Q: We want to order about 25 heating units for our apartment and have them shipped to us in Israel. Are there any special requirements we should know about ahead of time? -Cold in Jerusalem

Dear Cold in Jerusalem: There are absolutely issues and requirements you must address before you ship them to Israel.
Customs will generally allow a private individual to import up to two of any item as a personal import. You will still have to pay customs duties as required by law, but you will be allowed to import the items without any need for special permits.
More than two units though may be considered a commercial shipment. If so, only a business can import a commercial import.
On rare occasions, if you can prove you are importing a large number of items for personal use, Customs may allow the import. I had one client import 30 meter-long brass trumpets. He claimed to be importing them for the Leviim to use when the Temple is rebuilt. Customs allowed the import. I like that story because it could only happen in Israel.
When importing anything that uses electricity and will be plugged in an electrical outlet, you must also receive a permit from the Standards Institute. This can take many long months. If you do not have this permit, Customs will not clear the shipment and you will have to pay storage charges until you receive the necessary permits and Customs clears your shipment.
Please be aware - the model number for which you receive a permit is the only model you can import. It does not matter that your approval for model 318 is exactly the same as model 319 - Customs will not recognize your approval of 318 as having anything to do with model 319. Don't let you supplier "upgrade" or send a newer and better model. I have had some clients relabel a product with the "old" model number to avoid problems!
Depending on what the item is, other government ministries may get involved and you may need other permits as well.
I hope this answer will help you get warm and toasty before the end of the winter!
All the best, Shmuel