Turning thermostat down = using more pellets???

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Jambruins

New Member
Hearth Supporter
Jun 26, 2008
57
Chazy, NY
I have been thinking that I may actually be using more pellets by turning the t-stat down. My reasoning is that the stove keeps the house at 64 from 7:45-1:00. It runs on high/low using settings 3 (3 lbs/hr) and then 1 (1 lb/hr). Then at 1:00 the t-stats calls for 70 so my stove basically runs on setting 3 constantly as it takes a while for the house to get to 70.

Do you think I would use less pellets if I left the house at 68 throughout the day. The stove would still alternate between setting 3 and 1 and it would definitely use more pellets between 7:45-1:00 but it would use a lot less from 1:00-3:30. I know the best way to find out is to try it and it depends on outside temp and wind but I was just curious as to what people thought in general. Thanks.
 
That's a very good question and has been discussed many times. Personally I've found that if you keep the stove set at one temperature, say 68°, it will do whatever it has to, to maintain that temperature. If you have the thermostat set to drop to let's say 64° at night and back to 68° during the day, it will use more pellets to get back up those 4°. But it didn't use any pellets when it lost those 4°, so it kinda 'breaks even'.

I think it's a matter of preference, but I feel that leaving it set at one temperature is the most efficient bet.

Steve
 
Jambruins said:
I have been thinking that I may actually be using more pellets by turning the t-stat down. My reasoning is that the stove keeps the house at 64 from 7:45-1:00. It runs on high/low using settings 3 (3 lbs/hr) and then 1 (1 lb/hr). Then at 1:00 the t-stats calls for 70 so my stove basically runs on setting 3 constantly as it takes a while for the house to get to 70.

Do you think I would use less pellets if I left the house at 68 throughout the day. The stove would still alternate between setting 3 and 1 and it would definitely use more pellets between 7:45-1:00 but it would use a lot less from 1:00-3:30. I know the best way to find out is to try it and it depends on outside temp and wind but I was just curious as to what people thought in general. Thanks.

setting it back for 5+ hours uses less total energy thanks to newton's law of cooling.
 
Shortstuff,
I agree that it won't use many pellets while it is losing 4 degrees but I have to think the pellets used to gain back that 4 degrees would be much more than the amount of pellets I saved when it lost 4 degrees.

Patrick,
I realize that is uses less total energy but is the total energy saved offset but the amount of energy used to get the temp back up? I think it is much easier to lose energy than it is to gain energy.
 
I think of it more from a comfort point of view.

In my house (other designs might vary) it take too long to recover the temp. to a comfortable level. I let mine shut down just for 4 hours during the day as I'm direct vented and I'm using a UPS for some peace of mind.

Other than that, I let it run constantly. I manually adjust the conv. speed based on outdoor temps. This year, I've run the stove full blast most of January and now into Feb. On the coldest mornings, the stove still cant keep up and the furnace kicks in to keep the house at a minimum temp (65).

In my case I'm undersized (Quad 1200) for what I'm trying to heat (2500sf), but I think I've reached a balance between pellet vs propane. In order to heat the house better, I'd have to burn 2 stoves. I don't think I'd be able to offset those costs, and pure comfort is not enough reason to spend more money.
 
Jambruins said:
I realize that is uses less total energy but is the total energy saved offset but the amount of energy used to get the temp back up? I think it is much easier to lose energy than it is to gain energy.

total energy is all of the energy expended over the day, it includes the ramp back up time.

The only reason you have to burn fuel is to replace lost energy, right? Otherwise when you got the house to 71 in November it would stay that way. We know that's not true - warmer bodies transfer their heat to cooler ones. In this case your house is the warm one and the outdoors is the cool one. How fast that happens depends on 2 things: the thermal properties of each body (what is your house made of, how well is it insulated, etc) and the temperature difference between the two objects. We burn fuel to replace the heat lost in that transfer and keep a steady temp inside.

newton's law of cooling applies to the second factor - the temperature difference between inside and outside. The law basically says that the rate of heat loss is proportional to the temp differential. If there is a 70 degree difference you may be losing 20KBTU of heat an hour, but when there is just a 50 degree difference you might loose just 15KBTU. The actual numbers are made up and depend on the construction of your house, but the principal is the same: the bigger the temperature gap the greater the RATE of heat loss. If you close the temp gap you lose heat slower and therefore have less total energy to replace when you turn the tstat back up than you would have slowly leaked out if you just left the tstat alone.

my example, if there is a 70 degree difference you are loosing 20KBTU an hour whether or not the stove is running. If the stove is running on medium or so you are essentially replacing those 20KBTU each hour and keeping it a steady temp inside. If however, the stove is off then the temperature inside begins to drop. Maybe you loose 20KBTU the first hour, but the second hour it is cooler inside (meaning a smaller differential between inside and outside) so you lose something less - maybe 18KBTU, the third hour it is cooler still - maybe 15KBTU.. let's say this is as cold as you want it to get so it stays at that temp for 2 more hours - losing another 15KBTU each hour which the stove replaces by burning on low keeping the inside temp steady and cool.

OK, so now its cool inside, and you've burned 30KBTU worth of pellets in the last 2 hours to keep it at the minimal temp. How much energy do you need to get back to the original toasty room temp? It's exactly how much energy you have lost - remember all we ever do is replace lost heat. That would be 20 + 18 + 15 = 53 KBTU. Add the 30KBTU the stove burned on low and you're back at room temp having spent 83KBTU.

If you had stayed at room temp the whole 5 hours, losing 20KBTU an hour, you would have used 100KBTU worth of energy. In each case the room is the same temp afterwards and all energy is accounted for. Your house is just more efficient at holding onto the heat it already has when the differential between inside and outside is less.

(again, the actual BTU rates are totally fabricated and depend on your house and the actual temperature differential, but the priniciple is right.)
 
CZARCAR said:
what about the burn efficiency factor of the pellets? i think a constant burning bed would burn more efficiently than a bed which suddenly introduces an increased quantity of pellets that might lose some energy in the pre ignition phase.

I'm not an expert on that front, but I think - as you say - that is more of a factor during the phase of getting the stove started. So its a good argument against turning the stove on and off several times an hour, or just setting it back for a few minutes while you run to the store - but over a 5 hour period which was the topic of this post (or overnight), the extra startup period is such a small proportion of the total time it cannot really change the fundamentals. (If it did, my self-cleaning Mt Vernon which shuts down every few hours to clean itself would be wildly inefficient - which it isn't).
 
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