Energy Savings from Thermostat Set-back and Set-up
Last Post 23 Jan 2012 12:12 PM by Dana1. 18 Replies.
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Lee DodgeUser is Offline
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05 Jan 2012 05:03 PM
In a separate thread, there has been a discussion about various things including the energy savings associated with thermostat set-back (during winter) and set-up (during summer).  That tread has disintegrated into claims that are not referenced to any studies or calculations.  I wanted to provide some published studies that have examined the subject. 

Concerning the effect of thermostat set-back during the winter and set-up during the summer to save energy, The Canadian Centre for Housing Technology issued a report NRCC-48361 by M.M. Manning, et al. that addresses the savings for a well insulated (R-2000) house using a conventional, gas-fired, hot-air furnace. These were experimental measurements on two identical test houses that presumably are wood frame, and that were equipped with older-style furnaces at the time of these tests, 80% efficiency, and permanent split capacity blower motors. The twin houses are located in Ottawa, Ontario, Canada, and were subjected to 8408 heating Fahrenheit degree days (4671 heating Celsius degree days) and 652 degree cooling Fahrenheit degree days (362 cooling Celsius degree days).

Relative to leaving the thermostat set at 72 F (22 C) year-around, they reported the following savings in gas for the winter and electricity for the summer:
Winter:
64 F (18 C) night-time set-back, 23:00 – 6:00: 6.5% savings
64 F (18 C) day and night set-back, 23:00 – 6:00, 9:00 – 16:00: 10% savings
61 F (16 C) day and night set-back, 23:00 – 6:00, 9:00 – 16:00: 13% savings
Summer:
77 F (25 C) day-time set-up, 9:00 – 16:00: 11% savings

The result for night-time set-back of 6.5% savings for an 8 F set-back is close to the old guideline of 1% per degree F set-back, and in fact, for a milder winter climate, the guideline would probably be met.

Some comments not included in the report:

1. For a heat-pump system that does not lock-out the electrical resistance heater, these winter saving are much less or zero, since heating by resistance is 2 or 3 times as costly in source energy as using a heat pump with a COP of 2 to 3.

2. For a furnace with electronically commutated motors (ECM) for the fans, these savings would be reduced by an unknown amount if during the morning recovery period the fans were allowed to run at high speeds, which is the normal case. In the case of the fans running at higher speeds, more electrical energy would be used and less gas energy for exactly the same total energy (NRCC-45419 by J. Gusdorf, et al.), but the source energy requirements for electricity are about 3.3 times that for natural gas.

3. In the limit of extreme mass systems (extreme heat capacity), temperature set-back is not important since the interior house temperature would change a small amount (say 1 F) during a night-time set-back, but would require a large amount of heat energy to reheat to regain that same small amount (1 F). This results in no net savings for the set-back strategy, since the temperature does not drop significantly during the night, and therefore, the differential temperature between inside and outside does not change significantly from what it would be for a constant thermostat setting. 

A report by Scott Prigg, "Electricity Use by New Furnaces, A Wisconsin Field Study," State of Wisconsin Division of Energy Report 230-1 shows that two-stage and fully modulating furnaces with ECM motors typically operate at at low firing rates 50% to 80% of the time, with maximum firing rates used only during the heat recovery period in the morning.  The high rate operation is important since high firing rates require high blower speeds, and pressure losses in ducting increase at the cube of the air flow rate.  Unfortunately, the data were not analyzed to determine the net effect on overall gas and electrical energy associated with set-back for these modern furnaces.   

The above means we still don't know if winter thermostat set-back saves source energy with modulating furnaces with ECM blowers.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
jonrUser is Offline
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06 Jan 2012 09:20 AM
The only thing I would add is that one needs to be sure to account for the fact that the 6.5-13% setback savings are full season and the figures often used for ECM fan efficiency reduction are just for the fan and just for that 1/2 hour period where the house is heating up in the morning. These figures cannot be compared. For example (not real figures), -50% fan efficiency may equal -5% furnace warm up period efficiency and -.5% full season efficiency. Best not to use the nebulous % and use btu, or preferably dollars.

My guess is that except in the case of electrical resistance heat (which should be disabled), 1/2 hour/day of moderately decreased total furnace efficiency isn't a big whole season effect. Also, most people don't have ECM fans.

