Let me try to visualize your system first.
- You have a high efficiency boiler that provides domestic hot water and another hot water tank that provides hot water to the baseboard system.
- That tank is heated by a heat exchanger in it, running on the hot water from the boiler?
First of all, the hot water tank would create additional lag time. There is no reason to use another tank; a straight heat exchanger between the boiler loop and the baseboard loop would be simpler and would eliminate a lot of lag time and storage losses of the second tank.
The other option for your problem is that the heat output of your baseboard system is simply too low. There are two components to this: the actual size of the baseboards and the temperature of the hot water running in them. Increasing either will increase the heat output.
- Radiant systems should be designed to operate on as low a water temperature as possible, which will result in a more comfortable home and higher energy efficiency.
- If your system is already installed, though, it is much easier to just raise the water temperature if it is underperforming.
To solve your problem further, you would need to measure how hot the water gets in your system and how long it takes to get to this temperature. If it reaches the target temperature fairly quickly, but the house takes forever to heat up, your baseboards are too small and the water temperature will need to be increased. If, on the other hand, it takes a very long time for the system to reach its target hot water temperature, we will need to look at the source side of the system.
For more information:
Also, read Bill Bradbury's Q&A "What materials will I need for a hydronic baseboard heating system?"