[Greenbuilding] cost of electric versus natural gas and gasoline

[Nick Pine]

Maureen writes:

>I thought we were encouraging people to build tight, ventilate 
>mechanically, and not use any unvented appliances. Am I missing 
>something?

 As Keith pointed out, there's a difference between "which is cheaper?"
 and "which is healthier and more environmental?" I like sunspace
 heating, but...

>Two other alternatives: a) burn natural gas in a $200 unvented heater 
>and use a dehumidifier or an $80 window AC with a humidistat indoors to 
>condense water vapor, and b) cogenerate heat and electricity with a 
>$900 Honda EU2000 generator, which might have a natural gas conversion 
>kit. An exhaust hookah can help remove and store heat, and the exhaust 
>can depressurize a small enclosure to avoid indoor air pollution, and 
>it looks like an EU2000 can be plugged into an active wall socket, 
>illegally of course :-)

 Unvented gas appliances have been used for many years with no serious
 health effects, and they are cheaper to buy ($200 vs $600?) and more
 efficient (100%) than (60%?) vented versions and they don't require
 chimneys. If everything is working OK, their major pollutant is water
 vapor, about 11% of the heat output. We could remove it with some sort
 of air-air heat exchanger, but it seems simpler and cheaper to condense
 it indoors, ie turn the latent heat into sensible heat. We can still
 build tight and ventilate mechancally.

 If gas costs $1.50/therm, a $600 vented gas heater might deliver heat 
at
 $1.50/0.6 = $2.50 per therm. If a $200 unvented version makes 30K Btu/h
 for 30K/100Kx$1.50/h = $0.45/h and 3300 Btu/h of that is in the form of
 water vapor and turning that back into sensible heat with a 10-SEER
 window AC (I bought one for $69 last summer) takes 3300/10 = 0.33 kWh 
of
 10 cent/kWh electricity worth 3 cents, the total cost is $0.48x100K/30K
 = $1.60/therm. Winter humidification is a side benefit, and the AC can
 be in a window in summertime. A $4K Empire Mantis 93% condensing gas
 fireplace can deliver $1.50/0.93 = $1.61/therm, with a lot more
 complexity.

 Small Honda cogen seems less economical, with backup electrical power 
as
 a side-benefit. Burning 1.08 gallons of gasoline or 1.35 therms of
 natural gas in 4 hours with good heat exchange might produce 6.4 kWh of
 electricity (EU2000's can be paralleled with no special wiring, so grid
 sync seems easy) and 113.2K Btu of heat, and it seems likely the EU2000
 would last 10,000 hours, which adds about $0.09/h for a total combined
energy cost of about 11 cents/kWh, or 16 cents, with $4/gallon gasoline.

Nick