What size boiler to get for forced hot water baseboard 2 zone 2 floor apartment

[WishfulThinker]

A friend has a rental property north of Boston. The tenant who has the combined 2nd and 3rd floor apartment with a total of 153 feet of baseboard and 8 rooms has had a lot of problems with the heating system going out. It's an old gas boiler in the basement that needs to be replaced. I think the old one was about 150 BTU but one plumber said he should get one a bit smaller 142 or so BTU. He said the old one has a lot of problems and has old style (?flare valves) and such. But he said the piping to the apt. is fine (3 pipes to 2nd floor and 1 pipe to 3rd floor). He said you don't want an oversize boiler, just within 10% of what you need. It's an old house and I doubt if it is well insulated.
Another plumber who hasn't see the boiler yet said he needs 200 BTU.
I googled "problem with oversizing" and found "short cycling can become a problem".
The second plumber is less expensive...
Any advice on how to choose a size? Thanks for any advice!

 

[Dj2]

There are websites where you can plug in your numbers (size, age of the house, etc) and find out what you need.

Also, you can get another plumber to bid. 3 are better than 2.

 

[Sponsor]

 

[hj]

There are many factors to consider when sizing a boiler . Floor area is only one of them, and a fairly minor component at that. Unless it was heating properly, the amount of baseboard may also give an erroneous result. An oversized unit may short cycle, but an undersized one may NEVER turn off and still not heat the area.