Thermostat Night Time Setback

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New Hampshire
Sorry for such a long post.

We moved to New Hampshire in September 2016 after living in the Midwest for many years. Our six previous homes in the Midwest had forced air natural gas heating systems. This has been my first experience with both fuel oil and a hot water radiator heating system.

But this is the third home where I have used microcontrollers and sensors to track the heating system and log data. I am doing this more extensively here than any other home, mainly due to heating costs in this New England area.

This is also the first home where I have built and installed a weather station, which I log data to use along with my heating system data.

The purpose of my post is to discuss what my data shows when setting my thermostat back at night. I’ve been using a setback thermostat in my previous homes for many years and have been happy with the results. Although I never analyzed any data to this extent, I’m confident I saved fuel in those forced-air heating systems running them at 68 during the day and 62 at night.

But I find that is not the case here with this hot water radiator heating system.

First some specifications: this is a 1970 raised ranch with 1,200 square feet on the main floor. It has Pella insulated windows and wood siding, except for the lower front, which is brick.

The house has the same baseboard radiators I have seen in other homes around here. We have 3 zones, main floor, lower floor, and hot water tank.

The Buderus G115 oil boiler is on the lower level in the garage. The lower living area of 550 square feet has a separate heating zone, which we leave shut down. Nonetheless, both the garage and lower living area stay around 56 degrees during the winter. It was running 57 degrees in the garage before I insulated the exposed hot water pipes in the garage ceiling.

A year ago I added 10 more inches of fiberglass insulation in the attic, for a total of maybe 13.5 inches. I think I have the house about as energy efficient as it’s is going to be, short of solar panels.

We have been averaging 527 gallons of fuel oil or $1,450 for a full year. That’s not too bad for this area. Plenty of people with larger, old homes burn twice as much. But it’s noticeable compared to our Midwest homes where we typically burned $500-$600 of natural gas each year.

I decided to try and find my optimum setback temperature based on furnace run times between 9 PM and 6 AM by comparing the number of seconds of runtime to the average outside temperature (taken every 3 minutes at my weather station) for the same time period.

I had done this sort of thing before on a daily 24-hour basis comparing the Heat Degree Days (HDD) for that same day. I even calculated my own location’s HDD using my weather station data.

The problem I found with that approach, as far as trying to find an optimum thermostat setback, was that things going on in the house, like showers, washing clothes, dishes, etc., would skew that day’s usage.

By working with just the 9 PM to 6 AM time period, all those factors are out of the picture. The only thing different from day to day would be the outside temperature, which does make quite a difference.

For this comparison, I’m using 4 data elements from my log files for 9 PM – 5 AM.

Main floor thermostat setting

Seconds the oil burner flame is on

Seconds the main floor hot water pump is on, meaning the main floor thermostat is calling for heat

Average outdoor temperature

So far, I’ve only analyzed the data this way the past 8 days. I have data logged going back the 3 years we have lived here, but not based on various setback settings.

I started by comparing my normal 62 degree setback to leaving the thermostat at 68 all night long. I was shocked to see I burned .3 gal more oil with the 62 degree setback than leaving it at 68. That’s 9 gallons more per month or around $25 more cost.

This is due to the amount of time it takes the oil burner to get the house back up to 68 in the morning. If the outside temperature is say 22 degrees during the night, it will take 90 minutes for the thermostat to get back up to 68 from 62.

Of course, the oil burner isn’t burning oil that whole 90 minutes, but it will come on every 6 minutes or so to maintain the hot water temperature. My winter aquastat settings are 205 High, 180 Low, and 15 Diff.

Anyway, at that point, I started setting the thermostat back to 67, 66, etc. and evaluating that data. I haven’t done enough days yet, to say positively, but it appears to me a setback of 67 degrees saves a little fuel. A setback of 66 burns about the same as leaving it at 68 all the time. And setting it back 66 and lower burns more fuel.

I could kick myself for not realizing this 3 years ago!


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Peace valley missouri
Might want to look into a boiler reset control that resets water temp by outdoor temp. 0*-180* , 60*- 140*. Baseboard needs 120* minimum water temp. If your using the boiler for domestic hot water still might be able to use a reset just a higher minimum temp. Check with your electric company to see if they offer a energy audit that includes a blower door test it checks how tight your house is.


Retired Defense Industry Engineer xxx
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New England
A more modern boiler will have the ability to be setup with a priority zone for the indirect...on many, that will make the boiler fire hotter than the 'normal' setting to get the water hot and then back to keeping the house warm. Many older boilers tend to be way oversized, and even modern, right-sized ones, you generally never notice that the space heating was off while the indirect was reheated. They may use an external controller, and that can be setup for a priority zone.
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