How to properly size a residential hot water boiler and radiators




How to properly size a residential hot water boiler and radiators

John White, Energy Marketing Service


A properly designed and sized hydronic heating system will insure your comfort and save many dollars on installation and operating costs.
There are a lot of internet sites offering boiler sizing information. Almost all use rules of thumb for sizing the boiler and only a few tell you how many feet of baseboard convectors or radiant piping should be installed in each room in order to assure even heat distribution throughout the home. Its not unusual to find a 140,000 btuh boiler along with 200 linear ft of baseboard convectors installed in a 2000 square foot home, although an 80,000 btuh boiler and 100 linear ft of baseboard may have been more than adequate. The savings in equipment alone could be well over $1000 if only a heat loss calculation is performed.

If you are replacing only a boiler and leaving the existing baseboard or radiant floor piping in place, then a simple whole house (block load) calculation will determine the new boiler size.


This load calculation indicates 53,687 btuh are needed to heat the sample home. Therefore a boiler with an output of at least 53,687 btuh must be chosen.

If you are replacing the entire system, boiler plus radiators or radiant flooring, then a room-by- room calculation must be performed. This calculation will determine the correct amount of baseboard or floor piping to be installed in each room.



The illustration above indicates 6223 btuh are needed to heat the kitchen. If the baseboard manufacturer's specifies 580 btuh/ft., then 11 feet of baseboard will be needed in the kitchen (6223/580 = 10.7). Likewise, the living room will need 7500 btuh to keep it warm, therefore 13 feet of baseboard will be needed (7500/580 = 12.9).

A heat loss/gain calculation is not rocket science. One can be preformed by anybody capable of calculating the square feet of a homes' walls, floors, ceilings and windows. Below is a link to an extremely affordable (only $49) HVAC load calculation program that will produce a heat loss calculation plus a heat gain calculation (sizes A/C) and operating cost estimator. You will be able to compare one system to another or one fuel vs another. Its well worth looking into.

EMS HVAC Load Calculator