My son purchased a house with a hydronicly heated family room.
At the present time when you need heat you have to plug in the pump to circulate the water and warm the room
I have researched the equipment needed to install a thermistat control.
The queston my son has is it now takes 4 to 6 hours after the pump is turned on for the room to start heating up. How will the thermostat keep the room constant without always playing catch up with the temp lag.

You are describing a hot water heated slab. It takes a long time to heat up a ton of slab. When you add a thermostat in the room and relay to control the pump, the heat will remain fairly constant. Slab heat is not meant to be set down to a lower temperature and then turn it up daily.

If you want to be able to set the temperature lower to save energy, control the slab heat with an outdoor reset control to keep the room temperature down to 60 at all weather. Add baseboard heat to the room to bring the temperature of the room up and down as desired with a setback thermostat and zone valve on the baseboard loop.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.