Thread: Boiler/Water Heater

Just for my general understanding, what is the difference between a boiler and water heater? (FYI, I am not in the trade, just find this to be an interesting forum.)

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boiler - used to heat water in hydronic heating system (baseboards)
water heater - used to heat up domestic water (sink, shower, etc)

BOILER. A self-contained appliance from which hot water is circulated for heating purposes and then returned to the boiler, and which operates at water pressures not exceeding 160 pounds per square inch gage (psig) (1102 kPa gauge) and at water temperatures not exceeding 250°F (121°C).

WATER HEATER. Any heating appliance or equipment that heats potable water and supplies such water to the potable hot water distribution system.

(from the IRC)

Zibby and Jerry, thanks. Your posts make the difference quite clear to me. Merry Christmas!

Now, to make things interesting, a water heater can be used as a heating appliance.

Only if its desihned for it (ex Rennai units) and at that point it would be a boiler

Originally Posted by Zibby Bujno

Only if its desihned for it (ex Rennai units) and at that point it would be a boiler

It would still be a water heater unless the pressure/temperature exceeded that of a water heater, in which case it would no longer be suitable for use as a water heater, then, yes, it would be a boiler.

open systems (outlets for use, regular flushing, replenishment) such as a potable system may use a water heater. closed systems, i.e. without distribution outlet require boilers. Stagnation and dead ends are unsanitary, and should never be present in a potable system.

Originally Posted by Darren Miller

Now, to make things interesting, a water heater can be used as a heating appliance.

Correct.

The water heater portion of the tank heats the water and feeds the domestic hot water system while the closed system of the tank (a coil located within the tank) circulates heated water through the heating system ... however, that is still a water heater, those models just have additional components which supply heated water to a closed heating system.

How about:
A boiler can heat water as well as boil it. A water heater can only heat water and should never boil the water.

So boilers are heaters : heaters are not boilers.

Originally Posted by Jerry Peck

...those models just have additional components which supply heated water to a closed heating system.

Not exactly. Its a boiler with heat exchanger and expansion tank (different designs) to heat domestic water. Again, its not a water heater, but a boiler

Originally Posted by Zibby Bujno

Not exactly. Its a boiler with heat exchanger and expansion tank (different designs) to heat domestic water. Again, its not a water heater, but a boiler

The ones I've seen posted here (have not seen one in real life ... heat - what is 'heat'?) ... are water heaters with internal coils for heating the water for heating systems - those are still "water heaters".

I suppose you could have a boiler with a coil for heating water taken to a hot water storage tank, but you would need to be concerned about the temperature of the water and the pressure of the system (the pressure of the system should be independent of the pressure of the boiler, but the temperature of the water would not be independent of the boiler).