DHW and Cast Iron Radiators with Tankless?

I haven't been able to find a great answer for this:



I have a space that has a heat loss of 24,000 BTU/HR, one shower, one bathroom sink and one kitchen sink. I want to use one system to heat the place and provide domestic water. I have 4 (2 per zone) cast iron radiators at about 8000 BTU/HR in size. My water coming in is about 40F.



The solution I cam up with is to run either a combo water heater with a built-in heat exchanger, or a tankless heater with an external heat exchanger. The idea is that you really only need hot water to shower for about 10-15 minutes. Since I have cast iron radiators the space can deal with no heat for at least that time. I would install a flow switch on the DHW supply which would disable the circulating pumps. Once the shower is done the heating system starts back up. I want to run a pump on the DHW side, a heat exhanger, a pump on the cast iron side, a differential bypass valve, two zone vales connected to thermostats and run everything with 1/2" pex. My concerns are as follows:



1)Can I use a tankless for this application. for 24,000 btu/hr I only need 2.4 GPM@deltaT of 20. I am in Ontario, so if I run the tankless at 120 I should be ok with estimating deltaT of 20? Most tankess will do 90deg temp rise at 5GPM, my water is well water at 40F.



2)Can I get a pump that is small enough to push 2.5GPM? I can probably go up to 4GPM (don't want the water to be too fast going through the rads) My longest run is 50 ft, so my head loss is only 2.5. Will the bypass valve maintain my GPM?



3) For control, I want the thermostats to switch the zone valves, and enable the pumps, I believe this is typical. I would love to find a thermostat that has unoccupied,occupied modes so I don't have to change all the programing when we leave for a long period of time ( it is an accessory building that is not always occupied and sometimes we just need to maintain minimum temp)



Can anyone help?