There is a third type called a combi water heater that can actually do both.
Closed hot water heating system.
Closed systems are often used in second homes or primary residences in areas prone to long power outages.
All water heaters waste heat energy even when the burner is off and the unit is sitting idle between heating cycles.
A closed water heater system means that the hot water cannot expand beyond the valves and mix with the municipal water.
The valves that are usually found in the closed systems are the check valve pressure reducing valve mixing valve backflow preventer and other.
Since on many hot water heating systems a key air bleeding or air vent point is at the air scoop or air separator closer to the boiler.
The down side is two heat sources.
Hot water chilled water tempered water process water and glycol filled closed loops are often ignored when considering a water treatment program which can be a costly mistake.
Unlike conventional water heating systems closed hot water boilers should only be flushed when operation problems occur.
A quick visual inspection can tell us if there is obvious leakage corrosion or clogging of some automatic air separator devices and fdor some automatic air purge valves a simple step of opening a valve cap.
Closed loops that are not properly treated can cause significant corrosion damage.
In modern systems closed tanks are preferred.
Closed loop water treatment.
If freeze protection is an issue than a closed system with anti freeze is a good idea.
Remember to give the system time to drain and fill.
One circulating heated water to the rooms and the other returning the much cooler water back to the boiler to get reheated.
These are for closed systems usually called hydronic systems and they are just for heating.
They can be used with radiators or radiant floor heating.
Hot water systems are closed meaning that the water constantly circulates through the pipes and into the boiler.
The expansion of water 4 7 from 0 o c to 100 o c can be collected in an open expansion tank above the rest of the system or inside a closed pressurized tank close to the boiler and safety valves.
Closed loops that are leaking can waste water and energy while causing corrosion.