A Cost-Effective Way To Heat Domestic Hot Water?

By John Ross, Sustainability & Renewable Research Lecturer at South Eastern Regional College

During the period March 2006 to September 2007 BSRIA carried out detailed monitoring of domestic hot water consumption in 120 dwellings across the UK.

The project was managed bythe Energy Saving Trust with thedata analysis contained in thereport ‘Measurement of DomesticHot Water Consumption inDwellings’.

The study had four principalobjectives: the real-timemonitoring of water consumptionand associated energyrequirement, the identification ofthe patterns of usage in terms oftimes and temperatures, thecomparison of the resultsobtained with BREDEM (BuildingResearch EstablishmentDomestic Energy Model)assumptions.

Although the volumetric usage ofhot water was in line withBREDEM it was determined thatthe average temperature rise ofwater from cold feed to hotdelivery was significantly lowerthan that currently assumed inBREDEM.

Therefore using BREDEM wouldactually overestimate the actualenergy consumption associatedwith domestic hot water use.

The report reflected an averageUK domestic house uses around122 litres of hot water per day,which needs around 9.86kWh ofenergy to heat from 10°C to 60°C,equating to around 3600 kWh perannum. This by any stretch is aconsiderable cost to any family:

Using Oil @360 litres (assumed100% efficiency) @ 62 pence perlitre = £223.20

The use of Electric @17.8 penceper kWh (NIE standard rate) =£622.80 per annum

Economy 7 tariff @7.3 pence perkwh = £262.80 per annum

Natural gas prices at @4.73 perkwh = £170.43 per annum at100% efficiency

Note fossil fuel devices averageefficiency if probably in the 70% to85% range at best

In a basic view of the costsabove, natural gas is thecheapest option, but the addedbenefits of the heat pumpsabilities can’t be ignored; in thesense of using heated air from anexisting room to greatly improvethe COP or the ability to providecooling, humidification whilstproviding hot water during thesummer months.

Also gas is a fossil fuel andtherefore is seen as a ‘nasty’ interms of pollution and climatechange and the losses throughsystem efficiencies need to befactored in.

Utilising heat pump technologyrepresents in my view the mosteconomical long term heating andcooling source. With their use,you can reduce your sanitarywater heating by much as 300%,leading to cost-effectiveness andextensive savings.

Most people own a heat pumpand don’t even realise it – yourrefrigerator is in fact a type of heat

pump. Heat is extracted from theinside of the fridge to keep foodfresh and is expelled through thecondenser grill at the back of theunit.

When used in the opposite way,heat pumps extract heat from anoutside source (air, ground orwater), and rather than the heatbecoming a waste product, it isconveyed to a point of use withinthe premises. In this case, thepoint of use would be a hot waterstorage cylinder.

The heat energy that has beenproduced by the heat pump istransferred to the water in thecylinder via a high efficiencyfinned copper coil – the same waya common gas or oil boiler would heat your hot water.

However, the heat pump coils inintegrated cylinders have a farlarger heating surface than thetraditional domestic boiler coilsthat are offered in copper orstainless steel cylinders.

This allows the coil to dissipate asmuch heat as the heat pump canpossibly produce, preventing theheat pump from multi-cycling.Principle: the heat pump recoversheat from the air to heat the waterin the tank – all in the sameappliance.

The average combination systemhas a capacity of around 300litres (smaller units are available),a volume suitable for a waterconsumption of a family of 4 or 6people. This incorporates anupper heat pump that allowsproduction of water up to 60°C.

Most have excellent COP’s from astarting range of around 3.5 to justover 4.0 (coefficient ofperformance according to EN355-3), around 4.0 the highest in themarket at present with companiesworking hard to improve systemCOP’s. This allows real savings ofup to 70% of energy consumed fordomestic hot water.

Intuitive and complete regulationgives access to different controlmodes allowing energy saving andcomfort i.e. “ECO” mode,“Day/Night” hourly programming,Holiday programming etc and otherchoices of operating programs.By fully utilising the economy 7tariff rates overnight to heat yourdomestic water this really cutoperating costs.

