HVAC Troubleshooting

A BSRIA Guide www.bsria.co.uk

HVAC Troubleshooting

BG 25/2014

By Salim Deramchi and Ralph Bell

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1 Introduction 1

2 Refrigeration 3

2.1 Compressors 3 2.2 Condensers 12 2.3 Condenser and evaporator fans 14 2.4 evaporators 15 2.5 Split systems 16 2.6 Air-to-air heat pumps 22

3 Ventilation 28

3.1 Air handling systems 28 3.2 Air handling unit fans 32 3.3 Heating and cooling coils 34 3.4 Thermal wheels 36 3.5 Air-to-air plate heat-exchangers 37 3.6 Run-around coils 37 3.7 electrode boiler humidifiers 38 3.8 VAV systems 40

4 Heating 46

4.1 Boilers 46 4.2 Gas burners 48 4.3 oil burners 50

5 Common items 54

5.1 Pumps 54 5.2 Motors 55 5.3 Variable speed drives 57 5.4 Pressurisation units 58

Appendices Appendix: A Refrigeration system troubleshooting flow charts 59 Appendix: B Troubleshooting with pressure gauges 68

Contents


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The intention of this guide is to impart a methodology and, hopefully, a basic logical sequence in identifying and solving problems on fundamental HVAC systems found in most commercial and some domestic premises.

The tasks outlined in this guide are based on the technician having a sound knowledge of the investigation/check procedures to be carried out. Control systems may need to be shut down during some of these, either as an individual loop or as a whole. This should not cause problems, providing it is carried out by someone with a good working knowledge of the system. All control systems should allow plant to be operated by hand, although it may be involved and require more staff.

This guide has been designed so that it can be used by engineers and managers with different levels of HVAC knowledge. The troubleshooting procedures have not been presented in a hierarchical fashion, but instead in order of the affected system or component.

How to use this guideFirst identify the system and component which is exhibiting symptoms – these are listed in the contents page of this guide.

Each section is colour-coded as follows:

Refrigeration

Ventilation

Heating

Common items

Second, identify specific symptoms that are occurring. These are third-level headings e.g. 2.1.5 Compressor short-cycles. For each symptom, a number of possible causes are given, and an action is suggested for each of these.

Two appendices are provided, both relating to the troubleshooting of refrigeration systems. The first provides a number of useful flowcharts, which are cross-referenced to the guide. The second provides guidance on using pressure gauges as a troubleshooting tool.

Each of the actions in this guide has been assigned a grade of one, two or three stars. These outline the level of ability required to undertake the action:

1 introduction

Grading Level of ability

* basic check – can be carried out by anyone with knowledge of HVAC systems.

* * system check – can be carried out by a competent member of staff or qualified maintenance engineer.

* * * invasive / intrusive investigation – must be carried out by a legislatively qualified technician with proven competence and ability to work on the system.


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2 refrigeration

Possible cause Action

Capacity control pressure switch set incorrectly or faulty

Check switch and reset or replace * * *

Timing relay set incorrectly or faulty Check relay and reset or replace * * *Capacity control system set incorrectly or faulty

Check individual components and wiring * * *


Control circuit is open Check if there is call for cooling * Check if a protective switch has tripped * * Check if anti-recycle timer is timing out. Wait for timer to complete cycle

* * *Check power supply to control circuit, fuses and circuit breakers * *Check if emergency stop switch engaged *Check oil failure control * * *Check if overload protector is tripped * *Check if low pressure switch is open * * *Check if high-pressure switch is open * * *

loose wiring repair wiring * * *Faulty compressor motor see motors in section 5

Faulty thermistor Check thermistors for proper functioning * * *low line voltage Check voltage * *Faulty motor relay Check electrical connections * *Motor overload Check power at the motor (causes of motor overload include

high condensing pressure or partial seizure of compressor) * * *

Faulty starter Check starter and repair or replace * *Motor shorted or seized Check motor and repair or replace as required * * *

Check for mechanical failure of compressor. repair or replace as required

* * *

Cont ....

2.1 Compressors

2.1.1 Poor compressor capacity control

2.1.2 Compressor will not start


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3 Ventilation


electric power failure Check voltage * *Control panel circuit fault Check if circuit breakers have tripped * *Isolating switches off Check for time switch error * *Duct blockage Check for blockages to coils, duct or filters * *

Check if regulating or fire dampers closed or cooling coil frozen * * *Impeller loose on shaft Check bearings and replace as necessary * * *


Wheel loose on shaft Check bearings and replace as necessary * *Belts loose or missing Tighten belt * *

3.1 Air handling systems

3.1.1 No air flow – no power at fan motor

3.1.2 No air flow - motor runs but fan does not run


Filters dirty Replace filter * leakage on pressure side of system Check for duct joints unsealed and access doors loose * * High system resistance Check main branch dampers * * Blockage in air handling system Check for blockages * * Running speed incorrect or belt slipping Check speed / slippage. If belt slack, tighten. If belt worn, replace.

If pulleys worn, replace * *

Check fault in variable speed drive (VSD) and control program * * *

excessive pressure loss in system Check air flow against fan static pressure. This will indicate whether the problem is at the fan or further downstream

* * *

Check damper position

System resistance higher than design. Check for blockages * * *excessive duct leakage Check ductwork integrity and access door fitting * * *Access door open Check and shut if necessary *extract fan speed too low Check for speed and compare to design * * *

3.1.3 Insufficient air flow


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Boiler thermostat is incorrectly set Check thermostat setting * *Poor combustion Check flues, sight flame and assess combustion quality * * *


System controls faulty Check for voltage, temperature settings and safety switches * * *Control thermostat failed and high-limit thermostat activated or both controls may have failed

Check temperatures – if higher than high limit setpoint, replace high limit thermostat

* * *


Incorrect combustion Perform combustion analysis and correct * * * Inadequate air flow for combustion Inspect boiler, chimney and boiler room fresh air intake for source

of blockage* * *

Test holes in flue unsealed or flue leakage Cover holes and check for leakage * * Boiler room fans stopped Check boiler room fans * *

4 Heating4.1 Boilers

4.1.1 Boiler fails to meet system load

4.1.2 Boiler and system overheat

4.1.3 Fumes in boiler house


Boiler internal controls failure Investigate and reset or replace * * *Boiler has previously overheated and the overheat cut-out has not reset

Ascertain cause of overheat. once rectified, reset * * *

Supply fuse blown or supply isolated establish cause of blown fuse and replace * * *no pilot flame Refer to section 4.2.6

Main burner does not light Investigate loss of fuel to burner, rectify and light * * *no ignition spark Refer to section 4.3.5

4.1.4 Boiler does not ignite


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