Heat Loading and Efficiency

[ddddddden]

I conducted an interesting experiment last winter. Adapting a proper flue to our old smoke dragon insert proved too difficult, ultimately because the insert's 5 x 15" outlet was overlaping the lintel by ~ 1". So I put a pair of $40 electric radiators in the room. These things run a 600W element, a 900W element, or both, for 2000, 3000, or 5000btu. Most days, I ran the 2 heaters @ 3000btu for a total of 6000btu. Guess what. This kept the room in the low 60's, which is all the smoke dragon ever did unless I shoveled wood into it on a cold night when the draft was good. Looking back now, I'm not surprised by the dragon's relatively poor performance. . .typical "slammer" install into an exterior chimney. I guesstimate it was running ~ 30% efficiency. What did surprise me was that 5000 - 10000btu was an adequate amount of heat most of the time, probably because I left them on almost continuously, so the house was always preloaded with some heat. So I got to thinking, and reading, and thinking some more. . .

 

[ddddddden]

. . .I read, probably on here, that the rate at which a house loses heat is a function of the difference between indoor/outdoor temp, so the warmer you try to keep the house, the faster it loses heat. There is only a given amount of heat energy in a load of wood, but it would seem that the most efficient heater is the one that releases the heat as steadily as possible to keep the house as close as possible to the target temp. Any excess heat, besides being waste, is lost more quickly. I guess I'm just trying to share my experiment and what I've learned here. Do I get a gold star? Anyone got numbers for the temp diff / heat loss rate?

 

[Battenkiller]

Anyone got numbers for the temp diff / heat loss rate?

It depends on a lot of variables, such as insulation, air-tightness, surface area of roof and outside walls, volume of living space, specific heat of building materials, outside air movement, method of heat transfer, etc. No easy numbers to just plug into a formula. Here's a start:

http://en.wikipedia.org/wiki/Heat_transfer

 

[SolarAndWood]

it would seem that the most efficient heater is the one that releases the heat as steadily as possible to keep the house as close as possible to the target temp.

That is always my goal, but you have to compensate for the variable output of a wood burn cycle vs constant output of an electric heater. I try to accomplish this by timing the reloads for when we want a little more heat in the living space. The challenge, if you are burning 24/7, is to leave enough time for a burn down so that the coal bed doesn't get out of hand. When it is super cold/windy, I throw some pine on top of the coals after about 2/3 of the burn cycle. It seems to burn down in about the same amount of time but allows you to put extra heat out at the end of the burn cycle.

As far as efficiency and formulas, my analysis is pretty simple: Process/store/burn enough wood to keep the ladies in the house happy. My wife was happy with low 60s when we paid a gas bill, but now she expects more.

 

[Highbeam]

Heat output from the stove (most stoves) is highly variable. Max btu output is not something you can compare to the constant btu output of an electrical heater. So what are you trying to say or accomplish? Heating your house to a cold temp can be done with a constant 5000-10000 btus? That's fine. How about holding 75 all day?

 

[ddddddden]

I was surprised at what 5000 - 10000btu could do. I think maybe the more variable heaters load the house in a more lossy way.

 

[SolarAndWood]

There is nothing efficient about paying for electricity.

 

[Slow1]

Interesting (if expensive) experiment you ran there. So now you know what it takes to maintain low 60's in that area. If you wanted to try again and see what it takes to maintain 10 degrees higher try again next year You will likely find it is a bit more than you expect. As you said, the heat loss is a function of the differential between where the heat is and where it is going (i.e. hot to cold side) which then is slowed by how it is being transferred (whatever is in the way insulation etc can slow it down). If you hold the insulation etc constant (don't upgrade) and the outside temperatures are on average the same (doubtful, but perhaps close enough?) year to year you may get a useful result. But in the end you will simply have more numbers.

What you do know, however, is what sort of stove output you likely would have needed last year to maintain the low 60's - increase that and you would have had a warmer room .

I agree with your conclusion, however - if you heat the room dramatically you will lose a lot more BTUs to waste than if you keep it at a comfortable temperature. However if you are going with wood then you aren't going to be able to avoid some temperature fluctuations (even with the best masonary heater). Looking at stoves a good heavy soapstone stove with a cat (like the FV) does give a fairly steady heat output but it still will fluctuate some - but does 10 degrees really make a big difference in overall efficiency in the home? I suspect that it doesn't for most of us - the difference in $ savings between wood and the electric you burned last year will more than make up for that, even just looking at wood vs wood I expect you will see more efficiency gains from practicing good burning techniques than from keeping your interior temps +/- 5* of some ideal temp. (I have rather arbitrarily picked a 10* range - I have no idea how wide a range it would take).

However, being a numbers guy it would be interesting to see - if you want to do the experiment with the electric I'd be happy to help design it

 

[ddddddden]

Well, the experiment was necessary because it was late Nov, and due to insufficient lintel clearance, I had to abort my other experiment for determining how well the smoke dragon could perform with a proper flue. Rather than slamming the dragon back into the fireplace, I covered the mouth of the fireplace with sheet metal and deployed the 'lectrics. I also sorta sealed the top of the exterior chimney, and tightened up the house a bit by doing a few things like cramming a piece of foam panel into the recess below the attic hatch. So this was not a 100% fair comparison for the ol' dragon, but I was pleasantly surprised to find that 5000 - 10000btu was suficcient. As for expense, it increased the power bill by about $100/mo. I know the lossy dragon would've eaten that much in wood(paid for in $ or sweat) to get the same amount of heat into the house. I appreciate the offer to collaborate on a more controlled experiment, but I plan to get back to wood burning this year, with a less lossy system.

 

[madrone]

Many wood burning sites and stove manufacturers describe a series of small fires that burn completely as the ideal way to heat with a stove. The obvious reason for this is the shorter cycle with a lower high room temperature, in addition to the increased efficiency. Running an EPA stove that way comes closer to a steady heat source like an electric heater. Loading a stove full and then damping way down gives you a longer, lower-temp burn at the cost of efficiency, while a full load run hot has a higher peak temp. So, if you're a wood burner, you have to decide between more loading, or greater swings in temperature. Personally, I hate the feeling of electric heat almost as much as I hate electric bills. I'm happy to waste a little free heat.

 

[ddddddden]

I'm not suggesting that anyone abandon wood heat, but the liquid-filled electric radiators feel nice, like a stove. What I now want is a heating appliance that burns wood in a way that simulates, as closely as possible, how a heater burns electricity.

 

[Slow1]

I'm not suggesting that anyone abandon wood heat, but the liquid-filled electric radiators feel nice, like a stove. What I now want is a heating appliance that burns wood in a way that simulates, as closely as possible, how a heater burns electricity.

I'm not really sure what the feel is that you are looking for but perhaps the "softer heat" that the soapstone mfgrs like to put in their marketing material? Combine it with an extended burn in a cat stove (ala Woodstock not running full bore) and perhaps that is what you are looking for? If it is you may be happy - but somehow I seriously doubt you will find "feels like a liquid-filled electric radiator" in the WS marketing material anytime soon!