I would like to see a thermostat what was smart enough to not set back so far when it is really cold out and the furnace won't have enough excess capacity to heat the house back up in a reasonable time. It could also vary the timing based on predicted cool down/warm up rates.
Lee DodgeUser is Offline
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06 Jan 2012 12:03 PM
jonr said:
"I would like to see a thermostat what was smart enough to not set back so far when it is really cold out and the furnace won't have enough excess capacity to heat the house back up in a reasonable time. It could also vary the timing based on predicted cool down/warm up rates."

I think that your request has been granted. My thermostat (Honeywell VisionPRO IAQ) has "Adaptive Intelligent Recovery," which means that it learns how long the furnace (or A/C) must run to meet the desired set-point depending on the indoor temperature (and possibly outdoor temp) and set-point, and it starts the furnace (or A/C) at the correct time so that the house temperature is at the desired set-point at the programmed time. For example, I program a schedule of 67 F (19 C) for 6:15 - 22:30, and 60 F (16 C) for 22:30 - 6:15. This time of year with an outdoor temp of 18 F (-8 C) the furnace will turn on about 5:15 when the indoor temp has dropped to about 63 F (17 C), and the house will be at the desired temp at 6:15 after the furnace has run continuously for an hour. Pretty darn convenient!

I may connect a Kill-A-Watt meter to the furnace electrical and operate with and without the setback to see what electrical penalty I am paying for running the ECM fan at maximum speed during this warm-up period, as opposed to a low speed during the night. However, I cannot directly measure the natural gas savings due to the lower temps during the set-back period. I suppose that I can record the indoor and outdoor temperatures and compute the differentials and estimate the gas savings close enough. This would make the assumption that the (condensing) furnace operates at the same efficiency (97.5%) independent of firing rate, which may or may not be a good assumption. Anybody know? I could not find data after an internet search.

In my case, this warm-up period is NOT a small fraction of the total furnace heat output.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
LbearUser is Offline
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06 Jan 2012 03:59 PM
61F and 64F seems a little too cold for my tastes for indoor temperatures during winter. This is for sure sweater and blanket temps. There is no way an elderly person or someone who has health problems, or someone with infants in the home, that can tolerate 61F-64F indoor temps.

My thermostat is set at 68F during winter and even then it sometimes feel chilly.
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06 Jan 2012 04:19 PM
Good to know about the thermostat being available.

With a Kill-A-Watt, readings of the gas meter and adjustment for degree days, I think you could get a pretty accurate comparison in a week of alternating setback/no setback every other day.

Low 60s is perfect for sleeping.
Lee DodgeUser is Offline
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06 Jan 2012 04:26 PM
Lbear-

Yes, actual thermostat set points are a personal choice. My focus was whether winter thermostat set-backs or summer set-ups save energy, and to quantify those values. The answer appears to be yes, unless you have a heat pump without a lock-out on the electric resistance heater during heat recovery time (in winter), or you have a two-stage or modulating furnace with an ECM motor, in which case the jury is still out. Also keep in mind that the "standard" temperature in the NRCC-48361 report was 72 F, so you are already saving 10% or more relative to that setting by using 68 F as your standard setting.

When my Mom got old, she used to "waste" energy in Houston by running her heat at about 74 F during the short winters there, but she used NO air conditioning during the summer... in Houston(!), unless a wimp like me was visiting.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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07 Jan 2012 09:38 AM
Low 60s is perfect for sleeping.
Yeah, I'm a bit worried about missing that when I go to live in a radiant floor environment where there is no setback.
Dana1User is Offline
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09 Jan 2012 02:31 PM
Posted By ICFHybrid on 07 Jan 2012 09:38 AM
Low 60s is perfect for sleeping.
Yeah, I'm a bit worried about missing that when I go to live in a radiant floor environment where there is no setback.

If micro-zoned, bedrooms can be maintained at a lower temp 24/7.  I keep my bedroom at ~62F in winter which is fine for any of my  uses of that room. I wouldn't use it as a the home office at that temp though.

Radiant floors in bedrooms end up heating the bed from below and can even lead to feeling OVER heated during periods of peak heating if you tried to keep the room warmer than 65F unless you get rid of some of the covers.
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09 Jan 2012 04:42 PM
Maybe we did the right thing by NOT putting radiant heat in the bedrooms after all. We don't spend a lot of time there.

