Achieving the elusive "overnight burn"

[CowboyAndy]

So, whats the secret? Last night was only about 40, and I loaded up the furnace at 11pm. At 5:30am it was just coals, and not alot either. The wood I was burning was sugar maple, 4"-6" rounds and a 6" oak round. The thermostat was set for 75, at 5:30 it was 70 in the house. Any tips or suggestions?

 

[Nofossil]

So, whats the secret? Last night was only about 40, and I loaded up the furnace at 11pm. At 5:30am it was just coals, and not alot either. The wood I was burning was sugar maple, 4"-6" rounds and a 6" oak round. The thermostat was set for 75, at 5:30 it was 70 in the house. Any tips or suggestions?

You mention 'furnace' I assume this means a hot air woodburning appliance. If a load of wood can't maintain the house temperature that you want for as long as you'd like, you have a limited number of options:

1) Make sure your wood is well seasoned. Dry wood provides more heat per cord.

2) Make sure that your furnace is properly adjusted - I can't help with this one.

3) Get a bigger furnace.

4) Get a more efficient furnace. There are boiler and furnace designs that can get substantially more heat out of a cord of wood than the traditional designs.

5) Insulate the house better.

That's about it.

 

[CowboyAndy]

1) Make sure your wood is well seasoned. Dry wood provides more heat per cord.

Okay, I have to admit that its probably not as seasoned as it should be, as we had alot of rain this year. How would you rank the importance of dry wood?

2) Make sure that your furnace is properly adjusted - I can't help with this one.

What kind of adjustments can be made?

3) Get a bigger furnace.

4) Get a more efficient furnace. There are boiler and furnace designs that can get substantially more heat out of a cord of wood than the traditional designs.

This is a brand new Newmac cl-86 wood/oil combo.

5) Insulate the house better.

The house is fairly well insulated... for being a 115 year old house, anyways. New windows, doors and new insulation in 80% of the house.


We set out tstat at 75 last night. Would turning it down a few degrees help the wood to last longer?

 

[Nofossil]

I'm playing in a different sandbox, so I don't have firsthand experience with some of the issues that you're facing.

I can say that it takes energy to boil the water that's in your firewood, and that energy is then carried up the chimney and out of the house. I'd guess that any hardwood that hasn't been cut, split, and stacked for a year probably has enough moisture so that it's wasting an appreciable amount of heat.

I have no experience with the type of furnace that you have. I have a gasification boiler which is a whole different breed of animal. There may be inlet and damper adjustments that could promote a more efficient burn. In general, any woodburning appliance is much less efficient when idling than when burning flat out. Prolonged idling may be convenient, but it requires more wood than a series of short, hot fires.

Turning down the thermostat will reduce the heat load and should make your fire last longer. I don't know how long a burn you can expect from your model.

 

[deerefanatic]

Cooler temperatures will always use less fuel.. But if you purchased the unit to keep your house at 75, then by all means you should expect it to do so. But, I notice the unit is only rated at 86,000 btu/hr. I would say that is your problem. It may not be keeping up well with demand. See, sure a furnace can put out 86,000/hr, and that is the most efficient burn it will have, but the firebox won't last long. On the flip side, an oversized furnace will last longer between loadings, but more of your wood will "simmer away" up the chimney, so in the end, you'll go through more firewood.

As for settings on the furnace, doesn't look like there are many.

What's the square footage of the house? Better yet, take a look at SlantFin's website and download the heatloss explorer software: (broken link removed to http://www.slantfin.com/heat-loss-software.html) Do it all out and you will know exactly how much heat your house sucks down for a given outdoor temperature (usually you put the coldest temperature of the year in this blank, although if you want to see what you used last night, then put last night's low temperature in there.)

After you do that, report back to us. We can then see if it's just an issue of a furnace that's a tad too small or something else.

 

[muncybob]

Is it reasonable to "assume" that a similar type of appliance but a boiler(vs a furnace) would be more efficient?

 

[Nofossil]

Is it reasonable to "assume" that a similar type of appliance but a boiler(vs a furnace) would be more efficient?

Not necessarily. I don't know how much efficiency variation there is between different models, but I'd expect there's more variation based on firewood condition and operating techniques than anything else.

The real jump in efficiency comes when you get into boilers or furnaces that are designed to provide secondary combustion. The best of these seem to use about 40% less wood than conventional units do to provide the same amount of heat.

 

[deerefanatic]

Well, yes and no.........

A boiler would be more efficient, but only because of the availability of gasification boilers. Without gasification, efficiency would probably be a little less. You have to remember with a boiler, you heat the water, then transport it through pipes to a fan coil to extract the heat from there.... All those conversions and pipes decrease efficiency a little... If you have any water storage (and lets face it, all boilers do to some extent) then you have standby losses....

So the answer is, no... Equal for equal, it wouldnt be more efficient. If you went to a gasification boiler, then yes, much more efficient. The advantage of a boiler is that you can put a heat storage tank on it, so even if it runs out of wood, the storage tank could ride you through until the next morning......

 

[deerefanatic]

Ah nofossil, you beat me agian!!

 

[CowboyAndy]

But, I notice the unit is only rated at 86,000 btu/hr. I would say that is your problem. It may not be keeping up well with demand. See, sure a furnace can put out 86,000/hr, and that is the most efficient burn it will have, but the firebox won't last long. On the flip side, an oversized furnace will last longer between loadings, but more of your wood will "simmer away" up the chimney, so in the end, you'll go through more firewood.

Correct me if I am wrong, but the 86.000 btu rating is for the oil side, isn't it? I was under the impression that wood stoves and furnaces don't have a btu rating... because its all about how much fuel you load into it. We went round and round with a few different companies looking for a furnace, and most said the same thing: you either need a big one, or a small one. Our house (2000 sq ft) seemed to be right on the cusp, and due to financial reasons and avalibility, we went with the smaller. But going by the firebox dimensions of ours and others on the market, seems that it should be good up to at least 2500 sq ft.

But, we may need not go any further, it's all becoming VERY clear at this point: my wood isn't as well seasoned as I thought it was. Seems like every question I ask, the respose includes some form of "seasoning". Most of it has been cut since march/april, but we had a rainy summer, and the more I think about it I didnt stack it in the most ideal location (near an eve). Live and learn, eh?

 

[deerefanatic]

Yah, that'll do you in.... I'm thinking of stacking my wood inside our abandoned silo......