Metal plenum with cast iron heat echanger - what is it ?

SJS55

I am a home inspector inspecting a large antique home in Cambridge MA



The house has a hot water boiler connected to two-pipe cast iron radiators



However it has six metal plenums with cast iron heat exchangers.

Hot water is supplied to the exchangers and is returned to the boiler.

Old spiral metal ducts provide gravity hot air thru registers to the 1st floor

from this plenum.  There are access panels into the plenum.  These plenums have to be approx. 100 years old



What is this system called ?

Mark Eatherton

That is an indirect heating system...

Back in the era of cheap energy, it drew its incoming air from outside (100%). I am guessing that their outside air connection has been eliminated, and they draw air from within the dwelling.



ME

Brad White

It also

could have been a mix of outside air and return air. Some houses of that era had 100% outside air to certain spaces such as nurseries and sick rooms for the health benefits. Others had entirely recirculated air.  Earlier systems, dating from the 1880's or earlier indeed had 100% outside air. As Mark said, some later thought may have modified the source of incoming air.



Properly done with ducts pitched correctly, it was very effective.  The radiators were sometimes essentially boiler sections with iron pins. Water or steam flowed within and air instead of combustion gasses flowed through the pins to the spaces served. Dampers were used for balancing and more sophisticated systems used bi-metallic coils to open and close dampers based on room temperature. Rare to see these survive.



You mentioned spiral ductwork which seems odd for that vintage. Spiral duct is of more recent manufacture (long coils for nearly unlimited duct lengths). One hundred years ago, I would have expected lapped seams with belled ends.



If you want to see some magnificent ductwork, see if you can get into the attic of Cambridge City Hall, designed in 1888.  There are 7-foot diameter ducts up there with hand-hammered rivets.  Beautiful work. Those tin-knockers must have had wrists the size of fire logs.