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        riddles >> easy >> Cold sweat
        
(Message started by: denis on Jan 26^th, 2009, 10:18am)

Title: Cold sweat
Post by denis on Jan 26^th, 2009, 10:18am

I have a house that is centrally heated with natural gas.

The furnace is a 45,000BTU/hr gas furnace with natural gas costing $0.36 per cubic meter. Assume negligible electric requirement for the fan blower and that the furnace converts the natural gas to heat at 90% efficiency .

On the north side of the house is a room where is my office/PC I spend most of my waking time. In winter the furnace is needed to heat the house but I only really need to heat the room I use during the day and I only need to heat the bedroom during the night. In other words, I can substitute the furnace with strategically placed electric base boards heaters in the areas I really use.

Assuming the electicity utility here charges $0.10/KWh, how many 1000W electric baseboards heaters can I use concurrently (assume 100% efficiency) until I exceed the break even point and should just turn on the central furnace instead?

I thinks all info required is here, but if something is missing, let me know and I will provide it.

Title: Re: Cold sweat
Post by towr on Jan 26^th, 2009, 10:36am

[hide]10⁶ Btu = 28.263682 m³ gas

(45000/0.9) * (28.263682/1000000) = 1.4131841 m³/h

0.36*1.4131841/0.10 = 5.08746276 KWh

So 5 1KW spaceheaters
[/hide]

Quote:

(assume 100% efficiency)

I doubt they'd be less efficient, because any loss in efficiency would be waste heat ::)
(It could be more efficient if it used a heat pump, but then it wouldn't really be an electric heater.)

Title: Re: Cold sweat
Post by denis on Jan 26^th, 2009, 10:59am

OK follow up question:

Suppose I'm in Canada where it's 20 below in winter. In order to keep the pipes in the basement from freezing and to keep the bottom floor at a "reasonable" temperature, I need to keep the furnace running for 20% or the time regardless of space heater use. How many strategically placed 1000W space heaters can I concurrently use if I keep the furnace running minimally 20% of the time instead of keeping it shut while staying below the break even point? i.e. How many space heaters + 20% furnace to keep below the cost of 100% furnace?

(actually looking at towr's third line above, the answer just jumps right out)

Title: Re: Cold sweat
Post by denis on Jan 26^th, 2009, 11:18am

on 01/26/09 at 10:36:59, towr wrote:

I doubt they'd be less efficient, because any loss in efficiency would be waste heat ::)

There could be some wasted energy, albeit negligible, that escapes as light (e.g. when the heater coil glows)

In light bulb it's the opposite situation, wasted energy shows up as heat.

Title: Re: Cold sweat
Post by towr on Jan 26^th, 2009, 12:54pm

on 01/26/09 at 11:18:44, denis wrote:

There could be some wasted energy, albeit negligible, that escapes as light (e.g. when the heater coil glows)

Only if you don't have the blinds down. ;)

I suppose some energy might also escape as 'noise' on the electrical lines.

Quote:

In light bulb it's the opposite situation, wasted energy shows up as heat.

And consequently, in the winter there's little reason to turn lights off to save energy. (If you have the blinds down)