Setback tstat or constant 24 hr temp as it relates to interior furnishings.

Paul Wolf · December 23

While just a DIYer, I have a pretty good understanding of hydronic systems. My question has to do with setback tstats, interior furnishings and efficiency. I've read several posts here, but haven't quite found what I was looking for. My question is at the last two lines of the post. Provisional information is in between.

I am hoping the responses will help me keep peace and domestic tranquility within my household.

My current system is PurePro AGDV5-HP (LP fired). Two heating zones (up and down) of finned baseboard and a separate zone for the Superstore DHW.No ODR. We also have a Mitsubishi mini-split 3-zone 30k system (HyperHeat) with one 15k air handler downstairs, and two 9k units in two of the upstairs bedrooms. We use the downstairs one for auxiliary heat down to about 27-29 deg F outdoors. The 2 upstairs ones are only used for AC. We have a wood stove but really only use it on very cold days, or during several-day cold spells. We only use any/all of these heating sources during occupied (daytime) hours. For heating (downstairs only) we set the daytime (occupied) tstat to 68. At night it resets to 62.

We keep upstairs zone (bedrooms & bath) at a constant 58. The house is 2x6 construction (built 1984) is very well insulated, pretty tight and has new, high quality, energy efficient windows. Low E & argon.

I understand the structural thermal loss calculations and the like pretty well. My question pertains to the thermal loss and reheating of the interior furnishings. For the sake of my question, let's assume that the structural thermal loss is constant and that I do not use any of the auxiliary heat sources.

Assuming a setback as above (68/62) or a constant 24 hr 68. Will I use more fuel with the setback, given that I need to replenish all thermal loss from my furnishings? Or will I use more fuel just maintaining a constant 68 and not have to reheat my furnishings? If the correct response is the latter, is the difference to keep at it a constant 68 significant?
Thanks.

bjohnhy · December 23

Assuming the structural thermal loss is constant? I think you mean ignoring solar gain and changes in wind factors. But the thermal loss (btu/hr) will be less at 62F inside vs the thermal loss at 68F. That's the whole reason for setback. For strictly gas supplied system without an outdoor reset, fuel use will be less with the setback schedule you describe.

I think you realize your system is more complex than that. And if your using the heat pump regularly, keeping at constant 68F may be more efficient. I would encourage you to use the ODR on the boiler (not with a deep setback however) and maximize the heat pump use when electric rates and outdoor temps are favorable for your HP model. And keep at constant 68 (+/- an hour or two set forward (69F) on your boiler, strictly for comfort when you wake up or having morning coffee)

Jamie Hall · December 23

Assk six people and you will get 8 opinions. In my view, 68/62 is a bit much, but not so much from the fuel usage standpoint.

The real problem — which most people don't have to factor in — is that some kinds of furnishings can be damaged, some rather severely, by continuous temperature swings. Most modern furniture doesn't really care — think IKEA and the like — but any furniture or other furnishings which are solid wood or perhaps even more so solid wood with veneers — well be damaged, if not destroyed, by relatively quick swings, and it may not take that long. I'm not talking normal seasonal swings here, or even most daily swings from changes in the weather, but the constant two times a day now you see it now you don't setbacks.

Joints loosen. Wood cracks. Veneers come off. It is possible to reduce a high end piano to unrepairable junk in a matter of a few years. I've seen it happen…

hot_rod · December 23

humidity levels in the home have a lot to do with wood movement.

Cabinets, flooring, furniture etc love to have a constant humidity level to prevent joints opening

Winters in the dry Utah desert, lots of static electricity and split fingers for me.

fentonc · December 23

I have pretty good data on this for my particular house, as I have an extensive monitoring system. Last year I had steady thermostats (68F on main floor, 62F basement, and I think 65F for upstairs, but might have been 62F). This year I switched the main floor to be 62F between 10PM and 6:45AM, and from 8:30AM-4:30PM during the week, upstairs and basement are still 62F steady. The main floor has lots of windows, and has the highest heat loss (but also gets lots of sun, as it has south facing windows).

My house:

Basement, 1st floor, 2nd floor, each with their own hydronic zone, each floor about 800 square feet
Located in Westchester, NY
CI natural gas boiler - nominally 140K BTU/hr, but underfired to 130K BTU/hr due to low gas pressure.
The worst I've seen the gas usage in the last 4 years of detailed monitoring, at the design temp of 12F, was about 25K BTU/hr.

Last year, for days with more than 30 heating-degree-days, the house averaged about 471 BTU/HDD-hr. So far this year I've been averaging 384 BTU/HDD-hr (18.5% lower). I also increased the aquastat differential (to reduce short cycling) and upped the gas pressure (to reduce excess air) from 120K BTU/hr to 130K BTU/hr, which will have had some effect, but I would bet at least half of that is due to the thermostat setbacks.

