Calculate my energy savings

[Rhea]

I’m interested in the answer to this, if anyone is interested in trying to calculate it.
How much oil did I save by not pouring out the hot water?

I boil a gallon (3.8L) of water to make spaghetti on a conduction stove in a 6-quart (5.7L) stainless pot.
It boils for about 15 minutes (0.25hr) (so there might be some loss in steam)
The kitchen is 16’x24’x9’tall (4.9m x 7.3m x 2.7m tall)
I pull out the spaghetti and serve to my appreciative children and their significant others.
I leave the water in the pot until it cools, because, damn it, I paid for that heat and I’m not going to pour it down the drain to just heat the yard.
Presumably, this prevents the need to run the oil furnace for a certain amount of time.

This is repeated nearly every day for whatever is being cooked in hot water. I pour no hot water down the drain, I always heat the house with it. But I can get satisfaction out of understanding just one pot of spaghetti.

If you need to make assumptions about furnace efficiency, or room insulation or anything, state your assumptions so that I can copy them into this OP for others to use (or argue about) the same assumptions.

 

[Rhea]

When I think about calculating this, I imagine simplifying by asking how much does one gallon of boiling water change the temperature of one 16x24 room. And the comparing how much oil is needed to change the tempertaure of that room by the same amount.

 

[rousseau]

We have a high efficiency furnace that is constantly keeping our entire house warm. During a cold month we might pay between 100 - 150 dollars CDN in energy costs. Divided by 31 days that works out to about 5 dollars a day on the high end.

Your pot of water likely heats up a small portion of one room for maybe 30 minutes. Let's say we have a 10 room house, and we're heating 0.5 of one room, for 1/48th of a day. Given the 5 dollar/day baseline cost, you're maybe saving yourself about a penny per day.

But then you need to factor in the contribution you'd get from the heat that would be dissipated when you poured it down the drain, which would be non-negligible.

 

[bilby]

The heat capacity of water is about 4kJ per liter, per Kelvin.

Assuming your thermostat is set to 20°C, the water provides roughly 80K x 4kJ/L/K x 3.8L = 1.2MJ of heat to the room as it cools from boiling point to room temperature.

The energy density of heating oil is about 37.3MJ/Litre; That 1.2MJ therefore saves you 1.2MJ/37.3MJ/l = 32ml of heating oil.

If you pay $4/gal for heating oil, that's about $1.05/litre, so 32ml of heating oil saved is about 3.4¢ worth - less any inefficiencies in your furnace.

That's only the saving over throwing the water out while it's still at boiling point though. If you didn't let it cool to room temperature, you would probably still "let" it cool a fair bit, while you were serving up the food and doing other stuff - presumably you wouldn't race to get the hot water down the drain as fast as possible.

And as the rate of cooling is far higher to begin with, than it is as the water approaches room temperature, you're only saving a fraction of that in reality. If the water would have cooled to 60°C before you threw it out, then your savings by keeping it would be half what I estimated, so one or two cents at best.

Water at 60°C still seems plenty hot, and could scald you; But it's already given up half of the heat it had when it was at boiling point, compared to what it will have at room temperature. Unless you were in a tearing hurry to dump the water, it would likely cool that much anyway.

If you want a more accurate estimate, you would need to measure the temperature of the water at the time you would consider disposing of it (if you weren't trying to save energy). And the actual room temperature, efficiency of your furnace, and price of heating oil would also be needed, in order to get a more precise answer.

I am gratified to discover that my answer is roughly the same as the figure @rousseau got via a completely different methodology; It gives me some confidence that the saving likely is in the order of one cent (US) per pot of water.

 

[bilby]

When I think about calculating this, I imagine simplifying by asking how much does one gallon of boiling water change the temperature of one 16x24 room. And the comparing how much oil is needed to change the tempertaure of that room by the same amount.

You can simplify further. You just need to know how much energy comes out of the water as it cools, and compare to how much comes out of oil as it burns.

The room remains the same size no matter how it is heated, so that factor isn't relevant.

 

[steve_bank]

The equation for temperature rise of a material is

q = m*c*dT
q = heat energy in Joules
m = mass of the material
c = the heat capacity of the substance
dT = temperature rise

Th heat energy stored in a material is C the heat capacity.
Cp water is 4182 J/kg °C

A calculator.

