Night Storage Heater, electric, Christchurch, New Zealand, University of Canterbury housing © Daniel FriedmanNight Storage Heaters & Heat Sink Radiators
Supplemental electric heat vs heat dump radiators on hot water systems

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Night storage heaters & heat dumping radiators:

This article first describes night storage heaters, including the definition of a night storage heater, where they are typically used, and recommendations for making the most effective use of a night storage heater.

Second we describe heat dumping radiators, also referred to as heat sink radiators or heat leak radiators used with hot water heating systems to provide water circulation and temperature control.

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Night Storage Heater & Heat Sink Radiator Installation & Operation

Night storage heater (electric) controls (C) Daniel FriedmanAlthough they are quite different in operation, night storage heaters and heat sink radiators are steel appliances that work to store and release heat in high-efficiency energy-conserving heating or hot water heating system designs. Here we describe and compare the operation of these two approaches to heating buildings and hot water.

Our photo shows the controls on a Dimplex™ electric Night Storage heater installed in campus housing, University of Canterbury, in Christchurch, New Zealand.

[Click to enlarge any image]

Article Contents

Definition of a Night Storage Heater

Night storage heaters combine an energy source (electricity at off-peak rates) and a thermal mass (such as heavy bricks or cast iron) to store energy in the form of heat so that that same heat can be directed into the occupied building space when needed.

Storage heaters are designed and controlled to take advantage of off peak electrical rates to store heat for later use. These heaters are typically used in climates that are mild enough that larger, high-capacity central heating is not required. The Dimplex storage heater illustrated here is installed in a home in Christchurch, New Zealand. We estimate that this unit has been in service since 1981.

In Scotland and other U.K. countries similar night storage heaters, also made of steel, are connected to hot water heating systems and form heat sinks that store un-needed heat. An example of a steel heat sink radiator on a hot water system heated by a biomass boiler is at LEAKS in STEEL RADIATORS.

How does a Night Storage Heater Work?

The night storage heater is charged (heat is stored in its thermal mass) usually at night during off-peak hours, a period when electric utility rates are lower than during daytime hours. For this reason, the heater will feel hottest in the morning - it has been charging or absorbing heat provided by electricity during the night.

Electrical meters for storage heater, Christchurch New Zealand (C) Daniel FriedmanDuring the day, as heat is transferred from the storage heater to the building's occupied space, the heater slowly cools off. If the heat output during this period is not sufficient the occupants can increase the heat output from the unit by adjusting its output control.

In order to take advantage of off- peak electrical rates, the night storage heater will be supplied electricity through a separate electric meter and wiring circuit used only for the heater.

Our photo (left) shows the separate meter for the storage heater - the left-most meter in the enclosure. The separate circuit for the night storage heater is labeled "night storage" and can be seen as a vertical wire between the two electric meters.

[Click to enlarge any image]

Typically the charging circuit will automatically switch on during off-peak hours and switch itself off during hours of peak electricity demand. At the end of the off-peak rate charging period after the charging circuit has switched off, the heater continues to release heat (from its stored thermal mass) over the course of the day.

Supplemental heating might be needed when cold weather combined with a high building heat loss rate may exceed the output capacity of the storage heater. This supplemental heat may be provided by other heat sources (e.g. by a pellet stove or by other electric heaters that operate during more costly peak rate times).

Night storage heaters give out some heat even when the heater is ‘charging’ during the night, thus assuring that the building receives some heat during those hours.

Recommendations for Use of Night Storage Heaters

The following guidance on using a night storage electric heater is adapted from information provided by the University of Canterbury housing department, Christchurch, New Zealand.

Watch out: prevent contact with a night storage heater surface by children, elderly or frail individuals: the surface of an electric night storage heater can be quit hot. In New Zealand the surface temperature of a night storage heater must meet the requirements of AS3103 that addresses the safety-covering requirements of electric heating appliances. In Europe, the night storage heater surface temperatures meet a similar standard, quoted from Dimplex:

The surface temperatures of this heater are within the requirements of EN60335-2-61, the European Standard covering the safety requirements for Electric Storage Heaters, and momentary contact with any part of the heater should not cause injury. However, in order to be effective, heaters of any type do get hot, especially around the air outlet grille. - Dimplex XLN / XLSN Operating Manual

At the temperatures specified by these two standards, momentary contact with the heater surface should not cause injury but the heater surface will indeed feel quite hot to the touch, especially around the air outlet grille on the heater face or on some models its top.

How to Set the Night Storage Heater Controls

Typical night storage heaters include two occupant-adjustable control knobs.

Temperature input (right hand knob on the night storage heater shown above): this knob is set to its highest position (9) to obtain the maximum heat into (and subsequently out of) the storage heater. In cool but not very cold weather, when less heat will be required, you can set this control to a lower number. Note that the University has removed the actual control knobs on this heater, leaving this unit set at fixed (and undetermined) settings.

Dimplex recommends the following Input Knob settings based on weather conditions:

Temperature boost (left hand knob on the night storage heater shown at above left).

Dimplex explains the settings of the temperature boost control as follows:

The University instructs occupants to set to this control as follows:

When set to (1) the heater's damper is closed and the unit delivers heat only by radiation from its surfaces and by natural air convection around the exterior surfaces of the unit.