DCContrarian · December 24

There are two questions here:

First is, does a setback cause a house to lose less heat? The answer is yes. The amount of heat lost to the environment is determined by the difference between the inside temperature and the outside temperature, with a setback the inside temperature is lower some of the time and loses less heat.

The second question is whether the setback will save you energy. With a combustion appliance the answer is yes, a BTU is pretty much a BTU. But with a heat pump it becomes more complicated, because a heat pump uses a variable amount of energy to produce a BTU. First, the colder it is outside, the less efficient a heat pump is. So if you're running the heat pump less when it's warmer and more when it's colder— like you would if you turned the thermostat down when the house was unoccupied during the day — the setback may not save you anything. Some heat pumps have supplemental heat, when the controller detects that the heat pump isn't meeting the heating load it energizes resistance heat to supplement the heat pump. The supplemental heat is much less efficient than the heat pump, typically about half as efficient. Often the way the heat pump detects whether the heat pump is meeting the heating load is by comparing the thermostat set point to the inside temperature. In such a system, a setback will guarantee that the supplemental heat runs regularly, which will increase your energy usage.

Generally the recommendation for heat pumps is not to use the setback.

fentonc · December 24

It looks like you have a ~130KBTU/hr boiler, which is probably extremely oversized for your actual heat loss - one advantage of such an oversized system is the rapid recovery from setbacks. Depending on what your control setup is, you could have the heat pump maintain steady-state at either 62F or 68F, but use the boiler to quickly recover from the setback. With programmable thermostats you might be able to do something like:

Boiler and heat pump are 62F overnight
At 6AM, boiler jumps to 68F and starts pumping out heat. Heat pump is still at 62F.
At 8AM, heat pump jumps to 68F (and the house is probably already at 68F), and boiler falls back to 66F so that it only kicks on if heat pump can't maintain heat.

Paul Wolf · December 26

WOW ! What great set of responses… Thanks folks. Great info to digest. I guess my wife wins this one, and I (and one of my (apparently *somewhat* knowledgeable ) HVAC friends) lose. I appreciate all the good info provided. Thanks again.

EdTheHeaterMan · 3:07AM

I believe that the magic number is 4 hours. anything less than 4 hours of setback is a waste of time in my opinion. that is because you do use a little more fuel recovering and that foosets the savings of the non operation time during the setback. That said if you setback 5° for 8 hours then there will be a savings since there will be 4 hours of lower heat loss due to the lower ∆T operation after the initial 4 hours.

It will not cost you more to set back for less than 4 hours, that just does not make physical sense. Running a burner for 30 minutes to recover 5 degrees in a given hour is the same as running for 10 minutes to maintain the temperature three times in the hour. So don't worry about actually spending more if you setback for short times, Just don't expect any savings for setting back for 2 or 3 hours while you take a nap or go to a movie.

Heat pumps with electric resistance backup or aux heat is a different story. It will cost more to recover anything in excess of 1.5° since the thermostat will operate the higher cost electric resistance heat for recovery for more time than is practical for the savings from non-operation for any amount of time less than 48 hours.

In the early 2000s Honeywell attempted to use something called Adaptive Intelligent Recovery (AIR) in order to keep the AUX heat from engaging on heap pumps by starting the recovery as much as 2 hours prior to the selected time for that higher temperature. There was an algorithm that would start the compressor and see how fast the temperature increased based on the previous three day memory. If it was taking longer than the previous three day average then after 15 minutes of operation the Aux would be allowed to kick in if needed to get the temperature back on track. This might happen as the average outdoor temperature was dropping. As the average outdoor temperature was rising then the compressor would reach the setpoint sooner than needed and cause the next day start time to be less than it was the previous day.

They also use AIR with gas and oil heaters to start the burners as much as 1 hour ahead of the selected time, and that hour would get reduced as needed based on how fast the room temperature recovered. Colder days took longer, warmer days took less time. Those thermostats actually saved a little more than the manual setback or older clock thermostats of the 1950s and 1960s the problem with those thermostats is that the homeowners didn’t like waking up in the middle of the night to a colder home, and stopped using the setback feature, so no one was saving any money if that setback was disabled. The tax rebate for setback thermostats were eliminated as a result of that practice. Only the “Smart Thermostats” would qualify for the tax credit and that meant that you needed to get a thermostat that works with the internet using your home’s‘wifi network

Setback tstat or constant 24 hr temp as it relates to interior furnishings.

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