The reason for this is to have the two water uses separate.
How to heat both my floor and sepate domestic water.
The two systems are basically tied together.
Normally the wood boiler is plumbed to a heat exchanger see drawing above.
The idea here is to prevent domestic water from getting contaminated by stale water that sits uncirculated in the heating system during the off season.
The same water that ends up in your hot shower or dishwasher for example has passed through the floor first.
One option is to keep domestic hot water completely separate from water that s part of the heating system using a heat exchanger.
As you can see this allows the boiler to heat a tank of potable water which in turn can provide domestic hot water and floor heating in an open or closed configuration.
Hot water from the boiler is circulated through radiators or a hydronic heating system to warm the ambient air.
The water heater to use with radiant floor heating should be a high efficiency unit.
The best kind is one that has a sealed combustion chamber and an external heat exchanger.
Even if only part of the home uses.
The traditional boiler water may also be used for domestic hot water by directing the water through a separate integrated water heating unit before routing it to a fixture such as a faucet or shower.