HOT WATER SUPPLY

Services and Utilities

Domestic Hot Water

Hot water for domestic purposes may be generated either:

Locally (at point of use)

or

Centrally (in a plant room)

Each of these may be either an instantaneous or a storage system.

Hot Water

Central

Storage Instantaneous Semi - storage

Local

Storage Instantaneous

• Central: hot water is generated at a convenient point within the building and distributed via pipework to the draw off points.

• Local: hot water is generated at or close to the point of consumption.

Central v LocalCentral

Mainly used for buildings with:

Very little and infrequent hot water demand, e.g. Hand rinse in toilets.

Hot water demand at remote parts of the building e.g. Workshops, laboratories , garages and out-houses etc.

In the right application provides Low capital, maintenance and energy costs, but appearance, performance and space requirement may be an issue.

Mainly used for buildings with:

Water demand on a regular basis. Domestic dwellings, offices, schools etc

Large scale systems, eg hotels. hospitals

Draw –off points arranged in clusters etc.

The most common approach for providing hot water

Local

Storage v Instantaneous

Storage : hot water is generated before it is required and stored in an insulated vessel until required.

Instantaneous: hot water is generated on demand as and when required, there is no hot water storage.

Note: The hybrid, semi-storage systems is a instantaneous system with a small amount of storage

Storage v InstantaneousStorage Instantaneous

Pros

Can deliver high flow rates.

Reduced peak energy demands.

Good for applications with regular demands

Cons.

Possible to run out of hot water.

Space & plant required to store the hot water.

Pros

Constant hot water availability

Minimises energy losses

Minimises plant space.

Reduced standing losses

Cons

Reduced delivery rates

High energy input required

ExpansionWhen water is heated or cooled it will expand or contract by up to about 4% depending on the increase or decrease in temperature.

Depending on the type and volume of the system this increase in volume may need to be accommodated. There are a variety of different strategies for doing this this.

Examples

Local, Instantaneous, hand rinse water heater,

Fuel: Electricity.Rating: 3 to 7 kW Water temperature: variable up to 40oC. Varies with flow rate.Connect directly to the cold water main. No special requirement to accommodate expansion.

Local, Storage, over sink water heater. 7 to 10 litres of stored water.

Fuel: ElectricityRating: 3kWWater temperature: variable or fixed up to approx 70oC Heat up period ; circa 10 minutes to 70oC.Connects directly to the cold water main. Usually no special requirement to accommodate expansion.

Local, Instantaneous shower,.

Fuel: Electricity

Rating: 7.5 to 10.8kW

Water temperature: variable up to

approx 40oC. Water temperature

varies with flowrate.

Connects directly to the cold

water main. No special

requirement to accommodate

expansion.

Central or local, storage, unvented*,

water heater.

Capacity: 30 to 100 litres

Fuel: Electricity

Rating: 1 to 6kW

Water temperature: up to approx 70oC.

Can be used for whole house hot water

or for a range of wash basins in

workshops, hairdressers etc.

Connects directly to the cold water

main.

*Note: This is an unvented hot water

storage vessel and must satisfy Building

Regulation requirements.

Central, instantaneous “multipoint”

water heater.

Fuel: gas

Rating : 25 to 35kW

Water temperature up to approx 40o C

Used to provide hot water to all points in

a domestic dwelling . (now largely

replaced by “Combi” boiler which also

provides central heating)

Connects directly to the cold water

main. Usually no special requirement to

accommodate expansion

Typical Combi Boiler

Arrangement.

