theory question

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kenny chaos

kenny chaos

Minister of Fire
Apr 10, 2008
1,995
Rochester,ny
Let's say on the coldest days, my oil boiler runs 24/7 to keep the house at 68. (We keep it at 65 to give the boiler a break.) That means I'm circulating 180+ water steadily. Now I incorporate one of these new and improved woodstoves and get the hot air to circulate fairly well. If the wood stove is keeping the house at 60, how hot will the boiler need to run to make up the diif?
Or I guess I want to know if I'm making coals, will it actually make the boiler work less, much less?
Can't get my head around it.
Thanks-
ken
 
A straight forward answer is that if your boiler operates based on a thermostat that measures air temp, and if you raise the air temp around the thermostat, the oil burner will not come on. So, if you add a wood stove or another heating appliance which heats the air above your thermostat setting, your boiler will not come on. How much this will happen is subject to heat put out by wood stove, heat loss in your house, circulation of heated air from the wood stove to the area measured by your thermostat. Air circulation is important with a wood stove to try to get fairly even heating, as the area around a wood stove will be warmed the most by radiant heat output and convection currents, and areas furthest from your wood stove may not be reached by those hot air convection currents.
 
Jim-
Thanks but not even close. The scenario is that the wood stove is keeping the house at 60 degrees. If the thermostat is asking for 70, how much heat would the boiler have to put out to make up the diif? Would it have to catch up to the current temp and then try to add more or just supplement it some.
 
The boiler has to make up from 60 to 70. If you left the thermostat at 60, the boiler would not come on at all. The boiler is only adding the extra 10 degrees.

Sorry I missed your question the first time.

It seems you have lots of heat loss in your home, or your boiler is very inefficient, or is grossly undersized for your needs. It seems almost inconceivable that a boiler can't make up 10 degrees without there being a big problem somewhere.
 
Yes, it's an almost 200 yr. old stone house that won't hold heat. I understand that it only has to make up the ten degree differance but how hard will it be? If it runs steady at 180+ to keep it at 68, how hot will it have to run to make up only ten degrees?
Like I said, I can't get my head around this.
Thanks-
Ken
 
kenny chaos said:
If it runs steady at 180+ to keep it at 68, how hot will it have to run to make up only ten degrees?
Click to expand...

Not very hot at all, just to make 10 degrees of difference in your air temp.

Depending on your boiler, you may have to keep the boiler temp above a certain level, to protect if from condensing flue gases that can be corrosive.

Joe
 
That last 10* probably accounts for 15-20% of your heat loss. That being the case your boiler should run about 20% of what it does without the wood stove. Heat loss is not a linear thing. If you graph out a heat loss of your house you would see that it's a curve that climbs at an increasing rate as the indoor/outdoor temp difference becomes greater. In effect you're base loading with the wood stove and letting the boiler pick up the extreme conditions.
 
heaterman said:
That last 10* probably accounts for 15-20% of your heat loss. That being the case your boiler should run about 20% of what it does without the wood stove. Heat loss is not a linear thing. If you graph out a heat loss of your house you would see that it's a curve that climbs at an increasing rate as the indoor/outdoor temp difference becomes greater. In effect you're base loading with the wood stove and letting the boiler pick up the extreme conditions.
Click to expand...

Heater, In your statement you say that heatloss isn't a straighline equation. I have never seen any other type of equation other than the straightline. I asked browning about it in this link. he told me he thought that my calcs were correct. Could you weight in on it. Its halfway down. Notice that I calcuate that I loose 14K btu Per 10 degrees regardless of the delta.

https://www.hearth.com/econtent/index.php/forums/viewthread/14678/
 
The reason for the upward curve on the heatloss program I use is due to the manual J based air infiltration factor included in the calc.
A program that looks at temp difference alone and uses a static air infiltration factor will usually show a straight line.
I think that's the case anyhow.

If heat loss was based solely on Delta T, indoors to outdoors, then it would be pretty much a straight line. The fact of the matter though is this. Air infiltration increases proportionally as delta T rises. The greater the temp difference the more "chimney effect" you get in any structure. Let me say it another way. Air infiltration is less at a 30* delta T than at a 70* diff. The R-value of a structure is basically the same whether at 45* or -10*.
 
Heater,

I really like the expanation. I really believed there must be more than just 14.2K per 10 degrees. The chiney effect rings true to me. And, also maybe a upward curve on any surfaces that loose a great deal of heat through convection sounds possible.

I thank you for the expanation.
 
