BACK FLUSHING | Procedure & Sequence

December 20, 2022 |

After the system flushing steps have been completed covering the Dynamic Flush, Chemical Flush, Clean Flush, and Passivation/Inhibition, the coil flushing, previously known as ‘back flushing’, should commence.

Coil flushing is conducted to ensure that all terminal units and peripheral equipment containing a hydrostatic coil and was bypassed during the main flushing works to protect them have been cleaned through. This is to remove any debris and materials that may have entered from the manufacturing and installation works, before being introduced and opened up to the system.

The process will also allow the cleaning of any pipework connected to the terminal units/equipment.

To read more on the flushing process and why it is completed, see our article 'FLUSHING & CLEANING | Pipework Systems Overview'

Do we really need to flush coils?

It should always be advised that every coil is flushed, but sometimes we need to nurture and discuss why with the contractors.

The main reason is obviously that there could be debris and byproducts from the manufacturing and installation process.

If an extensive system and only replacing one coil, there is probably no issue with not doing it, to be fair. But this is rarely the case; usually, we install multiple coils and sometimes thousands on a large project.

The considerations that need to be made when contemplating to bypass the process should be:

  • Size of system / how many units installed
  • Are the coils full of water, and have been for some time. Coils should be dry until the final pressure testing is conducted after the flushing process.
  • If the coils are dirty, multiple how many coils and their water volume vs. system size and think about this volume that will be released into the clean system.
  • What happens if it all goes wrong, do you have time to reflush the complete system.

We would always flush the coils, no matter how big or small the system was. This makes it a lot easier in the future, if there are issues, that all steps were followed. Rather than try to explain that not flushing the coils did not affect the system.

To see our full example of a method statement for the Flushing & Chemical Cleaning works, including the back flushing click here
Flushing & Chemical Cleaning Method Statement [MS Word]

Here is our Open Copy of our Flushing and Chemical Cleaning Method Statement in MS Word. With instructions




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What types of equipment is flushed?

The following items would usually be backflushed:

  • Water Cooled Chillers
  • Air Cooled Chillers
  • Plate Heat Exchangers
  • Boilers
  • Air Handiling Units
  • Fan Coil Units
  • Any Remote Water Coils

At what stage is coil flushing completed?

As noted above, once the Chemical Engineer has confirmed that the dynamic, chemical, passivation, and inhibitor has been completed successfully for the system and the system dosed – the Back Flushing would then commence on the equipment.

Are the coils back flushed or forward flushed?

Both methods are used:

  • Forward flush the coil
  • Backflush the return pipework through to the drain cock [see following diagrams], to ensure it has been cleaned.

What are the roles and responsibilities for the coil flushing?

CxA | GENERAL CONTRACTOR | FLUSHING SPECIALIST | CHEMICAL ENGINEER

General/Main ContractorThe general contractor should ensure that the coil flushing works are fully coordinated, the programme is being kept, and everything is documented. They should also oversee the flushing and chemical specialists and that they are performing their tasks properly.
Flushing & Cleaning SpecialistProvide advice, knowledge, expertise, resource, and equipment to ensure that this step is completed correctly and successfully.
Chemical EngineerProvide advice, oversee laboratory tests, and supply the chemicals needed, if required, during the process.
Commissioning ManagerThe Commissioning Manager [CxA] will, during this stage will be responsible for the review, check, and witness of the works to ensure that they are being completed in line with the approved documentation, method statement

Tools and Equipment that will be needed

To ensure that the works are successful, there will be a requirement for tools and instruments to be available and used.

Below is a list that can be used as a base of what would be expected to be used:

RefType of Instrument
1Tarpaulins / protection materials / rags
2Flexible hose
3Bucket to hold water
4Hand pump
5Hand tools

How long to flush a coil?

Depending on the size, we would allow 20 minutes per coil, It can be done quicker but will depend on:

  • Access needed
  • Equipment needed
  • Above ceiling or open ceiling
  • Space between coils for moving materials and equipment
  • Using system water or handpump

How do you flush a coil?

The coil flushing is quite an easy process but can be time-consuming, depending on the number of coils that need to be flushed on a project,

There are two main methods, depending on the water supply provided.

  • [1] Use the system water to provide flow through the coils.
  • [2] Use a hand pump to provide flow through the coils if system water is not avaliable.

[1] Flushing using system water…

The below is based on flushing a fan coil unit; other equipment will use similar steps for when the system water is available:

Step 1.1 – Ensure the pre-requisites have been completed

Refer to the pre-requisite document in our article 'DYNAMIC FLUSHING | Sequence & Procedure'

The main considerations for this element of works are:

RefDetails
1System confirmed by the Chemical Engineer/Facilities Team that the main system flushed and cleaned properly.
2Feedwater is available for the system, to replace the water that will be drained from the coil flushing process.
3When flushing the coils and pipework the flow rate being removed should be equal to the water being filled to minimize any issues with air being brought into the system etc.
4Appropriate chemicals should also be available to dose the raw water filing the system, usually, the system dosing and chemical control systems should be operating.
5Coils are isolated from the main system and bypassed, and set up as per drawing.
6The area is protected from any potential water damage if spillage. This work is usually conducted towards the later stages when carpets and finishes may be fitted.
7Main/flushing pumps are ‘On’ circulating water through the bypass loops. They can be off [as long as have enough head pressure to push the water through the coil].
8Ensure all documentation is approved including permits and method statements.