In case you have a greatneed for hot water (eg visitingfriends or family), there is accessusing internal emersion heaters forfaster thermodynamic heating.Most of these systems could beinstalled within one day withoutany fuss into an existing house,easily upgrading the hot watersystem.

These modern systems canprovide constant hot water at 60°Cwith the unit working constantlyand quietly, with total efficiencyand economy with storagestanding heat losses of around 1 -2°C over a 24 hour period.

Figure (see right) – The operationalCoolwex system installed in theSERC campus EnvironmentalSkills Centre in Newtownards fortraining and design aspects.

This system uses a 0.8kw heatpump air source to supply 3kwoutput and is installed to reflectseveral running heating andcooling modes.

These systems can in most casesbe augmented with solar panelwater heating or utilised withanother energy source through asecondary coil in the cylinder.In general these systems have/aredesigned to have at least oneother form of heat input to thecylinder as a backup, for example,immersion heater, gas/ oil boiler,Biomass or any other heatapplication.

The secondary system must becapable of raising the temperatureof the stored water to around 60 -65°C, although most heat pumpscan achieve this temperature ontheir own.

The heat pump applicationcombined with solar heating cancreate a very environmentallyfriendly and highly efficient methodof heating your hot water.

The Coolwex system has a hybrid arrangement that uses internalimmersion heaters to boost supplyThese heat pump systems requireinstallation in locations that remainin the 5.0º–32.0ºC range all year­round.

Cool exhaust air can be ducted toseveral or all rooms or outdoors. Ifthese systems are installed withducted access the excess heat ina utility room or basement thenusing this energy as the source,boosts considerably the workingefficiency of the unit.

Because they remove heat fromthe air, any type of air-source heatpump system works moreefficiently in a warm climate, whichto a degree can be designed intothe system plan.

Integrated heat pump systems willnot operate efficiently in a coldspace where extraction is takingplace; they tend to cool the spacesthey are in and this must beconsidered in the design.

You can install an air-source heatpump system that combinesheating, cooling, humidification

and water heating.

These combination systems canpull their heat from the indoor air inthe winter and from the outdoor airin the summer to enhance C.O.P.or vice versa.

Domestic water heat pumps usingair or exhaust air as the heatsource also offer two otherinteresting and useful functions.They can take the warm, moistexhaust air from certain rooms,like the bathroom, and thenrelease the dry, cool air intoanother room.

So you can cool a pantry, forexample, or supply dry air to alaundry room to dry washing. Inwinter the waste heat from theboiler can be used, for example.With a domestic hot water heatpump you will make an activecontribution to protecting theenvironment and to saving energy.Emission-free heat pumps alsoimprove the microclimate, whichmeans health benefits for you.

Heat pump water heater systemstypically have higher initial coststhan conventional storage waterheaters. However, they havelower operating costs, comfortprovision which can offset theirhigher purchase and installationprices (as per costing’s above).

Before buying a heat pump waterheating system, you also need toconsider the following:

Proper installation depends onmany factors. These factorsinclude existing fuel type (gasarea), climate, local buildingregulation requirements, andelectrical safety issues.Therefore, it’s best to have aqualified heat pump systeminstaller/designer (MCSregistered) install your heatpump. The use of ducted airsupplies have to be thoughtfullydesigned and agreed.

Periodic system maintenance cansignificantly extend your waterheater’s life and minimize loss ofefficiency.

The details in the owner’s manualfor specific maintenancerecommendations should bestrictly followed. Properinstallation and maintenance ofyour heat pump water heatingsystem can optimize its energyefficiency.

You can purchase a stand-aloneheat pump water heating systemas an integrated unit with a builtinwater storage tank and back-upemersion heating elements. Youcan also retrofit a heat pump towork with anexisting conventional storagewater heater (cylinder).

Other advantages of domestic hot

water heat pumps:

• Easy to install
• The plug-in units are easy to set up
• Little space required
• Hot water up to 60°C in heat pump
• mode – low current consumption
• Can be combined with solar or other heating systems
• Cascade-type configurations also possible for larger buildings