High calcium in ground water builds up on all faucets and bumsprayers which I regularly soak in an acid solution, but how to clean/dislove the calcium build up inside my on demand hot water heaters?? The only way I can think of would be to disconnect plumbing, place a pump in line and recirculate water from a plastic bucket.

That doesn't sound like much fun to do and could I do any damage to the hot water heater.

the acid I have used is vinegar [slow] or hydrocholoric acid [fast, but dangerous].

if you think that heater doesn't work properly, only than disconect from plumbing and fill with vinegar for a day, if needed repeat.

Suspect you could use a simple siphon pump like for siphoning gas from a car?

Is it possible to remove the heating element. If it is clean your water heater will be as efficient as when new. Calcium build up on the tank it’s self should not cause problems. If the tank is not stainless steel I wouldn’t use any acids.

How about "Lime-A-Way"?

How about "Lime-A-Way"?

There's a product called 'Calgone' saw it in Homepro

Have you seen Lime-A-Way here? Used to use in South Florida and it does work well. But I have not noticed any lime build up here in Bangkok (river water) but suspect upcountry wells may have.

True Value hardware shows they have it here

"True Value hardware"

Most of the prices on the website indicate "0". However, a can of Lysol air freshener is 540THB. Ouch! Using a local product would probably prevent you wallet from hemorrhaging.

Many of the products at the "True Value" hardware stores are imported from the USA and they are not"True Values" but the poster asked where to get the Lime Away not how much it cost.