I had to look this up, but there's actually a difference between a boiler and a hot water heater.
A hot water heater heats water for use in showers, taps, cooking and cleaning. It's meant for drinkable water. It's an "open" system, meaning water leaves the system and gets replenished with fresh new water.
A boiler is a closed loop system that boils the water for use in radiators that can heat whole complexes. It can also heat potable water running in separate coils through the boiler, for drinking or showering purposes. The water in a boiler is constantly reused, which is why the system is more efficient.
So, boilers tend to be large and complex and serve big buildings. Hot water heaters are generally in individual residences, though some hot water heaters can now compete with boilers.
Ergo, most of those solar things on Israeli roofs are probably hot water heaters, not boilers. I understand that there are some that can now boil water and keep a house warm, but they're still the exception. See https://www.israel21c.org/new-solar-device-keeps-homes-warm-even-in-cold-climes/
I was marked wrong for translating דוודים as "water tanks." Really, DL should check other dictionaries. Morfix, the one I usually test reality with, says "water tanks" is correct. And so do my bilingual (Heb/Eng) RL friends. I live in Israel and am trying hard to gain more of the language.
I agree with Dov's definitions of boiler and water tank, and think that a person can't have a lot of boilers. It's even a stretch to have a lot of water tanks, but I'll take that over boilers.
and I like the translation "hot water tanks" even better. I gave lingots to two people above who 1) first mentioned them and 2) confirmed that it's what Israelis use.
I've seen them here in Jerusalem on top of roofs (solar and electricity-run) and inside apartments (electricity only).
I repeated this lesson in hopes of being able to report an odd pronunciation I heard when doing this on my smart phone. The man read a sentence with the singular of "boiler" which, I would swear, he pronounced like uncle ("dod" with a kholam instead of "dood" with a shuruk, even though DL actually gave us the vowel dot on this rare occasion. But this time through I didn't get the same sentence, only this one with the plural, "d'vadim" where the vav becomes a consonant. BTW, my dictionaries (all old) give, in addition to "boiler," such meanings as kettle, pot, vat, cauldron. I tested this sentence with kettle, and DL marked it wrong. Is it no longer used in modern Hebrew for kettle? I see another comment (Mazzorano) giving "kumkum" for kettle, but I thought that was small, specifically a teakettle. He also gave "m'khamem" (plural "m'kham'mim"), but Alcalay (1965) translates that as "heater" which you can see from the root. Note, the man's sentence I heard earlier was: הדוד נמצא על הגג
I guess boiler vs hot water heater is very regional usage. I live in NY and we have a boiler in the basement. That’s what we call the tank that heats water for the house. It’s not a “closed system” as described in another comment. The water heats and we get it in our sinks and showers/bathtubs. Then down the drain it goes, out into a sewer system. I guess it could be called a water heater, but in this area, we use the term boiler.
The tiny tankless water heaters here do not replace the "dood shemesh" (דוד שמש) as far as I can tell. Especially the one that goes in the shower doesn't get hot enough to go through the entire winter (like right now, during the coldest days of winter). My dood shemesh has an electric switch so that I can heat water even when it's cloudy and raining or snowing. The tiny water heaters, in my experience, are most useful and economical during spring and fall.
We just replaced the one in our shower; we were told that it's called an ATMOR, after the company that makes them - printed only in Latin letters and not in Hebrew. If they did, אטמור perhaps?
If a doctor said “I have a lot of patients”, someone hearing her might think that she said “patience”. All languages have homophones, words with the same sound but different meanings. Context determines the meaning. Dodim means uncles or boilers depending on the context.