Best Of

I have been in your situation so I would recommend the following:

• Try to get your customer to agree to a less expensive radiator like euro panel radiators
• Once you have the radiators selected, do a room by room heat loss calculation and select the proper size radiator for each room.  
  • They will most likely be in the same location as the previous radiator but a little smaller
• Determine the riser and return pipe size for each radiator
  • Since the old system was most likely gravity and the new system will be using a circulator the pipe sizes will be smaller
• The least amount of piping work will be to use a Reverse Return parallel design.  That looks like this:
• You start at the boiler with 1” or 1¼” copper, depending BTU requirements of the zone.  
  • 1” piping will move 80,000 BTU of heat to the radiators. 
• You can then select the pump required to move that much water thru the radiators.  
• At some point you can use a common return for both zones and increase the shared piping as required 

These steps will keep your cost down so you don't need to spend more $$$ on oversized pipes and extra pipe runs that will be doing the same job right next to each other.  Example below:

You can see in the first diagram on the left there is not enough heat in 3/4" copper to get all the baseboards hot

You can fix this with two zones as illustrated in the center diagram, with two sets of ¾” copper running the length of the house to accomplish this task.  

The left diagram is a less labor and less material cost is illustrated by using a 1” copper shared return from the far side of the house. 

PS: EDIT

I have actually done that job in Wildwood NJ many years ago. A nice elderly lady on a fixed incom asked several contractors for a price of a new boiler because the old boiler was not big enough to heat her home.

The replacement boiler prices she received were way out of her price range and the replacement boilers would not fix the problem anyway. I offered to put a 1" copper return pipe and common supply pipe from the center of a baseboard loop back to the boiler room for less that 15% of the price of the lowest boiler estimate. Took about 5 hours from start to finish and she had great heat in the entire house. No new boiler needed!

Save yourself and the customer some $$. The old system was probably oversized. Have (or do) a heat loss calculation room x room.

Once you know the heat loss x room

1. Pick the radiation that will heat the house at 180 deg water (you can go to a lower water temp with more radiation and save some fuel)
2. determine the water flow in each zone (1 gpm will deliver 10,000 btu)
3. Size the pipe by gpm 3/4=4gpm, 1" 8 gpm, 1 1/4 12gpm etc
4. make piping layout and select circulators.

These are the basics. You can get in the weeds and design all kinds of systems.

you asked for our advice

You were given that advice
Yet you’re going to ignore it

It’s your equipment, your responsibility