The Hot water- Chilled water Air handler

I wrote a handbook for my company when I was a commercial service contractor for about 20 years. Here is an excerpt from that handbook that will probably always be useful because it is about some basic engineering concept. Additions and corrections are welcome.

INTRODUCTION

In buildings supplied from central plants, one service responsibility is the air handler that is used to condition the space, the control system and the piping, usually to the main hot water and chilled water supply and return piping.

The air handler is a box with a fan and motor, supported on spring mounts, helping to isolate the vibration to within the machine, and not transmit it to the building structure.  The components in the box may include an economizer, filters, hot water, steam and/or chilled water coils, and a condensate pan and drain system.  Please Press "Read More"

Controls

There could be several semi independent control systems for it or a single controller integrated into a building management system.

The Economizer

The economizer is an outside air duct and damper intersecting with the return air duct and damper and its controls and actuators.  It is used to cool the conditioned space with outside air when cool outside air is available. 

The hot and chilled water coils come after the economizer and filters.

The Chilled Water Coil

The chilled water coil is thick, commonly six passes.  A four-pass coil may be incorrect.  If you see one, and there is a need for additional capacity, check with the plans to see what was specified. Four-pass coils in place of a specified six-pass coil will have a low ΔTw and a low ΔTa. See that the supply water enters the air leaving side of the coil. This is an important capacity issue, the coldest water should interact with the coldest air.

The Hot Water or Steam Coil

The hot water or steam coil is often first in the air stream, after the economizer and filters. If it is not, ask the question about that and get clarification.  If it is installed wrong, and that is something that happens, it may become a freeze hazard.  The hot coil is usually thin, typically one or two passes.  This is because of the greater ΔT between the hot water and the air, than the chilled water and the air.  Additionally, the internal heat gain in the buildings, often primarily from the lighting is subtracted from the heating load, while it is added to the cooling load.  It is first because it freezes more slowly and protects the chilled water coil.

The Condensate System

The condensate system catches the water that condenses on an active chilled water coil.  It then carries it away from the unit and disposes of it properly.  The system consists of a pan that the chilled water coil sits in.  It has a drain connection on one side.  When the unit is level, the drain side of the pan is the low side.  There is a trap in the drain line.  The trap prevents the negative pressure of the fan from pulling air through the condensate line.  When the unit runs without a trap the fan sucks air through the condensate drain.  This prevents the water from leaving the unit.  This causes leaks from the unit.  The depth of the trap must be greater than the negative pressure of the fan in inches of water column.  A vent is sometimes installed after the trap.  If a long shallow drain line is a problem, a condensate pump could be installed to eliminate condensate-draining problems.

The Fan Drive and Blower

After the coils and condensate system is the supply air fan.  The fan usually has a belt drive and an adjustable sheave on the motor side.  The sheave is adjusted to control fan speed and therefore airflow and motor current.  Airflow is adjusted to maintain the proper air and water temperature splits.

The fan must run in the proper direction.  The type of supply air blower used in many commercial air conditioning applications is called a forward curved blower.  The forward-cupped veins in the blower wheel must be driven in the direction of the airflow.  A fan running backwards is a reason for low airflow.

This is the type of unit that could easily have serviceable bearings, meaning needed lubrication. 

Life/Safety Adaptations

Some areas require smoke detectors; they may be installed in the supply and/or the return air duct.  The smoke detector is usually wired in series with the freeze-stat to shut off the fan and close the outside air.  Sometimes this system becomes part of a smoke evacuation system.  Always try to know and understand all the parts of any system you work on. In most cases, the life/safety considerations take president over comfort when those objectives come into conflict.

The Air Distribution System

Supply air ducts are not usually a service issue; however two ductwork related issues are:

1. Flex duct sometimes is blown off of the collar.  It must be re-installed with duct straps, screws and duct tape.  Flex duct is not intended for long runs.  Typically runs longer than five to ten feet are not allowed by specifications in the blueprints.

2. Balancing dampers are used to balance the airflow from a diffuser to meet local requirements.  Balancing dampers are either at the diffuser or at the branch duct take-off from the main trunk.  The adjustment lever may be covered with insulation at the takeoff.  If they are not in the diffuser they are probably in the takeoff.

THE HOT WATER- CHILLED WATER AIR HANDLER

Normal problems in hot water-chilled water air handlers include condensate leaks, dirty coils, control malfunctions, water flow problems, and control, piping, and airflow design problems. In the case of a steam coil, the steam trap could be a problem.