Energy Accumulated in Heated Water - kWh

The amount of thermal energy stored in heated water.

www.engineeringtoolbox.com

In isolation air temp rises IAW q = mcdT'

Air - Specific Heat vs. Temperature at Constant Pressure

Online calculator with figures and tables showing specific heat (Cp and Cv) of dry air vs. temperature and pressure. SI and imperial units.

www.engineeringtoolbox.com

Calculating the actual temperature rise of a room is more complicated and requires modeling. Energy is being lost to the outside world. There are convection currents in the air. Regardless of the form the rate of energy transfer is proportional to the potential energy difference of source and sink, water to air temp.

Thermal resistance is common. Voltage becomes temperature and current becomes heat. Thermal resistance is degc/watt.There is thermal resistance from the water to the room air, and from room air to the outside world.

Energy is transferred form the water to the outsde world through a resistance from water to air and from air to the outside. There is a temperature difference across each resistance.

Thermal conductance and resistance - Wikipedia

en.wikipedia.org

There may be calculators on line.

Boil a big pot of water. Put it in a small room like a bathroom with an electronic thermometer and measure the temperature rise. Separate the pot and the thermometer. You have Joules/kg of water degC per degC temperature rise for the room.

1 watt is 1 Joule per second. You can calculate the equivalent kilowatt hours as would be measured on your utility power meter. There should be a calculator for that.

Kilowatthour (kWh): A measure of electricity defined as a unit of work or energy, measured as 1 kilowatt (1,000watts) of power expended for 1 hour. One kWh is equivalent to 3,412 Btu.

 

[bilby]

A typical human puts out about 80W of heat; You could achieve the same heating effect from having a visitor for about four hours, as you would get from letting the water cool before disposal.

Of course, they're going to let some heat escape from the house when you open the door for them, so maybe let them stay for five hours (or invite a couple for two and a half hours).

This strategy may backfire, if they eat more than 1¢ worth of your food while they are there. Remember to lock away the cookie jar, and only serve tap water (you may have a pan of room temperature water handy, that would otherwise have gone to waste...)

 

[bilby]

The OP problem is interesting in regards to what factors are not relevant, that one might imagine would be.

The amount by which the pan increases the temperature of the room is irrelevant; The size of the room is also irrelevant. That's because the water and the furnace are both heating the same space. All that matters is the energy lost to the environment by the cooling of the water, as compared to the same amount of energy being added to that exact same environment by burning additional heating oil in your furnace.

The amount of oil saved is directly proportional only to the volume of water that is being cooled from boiling point, for a given room temperature. Nothing else needs to be factored in; we just need to know a handful of constants (the efficiency of the furnace, the price of oil, the thermostat setting, and the heat capacity of water), and we can calculate the cost saving per litre of water.

I find great pleasure in boiling a problem down to the fewest possible relevant factors (pun intended*).












* But you knew that. This isn't your first day...

 

[thebeave]

A typical human puts out about 80W of heat; You could achieve the same heating effect from having a visitor for about four hours, as you would get from letting the water cool before disposal.

Of course, they're going to let some heat escape from the house when you open the door for them, so maybe let them stay for five hours (or invite a couple for two and a half hours).

This strategy may backfire, if they eat more than 1¢ worth of your food while they are there. Remember to lock away the cookie jar, and only serve tap water (you may have a pan of room temperature water handy, that would otherwise have gone to waste...)

I think I finally found a good reason to invite guests into my house.

I do a similar thing, except with my oven, not boiling water. I often make bisquits in the oven, and if its a cold morning, I'll leave the oven door open after I turn it off to help heat up the kitchen. Learned that trick from my cheap-ass, frugal dad.

 

[bilby]

Leaving the oven door open (once it's switched off) might heat up the kitchen faster, but it won't save any heating oil, all things being equal. The open door just means that the oven will be cold sooner, leaving your furnace to make up the difference.

The heat from the oven is going to end up warming the kitchen either way - unless you were planning to take the oven outside to finish cooling down.

The energy saving in the OP comes from not flushing the heat down the drain.

 

[Rhea]

The heat from the oven is going to end up warming the kitchen either way - unless you were planning to take the oven outside to finish cooling down.

