Saturday, 20 July 2013

Masonry Heaters - 2

The design of masonry stove I originally specified to the engineers was the "Missouri Designed Masonry Stove" (1) developed by the Energy Extension Service at the Missouri Department of Natural Resources. This stove, as described at (1), with a firebox exterior of 27 x 36 inches, is large enough to adequately heat a 1200 to 1500 sq.ft (112 to 140 sq.m) traditionally constructed residence, when well insulated and double glazed.

Example of masonry heater courtesy of (1)

Although I intend to position the heater at one end of the main living space rather than in the middle of the house, which is generally recognised to be the optimal heating position, (as I wish to retain the look and feel of a large industrial space within the house), this should be fine for my purposes given the additional heating sources (a range cooker at the other end of main living space, a small wood burning stove in bedroom for exceptionally cold nights and the large solar window for passively heating the high thermal mass) that will also be present in the house.

Example (side elevation) of masonry stove courtesy of (1)

 Whilst the  square looking Missouri masonry stove design would probably meet my heating requirements the general design vision I had was that I the heater should be "flatter" and "spread" across and "up" the western end of the living space.  I realised that I could start from scratch and custom design one that more suited my vision but decided to check out what I could find that met my requirements and had been tried and tested.Over the winter I had time to thoroughly search the internet and found that "The Double Bell Heater" was widely regarded as highly efficient, with excellent heat retention, was a well proven design and offered flexibility in design.

The double bell heater design is based not on the use of flues as outlined previously, and used in the Missouri heater, but on what is called the system of free gas movement. The general idea here is that gases will fill any volume (bell or chamber) completely and will naturally stratify by temperature with the hottest gases naturally pushing their way to the top of the chamber. The coolest gases will naturally be displaced to the lowest part of the chamber. The reference to "free gas movement" is because no external force is required drive this stratification which occurs freely due to gravity.

Inspirational example of possible "industrial" look for
double bell heater courtesy of Catherine Croft's
"Concrete Architecture" 
Chimney draft, the driving force in all other masonry heater design, has only one purpose in a system of free gas movement and that is to pull the cooled by-products of combustion out of the system. The final chimney connection always occurs at a location low in a given volume so that the exhaust is always cooler than what remains in the heater. While in the conventional draft powered system all the gases, hottest and coolest mixed together, are moved through the whole system, the double bell system naturally assures that the coolest gases always remain separate from the hottest gases. Only the coolest gases at the bottom of any given bell are able to move to the chimney exit or entrance to another chamber. Because of this separation and the inherent fact that the only the coolest gases move onto the chimney it is generally believed that the double bell system is superior to the flue based systems in overall efficiency and heat retention.

The fact that combustion gases will fully occupy a volume of any shape or size the system of free gas movement offers unending design possibilities. The natural temperature stratification of hotter and cooler gases within any provided space assures evenly heated surfaces regardless of the configuration.

(1) "The Missouri Designed Masonry Stove" published by The Energy Extension Service, Missouri Department of Natural Resources.