To obtain additional heat output from the storage heater, the temperature boost knob can be set to a higher or even its highest number. Normally the temperature boost is set up to a higher number when occupants are active in the building and is returned to its lowest setting (1) when occupants retire to bed for the night. This approach helps assure that the night storage heater will continue to deliver heat to the building interior (though at a lower rate) throughout the night.

When you open the damper (increase the temperature boost of the night storage heater) the open damper increases the air-flow through the unit, releasing heat more rapidly into the occupied space.

Watch out: if you leave the temperature boost on continuously (that is, during the day when less heat should be required), the heater will have less heat stored for release during colder night time periods.

Watch out: The controls an a wall-mounted night storage heater are (or should be) placed on the unit's top edge where they are visible and accessible by an adult but where they will not be readily apparent (and tempting) to toddlers who like to twist knobs.

Night Storage Heater Mounting

Watch out: because a night storage heater is heavy (earlier we mentioned the need for thermal mass in the form of bricks, cast iron or other heat storage media), it must be securely mounted to the building wall, preferably to wall studs or using suitable wall mounting hardware. Don't remove or disassemble a night storage heater from its installed position.

The night storage heater shown here is installed with its feet resting on the (carpeted) floor, and with an 8-inch clearance to combustibles at its left and right sides and also at its top surface.

Night Storage Heater Cleaning

Simple dry dust cloth wiping of the heater exterior surface is recommended during the heating season if dust is accumulating there

In warm weather or when the heater is off completely, damp-wipe the heater's exterior surfaces to clean them. Do not use abrasive cleansers and do not use furniture polish on the heater surfaces.

Hydronic Heat Sink / Heat Dump Radiators on Hot Water Systems

Steel heat sink or night storage radiator on a Scottish biomass boiler hot water heating system - leak is circled in red (C) 2016

The steel radiatior shown above is the heat sink radiator for a hot water heating system in Scotland that uses a biomass boiler that burns wood pellets. The radiatior in this particular system is connected to a lagged (insulated) hot water storage cylinder or tank. Other radiators in the home are controlled by thermostatic radiator valves. This one is not, and cannot be controlled by a valve, as we explain below.

A leak at this heat sink radiator radiator (circled in red) is discussed at LEAKS in STEEL RADIATORS.

Heat sink radiators are used with hot water cylinder systems and typically with an energy-efficient heat source that one wants to keep in operation for extended periods of time, typically with solid fuel heating boilers such as wood stoves or pellet stoves.

A heat sink radiator is an essential part of the system to which it is attached as the radiator provides the source of both circulation of hot water through the system and a temperature control to avoid overheating of the system.

How a Heat Dumping Radiator Uses Natural Convection to Circulate Hot Water

In the U.K., traditional solid-fuel hot water heating systems use a gravity-circulated domestic hot water piping circuit that circulates hot water from the wood stove, coal, peat, or pellet stove to a large hot water storage cylinder using a circulating pump. The system that we illustrate here uses a heat sink radiator to cause hot water to circulate by convection.

The hot water in these traditional heating systems circulates by "gravity" or for more technically-minded readers, by convection. That means that we take advantage of the difference in density between hot water and less hot (or cold) water (cold is heavier) to cause water to circulate through a piping system.

Because unlike the domestic hot water cylinder (hot water tank) the heat sink radiator continuously releases heat into the occupied space of the building some soruces call a heat sink radiator a heat-leak radiator or a heat dump radiator. Because it is always losing some heat into the room where it is installed, a heat sink or heat dump radiator will always be at a lower temperature than the hot water flowing through it.

This lower temperature across the radiator cause hot water to circulate through the water heating system by convection. Larger heat sink radiators or a heat sink radiator installed in a colder location will cause faster water circulation through the system. U.K. experts (cited below) note that the radiating capacity of the radiator (basically it's size) should be at least 10% of the KW output of the maximum heat output of the source heat - in this case a biomass wood-pellet stove. Some designs use 20%.

Heat Sink Radiators Allow Solid Fuel Heaters to Avoid Shutting Off

By keeping the heating system water from becoming too hot the heat sink radiator allows the wood-pellet stove to keep operating without having to shut off due to excessive temperatures in the system. Without this feature the room where the pellet stove itself is located could become chilly when the stove shuts down because the thermostatically controlled hot water radiators in other rooms in the building are too hot.

The heat dumping radiator also prevents water in the heating system from boiling and avoids sending scalding water to the building fixture taps. These radiators are also used with cookers that also produce heat for storage including heaters produced by Aga and Rayburn.

Watch out: proper piping and venting to the atmosphere and proper location of (or prohibition of) control valves are essential for the safe operation of these heating systems.

Below are additional details of the radiator piping at this installation. While you see what look like control valves at the ends of this radiator, they're not. They're safety devices. No control valves can be installed at the heat sink radiator since it must be always-on whenever the heat source (wood pellet stove for example) is on.

Controls at left end of steel hydronic storage heater, Scotland (C) InspectApedia MC

Notice rust traces along the bottom of the heater.

Steel storage heater leaks, Scotland (C) InspectApedia BC 2016

Typical locations for heat sink radiators are in rooms where the heat-always-on operation is not an issue, cold locations such as landings or locations where you like a bit of extra heat such as bathrooms.

Night Storage Heater & Heat Sink Radiator Sources & Information


Continue reading at ELECTRIC HEAT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLOCKBED RADIANT FLOORS - SOLAR DESIGN for an alternative design approach to storing heat



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