Sizing a boiler with a tankless heater option

DK27

I'm in the process of verifying that my boiler is properly sized for my heating system.  From reading all the other great posts I understand how to calculated the EDR of each radiator in my house and compare this to the net steam sq ft my boiler produces.  My boiler setup utilizes the tankless heater option to heat a radiant floor in an addition the previous owner added. My quetion is how do I account for the tankless heater option connected to my boiler in determining the correct sizing of the boiler.  Appreciate the help in the advance.

Jason Quinn

Tankless GPM

I believe one way is to find out the gallons per minute output of your tankless coil. Each gpm requires 62,500 btu for a specific temperature rise. This should get you in the ballpark.

kevin_58

boiler size

Size boiler for steam only. You can only use pickup factor for tankless. Hot water zone can not exceed that.

DK27

Tankless heater

Thank you jason and kevin for the responses. I think if i have this correct the responses sort of say the same thing, but basically kevin is saying that the btu draw of the tankless heater option is already accounted for in the pickup factor?  Do i have this right?

kevin_58

boiler size

When sizing steam boiler with tankless coil for heating, Size the boiler for steam only. For the heating from the tanklesscoil, you can only use up to the pickup factor of the boiler. The pickup factor is usually 1 third of the steam load of the boiler. A pickup factor is an extra allowance for steam heat loss in piping between boiler and radiators. If you use more than the pickup factor the steam side will suffer. If you size the boiler bigger for the coil the steam side will be oversized.