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Are you allowed to refill a percolator or hot water urn on Yom Tov?

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  • If you can do it without causing it to turn on, then yes. If not, then no. You need to add water to it in small enough quantities that you are not cooling the urn down to the point that the thermostatically controlled heating element kicks in.
    – Seth J
    Mar 15, 2012 at 18:39
  • You are allowed to do Bishul on Yom Tov, so who cares if the heating element kicks in?
    – Gershon Gold
    Mar 15, 2012 at 18:49
  • You can do Bishul, but you can't turn on an appliance to do it. Adding cold water to hot is Bishul. Kick-starting the element is an entirely separate problem (Mahloketh, though, which Melachah it is).
    – Seth J
    Mar 15, 2012 at 20:35

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The Federation Beis Din here in London have an incredible department called 'Fed Tech' which explores various appliances and their halachic status on Shabbos and Yomim tovim etc.

Their article on urns here addresses your question:

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.

Definitions:

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.

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.

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2 issues:

1) Does the light go on when the water cools down?

2) Cooking on water on yom tov (where cooking it in advance doesn't affect its freshness), is only permitted if it you were unable to do so before yom tov (e.g. the urn was full already or there was no time).

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  • talking where a light goes on when hot.
    – Gershon Gold
    Sep 29, 2010 at 19:33
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    Sorry for not getting back before yom tov. Electricity on yom tov is a hotly debated issue. While many poskim would permit your case (Grama on a derabbanan letzurach ochel nefesh), most contemporary poskim would say no. I am not familiar with all the back and forth on this. If anyone would like to clarify (or re-ask the question in terms of using electricity on yom tov) that would be great.
    – YDK
    Oct 3, 2010 at 1:39

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