Common problems encountered when working on these air handlers include

Controls.  The modulating control system has the temperature controls and the operating controls combined in an integrated system.  There are two basic electric modulating control schemes, one that uses mechanical thermostats with potentiometers and relay logic and the other is microprocessor based.  They both do about the same thing.  Another system is pneumatic; it uses modulated air pressure to operate valves and dampers in response to temperature.

Water flow.  When dealing with water flow problems, even more than some other types of problems, it is important to always do your maintenance first.  Many water flow problems, in equipment that did cool the site at one time, are resolved by proper maintenance.

Airflow.  Airflow problems through the hydronic air handler are the same as the DX air handler.  The filters and coils get dirty.  The fan motor, belt, bearings, etc. can wear or break.  Return air can be restricted.  Additionally, return and outside air dampers can be out of position for a variety of reasons.

TROUBLESHOOTING AIR FLOW-WATER FLOW PROBLEMS

The central plant supplies chilled water.  The required amount of water flow is specified on the blueprints, or can be determined with enough information about the unit, coils, expected inlet and outlet water temperature, and the heat load in the space.  Water flow is measured in gallons per minute.  There must be a device installed in the water line that allows water flow measurements (i.e. Bell and Gossett Circuit Setter) if the actual flow, in gallons per minute, is to be estimated.  This calculation requires special tools and information. 

The relative amount of water flow, airflow, and resistance to heat transfer is determined by some simple rules.  The critical measurements needed to analyze problems with the heat transfer capacity of the chilled water system are the entering and leaving water temperature and the entering and leaving air temperature.

Chilled water air handler temperature limits

                                                            Minimum °F      Maximum °F

Supply water temperature                            43                    48

Water ΔT                                                     8                    12

Air ΔT                                                        15                    20

Mixed air temperature                                 50                    58

The entering water

Entering water should be between 43°F and 47°F.  Entering water warmer than that indicates a capacity problem in the central plant, consult with the central plant. 

Water temperature drop or “ΔTwater”

This is the temperature of the supply water subtracted from the temperature of the return water.  It is a measure of the relative amount of heat absorbed by the water in a Chilled Water Unit.

Generally, we want to see - 8°F to -12°F water temperature drop across the chilled water coil.

Example:  45°F inlet water, with 55°F outlet water temperature means that the ΔTwater is 10°F.

Return air

Entering air temperature should be near to but warmer than the space temperature.  Plenum-return air may be as much as 5°F warmer than the conditioned space temperature.  If the return air temperature is excessive, meaning more than 5°F above the space temperature, look into economizer (excessive warm outside air) problems, or return duct leaking in hot air. 

Supply air temperature drop or “ΔTair”

This is the temperature of the supply air subtracted from the temperature of the return air.  This and dehumidification, filtration and ventilation are the products of the machine.

Generally, we want to see a 15°F - 20°F temperature drop across the chilled water coil

STANDARDIZED CHILLED WATER COIL TESTS

Clean The Coil. (Check for Under-sized Coil)

Case 1:  Chilled supply water temperature is not high   -and-   ΔTair is low 

-And- ΔTwater is low

Cold supply water temperature (less than 48°F) and a low air temperature drop across the coil (less than 15°F) and a low water temperature drop across the coil (less than 8°F) if detected, indicates a dirty chilled water coil.  The coils must be cleaned.  Check for dirty filters.

Under sized coils, such as four-pass coils will have the similar characteristics to a properly sized dirty coil.

Speed-up Supply Fan

Case 1:  Chilled supply water temperature is not high  -and- ΔTwater  -and-   ΔTair is high

Cold supply water temperature (less than 48°) and a high air temperature drop across the coil (more than 20°F) and low water temperature drop across the coil (less than 8°) if detected, indicate the fan is running slow.  Speed up the fan to no more than motor nameplate current (75-90% of nameplate current is often where it should be, 50% nameplate current is usually low).  Look for return air or supply air restrictions.

Low or Restricted Water Flow

Case 1:  Chilled supply water temperature is not high  -and- Water ΔTwater  is high  -and-  ΔTair is low

Cold supply water temperature (less than 48°) and a low air temperature drop across the coil (less than 16°) and high water temperature drop across the coil (more than 16°) if detected, indicate water flow is less than required.