I’ve had this discussion with my hubbin, and I posit that the heat will stay longer nd doo more in the kitchen than the oven because the oven is on an exterior wall. So getting the heat into the kitchen, then closing the oven door and returing to two layers of insulution between us an the outside has a net benefit.

 

[Rhea]

Wonderful inputs, thank you all. I enjoy the range of detail and assumptions.

So if I use one pot of water per day,, on average, I’m thinking it’s probably a little higher than Bilby and Rousseau calculated due to the low efficiency of the furnance and its air delivery system in this old house.

Enjoyable to think about the alternative heat from friends visiting. Multiply that by a bit higher since most of my friends are menopausal, so the amount of body heat on surface skin is just slightly higher, and tehy tend to wear tank tops in winter at least part of the time, exposing more skin to the air.

So I may only get a nickel or less a day, and save about 32ml of oil, which on a 6-month heating season comes out about 1.5 gallons a year,

BUT, and hear me out, here,

I will avoid the muddy spot in the yard during winter over the septic tank where the household temp water always keeps the ground above freezing.

 

[Rhea]

I still feel (i use the word literally) that that the heat of a pot of water is more significant than people think. I sometimes put the pot under my desk and my feet feel FABULOUSLY warm.

 

[bilby]

The heat from the oven is going to end up warming the kitchen either way - unless you were planning to take the oven outside to finish cooling down.

I’ve had this discussion with my hubbin, and I posit that the heat will stay longer nd doo more in the kitchen than the oven because the oven is on an exterior wall. So getting the heat into the kitchen, then closing the oven door and returing to two layers of insulution between us an the outside has a net benefit.

Maybe. A lot depends on how well insulated that exterior wall is. And of course, some people (like us) have their oven on an interior wall.

 

[Rhea]

More detail on why this is interesting to me is that we have an extremely inefficient house with dual heat. 50% of our heat comes from an oil furnance with forced hot air, but the hot air is only sent to 5 of the 10 rooms, and it first traverses an uninsulated basement. The other 50% of the heat comes from a woodstove with a small computer fan in a doorway to distribute, and natural convection to the second floor via the stairway, with one room heated by virture of the woodstove chimney going through it. Temperature variation between rooms can be as much as 20°F, with the woodstove room warmest and the unheated bedroom or library/den coldest. The thermostat is in the kitchen, so any small change in temp there will affect whether the furnace turns on or not.

 

[steve_bank]

You raise an idea I have not thought about or heard about. Storing waste heat from a stove.

When you bake put bricks in the oven....put them around the living room. But then it would take more energy to bring the stove to a temperature.

There is no such thing as a free lunch, according to thermodynamics.

Every square foot of body gives off heat of about 19 matches/hour. Du Bois area: The surface area of skin of an "average" adult is 1.8 m2 (1.8 x 10.76 = 19.368 ft2) The total heat production of an "average" person at rest per hour is 58.2 x 1.8 = 104.76 = 105 watts (18.4 x 19.368 = 356.37 = 356 Btu's per hour).

Invite people over without feeding and you get free heat. In the summer your air condition has to work harder.

 

[Rhea]

We don’t have an air conditioner, so that’s free.

 

[steve_bank]

Have you thought about wearing those fuzzy furry slippers? Looks like they might be warm around the house.

FURRY SPLIPPERS - Google Search

 

[bilby]

You raise an idea I have not thought about or heard about. Storing waste heat from a stove.

When you bake put bricks in the oven....put them around the living room.

Congratulations, you have invented the storage heater. We had one in my home in the UK back in the 1970s, so it seems unlikely that you will be granted a patent.

https://en.wikipedia.org/wiki/Storage_heater

 

[thebeave]

Leaving the oven door open (once it's switched off) might heat up the kitchen faster, but it won't save any heating oil, all things being equal. The open door just means that the oven will be cold sooner, leaving your furnace to make up the difference.

The heat from the oven is going to end up warming the kitchen either way - unless you were planning to take the oven outside to finish cooling down.

The energy saving in the OP comes from not flushing the heat down the drain.

It's definitely more beneficial to open the oven door and have it heat the kitchen fast, since I leave the house within an hour of eating breakfast, and turn off the wall heater as I leave. It does me no good to have the oven slowly warm the kitchen after I'm long gone for the day. I have natural gas for both my wall heater and oven, so having the oven door open helps lessen the burdon on the wall heater a teeny tiny bit.