Does cooling your house down at night really save energy?

[AC_Hacker]

Originally Posted by RobertSmalls

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?

My thoughts too.

I was tempted to refer to the CHP (Combined Heat & Power) thread. I had in mind a natural gas driven engine.

-AC_Hacker

[Xringer]

It got cold here last night. Average of 28.8°F between midnight & 8AM.
The Den was left open slightly and it dropped down to about 62F.

Sanyo was again left at 21C overnight. It used 5.08 kWh. ($1.06 USD) during those cold 8 hours.
Ave of 653 watts per hour.


Yesterday Sanyo 24hr usage was 12.9 kWh ($2.70 USD). Ave temp was 34.1 °F.
That's an ave of 537.5 watts per hour.

Today(Dec,5) it's going to be colder, I'm predicting at least 15kWh.

~~~
Looking at spending $1 to keep the house toasty all night,
strikes me as funny when I think of the money we spent on our
daily 1/2 hour oil burn (at 6:30AM) for hot water..

~~~
Edit 12/6/2010:
Well, yesterday wasn't bad. 13.6 kw ($2.85) with an ave temp of 30.20 F.

This morning, between midnight & 8AM the Average was 24.9°F
During those hours (Sanyo at 21) we used 5.20 kWh. (650 watts per hour)
Seems about like yesterday, but it was 4°F colder on average.
The den may have been isolated a little more, since it got down to 54F, vs 62F yesterday AM.

[Ryland]

Going off of the idea of running a natural gas engine to run a heat pump it looks like you would get heat from your heat pump as if you were paying 5.5 cents per kwh for electricity, this is based off of 1 therm being 29.3 kwh worth of electricity, so figuring that because the heat pump is pushing that otherwise wasted energy in to your house at 100% (not accurate) then adding to that by pushing heat from the out doors in to your house as well you are then dropping your energy rate in half by skipping the natural gas power plant to supply energy for your heat pump.

The other idea that we had today was, some friends have a whole hillside of PV at their house, if you insulated around the edges of them with bales of straw (use it for mulch in the spring) you could then tap off the heat from the PV in the mid west, as the PV get hot and the ground under them (about 2-3 feet) tends to then stay warm as well.

[Patrick]

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. 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?

On my heat pump the pump and the aux heating elements are independently controlled by the thermostat. I can set the differential to what I want (up to six degrees and one hour of delay, IIRC). I.E., if I set the temp to 50 and the differential on the heat pump to 3 degrees, the heat pump will come on at 47; if I also set the differential on the aux elements to 6 degrees and one hour of delay the aux elements will only come on if the temp drops to 44 and stays there for one hour. This gives the heat pump a chance to do its thing, but if it won't cut it, the aux elements kick on. I can even turn the heat pump off and just use the aux heating elements (in emergency mode).

The problem with disabling the aux heating elements is that on really cold days the heat pump may not be able to keep up with demand. With no aux elements, you're going to be cold.

[strider3700]

The manual for my honeywell th8000 thermostat is beyond useless. It states that on select models I may have advanced heatpump controls. It gives no details on how to get to them or what they may be. I'll have to walk through the hidden config settings and see if anything jumps out at me as being correct. It would be nice to be able to tell it not to turn on the aux heating unless temps are below a certain point or I'm asking for a 4 or 5 degree jump...

OK I just walked through there and found only the options listed in the manual so no special settings. The tstat does however control the back up heat... maybe there are jumpers on the back or something?

[RobertSmalls]

How about a toggle switch on the thermostat wire going to the electric resistive heater? Hopefully, you'd only have to use it a few times a year.

[Patrick]

Originally Posted by strider3700

The manual for my honeywell th8000 thermostat is beyond useless. It states that on select models I may have advanced heatpump controls. It gives no details on how to get to them or what they may be. I'll have to walk through the hidden config settings and see if anything jumps out at me as being correct. It would be nice to be able to tell it not to turn on the aux heating unless temps are below a certain point or I'm asking for a 4 or 5 degree jump...

OK I just walked through there and found only the options listed in the manual so no special settings. The tstat does however control the back up heat... maybe there are jumpers on the back or something?

Check out this manual: http://www.hydro-temp.com/help/drawi...069-1894ES.pdf

On page 9 it shows that you can turn off the Adaptive Intelligent Recovery.

Apparently the thermostat doesn't give you the ability to select temperature differentials, but you might be able to accomplish the same thing by careful selection of the maximum number of cycles for the different modes. I.E., if you select a low number for the aux heat (page 8), say 1 or 2 cycles per hour, the thermostat would run it until the target temperature was reached, then not run it for a long time which would let the temp drop way back, effectively creating a large differential.

