Controlling Legionnaires disease in Domestic Hot Water (DHW) Systems by Bob Wilson BSc M.W.M. Soc (Technical Director SafeSol Ltd)

Abstract

Government figures show that 1,953 MTOE[1] is used for heating Domestic Hot Water in Commercial buildings. This is equivalent to 2.3% of all the energy used in the UK (excluding transport).[2] Heating DHW has a major impact on the UK’s carbon footprint.

This paper asks a very simple question. In the light of ever-increasing energy prices and UK’s desire to be carbon neutral by 2050, can we justify heating domestic hot water (DHW) in commercial buildings to 60o C to control Legionella, when the Health and Safety Executive offers a lower carbon alternative?  Reducing DHW temperature to say 45o C (or lower) requires biocide addition and this option is rarely used.

This paper shows that there are considerable savings to be made by heating water to 40o C instead of 60o C even allowing for the cost of biocide addition.

Water heated to a lower temperature loses less energy as it circulates round a building.

The question posed is one that consultants, architects, building managers and the water treatment /water hygiene companies dealing with the control of Legionnaires’ disease need to consider carefully if we, as a nation are serious about a lower carbon future and the sustainability of our buildings.

[1] MTOE is the acronym for million tonnes oil equivalent and is a method for normalising the energy output of the different fuels used. 1 TOE is the amount of energy released by burning 1000 kg fuel oil = 4.1868 X1016 Joules

[2] https://assets.publishingservices.gov.uk

Introduction

The present H&SE Guidance [1] offers two methods for controlling Legionella in domestic hot water systems. The first and overwhelmingly the most popular is to heat water to 60o C (or even higher) and circulate it so that the water returns to the calorifier at a temperature above 500 C (550 C in healthcare premises) This technique has been widely adopted by consultants, architects, and the water treatment industry.

Water at 60o C is accepted as a scald risk so an industry requiring the supply, installation, and servicing of thermostatic mixing valves to ensure water reaches the user at a comfortable/safe temperature has been established.

The second means of controlling legionella is to dose an oxidising biocide into the DHW circuit. This method of control eliminates the need to heat water to 60o C so water can be circulated at a lower temperature. This method comes with an additional cost – There is the cost of a dosing system and an ongoing chemical cost, but biocide treatment should eliminate the need to fit TMVs

Huwa-San, silver stabilised hydrogen peroxide (Para 2.116)is one of the treatments that is offered as an alternative. This treatment is effective, and is mentioned as possible continuous treatment of DHW in HSG 274 Part 2 (Para 2.85 Biocide Treatments).

Huwa-San is registered under the Biocidal Products Regulation (BPR) for product type 2-5 including the disinfection of Human and Animal Drinking water. In an earlier blog we explored the use of Huwa-San technology for constant dosing. This blog provides information on the equipment required and suitable dose rates.

Apart from hospitals and other buildings with complex DHW pipe runs, the continuous biocide dosing method is seldom used. In hospitals constant dosing is used with temperature control as a belt and braces treatment as the NHS still regard temperature control as the Golden rule.

At a time when reducing energy use, carbon footprint and sustainability have become important issues for building owners it is, surely, time to revisit the methods for controlling legionella in DHW systems.

[1] HSG274 Legionnaires’ disease: Technical guidance Part 2: The control of legionella bacteria in hot and cold-water systems. This was not new guidance as the ACOP L8 2001: Control of Legionella Bacteria in water systems presented similar guidance.

Issues we need to consider

  1. Does a change from temperature control to continuous Huwa-San dosing really save energy?
  2. If the answer is yes, how much energy is saved now and in the future?
  3. What changes need to be made and is it worthwhile making these changes?

Let’s consider the questions in turn.

  1. Does a change from temperature control to continuous biocide dosing really save energy?

A diagram of a typical DHW system to the right. Water leaves the calorifier at 600 C and circulates under pump pressure, round the ring main. Water is used at outlets and the DHW temperature falls as it circulates round the building  before it re- enters the calorifier.

