Tstat setback

Andy Morgan_2

Is there a chart or something available that shows the average (theoretical) fuel savings just by lowering the thermostat a couple degrees? I know that after a certain point, the savings stop. I'm in a friendly debate with a customer!

Thanks


Andy Morgan

R. Morgan Mechanical, LLC

Einsiedler_2

setback savings

Savings with a setback thermostat can cut your heating costs from 20 to 75 percent. In summer, such devices may shave your cooling costs by 15 to 25 percent. (Your actual savings, of course, will depend on such factors as the climate, amount of insulation, temperature you set your thermostat to, and the rate structure of your utility company.
Rough rule of thumb: energy savings are 1% of the total heating bill for each degree of setback for an eight-hour period.

hope this helps.
EIN

jackchips_2

Why would

the savings stop? Wouldn't a btu not burned be a btu not paid for?

I also thought that two major considerations were:

1) recovery time (my WARM air system brings the house back up within an hour from 62 to 68);

2) a potential for frozen pipes in the heating system or domestic water.

R. Kalia

> Savings with a setback thermostat can cut your

> heating costs from 20 to 75 percent. In summer,

> such devices may shave your cooling costs by 15

> to 25 percent. (Your actual savings, of course,

> will depend on such factors as the climate,

> amount of insulation, temperature you set your

> thermostat to, and the rate structure of your

> utility company. Rough rule of thumb: energy

> savings are 1% of the total heating bill for each

> degree of setback for an eight-hour

> period.

>

> hope this helps. EIN

Tony_8

I respectfully disagree

From my own professional and personal experience and experiments, I found that setting back more than 5F for 8 hours used more fuel. In fact, when setback 10F for 8 hours my consumption went up almost 30% ! My boiler ran for 1-1/2 hours straight ! Five degrees or less used virtually the same amount as constant setpoint. I logged it and compared usages to average daily temps for a given month against months with the same avg temp. I have had customers try this with both hydronic and air systems and the results usually bear me out. I think the analogy I use the most is, " if you drive for 3 hours on the interstate at a constant 65 MPH, will you use less fuel than if you vary your speed between 50 and 65 MPH ?" I think we all can see the similarity.

Personally, I think setback for residences is a fallacy sold by saying if it's not on it's not gonna cost you anything. The price of recuperating from setback can be high.

As far as results varying because of factors such as climate, insulation, and setpoint of stat... aren't those some of the factors we use to determine heatloss/gain and equipment spec'd ? I think for setback to really be effective the design temp difference would have to be only 20F or less. Where it is 70+ difference recuperating can take hours from a 10F setback.

I'd like to hear other opinions from "experimenters"

Tony

Tom Meyer

Setback thermstats

IMHO - Setback thermostats work very well with agile, low mass, fast recovery systems. However, a tube in 4" of concrete, for example, is far from an "agile, low mass" system. Before the BTUs in the water are delivered to the conditioned environment, they must first heat the mass in the boiler, then the water and associated piping in the distribution system (circuits, manifolds, etc), then the mass (concrete, gypcrete, aluminum plate, etc), then the BTUs are transferred from the mass to the conditioned environment. As we know, heat goes to cold, always. So anytime a surrounding material is colder than the mass or the water in the distribution system the heat will bleed away from the hot to the cold. So, if the conditioned environment is 65 deg F, heat will continue to go from the mass and delivery system to the environment until it also is at 65 deg F. Now, when you take the thermostat off setback and want to heat the environment to 70 deg F, you have to bring everything up to temperature again. The boiler mass, the delivery system, the radiant panel, and then you start sticking BTUs into the room. Depending upon how deep the setback is, and how large the mass is, it could take several hours to get up to temperature at constant firing.

Comparing warm air systems to hydronics system by system agility isn't a fair comparison. Absolutely a warm air system, being extremely low mass (air), is more agile than a large mass 4 inch concrete pour.

You have to design the system to do what you want it to do.

Tom Meyer
Senior Designer/Trainer
Precision Hydronics Corp
www.precisionhydronics.com

Uni R

EIN if...

If your rough rule of thumb is that energy savings are 1% of the total heating bill for each degree of setback for an eight-hour period, how could the savings ever range from 20-75%. I could see an extreme case being close to 20% but for the normal joes and joannas with an hydronic system that gains temps slowly and a moderately tight home that loses temps slowly who sleep in their homes 7 hours a night and are gone from 8:30 to 5:30, this just isn't the case. Also, if the setback is more than 5°F daily the thermal expansion cycles are going to affect flooring, drywall screws, fine furniture etc., not to mention the fact that even the high end Tekmar controllers would have a hard time trying to calculate when to start heating so that you can actually take your jacket off when you get home.


BTW, my boiler system recovery rate at colder temps is maybe 2°F tops per hour with water temps maxing at about 140°F. Is there some logic that I should boost the outdoor curve for 180°F so that I can extend my setback and save more fuel running higher temps?


http://www.eere.energy.gov/consumerinfo/pdfs/thermo.pdf

Uni R

Comfort

This is interesting... Never thought about this.


"If you use a set-back thermostat to turn down your furnace operation during the sleeping hours at night, you will experience even less humidity in your home. This can cause even more discomfort and aggravate allergies." This may be much truer of scorched air.

http://www.energyserv.com/html/q2_-_can_t_reach_sufficient_hu.html


Out of personal curiosity I tried to look up some stats that actually cite the real studies. No real success but it seems like these statistics may be quite old (since the numbers and text seem to be immune to time) and all seem to be based on scorched air with the caveat "NOT FOR GEOTHERMAL" since the resistance heat would click in on recovery.


California says 20-75%, the US Dept of Energy says up to 10%, some cite "numerous studies", but nothing more definitive than that. I have a feeling that 75% savings number came from a mild winter climate and is the sum result of both setting back the evening hours to 68°F from some much higher number and then having it 55°F overnight and probably much of the ensuing day with no morning ramp up.


My concern would be this. If the boiler is actually correctly sized for the home, how is the house temperature going to recover in a reasonable time to get back to comfortable temps?

jp_2

please explain

I'm curious to know how you arrive at knowing you use 30% more fuel than you would have if you'd left the t stat alone?
what is your control?
tony, don't take this as criticism, the more I'm thinking about all this the more it makes me think!

Tony_8

simple

Our gas bills show CCF's used, days in cycle, and average daily temp. My control of the system is just that, my control. I alone controlled setpoint and envelope conditions. The house has CI rads on monoflo tees fired by a std CI boiler. Number of residents was constant. I, personally, am a creature of habit. Almost to obsession I had numerous months with which to compare.

No offense taken.