Check the chilled water valve and hand valves to be sure they are open.  Three-way valves are open with the stem DOWN (the opposite of the more common two-way valves).  Look for a wye strainer (on the supply line) to clean.  There usually is a “circuit setter”, a device used in balancing the system to limit flow (on the return line).  Setting the water flow is something done to a properly running system.  If after a quick analysis and it appears that there is a water flow problem, learning the policy around the circuit setter is the next question to ask.  Either you need to be allowed open it or someone will come with the tool to set the flow to the units’ specification for flow.

Check the temperature drop across a three-way valve.  If the water returning to the central plant gets colder through the valve, supply water is bypassing.  Adjust the valve stem button to close off the by-pass, or close a by-pass line hand valve to stop all supply water bypassing. This test is testing the control valve for internal leakage.

If the central plant is not delivering enough chilled water, that will have to be proven, perhaps to a skeptical audience.  Resist the urge to slow down the fan, to lower the airflow to match the restricted water flow.  Check the installed pipe size against the blueprints.

High Supply Water Temperature

Case 1:  Chilled supply water temperature is high.

If the entering water temperature is high, go to the central plant and ask if they are having a problem maintaining supply water temperature.  Note the reason for the problem and the estimated time for repairs.  If they are not having central plant problems, ask an engineer to look at the problems you are having with you.

COMMON PROBLEMS AND POTENTIAL SOLUTIONS

Chilled Water Coil Piping

The entering water must enter the coil at the air leaving side, and leaving water leaves from the air entering side.  The reasoning is that the coldest water must interact with the coldest air, and the warmest water interacts with the warmest air.  Carrier estimates that units with backwards water flow suffer from a 15% loss of capacity.

The piping to hot water and chilled water air handlers are usually all right.  Aside from surveys and first visits, low water flow issues and low capacity are usually the only reason to look into pipe size or design.

No Mixing

No mixing is when the outside air can reach the coils and the freezestat without mixing with return air.  This happens when the outside air enters the mixing box near the coils or when outside air and return air enters at opposite sides of a mixing box.  The cold outside air should meet the warmer return air at right angles at least 3 diameters of the duct away from the coils.

If the central plant is not supplying chilled water because the system is in “winter mode” and a freezestat trip occurs, and no mixing is the cause, one tactic I have used successfully is to drain the water from all coils and jump the freezestat.  This will give dependable outside air cooling and it will remove the possibility of freezing the coil and give good dependable cooling until the problem is repaired. The alternative is to disable the outside air and leave the coils filled.  This will supply heating and mechanical cooling, when available.

The air should be filtered after it is mixed.  The hot water coil comes next in the air stream followed by the freezestat, the chilled water coil, the fan, then and the supply duct.  If any component exists in the system outside of this order it may be a problem

Stabilizing a Job

When stabilizing a job, the usual strategy to deal with economizer problems short term is to drain the coils and jump the freezestat.

Some buildings rules will not allow the coils to be drained, if this is a problem the choices are fix the problem now, leave the unit off until the problem is fixed, or get a waiver of the mall policy from the customer.  If a freezestat tripped and you didn’t repair the defect and you put the unit back into service with water in the coils the best that can happen is the freezestat will trip again and you will have a call back.  If we are unlucky and the coil does freeze and burst it can cause lots of damage, a major repair job and maybe a lawsuit.  Always be sure to leave all units without an unreasonable risk of freezing. 

(The following is something I normally would have edited out of the public version of the story, but here is what I would tell my employees that work in cold climates about this situation – “Frozen coils are the most common, high consequence failure we have.  We cannot do enough to avoid them.  Always call your field supervisor to discuss these types of situations before acting or leaving the job.”)

Repairs after a Frozen Coil

Repairing a hot water/chilled water air handler after freezing has two goals.

1.         Stop the water leak

2.         Solve the problem that led to the frozen coil.

Stopping the water leak always involves making a decision about whether or not to replace the coil.  Everyone wants to repair the leak without replacing the coil.  It’s easy to make a mistake where you spend a lot of repair time (money) and still end up having to replace the coil anyway.  You also increase the unit’s downtime because the new coil isn’t ordered until it becomes obvious it’s needed.

At my contracting company, we generally try to repair leaks on return bends.  We replace coils with leaks in the body of the coil. There are times when this overly simple rule is wrong; when it’s really wrong, repair the coil.  This rule is not usually wrong.

