Most cost effective thermostat setting: consistent temperature vs. lowest temperature possible

Question

The problem: Our heating bills are about 25% higher than our average neighbors according to our bill, yet our temperature is always as low as we can stand it (mid 50's), we have excellent insulation throughout the house, we don't have drafty windows, and our furnace is a brand new natural gas system. The WAY we are heating is one of the last variables to explore.

The Set Up: We have a natural gas forced air heating system, set up with two zones. Zone 1 is where we sleep. Zone 2 is the rest of the house. We live in New England, so we do have freezing temperatures in winter.

The Question: What would you suggest as our timed pre-sets for Zone 1 and Zone 2, especially in the winter?

Here are some specific scenarios I'm trying to figure out:

• If the temperature outside is above freezing, do we need to really keep the interior heat of the whole house at 50-60 degrees F in order to prevent freezing pipes, like it seems every website suggests?
• Is it cost effective to turn off the heating (or turn very low) for the night in Zone B (where we don't sleep) if it's not going to go near freezing that night, then turn it back up to normal room temperature in the morning? Or is it more cost effective to keep it within 10 degrees F of room temperature, so the furnace doesn't need to work as hard in the morning when we want the temperature to come back up?

And are there any other factors to consider?

Answer 1

An Old Debate

There are is a very old debate about whether it's better to leave the thermostat at a constant temperature or to turn the temperature down when unoccupied and up when occupied. I am in the heat-as-needed camp that believes in turning the heat up and down.

One Argument

From a theoretical perspective, I think about it this way: Your furnace runs to replace the heat that escapes your house in cold weather. The greater the difference between the outside temperature and the inside temperature, the faster you lose heat. So with the temperature down when possible, the difference between outside and inside is less at those times, and you lose less heat over the course of the day.

There are those that say the furnace operates less efficiently this way. There are furnaces that run in two modes, one less efficient and faster, another more efficient but slower. I don't think this counters my argument. If this is the case, the right thing to do is start the heat up earlier and run at the slower / more efficient rate, not abandon the idea of changing temperature during unoccupied periods.

Actual Results

From a practical perspective, I have installed many programmable thermostats to lower heat in homes so that the temperature goes down to 60 all day when everyone's at work and up to 70 when everyone's home, with consistent results - lower heating bills. I don't have detailed data matching weather conditions to the bills but I am confident in the results - there are plenty of people with large commercial buildings that have maintenance staff that monitor these things very carefully, and looking at all the data have come to the same conclusion.

The Set Up: We have a natural gas forced air heating system, set up with two zones. Zone 1 is where we sleep. Zone 2 is the rest of the house. We live in New England, so we do have freezing temperatures in winter.

The Question: What would you suggest as our timed pre-sets for Zone 1 and Zone 2, especially in the winter?

I'd turn set both zones to 55-60 during the day, weekdays, if everyone's out at work or school.

I'd set the living area zone to 68 from an hour before people get home from work or school - maybe 4PM? to bedtime, say 10PM.

If the bedrooms are only occupied overnight, I'd set them to 68 from an hour before bedtime, whatever that is in your household, until 8AM or whenever everyone's up and out. If there are kids using the bedrooms after school before bedtime, you might want them up to 68 from 4PM to 8AM.

Don't forget, it's no big deal to turn up the thermostats for a temporary override if you happen to be in a zone outside of the usual schedule, so it's no big deal if there's a holiday or something.

I'd definitely want thermostats that allow a different program on the weekend unless the household is occupied the same hours on weekends as it is during the week.

How Low Can You Go?

If you want to take this to the extreme you could turn the heat all the way down when unoccupied. That would be a bit extreme for a house you come home to every day, but it's not uncommon for vacation homes. Of course you'd want to heat to a level temperature where the pipes won't freeze, and the temperature at the thermostat is no doubt warmer than the temperature where pipes are in outside walls - so 32°F is not a safe setting. The lowest safe thermostat setting would depend on how your plumbing runs, how your walls are insulated, and the outside temperature. I know in my area many people set the thermostat in their vacation home to 40°F or so off season when they might only be occupied one weekend a month. When there are extreme cold snaps, a few people will find they were a little too aggressive with the setting and wind up with burst pipes.