[strider3700]

thanks, I should have guessed there was a different manual for installers then for users... looking on page 8 it gives a temperature lock out for the aux heat. No differential as you stated but this will at least stop the electric from kicking on when it's 10C outside and I want it to go from 17 to 20 inside...

I just need to figure out exactly what model I have now. It's probably on the back.

[osolemio]

As long as the heating source has a linear energy consumtion, it would definitely pay to lower the temperature.

In the case of heat pumps, it may not, however.

As mentioned above, one can measure the COP. A good place to try it, without using a calculator manually, is here:

Cool Chips(tm) COP Calculator

The blue column in the middle is a conventional heat pump. As you can see, the less you have to raise the temperature, the more efficient the heat pump (higher CoP).

So if you allow your house to cool off for a period of time, and then heat it up again later, you will place a higher demand on the heat pump. You will have to ask it to generate a higher than normal temperature or run it for a longer time.

So it depends upon how quickly you demand the house to be heated up again. Lets say you turn the heating off in the evening, to have a cooler night temperature, and then heat it up early morning, to have a comfortable temperature when you wake up, then the earlier you start heating, and the slower the heating, the more efficient the heat pump. But then the period of a lower temperature differential is shorter, meaning less savings.

The conclusion is, that if you have a heat pump, you might not gain anything for a short period of lower temperature. You might even use MORE energy, as a slow, constant heat will require less strain (and thus higher efficiency) on your heat pump, than a sudden burst of a high energy requirement. You should also consider the life span of the heat pump, and possible ice build up and defrosting, if you use the heat pump this way.

If you have a system with a linear heat efficiency, like a direct electric heating system where energy used is always 1:1 (CoP = 1), it might pay off even with a shorter period.

Being away for several days or weeks would most probably be worth while no matter which heating system you have, but be sure not to run it so cold as to have water pipes freeze, or you could end up with serious water leaks and subsequent damage. One could mitigate the threat of a heating system failure by placing one or more simple electric heaters with the thermostats set at a temperature just below the lowered one. Lets say you let your house cool off to around 10 C (50F). You could then set the electric heater(s) to 7 C (around 45F), and if there is a failure of the main heating system, these will kick in and prevent damage. If the failure is electric - grid blackout - you will have had it in either case, tough luck! Unless you have a battery backed-up oil/gas/pellet furnace.

MY approach is to change the heating system so that it heats as large a mass in the house, as possible. The larger the mass you heat, the less temperature is required. The energy used would roughly be the same ("lower temperature but more of it"), but just about any heating system will run more efficient if the temperature required to heat it, is lower. Especially in this case, only longer periods of lower temperature will pay off, as the house heats off so slowly. But it is a great system - the huge heat capacity, and low heating temperature means that you can suck out heat - and store it - from a solar collector that would otherwise be too cold to provide any useful heat at winter. And it will last for days after a good sunny day, if your solar collectors are large enough compared to the requirement.

On that note, I am eventually changing the house (three floors) to an all underfloor heating system (2nd floor is only heated by residual heat from 1st floor). I am even considering to build in "underfloor heating" type tubes in some of the lower walls in the house. The more mass you heat in the house - the more capacity you get from when the sun is shining and energy is "free" - and the more area you heat (floors - walls), the less the temperature of those surfaces need to be. Heating the walls with built in pipes DOES require that you somehow protect the pipes from perforation. Imagine driving a spike or screw into the wall, just where the tubing is ... but there are ways of fixing this as well.

These last considerations are much more important to the heating efficiency of a building, than lowering the temperature temporarily for a shorter period.

[Piwoslaw]

Originally Posted by osolemio

The conclusion is, that if you have a heat pump, you might not gain anything for a short period of lower temperature. You might even use MORE energy, as a slow, constant heat will require less strain (and thus higher efficiency) on your heat pump, than a sudden burst of a high energy requirement. You should also consider the life span of the heat pump, and possible ice build up and defrosting, if you use the heat pump this way.
[...]
Being away for several days or weeks would most probably be worth while no matter which heating system you have

So you just have to find out how much of a temperature drop, and for how long, will make it worth while. Two possibilities are:

• have a heat pump that keeps the temperature stable, and another heat source for when you go from low to high,
• make the low temperature intervals longer.

Raising the whole house's temperature just for an hour in the morning kills efficiency, so maybe it would be better to have a higher temperature only in the evening, while in the morning heating only the kitchen and bathroom, without heating the rest of the house. That way you raise the temperature only once per day, instead of twice, which should halve efficiency losses.