The Technical and Halachic Story

Part 1 – What makes them tick?


FedTech, the Federation Beis Din’s department for Shabbos compliance, was launched last year; its focus – domestic ovens, fridges and freezers. Since its launch, we have received countless requests for assistance with a wide variety of shailas relating to the use of technology in Halocho. Shabbos kettles have featured high on the list of Frequently Asked Questions.

Responding to the need, FedTech have undertaken research into some of the appliances commonly available here in the UK. Information on specific models will be available on our website at This week, we launch a new series of articles which will shed some light on how Shabbos kettles work and the halachic principles which underpin their use.


I have noticed that the red light on my urn goes on and off intermittently. How does the urn operate?

An urn, like other kettles uses a heating element to heat the water. Unlike your weekday kettle which heats the water until it has boiled and then switches itself off, the urn continues to heat the water over an extended period of time; yet, it must also ensure that it does not boil dry. This balance is achieved by operating the heating element in on/off cycles. There are two ways in which these heating cycles can be controlled:

  1. Simmerstat– When using a simmerstat, the lengths of the cycles are determined by the setting which the user has chosen and will repeat in a uniform manner. A dial is provided to controls these timings. Changes to the heat of the water will not affect the length of the heating cycles, as all control of the heating elements is purely time-based.
  2. Thermostat– The length of the heating cycles will be determined by the heat of the water at any specific time. The thermostat reacts when the temperature dips below the tolerance level set by the user, and the heating element will be activated. When the desired temperature has been achieved, the heating element ceases to operate. It is unlikely that the heating cycles will be accurately predictable in their length and frequency.


My urn’s light seems to be lit all the time. How is it controlled?

Some urns are not controlled by on/off cycles. Instead, the amount of power used by the heating element is regulated to control the heat of the water.

In some cases, the kettle will have a dial. The dial is turned to high to boil the water and then turned down to a level that will maintain the temperature of the water. This manual control of the temperature can be inconvenient, and users will generally prefer an urn which automatically regulates its own temperature.

This automation can be achieved by providing two heating elements – a high-powered boiling element and a lower-power maintenance element. A thermostat is set to operate the high-powered element only when the water is particularly cold.

When the water is cold, the urn’s thermostat will instruct both heating elements to operate. Once the water has reached boiling point, the main heating element is switched off. The low-power heating element will continue to operate and maintain the temperature of the water. It is rare for the high-powered heating element to trigger on once the urn it has been initially boiled. The lower-powered heating element will generally suffice to keep the water hot for a long time. The frequency of these occasions when the kettle boils itself will vary depending on the kettles design. In some kettles, the water will boil very rarely – in fact as rarely as every few days. In other cases, this will take place a few times a day.


How can I tell if my urn is thermostatically controlled or if it uses a simmerstat?

Our experience has demonstrated that an appliance’s paperwork does not necessarily provide an accurate indication of its method of operation. Even discussions with sales teams or customer services may not yield results.



Part 2 – How should they be used on Yom Tov?


The last article uncovered the various means sued by manufacturers to regulate the heat used to boil water in Shabbos kettles. Some devices employ a simmerstat which controls the on/off of the hot water using a timer. Thermostatic kettles control the heating element turning it on and off based on the temperature of the water.



Can water be added on Yom Tov?

An urn which is simmerstat-controlled carries no concern with regards to adding cold water on Yom Tov as the cold water added will not affect the heating element’s operation.

When adding water to a thermostatically-controlled appliance, however, there is greater concern. This is because when a large amount of cold water is added to the urn, the heating element’s operation is changed and will come on sooner than it otherwise would have.


How do FedTech recommend that hot water be added on Yom Tov to an urn?

For urns which are thermostat-controlled, FedTech recommend adding water using one of the following four approaches:

  1. Add water when the device’s indicator light signals that the appliance is currently heating
  2. Add hot water
  3. Add small amounts of water so that the device will not register an immediate change. (You should try this before Yom Tov)
  4. Set a time switch to remove all power from the urn for a set period of time on Yom Tov. During this period, the urn should be filled. When power resumes, the urn will boil.