Notes: a. Water can be heated by LPHW supplied by a boiler, a direct fired heater (Andrew’s Type), or

by a heat pump.

  1. The DHW circuit can be complex involving many outlets over several floors in

the building. The longer the circulation loop, the lower the return temperature is likely to be.

  1. All DHW pipework should be insulated to minimise heat loss.

Table 1 shows the daily circulation rate of DHW through various pipe diameters – These are average flow rates at a pressure between 3 bar and 6 bar. DHW circuits can be very complex so an ultrasonic measurement of the flowrate in the system will be required to give an accurate result. These instruments are capable of measuring water flow at temperatures up to 800C and can be hired.

Table 1: Flow Rates through pipes of different diameters
Nominal Pipe DiameterLinear Flow through Pipe

(Feet per sec)

Assumes 10psi-20psi

Assumes 15 psi pressure drop through system

Tonnes/ day

50mm (2″)13-18804
65mm (2.5″)15-201400 (derived)
80mm (3″)16-242368
100mm (4.0″)19-285068

Most UK domestic houses now have a 28mm main. Commercial buildings – Colleges, schools offices and hotels will have larger diameter pipes. Flowrates depends on pump pressure, pipework configuration and the materials used so the table is only indicative to show the large volumes of DHW which need to be continuously reheated.

[1] Hydrodynamic design, Part 2 Flows through a Pipe Chubb Michaud C.F wcpconline.com 03.02 2003.

First Consideration: Does Reducing DHW temperature save energy? 

While reducing DHW temperature from 60o C to 45o C should save energy, there are two aspects which must be considered.

The first one we need to consider is – If the temperature drops from 60o C to 55o C and water must be heated to get back to temperature, DHW at 45o C will also lose temperature as it circulates and must also be heated back to its original temperature. The example below shows that there is an energy saving to be made by heating DHW to the lower temperature. This calculation was supplied by Teesside University as part of their investigation into energy savings in building.

Note: – Water leaving a calorifier at 60oC and returning at 55o (ΔT= 5.0oC) in a building would equate to a return temperature of 42oC if the water were heated to 45oC (ΔT=3.0oC) and 37.5oC if the water were heated to 400C. (ΔT=2.5oC). These small temperature differences coupled with the large DHW circulation rates in buildings mean significant energy savings.

Second Consideration: Does Reducing DHW temperature save energy?

The second consideration is that when water is used, for instance in running a bath, less untreated water at 60o C is used than when using water at 45o C. The same principle would apply to showers more water at 45o C would be used than water at 60o C.

This calculation shows that in buildings where TMVs are used for temperature control, the energy used to heat the lower volume of water to 60o C will be the same as heating the higher volume of water to 45oC.

These calculations show that the main use of energy in a DHW system is in maintaining water temperature, as the water circulates round the system. DHW use in buildings should be minimise to optimise energy savings and minimise biocide use.

How Much Energy Can be Saved?

Dr C. J. Ennis of the Clean Environment Management Centre (CLEMANCE), Teesside University prepared an interactive spreadsheet[1], that allowed energy savings to be calculated for the DHW circuit in any building.

The variables that need to be inputted are

  1. Energy cost: – Most systems use gas which is cheaper than electricity – Costs are available on Building energy bills.
  2. Tonnes of DHW used each day – The amount of water used dictates the volume of chemical that must be added to treat the system- Most buildings will have water use figures. DHW use can be estimated from Table 4.
  3. Tonnes of DHW circulated – The flowrate through the system should be determined accurately using a calibrated ultrasonic meter or equivalent.
  4. Return temperature before treatment. Available by measurement or by meter on return pipework.
  5. Ambient temperature in the building – The average room temperature should be established.
  6. The spreadsheet also lets the user to input concentration of oxidising biocide, and cost per litre of biocide to generate an annual biocide cost.

The spreadsheet calculates daily or annual energy savings for the building depending on the inputted information.

 

The following section investigates potential savings by switching from a temperature control regime to biocide control in 4 different scenarios. A micro (small office), small (large office), a medium business (office block) and a large business (large hotel /student) accommodation.