One guest bedroom has an electric wall heater. If the guests want to burn some kilowatts to be comfy after they see how we tried to save energy in the rest of the house, well, that's the beauty of electricity, isn't it? :-)

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10 Jan 2012 06:38 PM
This article classes staple-up and transfer-plate radiant floors as low-mass and, presumably, acceptable candidates for setback. http://www.healthyheating.com/downloads/Geoff%20McDonell%203.pdf I plan to program my upstairs rooms in reverse (i.e. 70 at night and 60 during the day.)

That said, the author, as do most people, forgets that thermal lag can also be your friend. Suppose you turn off radiant in a high mass house from midnight to 8 a.m. Yesterday, along the Mason Dixon line, that would have been eight hours averaging 27.5 degrees fahrenheit with a low of 25 at 6 a.m. Assuming three hours of recovery, the heat would have been made up at an average temperature of 36. With the flexibility of a house that cools slowly, you could have used setup to make heat yesterday aftenoon in plus 40 degree weather, which in the case of COP gains in a heat pump, would make setback seem like small change. You'd need oversized hvac, and you'd have to accept some variability of temps in hours of residence, although nothing as radical as 61 or even 64.

Lee, there is a study of setback in a high mass house, done in a comparison of four houses in Gaithersburg, MD. Jonr doesn't believe it because it said the setback penalty was modest. In addition to the plus side of thermal lag, setback would ramp up the dynamic benefit of mass in the shoulder seasons by relaxing the definition of what's comfortable. (Likewise for setback.) It seems like I will have plenty of gray days to measure kwh consumed by an electric tankless.


Lee DodgeUser is Offline
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11 Jan 2012 04:24 PM
toddm-

My comment about high-mass homes was limited to saying that in the limit of extreme high mass where the indoor temperature does not change significantly (say less than 1 deg.), that set-back and set-up are not interesting because the indoor temperature is almost constant. You point out an exception if using an air-source heat pump, and are willing to tolerate a little indoor temperature swing to do the heating (or cooling) when the temperature difference between indoors and outdoors is small. I agree with your exception.

I also do not question that when the outdoor temperature swings diurnally above and below the desired indoor temperature, that high mass buildings could reduce heating and cooling requirements. That discussion is outside the scope of the thermostat set-back and set-up that I was intending for this thread.

toddm said: "It seems like I will have plenty of gray days to measure kwh consumed by an electric tankless."
It is another sunny day today here in the Colorado mountains, with solar PV collection yesterday of 4.74 kWh per kW DC panel rating (14.94 kWh with 3.15 kW DC PV system). Another winter day running the electric meter backwards (but not making up for natural gas burned on cold days)!
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
Dana1User is Offline
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11 Jan 2012 05:39 PM
With a mini-split heat pump it's less obvious (independently of the mass of the house.) Yes, the COP in heating mode is higher when the outdoor temps are higher, but that is easily and severely undercut by dramatically LOWER COP when running the compressor & blower at max speed during the recovery ramps. While it's possible to invent scenarios where there might be setback savings, those scenarios would require fairly large (and optimally timed) diurnal temperature swings.

Manufacturers of mini-splits generally recommend "set and forget" constant temperature operation rather than a setback strategy to achieve best average efficiency (in either heating or cooling mode.)

With single-speed heat pumps and an "up at the crack fo dawn" schedule setback with bang-bang single speed heat pump would also be a loser since an AM recovery ramp would generally occur near the coldest hours of the day. In a high-mass/low loss house one could arguably schedule the heat pump for the warmer afternoon hours and intentionally overheat a bit, but automating that for optimum human comfort wouldn't be trivial.
Lee DodgeUser is Offline
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13 Jan 2012 02:20 PM
I would conclude that for most single-speed, single-power heating/cooling systems, there is an energy advantage (reduction) for thermostat set-back or set-up. For two-speed or modulating furnaces, the obvious advantage of less temperature differential available with set-back or set-up strategies MIGHT be somewhat or completely offset by the lower efficiencies of running the HVAC at maximum power/speed during the recovery period (as opposed to lower power settings to maintain constant indoor temps). However, there appears to be a lack of published results to quantify the case for multiple-speed systems.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
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16 Jan 2012 06:15 PM
Along this same line, I just built a house and put in a Penzotti propane water system. I am new to this type of system. I am wondering about the actual temperature settings to use. I am hot blooded so I don't mind it cold at night. I set the main house temp down to around 58 at night and up to 65 if I am home alone or 68 if wife or company is home. The bedrooms stay at 60 all day and all night.It's not so much that I am cheap, just broke after building this house! My question is: am I really gaining any savings by going that low and then have to warm the house back up in the morning? I live in Maine so it gets nippy around here. I have been told that this kind of heating is efficient enough to keep it relatively warmer instead of having to heat the whole mass up so much. Any thoughts or comments would be greatly appreciated.
JB
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16 Jan 2012 08:51 PM
Even in Maine you can use a mini-split to keep one zone or room warm, 24/7 for most of the season at a significant cost savings over heating with propane. In coastal Maine you'd average a COP of 2.5-2.7, but you'd still need the propane burner when it's well below zero out. In central or northern Maine well away from the coast your average might be as low as 2 if you're running it hard when it's sub-zero out a lot.

