Does cooling your house down at night really save energy?

[Piwoslaw]

It started with Xringer's innocent question:

Originally Posted by Xringer

Maybe letting everything cool down at night, and then trying to heat it back up (fast) in the morning, isn't really that efficient.?.

I wanted to reply there and then, but thought that maybe the answer deserves its own thread.

So, does cooling your house down and heating it back up again once or twice a day really save energy? Why? This is, of course, only during heating season.

My answer is below, but I would like someone to verify it, maybe with some numbers.

At night, or when you're not at home, the lower temperature differential between indoors and outdoors allows less heat to be lost. The longer the time with lower temperature, the less heat is lost, which makes up for any losses during warm up. The instruction manual for my programmable thermostat advises not to set it for night-day-night-day temperature changes every hour as this supposedly doesn't reduce energy usage.

Raising the temperature again allows the (gas or oil) furnace to work at close to maximum load, when it is most efficient. I'm not sure whether this is true for heat pumps electrical heating.

[RobertSmalls]

Originally Posted by Piwoslaw

At night, or when you're not at home, the lower temperature differential between indoors and outdoors allows less heat to be lost.

That's it exactly. Here's Fourier's Law of Conduction, which states that energy loss is proportional to the temperature gradient:



If the temperature differential from the inside of your wall to the outside is twice as large, you'll lose heat twice as fast through the wall once transient conditions stabilize.

No matter what you're heating with, less heat will be required if you let the house cool down overnight and while you're at work.

Furnaces are horribly inefficient when they first turn on, as you spend lots of energy heating up the furnace and its ducts. Running the furnace all at once, shortly before you get home / wake up would be ideal.

I've heard several reports (one of them carefully controlled and documented) of heat pumps using more energy when allowing the house to cool at night and when noone's home. I know that some heat pumps will engage the auxiliary electric heater (30-50% as efficient) if they see more than one degree difference between the temperature and the set point. What I'd like to know is, if you were to manually disable the aux. electric heater, could you then use a programmable thermostat + HP to save energy?

[Xringer]

Here's the last 7 hours in weather history, recorded by a station less than 1 mile away.
12/3/10
Current: High: Low: Average:
30.1 °F 30.4 °F 27.9 °F 29.0 °F



The Sanyo ASHP has been set at 21C (69.8F) since yesterday.

It's been 7 hours since midnight and the meter shows 1.8kWh. (38 cents US)
That would be a continuous 257 watts, if it ran without stopping.
(Just a guess, but 257w might generate 3,000 to 4,000 BTUh @29F)..

Can that be right?? It seems way too low for a house built in 1956..

257w x 24h = 6.168 kWh per day..?.

That doesn't jive with data from the last week..
But, it's pretty close to the KWhs from the last couple of days..
date - kWh
12/3 - 11.13
12/2 - 7.52
12/1 - 7.11
11/30 - 7.77
11/29 - 9.52
11/28 - 12.4
11/27 - 11.4
11/26 - 12.4


There are a few things I need to think about..

1. My Den is closed at night. With only a small amount of heat allowed in.

It's 16'x18' and has a lot of glass and poor floor insulation. (No basement out back).
We open the double doors back up every morning, adding more load to the Sanyo..
That might help account for the unusually low over-night power use..

2. At night, the exterior doors aren't being opened and closed..
As opposed to maybe 20 to 30 times during a sunny day.?.
So, no open-door heat loss at night..
(Does open-door heat loss during the day, cancel out our solar gain)?