The results given in Table 3 are based on the energy prices given in Table 2, the flowrates given in Table 1, a calorifier outlet temperature of 60oC and a return temperature of 55oC

The table is purely illustrative, as every building will have its own unique energy cost, DHW use, circulation rate, and calorifier flow and return temperatures.

[1] This spread sheet which allows energy savings to be calculated is available by contacting SafeSol Ltd

Energy Costs

Cost comparison websites like Power Compare list the following average prices for commercial gas.

Table 1: Flow Rates through pipes of different diameters
Business SizeAverage Annual Usage (kWh)Average Price (per kWh)
(p)
Projected Price
(+10%)
(p)
Projected Price
(+25%)
(p)
Micro Business5000-150004.04.45.0
Small Business15000-300002.472.73.08
Medium Business30000-650002.322.322.90
Large Business65000-1000001.941.942.43

Note 1. Cost comparison sites like www.moneysupermarket.com add at least 1p per kWh to the prices given in the table. The prices given are therefore conservative. Price increases are also conservative as price increases of 40% and above are being forecast on some sites.

Table 3:Typical DHW use in a building
Type of BuildingConsumption per occupant
litre/day
Consumption per occupant
gal/day
Peak demand per occupant
litre/hour
Peak demand per occupant
gal/hour
Factories (no process)22-455-1092
Hospitals, general16035302
Hospitals,mental11025225
Hostels90204510
Hotels90-16020-354510
Houses and flats90-16020-354510
Offices22592
Schools, boarding11525204
Schools, day15392

The use of an Oxidising Biocide will introduce other additional costs that must be taken into consideration.

  1. If the temperature of DHW is reduced to 45oC or less, there may be a need to install booster heaters to elevate water temperatures for kitchens or laundries. While this was a concern a decade ago when energy prices were low and stable, most washing machines and dishwashers today tend to operate at lower temperatures – most machines only have a cold-water feed. Booster heaters may still be required in certain buildings and the cost of installation and operation must be considered in the overall energy calculation.
  2. It is important that if a system changes from being temperature controlled to being controlled by biocide addition, then the building owner must demonstrate that legionella control is not compromised by the change. HSG 274 para 2.87 indicates that reduction of hot water temperatures should be carried out in stages and temperatures only reduced when efficacy against legionella is confirmed. Monitoring for legionella and biocide levels in the water system should be carried out at each stage. This means that each sentinel outlet should be sampled for legionella as the calorifier temperature is reduced. Normally temperature reduction would be 5o C in 3 separate phases if calorifier outlet temperature is to be reduced from 60o C to 45o C.  If Legionella Test results are acceptable then the biocide control regime can be introduced at the dose level being used. Legionella testing thereafter should be like that employed for temperature control. It is important that the level of chemical treatment is maintained throughout the system. This is easily achieved by weekly dosing pump inspection and on-site chemical residual checks.
Table 3:Typical DHW use in a building
Building SizeAssumed Pipe Diam. (mm)Avg. DHW Flow
(t/d)
DHW Usage Tonne Per YearEnergy Cost (p/kWh)Annual Biocide cost (£)Annual Energy Saving (£)
45C
Annual Energy Saving (£)
40C
Energy Saving
(£)
45C
Energy Saving
(£)
40C
Micro Business
Small Office
508045004.015014011175141386117364
Micro Business
Small Office
4.415015412192651526219155
Micro Business
Small Office
5.015017514218931736421743
Small Business
60 people office
65140015002.4790015141189261424118026
Small Business
60 people office
2.7090016551206881565119788
Small Business
60 people office
3.0890018880236001798023700
Medium Business
Office block/Hotel
80236894002.32282036061450763324142256
Medium Business
Office block/Hotel
2.55282039636495453681646725
Medium Business
Office block/Hotel
2.90282045077563464225753526
Large Business
Hotel/ Student Accommodation/ Prison
1005068457401.941029043024537803273443490
Large Business
Hotel/ Student Accommodation/ Prison
2.131029047237590463694748756
Large Business
Hotel/ Student Accommodation/ Prison
2.431029055000687504471058460

