I think I found the limit of my furnace

[H-Ranch]

So when it was -15F and windy here a few nights ago I awoke to a house that was at 59F. We have the thermostat programmed for 62F overnight so it was not a big deal. The OWB had burned through the wood I put in before going to bed and the water temperature was down to 118F. Plenty of coals to get another load started and in a short time the water was right back up to 175F. The furnace fan continued to run trying to bring the house up to temperature. Slowly, over the next 6-7 hours, the house temperature was raised from 59F to 68F. In that time the outside temperature increased to -5F, so really the house was only tracking to the increase in the outside temperature. The OWB was cycling on and off between 165F and 175F so it seems it was able to keep up, but the warm air distributing through the ductwork was only enough to maintain the house temperature.

I'm thinking that would mean that running the furnace on propane would have meant a 100% duty cycle - in other words A LOT OF PROPANE! Looking at a few charts it seems that a standard house the size of mine in Michigan should have a 120,000 Btu furnace. With the 25' cathedral ceilings with only a few inches of foam insulation and some big windows it should probably be a bit higher. We only went one winter heating with propane here so I really don't know how the furnace has performed, but it's only about 100,000 Btu. Actually never had an issue with it so it did seem adequate.

The rated maximum outlet air temperature of the furnace is 175F, so I'm probably a bit less than that with the OWB heat exchanger only getting to a maximum of 175F on the water side. I could have bumped the water temperature up more - I'm kinda hoping I don't have to do that any time soon though as I've had quite enough of the below zero temperatures for a while. I did find it interesting that the water could be kept up the temperature with the OWB without appreciably raising the air temperature in the house. At -15F I'm only getting a 6-7 hour burn time when fully loaded.

 

[Mad Professor]

I think you need a woodstove too : )

 

[Dusty Rhodes]

With your cathedral ceilings and your rated 100,000 btu unit it sounds like you are a bit undersized on your OWB. If you should have a 120,000 btu unit even without the cathedral ceiling it sounds like you did well to maintain what temperature you did during the recent cold snap. I don't wish to discourage you, but personally I would not be happy with the 6-7 hour burn time. That would cut into my sleep time. Who fires the unit while your at work? If I did not get a 10 -12 hour burn time out of my OWB I would be looking for a bigger unit.

 

[legg28]

"Alot of propane" that was the same thing i was thinking as my furnace blower ran almost constant the last couple days . My burn times dropped a little too because of the cold snap

 

[H-Ranch]

With your cathedral ceilings and your rated 100,000 btu unit it sounds like you are a bit undersized on your OWB. If you should have a 120,000 btu unit even without the cathedral ceiling it sounds like you did well to maintain what temperature you did during the recent cold snap. I don't wish to discourage you, but personally I would not be happy with the 6-7 hour burn time. That would cut into my sleep time. Who fires the unit while your at work? If I did not get a 10 -12 hour burn time out of my OWB I would be looking for a bigger unit.

The propane furnace is rated for 100,000 Btu so I'm saying it's probably a bit undersized. I built the OWB so I don't know what it's rated to, but I would say more than the furnace is capable of since it was cycling and the furnace fan was not. This was by far the coldest temperature the OWB has seen. As I said, I could have raised the water temperature another 10 or 15 degrees just for the below 0 spell and I may experiment with that next time. When it's not -15F I have no problem getting 12 hour burn times - in fact, most days it doesn't get a full load of wood and it doesn't get hickory/oak. I'm OK with a 7 hour burn time a couple times a year - I didn't miss any sleep so it may have even been 8 hours.

I'm not discouraged in the least - the OWB was the best thing I've ever built and it makes me smile every time I load it.

 

[H-Ranch]

I think you need a woodstove too : )

I did have a fire in the fireplace on Monday during the day. Doesn't provide much in the way of heating the house but it does help to dry out the wet gloves/hats from plowing the driveway!

 

[stihly dan]

Its not the furnace size, but the coil size and temp. You could run a 500, 000 btu furnace with the same ducted coil and water temps. Nothing would change. Many applications need 200*-220* temps in the winter to keep up.

 

[Dusty Rhodes]

If you were getting 10 -12 hour burn times and using lessor BTU hardwoods in your normal winter weather then I stand corrected. I would be using the primo wood when its really cold. Makes a big difference for me during the really cold weather. I can't say about the hotter temperature on your unit as I run an all water system so I am running 160 -180 F. temps. Glad to hear your not normally running out in the middle of the night to fill the furnace

 

[Cheesecutter]

I normally get 10-14 hour burns out of my OWB heating 2 houses. With the extreme temps I was burning cherry, oak, silver maple and elm mixed getting 8 hour burns and was quite content with that. No gas bills for me.

 

[H-Ranch]

Its not the furnace size, but the coil size and temp. You could run a 500, 000 btu furnace with the same ducted coil and water temps. Nothing would change. Many applications need 200*-220* temps in the winter to keep up.

Right - I was limited to the heat exchanger size based on the existing furnace plenum. So I'm guessing that propane would not have done any better at keeping up than the OWB did. I'm not planning on finding that out though because I don't like burning propane. I doubt I'll get my water temperature up to 220F (at least intentionally) but I may try 185F or even 190F next time it's below 0.