Edit:
~~~~That was then, this is now. 12/4/2010~~~~

Made one change. One of the double door on the den remained open.

Current: High: Low: Average:
30.6 °F 35.3 °F 30.6 °F 33.8 °F (It's 5F warmer than yesterday)!

At 7 AM KWH = 4.15 (87 cents). (We used 1.8kw yesterday)!

593 watts per hour for 7 hours. (Yesterday, it was 257w per hour).

Den partly open fell to 17.1 C

It was 5deg F warmer outdoors and it took 2.35 kw (49 cents) more power to keep the Den at 17.1C
More than doubled the bill !!

We did bad overall yesterday. Total of 11.13 kw ($2.34)..
Why?? Because it was Netflix night! I cranked up the Sanyo to get more heat in the Den!
I have to admit, it was nice to watch my movie (Get him to the Greek) in a warm Den,
and I new it was gonna cost me.
But I should have looked at the forecast before leaving the den open all night.
We are going to get some solar gain in there this morning..

[Ryland]

You also need to remember that if you have cooler air going in to your furnace to be heated that it's going to be able to pull more heat out of the furnace heat exchanger, so in theory at least your furnace is going to be more efficient with cooler air going in.
At work we have a modern coal stove, it has a bin like a pellet stove that you fill with 50 pound bags of coal, there is also a programmable thermostat and I can see the change in fuel use over the course of a few days if I turn the night temp down to 47F instead of 50F, with a day time temp of 55F, I don't have solid numbers on it but it's clearly using less fuel and the fuel used to warm up isn't all that great compared to the fuel used to keep it warm.

[Xringer]

Originally Posted by Ryland

You also need to remember that if you have cooler air going in to your furnace to be heated that it's going to be able to pull more heat out of the furnace heat exchanger, so in theory at least your furnace is going to be more efficient with cooler air going in.

It seems like sucking in real cold air to heat up, means you are going
to need more energy to get the job done. (Until the intake air is warm).

If the intake air was already a little warm, it's going to be easier
and faster to get the input air up to the requested setpoint.


My SUV gets 8 MPG on the hill near my house. But, on flat ground,
it can get 40 MPG (at 40 MPH).

It seems like any system is going to have to 'climb the hill', if it's
starting off trying to heat a real cold space..
But, if that space is not so cold.. There's not really a hill to climb. Just a bump.
So, you are driving on flat ground and getting good MPG, in short order.


~~

In these old oil burner forced-baseboard-hot-water heated houses
around here, when the burner comes on, it pulls cold outdoor air
into all the leaky windows in the basement.
And maybe a few upstairs. Kinda like when we use our electric clothes dryer.
All the air being pumped out, has to come from somewhere.


I like the Sanyo Mini-split ASHP, because it does not need to bring in any outside air.

Burning Oil is so dumb!! My oil burner is running 1/2 hour per day..
And making that hot bathing and washing water with oil is probably around 60 percent of my total energy bill. (Oil+Electric).
In this kind of weather of weather anyways..

[strider3700]

My heat pump/controller is one of those that it's less efficient to let things get really cold then heat them up rather then let it coast along. It's entirely the controllers fault though, it wants us to get that instant heat and uses the backup electric furnace to do it.

In it's defense it does have a learning ability that would let things cool down at night and then it would figure out how early it needs to come on to get things up to temp for when we want them. This takes a week or two of consistent programming for it to figure out and I treat the heat pump as back up heat for the woodstove so it never figures things out.

With a smart controller and consistent usage the heat pump would be just fine for the view of let it cool right off over night and then heat things up uses less energy then keep it warm all night.

[AC_Hacker]

Originally Posted by RobertSmalls

...I've heard several reports (one of them carefully controlled and documented) of heat pumps using more energy when allowing the house to cool at night and when noone's home. I know that some heat pumps will engage the auxiliary electric heater (30-50% as efficient) if they see more than one degree difference between the temperature and the set point...

Mini-Splits are an Asian development. They come from the land where energy is very expensive, and the pool of engineering talent is absolutely huge. So there has been so much technical focus on improving mini-split efficiency, that it might not be a good idea to lump them in with the category of all air-source heat pumps any more. It would be like making estimates of personal automobile fuel costs based on national fleet mileage averages, when in fact you are driving an Insight hybrid car.