Is the "chimney effect" the same as increased air pressure in the house due to higher temperature vs less pressure with colder temperature?
 
jebatty said:
Is the "chimney effect" the same as increased air pressure in the house due to higher temperature vs less pressure with colder temperature?
Click to expand...


MMmmmmmmmm.........It has more to do with the fact that the greater the temp difference, the more these two things happen.

#1 is that the greater the temperature differential there is between two objects the faster heat will radiate from one to another. As an example, take a hot cup of coffee outside tomorrow morning, set it down and stick a thermometer in it. Time how long it takes the cup to lose the first 15* for example. Then time the last 15*. You'll see quite a difference.

#2 has to do with the air or convective side of things but the same principle applies. The greater the temp difference between two masses of air, the more the warmer. lighter air wants to rise while the colder air wants to seek the lowest areas of your house.

Hope that helps :)
 
I'm not a big fan of the Manual J method for calculating air infiltration. Most infiltration is mechanical in nature, rather than the result of thermal differences. Wind pressure, exhaust fans, combustion air used by appliances (etc.) all account for much greater effects than anything related to temperature. In modern construction, the buildings are so tight that infiltration is often a controlled factor, resulting from air exchange systems (HRV's and the like), so it can be easily accounted for, and doesn't change as a result of temperature.

Joe
 
with a 200 year old stone house, I suspect you have some infiltration issues. How old are the windows :) A blower door test would be a great way to see where to leaks are.

hr
 
Thanks for the reply Master. I don't think it's so much an infiltration issue as much as it is an absorbtion (?) issue . The glass is pretty much all original. Most of the windows were painted shut when we moved in and I left them that way except for a few strategically placed ones for that summer breeze. It is not at all drafty but I suspect it's way too easy for the windows, and to a much larger degree, the stone walls, to want to find their equilibrium with the outside air temp. This is why I'm having a hard time getting my head around how hard the boiler is going to have to work to make up that extra 10 degrees. Ten degrees doesn't sound like much but then ..........

So guys- if I'm " base loading with the wood stove and letting the boiler pick up the extreme conditions," is that furly efficient or do I demand better? Cost me $3500 in oil this winter and if I could cut that in half without switching careers to wood cutter, that would work.

thanks again-
ken
 
An infrared camera is another great way to "see" where all the energy is going. I've used them on brand new homes with some alarming results. You can rent them now a days, split the cost with some friends and neighbors.

Many utility companies have infrared cameras for line maintenance. Or you may find your local fire department has one.

hr
 
Master- With all do respect, pay attention! I know I'm losing heat thru the old windows and the walls. There's not much I can do about that. I could use a door blower or infrared or incense sticks or a bunch of other things to verify what I know. What I really need to know is how much effort will it be to gain that extra 10 degrees?
Thanks alot=
Ken
 
Gosh Ken, Jim, Joe and a few others have answered you question early on in this post. Maybe you need to break out the engineering books and discover the answer for yourself?

it's not about "effort" or quanity of temperature, it's about how much the structure loses based against how much your emitters can produce. It changes. the colder or windier the more the boiler needs to make up the difference. There isn't a one number answer to your question.

However two words could answer your question "It depends"

hr
 
Please bear with me. They don't call me Chaos for nothing.
Okay, here's what I'm thinki; Maybe I could do the wood boiler and storage thing small and cheap if I'm just backing up the primary stove. I gotta funny feeling that yes," it depends."
Thanks guys. I spend way too much time reading on here. Very cool stuff.
 
Excellent idea. A small efficient boiler with 200- 350 gallon of storage. Burn hot fires, in a small fire box so over-heating will be less of an issue. Use 120- 240 gallons of storage for a little buffer and overnight storage, to take up the slack.
 
Thank-you Master. Is it okay if I call you Master?
Would it be a waste of time to consider stuffing a big copper coil in a stove I have ( uninsulated stove) put it in the barn and circulate thru a 1000 gallon tank twice a day? I can get a good hot controllable burn out of this stove for about 4 hours. And about reading your engineer books, I would've a long time ago if I could've.
thanks dude-
ken
 
If I go camo color, the wife would drive right by. Ummm, I really think that would work. I could replace the boiler with a couple of twenty year olds:0
 
Pook said:
kenny chaos said:
If I go camo color, the wife would drive right by. Ummm, I really think that would work. I could replace the boiler with a couple of twenty year olds:0
Click to expand...
1 fat woman will keep u warmer than 2 skinny ones i think.
Click to expand...

like I keep reading on these forums, "it depends." Now I understand!
 
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