Comments on: If You Think Thermostat Setbacks Don’t Save Energy, You’re Wrong!

By: eric

eric — Thu, 23 Oct 2014 09:26:27 +0000

After reading skimming your
After reading skimming your article. You said you heat to 68 but dididn't specify what you suggest setting it at when away from the house? You did use 8 degrees in your equation.so I am guessing you set it to 60 when away.

By: Donald B.

Donald B. — Mon, 27 Feb 2012 01:01:05 +0000

If someone's home loses heat
If someone's home loses heat at an amazing low rate, they can't save 5% using setbacks. Why? Because they've spent so much money on tightening and insulating that they've prevented "normal" setback savings.

For extra credit (or as homework for tonight) rerun the calculations for a house that drops only 1°F per hour. Just so everyone comes up with the same answer, assume that this house can recover 3°F per hour because it's furnace is matched to the home's needs.

By: Donald B.

Donald B. — Mon, 27 Feb 2012 00:44:05 +0000

Scenario 3: An 8°F
Scenario 3: An 8°F Setback with a real world recovery.

I'm OK with the drop rate in your example, but you failed to allow for the time it takes to recover. Say, for example, that the furnace would take 1 hour to recover from a 8°F setback. (Some more, some less, depending on sizing, climate, etc.) During that recovery hour, the average ΔT would be 36°F. Heat losses including that recovery time are shown below. (I hope my columns come through unscathed.)

t (hrs) ΔT (°F) Q (Btu/hr) Qt (Btu)
1 38 11,400 11,400
1 34 10,200 10,200
5 32 9,600 48,000
1 36 10,800 10,800

Total heat loss = 80,400 Btu

Still a "win" for set-backs, but not quite as much as Scenario 2.

Even this scenario is slightly flawed in that the drop rate and recovery rate would depend on the ΔT. The savings are found in the area of the temperature curve below the steady state temp line.

By: Allison Bailes

Allison Bailes — Wed, 22 Feb 2012 01:43:37 +0000

Paul M.:
Paul M.: If you thing this is long, check out my article with 58 comments over at GBA:

Who Knew the Stack Effect Could Be So Controversial?

By: paul mcgovern

paul mcgovern — Wed, 22 Feb 2012 01:05:50 +0000

when is this coming out in
when is this coming out in paperback???

By: John Ring

John Ring — Sun, 19 Feb 2012 21:10:59 +0000

This has been one of the most
This has been one of the most interesting discussions yet on this blog! However, in reading the posts much of the time I can't help but think of the tale of the blind men describing the elephant. One problem for me is that I have no idea where all the writers live, and climate zones make a huge difference in this matter. It would be great if each writer in some way gave an indication as to the climate they primarily deal with.

By: Kelly Merke, P.Eng.

Kelly Merke, P.Eng. — Sun, 19 Feb 2012 09:33:18 +0000

We have ten programmable t
We have ten programmable t-stats (elec baseboard heat) in our modest 1260sf house (+ basement) only to minimize the temp swing – we don’t set back – we hold steady at 71F in winter. If we did set back, the humidity would rise as the temp falls – causing the HRV run to react and over-compensate – quickly chasing the temp down to the new setpoint. Then when it re-heats, the RH would drop far below the min setpoint. That’s nuts. Ok, so now maybe you can guess that this is a fairly efficient place… the heat loss is 10kW in Zone 7 (!) as a result of our recent DER project. We would not only risk comfort, but potentially durability due to condensation. All to save 5% (at most) of $1000 annually. If that’s the price of comfort and I get to maintain my sanity too – so be it. So in our case, I have to agree with Kidd… and I would not recommend set back to ‘everyone’.

Really good discussion BTW – topic is dear to my heart.

By: Debbie Banks

Debbie Banks — Sun, 19 Feb 2012 06:20:54 +0000

thanks Allison for a great
thanks Allison for a great article.

By: Kim

Kim — Sun, 19 Feb 2012 05:40:44 +0000

Allison, Can you or anyone
Allison, Can you or anyone else reading this suggest a relatively easy to use programmable thermostat? I work with Affordable Housing builders who require the most user friendly products available.
Thank you,
Kim

By: rob brown

rob brown — Sun, 19 Feb 2012 00:01:18 +0000

I think my biggest problem
I think my biggest problem with setback methodology is twofold. Admittedly, my problems are with only a specific subsect of heating systems out there:

1. Outdoor reset vs Setback. We deal with hydronic systems, and often the savings from setback are used as an argument against outdoor reset water temperatures on modern boilers and the like. The problem is that reset can usually save at least as much, if not more than setback, especially in a modern, well constructed home. And when running it generally provides superior comfort. in the Air world I assume there is an analog to outdoor reset with modulating equipment but being a hydronic guy, I don't know for sure.

2. Comfort. You may get a thermostat back to room temperature at 6am when the owner wakes up... but you are not going to get his walls, floor and ceiling to the temperature they were when you went into the setback period for at least several more hours. In fact, I've lived in slab homes in which the nighttime setback schedule basically meant that the building surfaces stabilized at an uncomfortably cool level constantly. Thus the phenomena of the "cold 70" thermostat setting and all of the fairly simplistic (and often misleading) arguments about radiant being more comfortable at 65 than air is at 68.

You can save the most energy by turning the thermostat down to the bare minimum for human habitation as well... but that kind of defeats the purpose of a heating system. So comfort must be addressed in this conversation.

If you have to come out of setback to a thermostat setting that is a few degrees higher to counterbalance that ( and I know people don't, because they don't understand why they are uncomfortable in the morning... but if you want to get apples to apples that's what you'd have to do) and then bleed off that extra heat over time what's that do to your savings? it may not eliminate them, but it certainly reduces them.

I guess I'd feel a lot better if we'd all stop making blanket statements like "setback saves money" and specify the circumstances that matter. Setback saves money when using fixed temperature, fast response and/or oversized equipment. It saves a lot more in leaky homes than in tight homes. and it saves money by sacrificing some comfort... perhaps not enough to ignore the setback, but maybe you'd be better off with a slightly lower, full time setting than a bouncing setback schedule.

otherwise, I suggest a new blog for ultra green living... SHUTTING OFF YOUR HEAT SAVES MONEY AND THE PLANET. Sweaters are the new vogue!