Heat pump schematic (C) Carson Dunlop AssociatesHeat Pump Inspection, Diagnosis, Inspection, Repair Guide
     

  • HEAT PUMPS - CONTENTS: Heat pump system inspection, diagnosis, repair articles for home owners and building inspectors. How do heat pumps work?How the Heat Pump Works When in Cooling Mode = Summer Mode. How a Heat Pump Operates to Extract Heat from Outdoor Air When in Heating Mode = Winter Mode. Troubleshooting backup heat problems on heat pump systems that provide both air conditioning and heating. How to determine heat pump capacity. What is the COP or Coefficient of Performance of Heat Pumps? What is the COP balance point for heat pump designs? Heat pump system components and parts and a comparison of air, water, and ground source heat pump designs. How does a Triple Split System Heat Pump System operate?How does a Bi-Valent Heat Pump System Work?
  • POST a QUESTION or READ FAQs about how to diagnose & repair residential heat pump systems
  • REFERENCES

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Heat pump system inspection, basics of operation, troubleshooting & repair guide. This article series answers most questions about heat pump system troubleshooting, inspection, diagnosis and repair. We explain how heat pumps work to provide cooling in hot weather and heating in cool or cold weather.

We explain the concept of coefficient of performance or COP, and how the COP balance point determines how much use can be made of a heat pump in different climates.

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Heat Pump System Inspection, Diagnosis, Repair - List of Articles

Heat Pump principles Carson Dunlop Associates

This article describes how to inspect residential heat pump systems (combination heating and cooling systems) to inform home buyers, owners, and home inspectors of common heat pump system defects.

The articles at this website describe the basic components of a heat pump system, how heat pumps are inspected, diagnosed, and repaired, and we discuss how to estimate the rated heating and cooling capacity of a heat pump system by examining various data tags and components

. The limitations of visual inspection of HVAC systems are described.

[Click to enlarge any image]

Heat Pump Operating Principles

The schematic of a heat pump system shown above and the sketch at left are compliments of Carson Dunlop Associates.

A "heat pump" is an air conditioning system that can work in either of two directions.

During the cooling season the heat pump moves heat from inside the building to outdoors by removing heat from indoor air - by blowing indoor air across the evaporating or cooling coil.

During the heating season the same equipment reverses the direction of its operation, scavenging heat from outdoor air and moving it into the building - by blowing indoor air across a warmed condensing coil, and by blowing outdoor air across the outdoor coil.

Controls in the heat pump reverse the operation and flow of the refrigerant in order to change the direction of heat movement between the indoor and outdoor areas.

Since most components of a heat pump system are identical with those of central air conditioning systems, readers should also be sure to review our air conditioning system inspection, diagnosis, and repair articles.

Below we expand the explanation of how a heat pump works in summer - cooling mode and how a heat pump works in winter - heating mode.

How the Heat Pump Works When in Cooling Mode = Summer Mode

Below we illustrate the heat pump operation when the equipment is in heating mode.

Heat Pump principles Carson Dunlop Associates

 

In cooling mode the heat pump works like a conventional air conditioner. The indoor cooling coil expands liquid refrigerant into a gas form, cooling the coil. Indoor air is cooled by being blown across the indoor cooling coil (left side of the Carson Dunlop Associates' image).

Outside the refrigerant gas is compressed to high pressure high temperature gas, then sent through the outdoor condensing coil where outdoor air blown across the condensing coil condenses the refrigerant back to a liquid for its return to the indoor components.

Even though the outdoor air may be hot during the cooling season, it is at a lower temperature than the outdoor condensing coil - so the system works to transfer heat from indoors to outside.

See the next two illustrations for more detail about this process when we switch to heating mode.

How a Heat Pump Operates to Extract Heat from Outdoor Air When in Heating Mode = Winter Mode

Illustrations below provided by Carson Dunlop Associates.

Heat Pump principles (C) Carson Dunlop Associates Heat Pump principles (C) Carson Dunlop Associates

Different or Additional Controls Required for a Heat Pump vs an Air Conditioner: Refrigerant Expansion Devices

All Electric heat pump illustration Carson Dunlop Associates

We have said often that a heat pump is pretty much like an air conditioner, with a few control differences to allow the system to reverse its direction of heat movement between the indoor and outdoor areas.

