[Greenbuilding] distribution of radiant heat

[John Straube]

I think all Canadian fireplaces need a supply of outdoor air by code or standard but I can not find the reference that makes this so.
When you put a front door on the fireplace, it approaches sealed combustion, but some doors can be quite leaky.  I hope all on the list know that fireplaces without front doors are very very inefficient, and can be unsafe in an airtight home (risk of CO poisoning).
If the house is very well insulated, it is actually difficult to use normal size fireplaces without causing overheating.  In my own house, I chose a fireplace based on, in part, the size of the fire viewing area.  The view of a fire is a big part of the reason for us having a fireplace.  The problem is, the fireplace is rated at 50 000 Btu/hr.  My house is well insulated and airtight with good windows in a 7500 HDD F climate and so has a peak heating load in the order of 12-15 kBtu/hr.  Hence, even on the colder 0 F (-17C) nights, we would not want to run the fireplace flat out because it would cause serious overheating. On more typical days (20F, -7C), we run the fireplace in bursts, and have to try hard to reduce the desire to throttle it down as it then does not combust as cleanly or efficiently.
The masonry heater option would be practically much superior.  We could fire one really hard and thus efficiently, and the mass would nicely smooth out the variations in heating output, this allowing for easy dialing in of the heat output I needed (either 6000 Btu/hr or 12).  This  was seriously considered by us, but we could not easily find a large glass door version, AND it would have cost about 15-20000 dollars more, AND the house did not have the foundation and structure to handle the mass.
So while a masonry heater would be technically superior, I dont think it is necessarily the best solution for Chris' house on an island.
As food for thought, I use an ECM airhandler on low speed (300 cfm) to move air through the whole house (and filter it and distribute ventilation air) which draws 30W.  This delivers less than 50 cfm to each of the room furthest (40 ft) from the room with the fireplace.  I dont think it makes much difference to temperatures at all (based in turning it off for a few hours at a time).  It is frequently 75F, and even 78F in the living room at one end of the house, and usually 68-70F in the far end of the house.  The heat flows by conduction through walls and natural convection through open doorways.  Close the doors, and the temperature in these far rooms drops more than 5 F, even with R40+ walls, triple glazed windows, exceptional airtightness.

The discussion is titled distribution of radiant heat, which is not really accurate.
The wood burning appliance IS producing a lot  of radiation, but the radiaiton absorbed by surfaces in the room, whereupon it is turned into heated air.  Like all heated air, (heated by radiation or conduction) it rises.  Hence why most of the discussion centers around moving air.  
The equation for heat movement by air is
Rate of heat delivery in Btu/hr = 1.06 * Airflow rate (CFM) * Temperature Difference (F)
e.g., for a 125 CFM fan, sucking 78 F air from a ceiling and delivering it to a 68F bathroom the heat delivered would be
1.06*125 * (78 - 68) = 1250 Btu/hr (as an engineer, I round off 1.06 to 1.00).
This is a respectable rate of heat deliver for a bathroom, and many will accept a 68F bathroom (and some wont)
Remember to provide a return air pathway, which in this case could be a grille in the door. At 125 cfm, a 1/2" door undercut would not be sufficient.

Just BTW, doe hydronics, the equation is 
Rate of heat delivery = 500 * water flowrate (GPM) * Temperature Difference (F)
e.g a 2 gallon per minute flow, with a difference of 10F between supply and return will deliver
500 * 2 * (10) = 1000 GPM.

As the discussion is radiant heat, another equation is for heat delivery by radiantly heated floor is
Heated floor delivery = 2 * Difference between floor temp and room temp * square feet
e.g., if a 150 square foot floor surface area is 73F, and the room is 68F
2 * 5 * 150 = 1500 Btu/hr of heat can be delivered by a radiant floor.




On 2011-01-08, at 7:34 PM, JOHN SALMEN wrote:

> 
> 
> I'm a little confused. I had no problem with the term fireplace as here they
> are typically sealed units and the term is used to differentiate from
> freestanding wood 'stoves'. I have not seen an 'open fireplace' built (that
> is the term that used to be used here) for over 20 years (and it was a
> rumford).
> 
> What I am confused about are what the calculations are in reference to? (I
> don't really understand them) and what the point is of making a comparison
> to propane? I get the point about embodied energy and financial cost but the
> question here is about distributing heat from a central source.

Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture
www.buildingscience.com