The Leaky Bucket

HeatingHelp

The Leaky Bucket

The question was in the Manchester Evening News, a British newspaper, and it caught my eye when I was going through my daily Google Alerts on things that have to do with heating. The reader asked, “Does it pay to turn the heat off if I’m looking to save money on my heating bill?”

Read the full story here

Al Brown

I really like the bucket analogy, but it still doesn't answer the question - will the homeowner save money (ie pour more water into the bucket OVER TIME). You're (or Mr. Lewis Carrol Epstein) are missing the secret ingredient to answer the question. How much more (or less) water will be poured into the bucket during any given TIME period. TIME is of the escense in answering this question. If you let the bucket get completely empty and leave it that way for an hour ( or two or 24) and then begin to refill it. How much more (or less) water would you have used in that hour (or two or 24) to keep the bucket full in the first place??? Yes the bucket is the same size and yes it still takes the same amount of water to keep it full, but over time, the amount of water used to keep it full will vary depending on how low you let the bucket get and for how long. Right?

CLamb

The bucket example only models the output of the heating system. It doesn't address the efficiency of adding water to the bucket at various levels or the energy wasted (or gained) in the startup and shutdown of the pump feeding the bucket.

IMO, the key to solving this problem with a heating system is knowing the efficiency when it is starting up and shutting down. The efficiency when it is running at operating temperature is pretty easy to compute. A difference between keeping the building at an even temperature and using a setback it that the former requires more startup and shutdown cycles.

nicholas bonham-carter

I think that comfort will be greater when the temperature is even, but lower than the higher settings on a setback program.
My advice would be to have no setbacks, but try different lower settings, until you have no comfort, and then increase the setting by a degree.
This assumes the system is even and well balanced, with a good location for the thermostat.
If you are going away for several days, then a setback will be beneficial.—NBC

Henry

My experience at home is that setback costs more. The same we have experienced with large mass buildings such as churches and temples. Once the mass is warm it is jus to keep it warm. With setback, the mass cools and more energy is needed to bring it back to warm.

RayH

It doesn't give a simple yes or no answer. Does it cost more to set back the thermostat ? For how long ? For 8 hrs while at work ? For a full day or two?

ConsultingGeotech

Of course it takes less energy if the heat is cycled up and down, instead of kept constant. In fact you can actually estimate how much less energy will be used if you cycle the temperature of the house. if you normalize the Run Time Ratio of the furnace by the difference in temperature between indoors and outdoors, the answer is almost a constant. For my home, for example the constant is about 0.008 per degree F. So if the temperature outside is 20 deg. F and the house is kept at 40 deg., the run time ratio is 0.008 x 20 = 0.16 meaning that the boiler runs 16% of the time. If I turn up the T-Stat to 70 deg. F and keep it there, the boiler runs at (70-20) x 008 = 0.40 or 40% of the time. I suspect the Normalized Run Time Ratio for my house (0.008) is slightly variable because of factors like wind and solar gain, but it still gives a reasonable estimate of the amount of additional energy that will be used.

gennady

I always answer this question with another question. (I'm jewish), Why cars have 2 MPG ratings on their stickers? One for city driving, another one for highway. Driver is the same, car is the same, fuel is the same. The only difference is the manner in which car is operated. Highway, constant speed, city stop and go. It is easier to maintain speed then constantly accelerate and slow down. Maintaining constant temperature normalizes body reaction to temperature swings and allows to maintain lower indoor temperature at the same level saving even more fuel.

westbye

A warm thing will lose heat faster than the same cooler thing.

CanadaKeith

Basic heat flow is Q=UAdeltaT = heat flow rate of envelope times area of envelope times delta T. The U and A are fixed. Delta T will vary, and increase after a period of time when the heat is off. It seems to be a linear relationship, so in this simplified form, there is no penalty for allowing the temperature to drop when in heating mode.
The “everything gets cold” has some bearing when talking about thermal mass. Warmed objects can give back heat when the surroundings are at a lower temperature. Complicated calculations that I can’t do! Thermal mass certainly has a bearing on heating on cooling. In my double-wythe brick house, with virtually no insulation values in the walls, there is about a 12 hour time lag between outside temp and inside temp. This allows us to run the AC at night in the cooling season, and the house stays cool all day.
Loss through air leakage and stack effect will be less in heating mode when the inside temp drops. So there is an additional saving there by keeping the heat off.

AMservices

I've been telling people for years its better to find a temperature your comfortable with and let your system maintain, rather then letting your house go cold and have to run for hours to catch up.