Notes:

  1. These calculations are for illustration only. Each Building will have its own set of parameters, and these must be inputted to obtain actual costs.
  2. The illustration uses 2” pipe flow for a micro business, 2.5” for small. 3.0” for medium and 4.0” for large and the corresponding flowrates given in Table 1
  3. Capital costs for dosing equipment, control equipment, and additional water heaters have not been included in this table. Dosing equipment costs vary considerably depending on the oxidising biocide being used. Dosing control systems for all oxidising biocides are available but also vary considerably in cost. SafeSol provide a simple dosing solution for Huwa-San technology and can provide advice on more complex dosing systems.
  4. The cost of testing, as calorifier temperatures are reduced will depend on the number of outlets

The spreadsheet produced by Teesside University also takes this into consideration and cash savings can be reduced accordingly. In general terms chemical costs represent 10% to 20% of the energy savings[1]. Chemical costs can, of course, be reduced by minimising DHW use.

[1] Chemical costs will vary depending on the biocide used and the supplier.

Summary and Conclusions

As energy cost rises and as building manager eco-awareness increases there seems to be no real argument against reducing calorifier temperatures. Temperature control is by and large the way it has always been done and water treatment/water hygiene companies may not welcome a change in control regime. It is much easier to monitor sentinel temperatures than to install a dosing system and ensure good chemical residuals around the system. A lower temperature / biocide monitoring regime places more responsibility on water treatment/water hygiene providers as they will be in total control of legionella in the DHW system. Currently they will report out of specification temperatures, and it will generally be the Building Manager’s responsibility to adjust calorifier settings.

Architects and Consultants need to consider that heating DHW to 600C, when there is a lower carbon, energy saving alternative, is not the sustainable option.

It is time Architects, Consultants, and the Water Treatment Industry looked at the alternative option offered in HSG 274 Part 2 (Para 2.85 and following) if we as a nation are serious about energy and carbon footprint reduction.

What Changes need to be made and is it worthwhile making these changes?

The answer to this question really depends on whether the UK is committed to being net zero in carbon by 2050. If we are serious about reducing our energy use and carbon footprint then we should not continue to control legionella in DHW systems by heating water to 600C when an alternative energy saving method is available.

New Builds

A low temperature biocide control system would comprise a calorifier that heats water to 450 C or 40o C. There is no requirement for TMVs. Additional water heaters may be required on the DHW in kitchens and laundries, but this is increasingly less likely as more washers become eco -friendly with cold water feeds and surfactants that are improved to clean in colder water.

Prior to changing to biocide treatment on a DHW system the complete water system should be disinfected with the biocide that will be used to treat the DHW circuit. In a well-designed DHW circuit this disinfection should ensure that biocide residuals are established easily when the system is commissioned.

Site personnel should monitor levels daily for the first week and thereafter 2-3 times per week for the first month. In accordance with HSG 274 Part 2 every week thereafter.

Existing Buildings

DHW systems that have been well maintained, have been temperature compliant and disinfected in the past should not present too many issues in terms of biocide addition and control. There will be DHW systems where it will be difficult and sometimes take some time to establish and maintain a disinfectant residual. Water treatment companies will spend more time on site, particularly during the commissioning of chemical treatment.

The H&SE recommend that temperature is reduced in stages so it may take up to 3 months to reduce the Calorifier outlet temperature from 600 C to 450 C (50C drop per month).  Additional monitoring will be required during the turndown period. On site legionella testing may accelerate the process.

It is essential that the building owner gets solid evidence that chemical addition achieves the same control of legionella as the Temperature control regime.

TMVs are no longer required but may be costly to remove. They will receive more DHW with a low temperature regime and much less cold water.

SafeSol have worked with Huwa-San technology for over 20 years and have more than 75 years experience in water treatment. We are happy to help with any queries and provide technical support .

Call 0191 4478008 or info@safesol.co.uk