 

[CTYank]

I did have a fire in the fireplace on Monday during the day. Doesn't provide much in the way of heating the house but it does help to dry out the wet gloves/hats from plowing the driveway!

IMHO, that would be a part of the problem. In really cold weather, typically below 20 deg F, any open fireplace cools the house, because of the large amount of (super-cold) makeup air from outside. A proper modern woodstove would serve you MUCH better. And be much more effective at drying gloves. (May I suggest "Atlas Thermal" work gloves? Then mainly forget the drying stuff.)

 

[H-Ranch]

IMHO, that would be a part of the problem. In really cold weather, typically below 20 deg F, any open fireplace cools the house, because of the large amount of (super-cold) makeup air from outside. A proper modern woodstove would serve you MUCH better. And be much more effective at drying gloves. (May I suggest "Atlas Thermal" work gloves? Then mainly forget the drying stuff.)

Eh, the fireplace is not for heating the house - it's there mostly for ambiance. I used it during the day on Monday and had nothing to do with the lowest temperature on Monday night/Tuesday. The fireplace serves my purposes just fine and I find no use for a "proper modern woodstove" since I have the OWB.

 

[greendohn]

I jacked my temps up to 170-180 and broke out the not-so-seasoned pecan and locust with some well seasoned cherry and mulberry. My burn times were shortened to about 8-9 hours, maybe more.
My furnace blower was running non stop keeping the house 70 degrees Sun and Mon. when the temps were well below zero. I backed the house temps down to 68 degrees and the furnace blower still ran pretty hard, Tues.
When it's below zero outside with wind chills in the negative 30's,,or more!! Stoves are gonna' burn wood and blowers are gonna' blow!!

 

[fuzz1500]

On a different note..I like your avatar ! What kind of loader is that ? When the pic was small...I thought it was an old Dynahoe...like my grandfather had ! When my uncle refers to the quickness of its digging capabilities...he calls it the "Dyna-Slow !!!
Anyway..I see now that its not a Dyna-Slow !! Just curious !

 

[KindredSpiritzz]

Wow, 25' cathedral ceilings are crazy. All your heats up in the rafters. I'd be curious what the temp difference is at 3 ft and 23 ft. My ceilings are 7 ft so it keeps the heat bottled up in a useable range

 

[woodguy105]

We have cathedral ceiling in our great room. Lots of wasted heat up along the ceiling...

 

[Whitespider]

Dad has a monster cathedral ceiling in his great room... he runs two ceiling fans backwards (pulling air up, which forces the heat down along the walls and you don't feel drafts). They're variable speed fans and when it's really cold he needs to speed them up just a touch. The room stays pretty cozy now... but before the fans it was always cool in there.

Mine is a indoor hot air furnace, not an OWB... but I did notice some difference during this past extreme cold snap. If I let the house cool down overnight it can take awhile to warm it back up. Say I crash early, so the box don't get filled later in the evening, and the house cools to 60°... well, when it's -10° outside it may take 2 hours to raise the house temp 5° (from 60° to 65°), but only 1 hour to go from 65° to 70° (also 5°). Normally I have the thermostat set to 67° overnight and the house will be 65-67° in the morning depending on how much the draft blower kicks in and eats fuel. Usually a half hour in the morning will bring it up to 70-71° so it's warm when the family gets up and around, dressing for school and such. Well... when it was -10 or -15° that wasn't working so good, the house would be cooler in the morning and it would take a lot longer to warm up, and having the draft blower running for an hour, or two, or more, will eat up a lot of firewood in a relatively short time. I ended up raising the overnight set temp to 69°, the house would be a couple degrees warmer in the morning... and raising the temp didn't require near the time or fuel (firewood). Even raising the temp from 66° to 68° required more fuel and time than it did to raise the temp from 68° to 70°.

Lowering the thermostat overnight may save fuel during "normal" temperatures... but when it gets flat-azz arctic outside I'm not so sure that remains true for all homes and heating systems. I know I used less by raising the overnight set point... in my old uninsulated farm house.
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[H-Ranch]

Lowering the thermostat overnight may save fuel during "normal" temperatures... but when it gets flat-azz arctic outside I'm not so sure that remains true for all homes and heating systems. I know I used less by raising the overnight set point... in my old uninsulated farm house.
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Ha! That's what I did Tuesday night and last night. It may or may not use more wood, but it doesn't take hours to get back up to temperature that way!

(See how I agreed with you there? Your birthday wish continues.... )

 

[H-Ranch]

Wow, 25' cathedral ceilings are crazy. All your heats up in the rafters. I'd be curious what the temp difference is at 3 ft and 23 ft. My ceilings are 7 ft so it keeps the heat bottled up in a useable range

I have tried using the ceiling fans before - both directions, all speeds, and combination of some on/some off. I had several thermometers in various locations in the house and couldn't make a difference in temperature anywhere. My guess is that with only a couple of inches of foam in the ceiling it doesn't stay too warm up there so I just save on the electricity and don't run them in the winter. Now in the summer I do use them blowing down to have some cooling effect of the moving air on your skin.

 

[Whitespider]

(See how I agreed with you there? Your birthday wish continues.... )

Ahhhhhhh....... yes..... it's good to be me‼
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