I don't know about the bucket analogy because depending on outside temperature and what temperature you're trying to maintain inside, The holes are essentially modulating.
You're a losing heat (or water) faster, at colder temperatures.
If it worked like the bucket, every person that install the programmable thermostat would see instant savings.

If you want to lose heat slower, then you have to keep a colder temperature inside. And most people don't like having to sacrifice their comfort for efficiency.

I say it's more stressful for the heating system to run for hours on end to make up a 10 degree temperature swing, Rather than only having to come on a few times A-day for maybe a 1/2 hour to change the temperature 1 or 2 degrees.

The size of the holes will say how efficient your bucket is.

I can change heating systems, not people's habits.

If someone wants to save money on fuel, the best result come from the best insulation and tight construction.
You can have a heating system that's 99% efficient, perfectly designed and not see a dime of savings if this year is colder then last, when you have poor insulation and leaky construction.

Even if there's no more savings maintaining a steady temperature over shutting the heat off during scheduled away hours, at least the owner is comfortable in there home and the heating system doesn't have to run a marathon when they return at night.

Roy_2

Mr Lewis Carroll Epstein has overlooked two crucial elements in his analogy. He needs to add something to account for the thermal mass and time. I have won more adult refreshments by timing the burner run time over a set period and again when running a setback and recovery time frame. The Shorter a setbacks 6 - 8 - 10 hours versus 2 - 5 days and bigger setbacks 5 -10 degrees versus 2 - 3 degrees cause the burner to run more trying to bring the house back to the same comfort level as started.
Don't believe me - try it measure the run time from the start of the setback through the next day at the same time it was started (24 hrs)
Then measure the run time for 24 hrs no setback. You can thank me at the adult refreshment store at the next trade show.

ronewold

The real "bucket" involved is often much more complicated. Imagine a worst-case scenario: a single-story house in a cold climate on a concrete slab with scorched air heating ducts run in the attic. I will torture the analogy- In this case, think of the concrete slab as a bucket inside the larger bucket of the house. In fact, the slab bucket is probably just about as large as the house bucket in terms of heat capacity, because concrete can hold so much energy, so the house is like a big bucket with a just slightly smaller bucket (the slab) inside. The heating system puts water into the big "house bucket", but the only way water can get into the "slab bucket" is by water leaking in from the house bucket through the little holes in the slab bucket. So if you keep the house warm (full of water), the slab bucket will gradually fill up over time. Likewise, if you keep the house cool, the slab bucket will empty. The big problem comes when the customer comes back after the house has stayed at outside temperature and drained completely of water, and he turns on the heat and warms the house back up to 70. The air in the house is now 70, and the house bucket is leaking like mad out of the holes because of the huge temperature difference between inside and outside. But meanwhile, you have a freezing slab making the occupants of the house feel miserable even though the thermometer says the house is 70, and thus SHOULD be comfortable. Even worse, the warm air in this nightmare house is likely to congregate near the ceiling, so that the slab is going to be very slow to benefit from the heat energy available there. It's as if the holes in the "slab bucket" are inaccessible, so that you have to keep the "house bucket" super full in order for water to trickle into the slab.
So, it's complicated.

GBart

I find when I don't drive my truck I get incredible mileage and when I turn off my heating/hot water system I save a ton of fuel, it's amazing. - sarcasm

hot_rod

GBart said:

I find when I don't drive my truck I get incredible mileage and when I turn off my heating/hot water system I save a ton of fuel, it's amazing. - sarcasm

The correct answer to most efficient boiler operation, the off position

Same with energy storage, is it best to store energy in a insulated tank of water, cast iron radiator, a slab, a rock bed, etc. The "best" place to store energy is the un-combusted fuel, be it gas, oil, firewood. Once the chemical energy is converted to heat energy it starts slipping away from you.

In a living space where comfort is part of the equation I would say modulate the boiler to the load and run at the lowest possible temperature. Modulating boilers on ODR or constant circulation would be ideal for hydronic systems.

Steamhead

The short answer is "it depends".

One winter I got a bad case of the flu- aches, chills, the works- so I disabled setback on my thermostat until I felt better. Several weeks later, I ran out of oil. After dumping my reserve can in the tank and re-starting, I called for a refill- oil company said I wasn't scheduled to be filled for another ten days. Apparently in my case, disabling setback did increase my consumption.

As has been said before, making the holes in your bucket smaller is the way to go. Here's a thread about doing that at my house- unfortunately the pics are gone, as this was migrated from an earlier version of The Wall:

https://forum.heatinghelp.com/discussion/108819/the-hole-in-my-bucket-gets-smaller-steamhead

Henry

In my house which has a ventilation system, I discovered that a 1F setback used less gas during the winter and less electricity for air conditioning. Anything more than 1F had no benefit after monitoring consumptions since 1985! For large churches or temples with the massive stone and concrete structures, setback increases gas consumption! The only exception was the 5 day during the week setbacks of up to 10F.