For instance, most minisplits being sold now, use variable speed fans and a variable speed compressor (AKA: Inverter Technology) to adjust their output to a particular load situation, rather than cycling on and off around a temperature set point.

I think that in time the Inverter Technology will appear in whole-house heat pumps, but under a different name. I understand that some new European construction uses continuously variable fans for ventillation systems.

Regarding the aux resistance heater, my mini-split doesn't even have one and I suspect Xringer's unit is the same.

There may be another reason that the resistance wires, in the unit you mentioned, switch on when a greater than two degree situation is sensed, that is that the air that ASHP's put out is not nearly as hot as fossil fuel (parched air) furnaces put out. So the resistance wire may be functioning to accelerate warm-up, even though efficiency is lowered. Cold people can be so impatient.

* * *

But going back to the topic of this thread, the set back timer on my minisplit only goes down to 60 F, otherwise, I'd set it down to 50 F at night.

Regards,

-AC_Hacker

P.S. As another indication of the amazing efficiency increases of mini-splits, if you were the kind of person whose world-view was broad enough to consider that operating cost is an important consideration of heating alternatives, but that CO2 output is even more important, consider this:

All heat pumps use electricity. In almost all cases that electricity comes from coal, the most used fossil fuel in the world. So, when a heat pump is used it may be well over twice as efficient as resistance heat, but the CO2 produced is at the rate of burning coal. This situation is made worse because not all of the energy from burning coal is converted into electricity (conversion losses), and not all of the electricity sent from the power plants arrives at our homes (line losses). So when we flip the switch, we're burning coal.

Now, using this very handy fuel cost calculator from Build It Solar, which takes into account typical conversion losses, and line losses, I entered current local Natural Gas, Propane, Fuel Oil, and Electricity costs and came up with this:

For the mini-split efficiency numbers, I used the Fujitsu Halcyon which has a Heating Season Performance Factor (AKA: HSPF) of 12. This converts to COP like this:

COP = HSPF/3.412 = 12/3.412 = 3.5169988276670574443141852286049

COP = 3.52

Efficiency = 352%

But look at not only the cost numbers, but also the CO2 produced in operating that mini-split. Even considering conversion losses, and line losses, a really good mini split only uses about 1/3 as much coal as if you burned it right in your own home. It is pretty impressive.

However, the Halcyon is only good down to 5 F, so it's not for everybody.

Regards,

-AC_Hacker

[Xringer]

Efficiency = 352% !!

Sweet!! But, you've got me thinking about burning stinkin coal now,
and that 930w of PV sitting in the back-yard..

Yeappers, mini-split efficiency during sunny days could be made a bit better with some PV..

[AC_Hacker]

Originally Posted by Xringer

...mini-split efficiency during sunny days could be made a bit better with some PV...

Yeah, in more ways than one.

There's an energy neutral house project in Portland called the "Rose House".

Space-age technology is certainly at the core of the experiment. Part of the house's south-facing roof is covered with 300 square feet of solar panels that should produce 6,000 kilowatt hours a year (a typical American household spends 2,000 kilowatt hours a year on lighting alone). A special system sucks in hot air from underneath the solar panels -- it's typically heated to more than 100 degrees -- and uses it to heat water and air inside the house.

What this is refering to is that they used a device like a minisplit to draw in the heated air from under the PV panels and split the heat output between the inside air unit (when required), and a water-heating heat exchanger.

LINKS:

• Rose House Takes Efficiency to the Next Level
• Enter the New American Dream House
• Rose House - System Components

Regards,

-AC_Hacker

[RobertSmalls]

Originally Posted by Xringer

Efficiency = 352% !!

This is equivalent to the statement "Coefficient of Performance (COP) = 3.52. Efficiency is measured in terms of heat MOVED by the heat pump compared to electricity consumed by it.


The ultimate efficiency would be to run a high-efficiency heat engine, recover waste heat from it, and use torque from it to drive a heat pump's compressor. 500% efficiency?

______

http://ecorenovator.org/forum/conser....html#post8215

By my math, a COP of 3.1 is required to match the CO2 output of a new furnace.