Removing Lime Buildup in HVAC Systems

You can use white vinegar. The acetic acid in it will dissolve lime build-up. If you need something stronger you can get a more concentrated version of acetic acid. There are household cleaners such as Lime-Away and CLR (calcium-lime-rust) that both will remove lime.

Clear tubing (like tygon) also can get clogged with fungus build-up. You'll see a fogging of the liquid in the tubing and perhaps cloud-like globules floating in the fluid. If you're seeing that, you'll need a fungus killer like a solution of bleach and water.

I have used this product to remove lime and scale in Arc Furnace Electrode holders and various cooling heat exchangers over the years. Link is http://www.scalebuster.com/

Best product for the application you seek to resolve

How big is your exchanger? Are you sure its lime?

Before selecting a chemical for cleaning, be sure that it reacts well with the deposits (you can check yourselves). Better to contact expert agencies in this field who can analyse and suggest correct method.

If exchanger is big (say above 2m in length), you should go for a chemical circulation system through tubes. Steam heating or electrical heater can be used to raise temprature of chemical solution up to 50-60 deg.C. Circulate for about 5-8 hours, rinse with plain water and then passivate it by circulating alkaly solution for about 1 hour. Then open the end covers and blow the tubes one by one with pressurised water jet. For smaller exchangers you can emerse the tube bundle in a tub filled with mild chemical solution and leave it over night. Next day morning blow the tubes one by one with pressurised water jet.

In my experience a mild sulphamic acid solution (5 to 8 % strong) may useful for it. Tri-sodium phosphate can be used for passivation.

....and check also that the cleaning agent is compatible with the wetted materials of the system, lest using it should inadvertently cause a corrosion problem elsewhere.