for that unit. The two main issues with this charging method are getting the required data and measuring the system airflow. Not all manufacturers publish the temperature rise for their equipment. Another potential problem is system airflow. The amount of temperature rise achieved is directly related to the amount of air moving across the coil. If the ACTUAL airflow is different from the manufacturer's stated conditions, the temperature rise will also vary. Increased airflow will reduce the temperature rise and decreased airflow will increase the temperature rise. For the temperature rise method to work, the system must be operating with the correct airflow.
Heat pumps are rated by AHRI for heat output at two outdoor temperatures, 47°F and 17°F. The indoor return air temperature rating point is 70°F. Most heat pumps produce a heat output of close to their "tonnage rating" at the 47°F AHRI rating point and approximately half of their "tonnage rating" at 17°F. Most units operate correctly with an airflow somewhere in the range of 400 CFM per ton of capacity. Of course these are generalizations.
However, a general idea of temperature rise parameters can prove useful. Using these generalized assumptions, let's develop some generalized temperature rise performance parameters. The basic formula to use is BTU = 1.08 x CFM x TEMP RISE. This can be rewritten as TEMP RISE = BTU / (1.08 x CFM). At 47°F the capacity per ton will be 12,000 BTU. Plugging this into the formula we get TEMP RISE = 12,000 / (1.08 x 400). This yields a projected temperature rise of 28°F at a 47°F ambient. On the other end of things, the assumed capacity per ton at 17°F ambient temperature is 6000 BTU. Plugging this in we get TEMP RISE = 6000 / (1.08 x 400). This yields a projected temperature rise of 14°F. You can readily see that the result of a 50% reduction in capacity yields a 50% reduction in temperature rise. Now let's test our mathematical extrapolations against some REAL data taken from a manufacturer’s performance specifications.
Heat pumps are rated by AHRI for heat output at two outdoor temperatures, 47°F and 17°F. The indoor return air temperature rating point is 70°F. Most heat pumps produce a heat output of close to their "tonnage rating" at the 47°F AHRI rating point and approximately half of their "tonnage rating" at 17°F. Most units operate correctly with an airflow somewhere in the range of 400 CFM per ton of capacity. Of course these are generalizations.
However, a general idea of temperature rise parameters can prove useful. Using these generalized assumptions, let's develop some generalized temperature rise performance parameters. The basic formula to use is BTU = 1.08 x CFM x TEMP RISE. This can be rewritten as TEMP RISE = BTU / (1.08 x CFM). At 47°F the capacity per ton will be 12,000 BTU. Plugging this into the formula we get TEMP RISE = 12,000 / (1.08 x 400). This yields a projected temperature rise of 28°F at a 47°F ambient. On the other end of things, the assumed capacity per ton at 17°F ambient temperature is 6000 BTU. Plugging this in we get TEMP RISE = 6000 / (1.08 x 400). This yields a projected temperature rise of 14°F. You can readily see that the result of a 50% reduction in capacity yields a 50% reduction in temperature rise. Now let's test our mathematical extrapolations against some REAL data taken from a manufacturer’s performance specifications.
Model
|
ΔT @ 47°F
|
ΔT @ 17°F
|
BTUh @ 47°F
|
BTUh @ 17°F
|
CFM
|
ERHQ18
|
26°F
|
13°F
|
18,500
|
11,000
|
700
|
ERHQ24
|
25°F
|
12°F
|
24,400
|
12,500
|
800
|
ERHQ30
|
27°F
|
16°F
|
32,200
|
18,800
|
1055
|
ERHQ36
|
26°F
|
13°F
|
36,000
|
19,500
|
1310
|
The manufacturer states that a properly operating unit should be + or – 3°F of these typical values. You will note that our extrapolated values are all within the 3°F margin of error.
But what if the temperature is neither 47°F nor 17°F, what temperature rise do you look for then? Another assumption is in order here; we will assume that the capacity reduction due to ambient temperature drop is even. Since the temperature rise changed from 28°F to 14°F over a 30°F ambient change, this would represent approximately a 1°F change in temperature rise for every 2°F change in ambient temperature. In the above table, we see a temperature rise change of 13°F in 3 out of the 4 units listed. This gives similar results. In our assumed system, 28°F rise at 47°F ambient and a 14°F rise at 17°F ambient, a temperature rise of 23°F would be expected at 37°F ambient based on the relationship of 1°F rise per 2°F ambient temperature. Clearly, manufacturer's data for a specific unit is more accurate than extrapolated benchmarks; however, extrapolated temperature rise calculations are considerably more accurate than blocking the outdoor coil and pretending it's summer. Remember: the best method for charging any unit is the method recommended by the people who made it!
But what if the temperature is neither 47°F nor 17°F, what temperature rise do you look for then? Another assumption is in order here; we will assume that the capacity reduction due to ambient temperature drop is even. Since the temperature rise changed from 28°F to 14°F over a 30°F ambient change, this would represent approximately a 1°F change in temperature rise for every 2°F change in ambient temperature. In the above table, we see a temperature rise change of 13°F in 3 out of the 4 units listed. This gives similar results. In our assumed system, 28°F rise at 47°F ambient and a 14°F rise at 17°F ambient, a temperature rise of 23°F would be expected at 37°F ambient based on the relationship of 1°F rise per 2°F ambient temperature. Clearly, manufacturer's data for a specific unit is more accurate than extrapolated benchmarks; however, extrapolated temperature rise calculations are considerably more accurate than blocking the outdoor coil and pretending it's summer. Remember: the best method for charging any unit is the method recommended by the people who made it!
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