Your unit is Tankless, meaning it heats as the demand for hot water is requested - by opening a hot water faucet or by a heat exchanger for household heating.
I suspect that your unit is undersized for the load it has during the winter when you are heating your home. Read further for details..
I do not know how your heating system is configured, baseboard, floor mounted radiators per room, under slab unit or an aquatherm in the air handler .. you did not say, however; all of these units whatever one you have for heating are calling for hot water and probably recirculating it in a loop type system, they have a flow rate and it might be on demand or continuous. The number and types present to you the equation for your radiated heat requirements (demand) and in order to obtain that your Tankless water heater will need to provide sufficient hotwater to keep up with that demand. Again the demand equation is based on the heat exchanger types and quantity (total volume) as well as the temperature setting.
I am thinking that your heater continually runs but at a low flow rate in order to maintain household temperature - I do not know this for sure as again I do not know which type of heat exchanging system you have. If we knew this we would have a more accurate answer.
One more thing to add
The unit should really have a method of controlling the water temp depending on load and where that water is going. For example max temp to the heat exchanger but a limiter on the household supply line If it was plumbed properly for that functionality.
I have an Aquatherm and I typically adjust my hotwater heater up about 15 to 20 degrees in the winter to accommodate for the load (it is a storage tank).