optimum setback temps & anticipation

Gordy

Yes the the greater the DT the greater the loss.......Which puts us back to the envelope, the greater the R value the slower the losses. See thread link below. I stand with Bill Nye, and Nrt. Rob's Theory on this. Plus the fact that I have done my own experimentation on this with undesirable result. I have older ceiling radiant. Medium mass compared to higher mass slab radiant.

Even though you have standing Iron with quicker response time. Your MRT of the objects in your home will be lagging your air temp once the t-stat is satisfied, because of the loss they suffered during set back. Thus colder objects make you feel cooler "cold 70" or what ever your comfort set point may be, until they equalize with the set point. A reverse of what happened during set back.

If you were to install a smart t stat that learns how far in advance the system needs to come out of set back to reach your comfort setting, rather than just come out of set back at the given time. Your set back time will be even less as I have found. Not worth it.

Gordy

Spudwrench

Optimum setbacks & temp swings

I was wondering what the optimum temperature set back is for fuel savings. Our utility is reccomending "up to a 10* setback." Other sources have argued that too large a setback is counterproductive.
Would the building construction and insulation make a difference? i.e. a different setback might be appropriate for a very loose, lossy building, stick built vs. masonry, etc. etc. Has anyone come up with definitive answers?
Along similar lines- most 'stats tend to allow a 1-2* swing around the setpoint, no? Would a larger swing, allowing longer cycle times be more efficient? (Especially during the hours of the day when a building is unoccupied and comfort is not an issue?) This could be crudely accomplished by switching to a different T-stat with the anticipator misadjusted, allowing lots of overshoot.
Curious to hear peoples thoughts on this one.
Thanks,
Nathan

Wayco Wayne_2

Hey Nathan

It depends.... What kind of system are we talking? Is this residential or commercial? What is the building material making up the space? How many people in the space and what's their make up and metabolism? (my wife is always cold) With Boiler systems it is better to set back the water temp and not the air temp. Concrete buldings take longer to warm back up because of the thermal mass involved. Like I said, It depends. WW

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Spudwrench

Wayne,
Well, I was specifically wondering about a stick-built, lossy house, but I am also curious to understand the principles behind setting back. One hears so many conflicting opinions. I guess you could expermentally determine the best thing to do in a particular house or building by comparing the total run time of the boiler on two different days with the same average outdoor temps and differing degrees of indoor setback when the house is empty. But it seems like somebody should know this without having to reinvent the wheel!

Gordy

Set Back

Deep set back is counter productive. Unless you are leaving the dwelling for extended periods (Days or Weeks). When you let the MRT drop in the dwelling it will take the same amount of energy or more to bring the MRT back up as it will to maintain it. All the thermal mass (walls, floors, ceiling, furniture ect.) in the dwelling has cooled.

Setting Back 10* while everyone is away for the day. The chance it will ever drop that much is very unlikley. If your home construction is that lose you need to invest in getting the envelope more energy efficient. Not trying to save dollars by setting back. Keeping the energy in the envelope is what leads to less energy consumption


To me in comparision set back is like driving in stop, and go traffic, or cruising the highway non stop. Which is more economical. Heating a dwelling has more complex variables though. Depending on the emmiter end of the system.

JMHO Gordy

Wayco Wayne_2

Gordy

Please explain MRT. WW

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Steve Garson

I beg to differ. The amount of energy used to heat your home is related to the difference in temperature between the inside and outside. The larger the delta, the greater the heat loss. If you can lower the temp when the home is not in use or while sleeping, you reduce the heat loss.

My home has a 10* setback at night and during weekdays. The temp almost never drops the full 10*. But the house warms up with a single steam cycle of my boiler. I've seen the fuel savings, accounting for degree days.

Bob W._3

Look at this thread. Mike T. Swampeast Mo posted a link to a study regarding setbacks in masonry homes.

http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=191644&_#Message191644

Unknown

There is more to it than delta T. If you aren't even getting to your setback temperature, you're basically just offsetting cycles that would have run over the course of the night and very marginally lowering your load in the meantime, I would think. If this offset is saving you any noticeable amount of fuel, I would have a look at your steam system. It may be oversized or otherwise running inefficiently and your "help" here may be related forcing the system to operate in more efficient, longer burn cycles rather than your actual setback.

I could certainly be wrong! But that's my hunch.

Gordy

Wayco Wayne

> Please explain MRT. WW

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Sorry been battlin with the leaves, Did not mean to ignore your question. MRT "Mean Radiant Temperature" The thermal mass of all the components in the dwelling. Basically anything that absorbs heat, and can store it for a period of time.

Gordy

Mike T., Swampeast MO

I've been trying to answer just those questions for years.

The biggest problem I find is that any studies of setback in actual structures are few, small and far between. The vast majority of data comes from simplified calculations.

There are a few points of agreement:

1) Setback savings BEGIN when the space is being maintained at the lower temperature.

2) The proportion of savings from setback increases with increasing outside temperture.

3) The magnitude of savings from setback increases with decreasing outside temperature.

Beyond that, things get contentious.

YES, setback is influenced by the structure, the form of heating and the weather. A leaky, poorly insulated frame house heated by air will have much greater setback savings potential than a tight, well-insulated house heated by large radiant panels.

most 'stats tend to allow a 1-2* swing around the setpoint, no? Would a larger swing, allowing longer cycle times be more efficient?

I don't believe so because you're still maintaining the same average temperature.