[Greenbuilding] hot water versus forced air--and window AC versus central

[John Straube]

This is a common question, and a good one. 

oth systems deliver heat, but the hot water radiator requires less electrical energy to pump than a forced air furnace requires to move the air.
However, radiators in general are high temperature devices, and as such the return temperature of the water is often too high to allow condensation to occur in the boiler, this limited the combustion efficiency to about 82-84%
A condensing furnace will always act in condensing mode (as the return air temperature is low and ensure condensation) and hence a 94-96% efficiency is reasonable.
The difference in efficiency tends to overwhelm the savings in electrical power.
If you can ensure that the temperature of the water returning from the cast iron radiators is 120 F or lower, then a condensing boiler (of 90-92%) will  generally beat a furnace.

Complicating this assessment is the need for ventilation air. If you need to run the fan for ventilation anyway, the benefit of the boiler becomes less. Of course from a noise and comfort point of view, many people prefer radiators and radiant floors.

RE: AC. Room mounted "window rattlers" with EER10-12 or so are very likely to beat whole house units of even SEER15-17. This is simply because people dont run them and mix the air in the house sufficiently. You are using a lower efficiency unit that is cooling significantly less of the house. Hard to prove this, but people I know who run public and rental housing say there is no competition: window AC always use less. Again, comfort is a different story, and people choose the quiet, worry free operation of the central units.



Ben Pratt wrote:
> I would like to know which uses less energy to heat a home-all else 
> being equal: forced air or cast iron radiators.
>     I have radiators now, and like the even heat, but feel wasteful in 
> that It is usually impractical to turn down the heat at night or when 
> leaving the house for work.  I could get a deal on a forced air 
> system (from a friend that pulled a newer one out of his house), but 
> do not have a clear understanding of whether my gas bill would go up 
> or down.
>     I was also wondering whether having window AC units, and only 
> using them in only certain parts of the house, makes sense versus 
> getting a central air system.
>      Certainly, the forced air and central AC is more convenient, and 
> takes up less space, but which is more efficient? In surfing the web, 
> it is hard to find a unbiased opinion.
> -Ben
> 

-- 
Dr John Straube, P.Eng.
Associate Professor
Dept of Civil Engineering & School of Architecture
University of Waterloo
Waterloo, ON Canada