Principal, Subordinate and Tertiary return loops – What does it mean? Are these Sentinels?

HSG274 Part 2: The control of legionella bacteria in hot and cold water services.

In 2014 the HSE released update legionella guidance including HSG274 Part 2. Within this document there was updated guidance on temperature monitoring. In this blog I will endevour to explain the changes to temperature monitoring of circulating hot water systems and the impact this has...

Historically temperature monitoring of hot and cold outlets was simply as case of recording monthly sentinel outlets and annual representative outlets. Sentinel points were the nearest and furthest points on a system and representative outlets were the remaining outlets in between. This was open to interpretation and I have seen monitoring regimes where only one near and one far hot temperature has been recorded from very complex hot water systems, which is woefully inadequate. The purpose of any monthly temperature monitoring regime needs to determine if hot and cold outlets can meet their recommended temperatures (<20oC within 2 minutes for cold and 50oC or above within 1 minute for hot) in all areas of the water system. Therefore a comprehensive monitoring regime should include recording monthly hot and cold water temperatures from all extremities of the system, i.e. several ‘far’ points. For example, imagine a hospital with all core services running through a central riser and wards either side on several floors. You would need to record temperatures from the ends of each ward (at the furthest point from the central riser) on each floor to determine that temperatures were compliant at the extremities. That’s common sense!

I believe the guidance was updated to prevent people doing the absolute minimim temperature monitoring, by ensuring more monthly hot temperatures are being recorded, although my personal view is that it was unnecessary. Any decent monitoring regime should have 'already' recorded enough sentinel points and a quality legionella risk assessment should have identified suitable & sufficient sentinel locations.

Here is what the guidance HSG274 Part 2 says regarding temperature monitoring the hot water distribution and my thoughts...

HSG274 Part 2 Page 27, paragraph 2.62 Non-circulating HWS: states “In branched systems, the outlets at the ends of significant spurs should be identified as additional far sentinel points".

MH comment – Therefore recording temperatures from only one near and one far hot sentinel point in large branched hot water systems will be inadequate! This was always the case, but now it makes it clearer.

For circulating hot water systems HSG274 Part 2 now refers to temperature monitoring of principal, subordinate and tertiary return loops. See page 27 & 28 paragraphs 2.63 and 2.64.

Page 27, paragraph 2.63 states “Where the system consists of several recirculating loops, the end of each should be identified as far sentinel points for monthly monitoring”.

MH comment – This is similar to the sentinel locations identified in non-circulating systems. Basically demonstrating that hot water has achieved satisfactory temperature at the far points on a system.

Page 27, paragraph 2.64 states “Many larger circulating HWS have additional loops consisting of smaller bore pipe branching from the flow leg of a principal loop to supply a group of outlets and connecting back to a return leg. In systems such as this, the smaller bore loops are the subordinate loops and the larger loops are the principals. Subordinate loops should be monitored ideally at a suitable return leg or from a representative outlet, in order to test all subordinate loops quarterly. However, large and complex HWS, e.g. in hospitals, often have localised loops that feed only one or two outlets and these can be identified as tertiary loops…”

MH comment – Subordinate loops are therefore branches off the main (principal) HWS pipework in smaller bore pipework. Larger systems many have a further localised loop called a tertiary loop. Subordinate loops should be tested quarterly either from the return leg pipework or the outlet. A detailed schematic drawing is necessary to identify all these locations. Please refer to my schematic drawing blog.

Page 31, table 2.1 Checklist for hot and cold water systems.

Under Hot water services for principal loops states “… temperature measurements may be taken on the surface of metallic pipework”

MH comment – There is no mention of taking the temperature from the outlet, however I believe this to be an omission as it says “temperature measurements may be taken …”, i.e. inferring instead of.

Under Hot water services for subordinate loops states “…temperature measurements can be taken on the surface of pipes but where this is not practicable, the temperature of water from the last outlet on each loop may be measured…”

MH comment – Subordinate temperatures can be recorded from the surface of the return loop pipework and outlet/tap.

Under Hot water services for representative outlets (single pipe system) and tertiary loops (circulating system) states “…to create a temperature profile of the whole system over a defined time period.”

MH comment – No defined time period for temperature monitoring of representative outlets and tertiary loops. The previous version of ACoP L8 referred to annual temperature monitoring and the current HTM04-01 refers to recording representative temperatures annually on a rotation basis over 5 years.

Watermark Compliance view.

The new guidance for temperature monitoring of cold water and non-circulating hot water is an improvement over the old ACoP L8 as it clarifies that more monthly sentinel points are tested in larger (branched) systems, which will help to demonstrate a system is working correctly.

The new guidance for temperature monitoring of circulating hot water systems is very complicated. If a customer has a complex circulating hot water system and wishes to fully comply with HSG274 Part 2 they will require a detailed as-fitted schematic drawing to identify all the principal, subordinate and tertiary loops. See schematic drawing blog.

To fully comply with the recommend temperature monitoring regime, principal loops should be tested monthly, subordinate loops tested quarterly (ideally on a monthly rota) and representative outlets on a rotational basis. We would advise testing representative outlets on an annual basis.

HSG274 Part 2 does give some leniency on Page 28, paragragh 2.65. This implies that if it is impractical to monitor every part of a complex system some form of rationalisation and prioritisation should be applied. Customers should always keep in mind that the purpose of any temperature monitoring regime is to demonstrate that the system is operating effectively.