Kimo

I agree this is one of the most misunderstood concepts in all of HVAC (along with turning the thermostat lower on the AC to make it cool faster).
Let me try to explain it how I understand it. First, the amount of heat loss from a house (or any building or anything really) increases with increased temperature difference. (This is the fundamental definition of heat: energy transfer due to thermal gradients.) The total amount of energy lost is the product of the heat loss rate and the duration. (Energy is power multiplied time.)
Now consider two graphs of temperature difference inside to outside measured bottom to top and time of day measured left to right. We start at midnight. Our first graph is the house with the thermostat set to 72°F all day and night. Draw a straight line across the paper at 72°F for all 24 hours. Below that draw a curve of the outside air temperature, 0°F at midnight, -2°F at 1am,...40°F at noon, etc. Now, get scissors and cut out the area between the line and curve. The area of this shape represents the amount of energy needed to heat the house the entire day.
The second graph has the same lower curve of outside air temperature throughout the day, but instead of the straight line all the way across at 72°F, we set back the thermostat to 55°F at 8:30 in the morning when leaving for work or school and let the house cool off at its own pace. Then we turn the thermostat back up to 72°F at 3:30 in the afternoon to get the house up to temp by 5pm, and do similarly at night when everyone is sleeping snuggly under the covers and reset it before dawn. Regardless of how slowly the indoor temperature decreases during the setback periods of midday and night, and regardless of how long the system takes to recover, what was a straight line in the first example is now a curve (the shape of which depends on the mass of the house, insulation values, etc.) below that 72°F line where the indoor temperature is allowed to decay during periods of absence and recovers before the time of return. Getting the scissors again and cutting between the indoor and outdoor temperature curves gives us an area of paper proportional to the energy consumed. Now all that needs to be done is to lay the second paper on top of the first to see that the area and consequently the energy usage is lower with the setback schedule.
Not only is heat loss reduced during periods of set back, heater efficiency in increased during periods of recovery because more heat is transferred from the fuel to the indoors due to the greater temperature difference between the combustion gasses and the colder interior air.

Jamie Hall

An interesting argument, @Kimo . Unfortunately, several bits of it just aren't true. To take the last sentence. It may be true for one type of heating: forced air. There the colder interior air is against the heat exchanger, and there is the potential for a slight increase in efficiency. It is not true, of course, for either steam or hot water, as the heat exchange medium for both of those is as the same temperature regardless of the interior air temperature.

Perhaps more important, it neglects the heat capacity of the structure and its contents. The heat capacity of the air is negligible, but the heat capacity of the structure and contents is not. Were this not the case, the interior temperature would drop rather rapidly to the lower set point, and rise as rapidly when the set point was raised -- but if one factors in the heat capacity, even in theory one can see that this won't happen. It will take time for the temperature to drop. It will also take time -- and energy -- for the temperature to rise again. We see this empirically, of course, but it is nice to know that there are theoretical reasons as well. Your argument does not appear to adequately account for the energy used during recovery.

In practice, of course, for any given structure and its associated mechanical systems there will be an optimum set back which will minimize the amount of energy used over time, but it will be different for every structure and system, and will also differ with external conditions.

The Steam Whisperer

Running modulating equipment at full fire to bring space temperatures back up can result in considerably lower thermal efficiencies. In addition, during recovery, most system are going to create a lot more hot air, which is naturally going to rise towards the top of the space, where increased temperature differentials are going to increase heat loss and increase air leakage.

Setbacks are not universally more efficient.

Now when it comes to many religious structures, which are not used 6.5 days a week, the recommended setback temperature is 45F from the former Interfaith Coalition on Energy. In my own church, when they began using this aggressive setback around 1978, we saved about 1/3 on the fuel cost. Basically, during the spring and fall, the heat only ran for a few hours. Heat only starts to be needed when temperatures approach freezing since at these low indoor temps, warmth from sunlight and heat from the ground provides significant heating of the structure. I have also seen the solar heating aspect in my previous home and its freestanding and insulated garage. The home was quite well insulated and had a large amount of southern glass. Holding the building at 60F instead of 70 reduced the mid winter fuel usage by about 2/3 or more. The garage with 3 large southern windows and insulated slab and footings never dropped below freezing for 2 to 3 winters with no supplemental heat.

Savings from setback are extremely variable.