One pipe boiler sizing

Jim Macdonald

An architect I work for redid a house with a new owner. One radiator (not the furthest) is not working and I know they rerouted the piping which is probably the problem.
The existing boiler is on its last legs and the plumber is insisting on increasing the boiler size "just because". Since the farthest radiator from the boiler is heating satisfactorily I believe the boiler size is OK. What, if anything, is the downside I can use to convince them that an oversized boiler is not the right way to go.

Tim Gardner

boiler sizing

The sizing should be based on the EDR. Your intuition is correct, it is a mistake to oversize the boiler.

I recommend the book The Lost Art of Steam Heating, sold on this site. It explains exactly how to do it and it is a fun read, and it will save you a lot of money by showing you the right way to do things related to steam heat.

Mike T., Swampeast MO

Is this steam or hot water? If steam the new boiler should be sized to the existing radiation and piping--there's no other way with steam.

If hot water, the new boiler should be sized to the heat loss of the house--again, there's no other way with hot water.

For whatever reason some? many? installers use the "nameplate" sizing method and frequently add in some extra "just because"... BAD IDEA ON BOTH ACCOUNTS!!!! Oversized boilers waste significant amounts of fuel! If your plumber cannot understand this and refuses to either size a steam system to the radiation/piping or a hot water system to the load, find someone else!

ALH_4

oversizing

Edit: I am assuming this is hot water.

Start with a heat loss calculation, and go from there. The existing boiler may actually be oversized. From your descroption, it sounds like you are right about there being a piping problem. Are the monoflo tees installed correctly? Is there something different about that radiator or its piping? An oversized boiler will tend to short-cycle, which results in reduced efficiency. A modulating condensing boiler may be an appropriate replacement. Thermostatic radiator valves would be a nice addition to the system.

Mike T., Swampeast MO

The major similarities between a hydronic space heating system and a domestic water system are that they both use pipes and both include some sort of mechanism to heat water. Other than that, they generally play by very different rules. Ask you plumber for references with regards to space heating work and check them--to include asking about what sort of work was done.