In an air conditioner the outdoor coil is used only to cool and condense high temperature high pressure refrigerant gas back to a liquid refrigerant, and the indoor coil is used only to expand the liquid refrigerant to a gas (through an expansion device such as a Thermostatic Expansion Valve (TEV) or a capillary tube (Cap Tube).

But because a heat pump has to be able to work in both directions, its indoor and outdoor coil have to be able to exchange roles. To accomplish this the heat pump will need two refrigerant metering and expansion devices, one at each coil. Only one of the expansion devices is operating at a given time - depending on whether the equipment is operating in a heating or in a cooling mode.

Illustration provided by Carson Dunlop Associates.

Reversing Valve and Defrost Cycle Operation Procedure for Heat Pumps

All Electric heat pump illustration (C) Carson Dunlop Associates

In addition to the two expansion devices, the system may make use of a reversing valve that changes the direction of flow of refrigerant gas and liquid in the system.

An additional set of controls for operation the refrigerant reversing valve and the operation of the defrost cycle for a heat pump are illustrated at left. During a defrost cycle

  1. The reversing valve changes direction of the refrigerant flow to send warmed refrigerant "freon" to the outdoor coil in order to melt ice that may have formed there.
  2. During the defrost cycle the outdoor fan stops - we want to stop moving outdoor air across the outdoor coil in order to allow the warm refrigerant to warm the coil.
  3. If the system includes electric heating elements, one stage of the electric BACKUP HEAT for HEAT PUMPS may be activated in the blower compartment to warm air coming off of the blower. If the system does not include electric backup heaters, the indoor air handler blower will stop during the defrost cycle so that we don't' blow wrong-temperature air into the occupied space.

Illustration provided by Carson Dunlop Associates.

What is an "All Electric" Heat Pump System? - Compared with Gas or Oil Backup Heat

All Electric heat pump illustration Carson Dunlop Associates

The operation of the outdoor compressor/condenser unit and the indoor air handler blower assembly unit is normally run by electrical power.

When the back-up heat for a heat pump operated building is also all electric we refer to the installation as an all-electric heat pump installation. All electric heat pump systems are suitable where electrical energy costs are low or where the number of heating degree days in the local climate is modest.

See BACKUP HEAT for HEAT PUMPS for a discussion of how a heat pump electric backup heat system is tested.

For locations subject to very cold winter weather with many heating days during which outdoor temperatures are below the balance point for heat pump operation, building owners typically use a fossil-fuel backup heat such as natural gas, propane, or heating oil, and may heat by an integrated furnace or by a separate heating system.

Illustration provided by Carson Dunlop Associates.

How does a Water Source Heat Pump System Work?

All Electric heat pump illustration Carson Dunlop Associates

The operation of a water source heat pump system depends on accomplishing its temperature change by using water from a well, or more generally from a pair of wells.

Some water source heat pumps use one or a cascade of water storage tanks. In a water storage tank design, well water is pumped into the storage tank and the heat pump continues to pump heat into the water or take heat from the water (depending on summer or winter mode operation) until the temperature of the water no longer permits an efficient energy exchange.

At that point the energy exchange available in the tank of water has been exhausted - the tank is pumped back into a discharge well, or in some older designs, into another destination.

More details about measuring the efficiency or COP and EER of ground water heat pumps are found
at HEAT PUMPS, GROUNDWATER

Illustration provided by Carson Dunlop Associates.

How does a Ground Source Heat Pump System Work?

All Electric heat pump illustration Carson Dunlop Associates

For a simple understanding of all heat pump systems, air, water, or ground-sourced, it helps to understand that we are always moving heat back and forth between the heat pump and the air, water or ground. The differences are in the medium of exchange, not in the basic system operation and controls.

The operation of a ground sourced heat pump system is described by the illustration at left.

Unlike the water based heat pump operation, the ground sourced heat pump system uses a loop of pipes buried in soil to exchange heat with the ground; a separate tank system or discharge well system is not required.

Illustration provided by Carson Dunlop Associates.

 

Sketch of a Triple Split System Heat Pump System

Triple Split System Heat Pump Carson Dunlop Associates

The sketch above illustrates how a triple split system heat pump is organized.

The compressor motor is installed and located as a separate component inside the building while the other system components (outdoor coil and indoor coil) remain in their traditional locations.