(Incidentally - if the outside temperature is above freezing, there's no way it can freeze your pipes. At worst, it could get as cold inside the house as the lowest temperature outside, but under normal conditions, even with no heat, it won't get quite as cold inside as it gets outside. There is always some insulation, some greenhouse effect, some appliances running inside, etc.)

Other Potential Problems

If your heat bills are higher than expected, there are lots of possibilities. If your heat bill is higher than your neighbors with similar homes by 25%, it's not because they have better thermostat schedules than you. There must be some other issue. For example, a common culprit is a hot water leak. Even a drip leak can drain a lot of hot water over the course of a month, and that's heat escaping your house - your water heater will have to run more to make it up. If your water heater and furnace are both natural gas, you may assume the culprit is the furnace, but the real problem is money dripping down a drain all day.

Answer 2

Setting back your thermostat to any reasonable temperature for any reason amount of time will only save 5-10% over the course of a season.

Apples to apples

You can't simply compare your bill to your neighbor's bill. There are very numerous reasons why this is impractical. Different construction, exposure, consumption, equipment and more. Even two houses built by the same builder with the same plans will have a different heat loss profile because construction practices are not perfect nor consistent.

Where's the heat going?

Advertised combustion efficiency for fuel burning heating units ranges from 80% to 98%. That's based on availability of heat energy contained in the fuel being burned. So 98% means for every $1 of gas purchased 98 cents worth gets converted to heat available to heat your house. 2 cents goes outside. That's assuming the equipment is setup properly. The heat in your home then gets lost to the environment through the walls, ceiling, ground and infiltration air.

How to set your thermostat

Set it low and with longer run cycles. The advertised rating is determined under laboratory conditions. It takes 10 - 15 minutes for a furnace or the like to reach steady state. If your furnace/boiler is oversized it will turn on and back off before reaching its optimal efficiency. This is a very common problem because installers and scared of getting the call that the furnace is running all day. An optimal configuration is one where the furnace runs all day and the set point is maintained.

No easy answer

Unfortunately there is nothing to point a finger at given the information provided. Setting back your thermostat will have limited impact on your utility cost. Hopefully this information can help you find the issue and address it.

Answer 3

Our heating bills are about 25% higher than our average neighbors according to our bill, yet our temperature is always as low as we can stand it (mid 50's), we have excellent insulation throughout the house, we don't have drafty windows, and our furnace is a brand new natural gas system. The WAY we are heating is one of the last variables to explore.

I'm going to assume we're only comparing heat costs, not Gas vs. Gas costs, which may include other factors like dryers, water heaters, ranges (stoves / ovens).

Do your neighbors have multi-stage systems? If so, do you? Note: multi-zone and multi-stage are two very different things: multi-zone means you can set different areas of the house to different temperatures, multi-stage means the system itself will only draw as much energy as it needs to heat, usually prioritizing using as little as possible.

As an anecdote: I have a multi-stage heating and cooling system. What does this mean? This means, as indicated in another answer, my system runs in one of three modes: low/efficient, medium/efficient, high/inefficient.

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If you have single-stage, but your neighbors have two+-stage, you'll probably always have a higher heating bill, all other things being equal.

Effectively, with any given heating/cooling unit, the air in your house is heated up slowly and unevenly. There are parts of the house that will naturally be warmer and parts that will naturally be cooler. Depending on thermostat placement, this can cause a significant change. If your thermostat temp varies wildly, your heat will run inconsistently.

When using a single-stage system, it always puts out the same heat. This means that there are several things that differ between single-/multi-stage systems:

1. Consistency. A single-stage system has less consistent temperatures in the house, because it can only heat at 1 rate. This means that it always heats at "maximum", even when the variation is very small. Thus, even though most of the house might be comfortable, until the thermostat says "turn off", it still puts out MAX heat. This means that when the thermostat finally does say "turn off", the overall temp will be higher because air is still mixing.
2. Frequency. Often, a single-stage system runs less frequently than a multi-stage system because of this variation. It overheats the house during each run.
3. Consumption. As obvious, a single-stage system runs at 100% energy-consumption when it is running. Multi-stage systems do not. This can be good and bad, depending on the weather.