Combination Boiler

22mm heating flow pipe

22mm heating return pipe

Boiler Typical combination boiler arrangement

DHW supply Gas supply

CW service pipe

Temporary filling loop

Commercial Centralised Non-storage or Semi-Storage Water

Heaters

Fuel: Gas Heat Input: 50 to 500kW Flow rate: 500 to 9000 litres/hour with 50oC temp rise. Can be installed as multiple units to increase flow rates

Direct Centralised, Storage Systems

Boiler

Service pipe

Cistern

Hot storage cylinder

Cold feed to cylinder

Open vent pipe

Distribution to appliances

Circulation pipes

Direct Cylinder

1. Drain cock 2. Cold feed pipe 3. Lower immersion

heater 4. Upper immersion

heater 5. Open vent pipe 6. Hot water distribution

pipe 7. Return pipe to boiler 8. Flow pipe from boiler

Direct System - Problems

• Domestic hot water passes through boiler

o Corrosion – dissimilar metals (copper cylinder and steel section boiler)

o Furring – temporary hard water leaves deposits when water heated above 60oC

o Cannot connect radiators to same system

Indirect Centralised Storage Hot Water System

Boiler

Service pipe

Storage cistern

Indirect hot storage cylinder

Cold feed to cylinder

Open vent pipe

Distribution to appliances

Heating system

Zone valve

Feed & expansion cistern

Open vent pipe

Cold feed to boiler

Indirect Cylinder

1. Drain cock 2. Cold feed pipe 3. Return pipe to boiler 4. Flow pipe from boiler 5. Heat exchanger coil 6. Immersion heater 7. Hot water distribution

pipe 8. Open vent pipe

The domestic hot water stored in the calorifier is heated by primary hot water.

With hot water heating the water within the boiler system is Primary whilst the water in the calorifier and delivered from the taps is Secondary water.

Modern calorifiers are highly insulated to minimise heat losses

Dead Legs!!Tendency for the water to go cold in the pipes when the taps are off ,

This wastes water, in large systems it wastes a lot of water and must be prevented by “Secondary Circulation” or “Trace Heating”.

Secondary Circulation.

Pipework is added which returns

the hot water to the calorifier so

that even when the taps are

turned off the water is

continuously circulated through

the calorifier to keep it warm.

Installation Using Solar Collector

Solar hot water is inevitably used in conjunction with other heat sources via a variety of configurations.

1.) Combined solar store and Domestic Hot Water Store

Note: Controls have been omitted to simplify the diagram.

2.) Separate Solar Store Pre-Heating Domestic Hot Water Store

Note: Controls have been omitted to simplify the diagram.

Temperature control arrangements to prevent loss of heat when solar collector isn’t receiving heat.

Renewable Heat Incentive

A financial incentive scheme designed to reduce the pay-back period by providing an income to help meet the additional capital costs of renewable technologies.

•Currently available for Non-Domestic Applications

•Current tariff for solar hot water is 8.9p/kWh

•RHI is currently available for both domestic and non-domestic solar installations.

Prevention and Control of Legionellosis (aka Legionnaires’ Disease)

Refer to:

• HSE ACoP L8: Legionnaires’ disease — The control of Legionella bacteria in water systems.

• CIBSE TM13: Minimising the risk of Legionnaires’ disease.

• IoP: Plumbing Design Guide

Legionnaires Disease Key Points for Domestic Hot Water Installations

• Legionnaires Disease is caused by a group of bacteria that occur naturally in water.

• Inhaling droplets of water that contain large concentrations of Legionnella bacteria can cause severe pneumonia type repertory illnesses known as Legionnaires Disease which can be fatal.

• In domestic water systems the main risks are via showers, spa baths, spray taps.

• Legionella bacteria can multiply rapidly in warm warm water (20 to 35oC) providing a source of nutrients are available

(scale, slimes, organic compounds).

Legionnaires Disease (continued)

• The bacteria is dormant below 20oC and instantly killed at about 60oC. So keep cold water cold (less than 20oC) and hot water hot, (heated and stored at or above 60oC. )

• Disinfect new installations.

• Keep systems clean

• Make sure all hot water has been heated to at least 60oC on a regular basis.

Safe Operating Temperatures

Keeping hot water at 60oC or above keeps prevents Legionnaires Disease but it is too hot to be used safely in many circumstances To prevent risk of scolding particularly for people “at rick” (the elderly, infants and infirm), the following maximum temperatures available at the draw off points are recommended.

If hot water is required to be stored at 650C, how is it prevented from scalding users?

By using a thermostatic mixing valve

http://www.horne.co.uk/Products/thermostatic-mixing-valves