Illustration provided by Carson Dunlop Associates.

All Electric heat pump illustration (C) Carson Dunlop AssociatesHow does a Bi-Valent Heat Pump System Work? Three Operating Modes all using the outdoor coil

The operation of a bivalent heat pump is not as confusing or intimidating as the name might sound.

[Click to enlarge any image]

A bi-valent heat pump installation adds this term to handle three different heat pump operating conditions:

  1. Cooling mold - the system works as the heat pumps described earlier in this article
  2. Heating mode 1. In moderately-cool outdoor temperature conditions during which the outdoor temperature does not drop below a preset level, typically 38 degF. the heat pump provides heat to the building without resorting to a backup heat system.
  3. Heating mode 2. In colder outdoor conditions when the outside temperature is below the set point (we're using 38 F.) an auxiliary or backup heating system (such as LP or natural gas burners) operates to warm the outdoor coil that in turn is used to deliver heat to the occupied space.

 

Illustration provided by Carson Dunlop Associates.

What is the COP or Coefficient of Performance of Heat Pumps?

The COP or coefficient of performance describes the ability of a heat pump to extract heat from outdoor air down to some low temperature, typically 25 degF. for modern equipment. The COP determines how effective a heat pump can be at providing heat during cool or cold weather.

Details including a definition of COP, an explanation of the heat pump balance point, and how to calculate heat pump operating cost more accurately by considering degree days are at HEAT PUMP COP - Definition, sources of variation in heat pump operating efficiency & cost

Heat Pump operating cost variables & COP Calculations

Where a heat pump is used to provide part of the building's heat requirements, the efficiency of the air-to-air heat pump will be less at lower temperatures. Spies (1971, 1977) [2] notes that heat pump efficiency when outdoorr air is warm is quite different from at cold temperatures, making its use of electricity more complex.

The coefficient of utilization may be as high as 3.0, falling to 1.0 as outdoor temperature approaches 10 degF. In 1971 when Spies wrote that note for the Small Homes Council, few heat pumps worked at temperatures that low, Also that this was in 1971, newer equipment is capable of efficient heat extraction from colder air. Spies provided a calculation to transfer heat pump efficiency or COP into electrical costs when comparing heating fuel type cost alternatives:

Table of Electricity Cost Divisors for Heat Pump Operating Cost vs Degree Days - Outdoor Temperature

Degree Days for Your Location Electricity Price Divisor Comments
8000 degree-day heating season 1.4  
6000 degree-day heating season 1.7  
4000 degree-day heating season 2.2  

Notes:

Henry Spies, "Fuels & Burners", Small Homes Council - Building Research Council Circular Series #G3.5, 1971. 1977

Example: If you live in a climate in which the average number of degree days in the heating season is 4000, then to compare heat pump operating costs (using electricity) to other fuels and heating methods,

divide your current electricity cost (say 5 cents per kwh) by 2.2.

5 / 2.2 = 2.27 cents / kwh

List of Heat Pump Inspection, Diagnosis, & Repair Articles

Also see AIR CONDITIONING & HEAT PUMP SYSTEMS where we introduce and illustrate the basic components of air conditioners and heat pumps.

Heat Pump Stopped Heating or Cooling - Diagnostic Guide

If your heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

  • A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
  • A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
  • AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
  • BACKUP HEAT: on heat pumps, types of backup heat; problems with backup heat; begin here if your heat pump is not providing enough heat or if your air conditioning system provides heat when it should be providing cooling.
  • COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units including noises and compressor hard-starting repairs
  • A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
  • Dehumidification Problems - Air conditioner cools but does not dehumidify
  • DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
  • Fire dampers, and Heating and Cooling Air Duct Controls such as manual and automatic duct dampers, zone dampers, and fire dampers are discussed and distinguished at DRAFT REGULATORS - barometric damper
  • LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system - the articles in this series also assist in diagnosis of lost heating capacity for heat pump systems.
  • OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
  • ZONE DAMPER CONTROLS . discusses manual and automatic air duct zone controls

 

Continue reading at DEFECTS LIST - HEAT PUMP or select a topic from the More Reading links shown below.

Or see BACKUP HEAT for HEAT PUMPS

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HEAT PUMPS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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