There are lots of articles about the differences, I'll point this one out specifically: THE DIFFERENCE BETWEEN SINGLE-STAGE AND TWO-STAGE HEATING

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To continue my anecdote: my multi-stage heating system actually costs me about 20% less than my neighbors, whom all have single-stage systems. I live in Toledo, OH, right next to Lake Erie, my weather / seasons are almost identical to yours. The upfront cost was higher, but my overall maintenance and heating costs are lower. (Mine is a 1970's install, all of theirs are 2010's and later, and I still pay less for my heating cost, and I have my temperatures about 3°F higher than them.) Mine is three-stage instead of two, but you should gather that this is a major possible reason for the difference.

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So, what do you do?

First: check if they installed a single-/multi-stage system. If they installed multi-stage, verify that your thermostat configuration(s) is/are appropriate. If it's multi-stage, make sure they installed a multi-stage thermostat. Make sure the thermostat is configured to use multi-stage mode. (Many thermostats have support for two-stage, but you have to turn it on, because it uses things very differently. If it's multi-stage, make sure that feature is on.)

Second: if it's a single-stage system, you might always have a higher heating-bill. It's worth evaluating if it's cost-effective to replace the furnace with a multi-stage system. It's also worth exploring further insulation, and any locations that air might be escaping or leaking in.

Third: you may want to explore pressurization systems. Some people report success with reducing bills by pressurizing the house/attic, some don't. It might be another option you can use to see if it helps keep the house more comfortable. (Generally, if the house is at high-pressure, heat will escape, if it's at low-pressure: cold will leak in. Whether or not high/low pressure is beneficial is something that seems to vary highly per-house, per-setup.)

Fourth: check doors / windows. Make sure your doors are well-insulated, make sure the windows themselves are well-insulated. You can try adding curtains on all the windows and closing them to see if you have poor windows. (Old windows were often more poorly insulated. With my anecdote: my windows are literally less than 1 year old, which is a major factor in where heat escapes.)

Fifth: do you have bathroom exhaust fans? If so, try plugging them if you can stand it and see if that helps. (My dad used to lose a lot of heat due to poorly-installed bathroom exhaust fans.)

Sixth: do you have a basement? Are any basement walls exposed? Is your heating system trying to heat the basement? This may be another factor.

Seventh: is your system properly-sized? If you have vaulted / cathedral / high ceilings in the house, make sure those calculations were taken into consideration when the system was installed. (I've seen really bad HVAC installers not take that into consideration and under-size system installation, which resulted in exactly your issue was well as the system running constantly.)

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Finally, you asked explicitly about scheduling: I personally don't use (nor recommend) it.

Generally, with scheduling, you would set the house to a more uncomfortable temperature when you're gone (hotter in the summer, cooler in the winter). This can actually do more harm than good, depending.

When you change the temperature like that, your walls, floors, and ceilings are significantly affected. You normally wouldn't notice the effect at a constant heat, because it takes a long time for those areas to change, but 8-10 hours (a typically workday) is plenty-long for those changes.

This means that when the heat comes back up, it'll take much longer for it to get the house to a consistent temperature, because the walls/floors/ceilings have to catch up. This can actually do more harm than good, though it's an experiment to try, I would not be surprised if it went poorly for you. (Again, this varies significantly by insulation / build quality / weather. One answer indicates success with this, so it might be worth exploring.)

Answer 4

And are there any other factors to consider?

There is one factor of note that you have attempted to consider:

we don't have drafty windows

Unfortunately, the idea that windows are a major factor in a house's draftiness (or more technically, air change rate) is a common misconception. The reality is that the vast majority of draftiness is completely hidden, out of sight, in the thousands of feet of seams & gaps hidden behind trim and in unfinished spaces, in holes for utility to pass through. The only way you can really know whether or not a building is losing a lot of heat by leaking conditioned air to the outdoors is by testing it with a blower door (though many of the flaws can be detected with a thermal camera if you have the experience to know where & how to look).

Answer 5

It is a myth that furnaces need to work "harder" if turned off for a while or if the room cools or heats too much. This has been tested and is recommended even by the government:

https://www.energy.gov/energysaver/thermostats

https://blog.powerley.com/mythbuster-using-a-thermostat-setback-for-energy-savings/

You should have your thermostat as cold as possible during winter and hot as possible during summer to minimize energy use. This applies whether you are at home or away.