A double -circuit boiler is more expensive than a single -circuit. Double -circuit is able to give the house and hot water with heating, it has additional components. But if you buy a double -circuit boiler of medium power, then it will not be able to warm the house equally well and heat water. For the maximum result, it is better to spend money and buy a good powerful boiler. By the way, for a comfortable water supply, buy a boiler boiler.