At the state average of about 16 cents/kwh and a COP of 2.5 the mini-split would deliver (2.5 x 3412=)8530 BTU of heat into the house for ~$0.16 which is ~53000 BTU/$

At the state average of $3.20/gallon for propane, in a 95% condensing burner that delivers (91,000BTU/gal x 0.95)= 86,450 BTU of heat for $3.20, which is ~27,000 BTU/$.

With a mini-split heat pump it's nearly 2x the heat per per dollar, so keeping at least one room cozy and letting the rest run cool is pretty cheap.

Using nighttime only setbacks you're probably saving 6-8%, but if you ran a mini-split it to heat any significant zone it would save much more than that. During the shoulder seasons when the average temps are in the mid-30s or higher you'll be geting signifcantly more BTU/$ with the heat pump, and when it's in the mid-40s or into the 50s it's more than 3x the BTU/$.

See:

http://blog.energysmiths.com/2011/03/out-with-the-old-in-with-the-new.html?cid=6a014e86fffc67970d01538dda139c970b#comment-6a014e86fffc67970d01538dda139c970b

and

http://blog.energysmiths.com/2011/12/living-with-point-source-heat.html

(there's more detail on other blog entries on that site, if you want to dig around.)
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16 Jan 2012 11:05 PM
Our third most favorite topic... We've assembled 22 research papers on this topic that ought to add fuel to the fire...enjoy. http://www.healthyheating.com/Programmable-thermostats.htm rb
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19 Jan 2012 02:28 PM
Good discussion of setback thermoststs on the Energy Vanguard Blog.
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21 Jan 2012 05:32 PM
Thank you Dana, some very interesting info. Not sure if this needs to go into another thread or not, but do you have any preferences on mini split systems? I will be heating 1 zone, living room and kitchen with it, less than 1000 sq ft. Sounds like it is very much worth the investment. Winter finally hit up here, anxious to get a system up and running!
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23 Jan 2012 12:12 PM
Mini-splits with continuously variable speed on the compressor as well as variable or multi-speed blower heads are more efficient than some simpler models. The "big 3" for heating in much of the US are Fujitsu, Mitsubishi, and Daikin, but there are literally dozens of vendors & models. (IIRC Fujitsu sells major components to many of them, even if the internal system design & controls are done by the nameplate vendor.)

On some of the tiniest units there's some 3rd party indication that Fujitsu may outperform the Mitsubishi on low temp (under +10F outdoor temp) efficiency, but it's a very small sample size (one) and only one pair of models tested, so the importance of that testing is somewhat limited. More important than the model is the access to local & regional support by distributors, and using vendor-certified installers that fully test the unit as part of commissioning it. They are deceptively simple to install, but can be "working mostly" at sub-optimal efficiency for any number of reasons that are easy to diagnose and fix, in the hands of a competent tech who has the right tools & working knowledge. Having good access to parts & technical support with one manufacturer and not another would be a deciding factor.

With any heating system it's important to perform a heat loss calculation (Manual-J or similar) on the zone it needs to serve if you want optimize cost, efficiency, & comfort. If your outside design temps are below -4F (probably are, in ME) you would likely need another source of heat for handling peak loads, even if the mini-split is providing most of the heat even at design temp. The output capacity (as well as efficiency) falls with outdoor temperature. The +47F rating for BTU output isn't very meaningful at the temperature extremes in New England, but it'll still be significant fraction of the rated number at +10F. (I think Mitsubishi rates them at their +4F or +17F output, which is a more useful number to work from in your climate.)

I have a growing number of family members who live in US climate zone 4 heating primarily with mini-spits (and raving about the cost/comfort.) I was perhaps late to the party, skeptical that it would work as well in cooler climates, but I'm well past that now. :-)
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