Burn times with thermostat lowered?

[ktm rider]

I read a post recently about a guy that got a long burn time in his OWB by turning down his thermostat to 55 degrees while he was away. Did he get this long burn time from setting the T-stat down low or was it just because he packed his OWB completely full of wood before he left??????

I have always wondered if setting the T- stat down has any effect at all?
Wouldn't the houses' temp fall from 55 to 50 just as fast as it would drop from 70 to 65? So the boiler would have to raise the temp in the house the same 5 degrees no matter what. 5 degrees is 5 degrees, Correct? This is assuming that the temp outside is way lower than 50 degrees, lets just say it is 20 degrees.

Besides the initial free fall from 70 degrees to 50 degrees, which probably doesn't take all that long, I don't see how this would stretch the burn times at all. Of course I am probably way off but this is one of the many questions that rattle around in my head..

Any opinions??

 

[windthrown]

Lowering house temp would reduce the heat demand from the OWB. Hence the damper would open for shorter times and farther apart, and hence the wood would indeed last longer. Looking at it another way, if say the t-stat was set at 70 and it was 40 outside, there would be a 30 degree differential. If he sets it at 50 and it is 20 outside, there would be the same 30 degree differential. From my experience with the house t-stat set at 70 when it is 40 degrees outside, the wood lasts a lot longer. Lowering the differential between the house temp and the ambiant temp will always conserve more wood, regardless of the heating device used.

However, and here is the caveat, once he returns home and sets the t-stat back up to 70 degrees, it is going to put a huge demand on his OWB and it is going to need a long hot burn to raise the house temp back up to 70 degrees. So while he may conserve wood and get longer burn times while he is away, the heat demand to raise the house back up to a comfortable level is going to burn up a lot of wood. Overall he would save more firewood that way though. Less heating will require less wood in the long run.

Another gotcha here though is that with a hydronic floor heating system, they recommend keeping the t-stat at the same temperature all the time. Or near the same temperature. This is bacause hydronic floor heating takes a long time to transfer heat. So if he has a hydronic heating system, the time between his coming home and the house being comfortable can be in the hours. Personally I used to set the t-stat down to 65 in the day and 70 at night. That conserved enough wood to make it worth while putting in a set-back thermostat, and it was not so uncomfortable to live in at 65 and warming to 70 during evening time when the atmosphere was sucking the heat out of the house at the highest rate. It also only took about an hour to raise the temperature up the 5 degrees.

 

[oneoldbanjo]

KTM:

I am that guy.....and here is the reason I use my approach.

1) The loss of heat from an object is directly based on the difference in temperature between the two objects. If my the inside of my house is 70 and the outside is 30 - there is a 40 degree temperature difference. If I turn my house down to 50 and it is still 30 outside there is a temperature difference of only 20 degrees - and the heat transfer is half the rate that it would be if my house was 70. Think of the temperature difference as if it were a pressure, and it is easier to understand that more pressure would increase the heat loss and less pressure would slow the heat loss down.

2) My house and garage are made from SIPS panels and is extremely well insulated - my house is basically a big stryofoam cooler. I only lose about 10 degrees a day if I turn the heat off... so when I have my house at 70 and turn the thermostat down to 50 I get about 2 days that my OWB won't be heating the house at all. The length of time that it takes to drop to 50 depends on the outside temp and if it is cold it will drop faster, and if the sun comes out and shines in my windows during the day it can help considerably. MY OWB is basically just spending the first two days keeping it's water temperature between the set points and not sending any heat to the house.

3) My garage is also made of SIPS panels and it does have radiant heat and the PEX tubing is in a 6" concrete slab. The temperature of the garage is very constant as the slab is a 156,000 pound thermal mass - it takes a huge amount of energy to raise the temperature up in the garage and when I move the thermostat up a few degrees the OWB really eats wood. I normally keep the garage thermostat at 50 degrees. When I was getting ready to go away for a week over Christmas I increased the garage temperature a couple of degrees every day until I got to 65 degrees - and then I shut the garage heat off completely when I left. Over the 10 days that we were gone the garage cooled down to 50 degrees - which is the normal temperature I keep the garage. The huge thermal mass of the concrete supplied the heat to the garage while I was gone.

4) When I return home and turn the thermostat in the house back up to 68 degrees it takes about 2 hours to get the house back up to temperature and it takes a few more hours for all the hard surfaces like countertops, cabinets, floors, walls etc. to stabilize. This period of time the OWB is working hard and it provides time to burn out all the creosote that built up in the OWB while it was not working very hard. It does not take a huge amount of wood to get the house warm again - but it does take more wood than a normal day would.

This method has proven to work very well for me. Prior to leaving I load the OWB up with as much good seasoned wood as I can, and I have been gone as long a 4 days and returned home to find the OWB still burning (The 3 times I have done this the weather was not below 30 for very long and it got up into the 40's during the day). My house is very well insulated and most people won't get the kind of performance that I get - however lowering the thermostat in any house will always require less energy and require less wood in your OWB.