radiant heat system with tankless water heater re-design

wykrrr

Hi,

I have a dedicated tankless water for my home radiant floor heating. The floor is concrete slab and the pipe in concrete is 1/4" or 3/8" copper pipe.
This is the current system:



This is the proposed system with additional components:


The changes are:
1. Add air separator and expansion tank on hot line discharging side.
2. Add pressure reducer (maybe plus a one way protector) for the city water injection
3. Move pump to hot line after expansion tank with two service valve: drain and purge valve.


My questions:
1. Can I use ball sized expansion tank? Like this one: ZEP-1. Because I calculate the expansion tank size from here, it shows I only need 0.2 gallons size for my system with less than 5 gallons total water raising from 70 degree to 130 degree. Or should I use the traditional 2 gallon sized expansion tank?

2. To start the system, my plan is to turn on drain value, connect to a garden hose to a bucket. Then turn on purge value, connect to a garden hose to an external pump in the same bucket to remove air in the pipe. For this purpose, is there any requirement for the external pump? Can I use the pump for normal water heater flushing?

3. Is it necessary to move the pump from cold line to hot line?

4. Is the Y strainer necessary to filter out debris in the pipe? As far as I know, some stuff in the water is produced by the chemical reaction between the water and copper pipe. These stuff is good for the system. I am not sure Y strainer will filter these stuff out.

Thanks a lot.

hot_rod

It always comes down to what size circulator is adequate to keep the tankless happy. In most cases it is grossly oversized for the radiant gpm requirement.

Some tankless manufacturers show the pressure drop across the tankless at various flow rates to help determine if primary secondary makes more sense for the piping.

Consider a combination 5461 air/ dirt, mag separator for a few more bucks.