Step 1.2 – Forward flush coil

RefDetails
1Protect area in line with method statement in case of water spillage & Bypass is closed
2[4]… Attach the temporary hose to the drain cock installed on the return pipework, the drain cock should be installed between the coil and commissioning station / double regulating valve.
3[4]… Run the temporary hose to the bucket where water will be caught.
4[2]… Open supply pipework isolation valve to allow water through coil and to drain cock.
5[4]… Open drain cock isolation valve to drain water through the hose to the bucket, there should be quite a good velocity of water to push any debris out the coil and pipework.
6[-]… Run until satisfied coil and pipework flushed through enough.
7[4]… Close isolation valve on drain cock.
8[2]… Close isolation valve on supply pipework.

Step 1.3 – Back flush pipework

This step will ensure that the pipework from the bypass valve to the drain cock is cleaned and flushed through.

RefDetails
1Protect area in line with method statement in case of water spillage & Bypass is closed
2[4]… Attach the temporary hose to the drain cock installed on the return pipework, the drain cock should be installed between the coil and commissioning station / double regulating valve.
3[4]… Run the temporary hose to the bucket where water will be caught.
4[6]… Open return pipework isolation valve to allow water through pipework to drain cock.
5[4]… Open drain cock isolation valve to drain water through the hose to the bucket, there should be quite a good velocity of water to push any debris out the pipework.
6[-]… Run until satisfied pipework flushed through enough.
7[4]… Close isolation valve on drain cock.
8[2]… Open supply and return isolation valves on pipework to allow water from the system to flow around the coil.
[1]… Ensure the Bypass isolation valve is closed

Step 1.4 – Move to next coil

RefDetails
1Remove the hose and drain surplus water into the bucket.
2Dispose of water.
3Remove protection.
4Replace ceiling tiles etc.
5Set up for next coil.

Step 1.5 – Complete all other coils

Complete all coils in the system that are required to be flushed.

Step 1.7 – Sample system water

Once all coils have been completed and integrated with the system, a site and laboratory sample should be taken in line with the sampling detail in the dynamic flushing.

This is to prove and close out the flushing process before moving to the water quality management up to handover.

Step 1.8 – Ensure all documentation is fully completed, witnessed, and signed

This is important as it will ensure that the activities can be proved and documented in case of issues later.

[2] Flushing using hand pump…

If there is no system water available or cannot be used, we have to come up with a different plan. Usually, that will involve using a hand pump, similar to what is used for pressure testing to provide water flow through the coil and pipework.

The below is based on flushing a fan coil unit; other equipment will use similar steps:

Note that this method may require additional connections on the supply and return pipework for the hand pump to connect to.

Step 2.1 – Ensure the pre-requisites have been completed

Refer to the pre-requisite document in our article 'DYNAMIC FLUSHING | Sequence & Procedure'

The main considerations for this element of works are:

RefDetails
1System confirmed by the Chemical Engineer/Facilities Team that the main system flushed and cleaned properly.
2Feedwater is available for the system, to replace the water that will be drained from the coil flushing process.
3Appropriate chemicals should also be available to dose the raw water filing the system, usually, the system dosing and chemical control systems should be operating.
4Coils are isolated from the main system and bypassed, and set up as per drawing.
5The area is protected from any potential water damage if spillage. This work is usually conducted towards the later stages when carpets and finishes may be fitted.
6Hand pump available.
7Ensure all documentation is approved including permits and method statements.

Step 2.2 – Forward flush coil

RefDetails
1Protect area in line with method statement in case of water spillage & Bypass is closed
2[4]… Attach the temporary hose to the drain cock installed on the return pipework, the drain cock should be installed between the coil and PICV / commissioning station / double regulating valve.
3[4]… Run the temporary hose to the bucket where water will be caught.
4[3]… Connect hand pump to supply pipework at the connection point and fill pump basin with water
5[4]… Open drain cock isolation valve to drain water through the hose to bucket.
6[3]… Pump hand pump to force water through coil and down into the bucket, there should be quite a good velocity of water to push any debris out the coil and pipework.
Pump till satisfied.
7[4]… Close isolation valve on drain cock at return.
8[3]… Disconnect hand pump.

Step 2.3 – Back flush pipework

This step will ensure that the pipework from the bypass valve to the drain cock is cleaned and flushed through.

RefDetails
1Protect area in line with method statement in case of water spillage & Bypass is closed
2[4]… Attach the temporary hose to the drain cock installed on the return pipework, the drain cock should be installed between the coil and PICV / commissioning station / double regulating valve.
3[4]… Run the temporary hose to the bucket where water will be caught.
4[5]… Connect hand pump to return pipework at the connection point and fill pump basin with water
5[4]… Open drain cock isolation valve to drain water through the hose to bucket.
6[3]… Pump hand pump to force water through pipework and down into the bucket, there should be quite a good velocity of water to push any debris out the pipework.
Pump till satisfied.
7[4]… Close isolation valve on drain cock at return.
8[3]… Disconnect hand pump.
9[2], [6]… Open supply and return isolation valves on pipework to allow water from the system to flow around the coil.

Step 2.4 – Move to next coil

RefDetails
1Remove the hose and drain surplus water into the bucket.
2Remove hand pump.
3Dispose of water.
4Remove protection.
5Replace ceiling tiles etc.
6Set up for next coil.

Step 2.5 – Complete all other coils

Complete all coils in the system that are required to be flushed.

Step 2.7 – Sample system water

Once all coils have been completed and integrated with the system, a site and laboratory sample should be taken in line with the sampling detail in the dynamic flushing.

This is to prove and close out the flushing process before moving to the water quality management up to handover.

Step 2.8 – Ensure all documentation is fully completed, witnessed, and signed

This is important as it will ensure that the activities can be proved and documented in case of issues later.

To read our article on the NEXT STEPS that should be taken see 'FLUSHING & CLEANING | Post Monitoring Of Water Quality'

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