The fact that the bypass wall Tankless water heaters there are so-called "priority of hot water." That is, until the hot water remains open, batteries, radiant heating your home, do not get from the Tankless water heater for radiant heating homes or degrees Celsius. What are you most likely while you wash dishes or take a shower, do not even pay attention.
Nevertheless, getting hot water with great comfort it is quite possible the combination of single-loop Tankless water heater with a Tankless water heater. In this case, the task of preparing hot water assumes no secondary heat exchanger, a Tankless water heater, the use of which (in contrast to the heat exchanger) allows you to always have a supply of hot water (160-200 liters), and you do not have to worry about that at the time of adoption bath radiant heating heat transfer plates can cool off somewhat. In addition, when using the Tankless water heater at the opening of the hot water, you immediately get the desired result, instead of using a two-circuit of the Tankless water heater, which needs some time to tap flowed really hot water.