HOT WATER SUPPLY Services and Utilities
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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
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