The next article will discuss the question of the use of the thermostatic Shabbos kettle on Shabbos.


Part 3 – But can they be used on Shabbos?



The current series of articles has its focus on the use of Shabbos kettles. This article covers the halachic discussion surrounding the Shabbos use of a thermostatically controlled appliance.


If I take water out from a thermostatically controlled urn, for example, to make a coffee, am I affecting the way in which the urn will heat itself?

Mr Dovid Gurwicz, who has been of enormous assistance in this project, provided some scientific background which is useful in addressing this question. There are two principles of physics which work in tandem and will affect the way the thermostat operates.

  • Surface area– the larger the surface area of the water, the faster the heat loss, as more of the body of water is exposed to the environment and its ambient temperature.
  • Insulation– if the body of water is insulated, its rate of heat loss will be affected. The more insulation, the less effect removing water will have on the rate of heat loss. Additionally, the water itself may provide insulation ensuring a slower rate of heat loss.

In an urn, this set of variables will work together in a complex manner to determine the rate of heat loss. Ultimately, making a scientific calculation of the effects of removing hot water from one’s urn is highly complex and effectively unpredictable – certainly not an activity to engage in whilst preparing an early Shabbos morning coffee.

Our own testing has shown that removing water from the urn can slightly speed up the onset of the heating cycle, although others who have investigated these questions have not been able to come to conclusive findings about the effects of removing hot water.


On Shabbos, is one allowed to remove water from an urn if it is controlled by a thermostat?

The questions surrounding the use of an appliance which is controlled by a thermostat have been widely discussed by the Poskim. Dayan Weiss in Minchas Yitzchok, R’ Shlomo Zalman Auerbach in Minchas Shlomo and many others wrote extensively on the topic. How are we to regard the use of a thermostatic Shabbos kettle?

In relation to the use of ovens on Shabbos, and in line with the rulings of Shemiras Shabbos KeHilchoso, FedTech’s advice is that one should refrain from opening the oven door unless the heating element is already operating. Opening an oven door when the oven is not heating is problematic as the introduction of cold air to the oven cavity will cause the element to cycle on earlier.

Despite his ruling regarding the use of ovens, Shemiras Shabbos Kehilchoso permitted the use of a Shabbos kettle even if it is operated by a thermostat. The more lenient status of a Shabbos kettle can be attributed to a few factors:

  1. As described above, it is not clear what the effects of removing water are on the kettle’s performance. It is possible that the appliance’s insulation, shape, size or kitchen environment will ensure that the kettle’s performance is not affected.
  2. The effects of extracting water on the appliance’s operation are more indirect. In a Torah journal printed in Gateshead in memory of Rav Dessler zt”l, the Gateshead Rov Rav Shakovitsky zt”l  argued that the forbidden status of grommo only applies if one’s action can be viewed as being directly engaged with the item used for the melocho. In the case of a fridge or oven, the air introduced by opening the door will directly interact with the appliance’s sensor/thermostat. The Shabbos kettle is different; for these appliances to be affected, first the water must cool down and only then will a new reading be received by the thermostat. Although grommo is forbidden on Shabbos, the long chain of reactions was viewed by Rabbi Shakovitzky as sufficiently distancing the actions of the person from the reaction of the device, as to be permissible.


There is an easy way to avoid the issue altogether. Some urns cycle on/off relatively frequently. When they do so, a light is displayed to indicate that the device is heating.  Removing water from the urn only when the red light indicates that the urn is already heating means that there is no chance of affecting the thermostat at all. (If one sets a relatively high temperature, the light will show relatively frequently. Note, however, that setting the temperature of an urn too high may cause it to boil dry.)

On Yom Tov there is no need to wait for the light to come on when removing water from the urn.


Should I buy an urn which is controlled by a thermostat?

The generally accepted halachic position on this matter is that one may use an urn on Shabbos even if it is thermostatically controlled.

When purchasing a new urn, it is worth noting that one which has a simmerstat control sidesteps the issues altogether. (See part 1 of this series for a description of how a simmerstat works).


For information on specific models which have been researched, click here.

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