Repairing leaks on return bends is simply finding the leak, pushing it back into shape, clean it with sand paper, and brazing it with 15% solder.  This will work on most leaks you can reach with your torch.  If you can’t braze it for some reason, we have used epoxy to repair water leaks.  It usually sets-up in a short time.  Sometimes it must set-up over night.

After You Repair or Replace the Coil

Whether you replace or repair the coil, the cause of the freeze must be found and repaired.  Common causes of frozen coils are no mixing, disconnected and propped open outside air dampers, miss-set minimum position rheostats, and especially, broken or poorly designed control systems.

(Again, internal directions to my employees – “You may not fill a repaired or replaced coil until the control problems are solved or the outside air damper is locked closed. Your field supervisor must approve all aspects of a frozen coil job.”)

MAINTENANCE OF THE HOT WATER-CHILLED WATER AIR HANDLER

Cleaning the Coil

Clean coils are essential. I have been successful using an electric power washer and cleaning from the air entering side of the heat coil and the air leaving side of the chilled water coil.  Sometimes removing the top of the unit allows cleaning between the coils.  In some extreme cases, because the face of the chilled water coil was plugged and there was no other way, we have removed the unit, disassembled it on the ground, cleaned the coils outside the unit, and put everything back together.  If the fan motor is external, you may be helped in cleaning the coil, by washing the coil with the fan running.

A good rule would be – “Never spray water on a coil with an internal fan motor running.”

You must do what it takes to clean the coils well enough to clear the fault, or all the effort you spend will be for nothing.

The standard tests for chilled water air handlers to apply when checking the effectiveness of coil cleaning effort. 

Measure the key temperatures before you start cleaning so you can gage the effect the cleaning has had.  If the occupied space cools down, and the standard tests no longer indicate a dirty coil, the coil has been effectively cleaned.  A properly running machine will have a comfortable zone with about a 55° supply air temperature, and an 8°-12°F ΔT on the water.

Condensate Leaks

Condensate leaks are a common problem in this class of machine.  Leaking may be caused by one of several different problems.  Dirt is a common cause of leaks.  Dirt can build up in the condensate pan, trap, or drain line.  The pan and trap must be cleaned thoroughly at coil cleanings, water leak service calls, and whenever you see dirt or slime building up.  Dirt on the coils reduces the airflow area.  This causes the air to flow through the coil faster.  The velocity can increase to the point where condensate is being thrown off the coil by the air velocity, filling the fan section.  Cleaning the units with a power washer will remove most causes of dirt related condensate leaks.

Long condensate lines are common on these air handlers.  If the condensate drain line is long and has a shallow pitch, a condensate pump may be required.  Pipe the pump’s output into the existing drain line.  Be sure to power the pump from a circuit that remains on 24 hours a day.

Chilled water lines, including the strainer and all valves should be insulated.  The condensate that forms on these parts may be the cause of chronic nuisance water leak problems.

Motors, Belts and Bearings

Maintenance of the motors, belts and bearings is the same here as in other air handlers.  Check belts and replace them as needed.  Overly tight belts will destroy bearings.  Listen for bearing noise, check for bearing play if you hear or feel something that makes you suspicious of the bearings.  Grease and oil non-sealed bearings as needed. Grease bearings sparingly; no more than once per year at the cooling start-up maintenance inspection.  Generally, installing grease fittings on bearings that don’t already have them is not encouraged.

Check the alignment of sheaves and repair as needed.  The surfaces of the sheaves where the belt rides are shaped like a “V”.  If the faces are shaped more like a “U” the sheave must be replaced as warn.  Install the guard over the belts whenever they are off, before you leave the job.

CONCLUSION

The hot water/ chilled water air handler is very similar to, yet very different from the DX air handler.  The controls are less well understood by the average mechanic.  The heat transfer rules are easy.  If the entering water is cold, and the leaving water is too warm, you need more water flow.  If the leaving water is cold, either speed up the fan or clean the coils.

The main risk of operating this kind of equipment is the potential for huge amounts of water coming through the ceiling in the middle of the night because a frozen and busted water-filled coil.  If there is a danger of that happening, lock off the economizer or drain the coils.  You may need heat.  If so, the only choice seems to be to lock off the economizer so that the heat coil may be filled.

The central plant is responsible to deliver the proper amount of chilled water. If they can’t or won’t the best course of action for a tech is to make documentation.  Get the evidence that will be needed when this problem reaches a point where it must be solved.  Be sure to document a clean and well maintained unit, the names of the people met with, the times and dates of the meetings and what was said.