Owb undersized?

[paladin]

We are looking at houses,and we looked at one yesterday that had a Heatmaster cs-4000. The tag on the burner said it was a 4000 sq. ft. model,it was heating a 3000+- sq.ft. total incl basement and hot water. The question is if I was to add a garage/shop 900-1200sq.ft. I would be over there rating what happends with these owb's. Do they not keep up or use alot more wood or will It be ok. Mainly the shop would just be kept at 40-45 degrees when not working in it. I curently heat with a stove it the basement but I figured the next house would have a owb,and I'm trying to learn about them and check this site daily. Thanks in advance.

 

[jeeptj19992001]

you burn more wood, less efficent

 

[Geez]

Most OWB manufacturers rate their units to heat twice as much as they can. That is, if you are heating 2000 square feet, you need one rated at 4000. Seriously.
Even the dealers will tell you this. That one will probably give you decent supplemental heating for the house but I think that's about it, at least from what I've read and learned.
For my shop I built an insulated box with heat exchanger and blower. It's designed so I can cover the heat exchanger opening with rigid foam board and the air inlet is constructed to hold two air filters. When I'm not in the shop I slide the air filters out and slide in a matching sized piece of 1 1/2" foam board, and put the piece in front of the exchanger. So when I'm not in the shop I'm only heating the inside of the box. Works great and only takes minutes to bring up the temp in the shop. Saves tons of wood. Just mention this because something similar might put less of a load on your apparently undersized unit.

 

[Slick]

I think that would be totally fine, won't take much heat to keep your shop at 40-45. I have a CB 5036, they rate it at 250,000 btu...I have more than 300K worth of heat exhanager hooked to it plus domestic hot water. House is 1200 sq feet with a 120k coil, shop is 2800sq ft with I think a 100 and a 160K coil, might be even bigger) keeps up fine, my dealer said it would keep up fine but that I would need to feed it more when it's cold out and he's right, when it's cold out my 2800 sq foot shop (probably heating 2000 feet of it) with just R6 insulation in most of it will really eat some wood, I can get 12 hrs if it's good wood, anything iffy in the wood department and it's 8+ hrs. It was in the 30's last night, just checked it and after 15 hrs I have half a load left....so my feeling is that furnace would keep up OK making heat but don't expect anything over 12 hrs burn times when it's really cold out. These things can really make the BTU's when burning, a bigger unit will give you longer burn times with an obviously bigger box...but that's a healthy sized heatmor your talking about.

 

[ericjeeper]

OWBs are the only thing rated this way

Everything else is rated BTUS. The way it should be..
Instead of buying a larger OWB. Tighten the envelope of the house. And build the garage as tight and as energy efficient as possible.. Better doors, windows and better wall insulation packages.

 

[trialanderror]

it's not that it wouldn't keep up.

it's the fact your limited by firebox size. just needs more wood. like slicks situation. he could hook up another 250k of exchangers, but just need to fire it more.

 

[mtfallsmikey]

My experience: I have heated my 800 sf shop, 1200 sf house with the CB5036, but could have used the 6048, more water storage, probably might have sucked up less wood. Realistically, take an OWB's BTU rating, cut it in half, the there's your real world rating. Do a heat loss on both the shop/house.

 

[Slick]

Where is everyone's "half of rated BTU's" coming from? I'm just not seeing it...at half the rated BTU's of my 5036 (so 125,000btus) I'd only be able to heat my house, not my shop and it's no two car garage, it's a 40x70 building with 16ft ceiling in half of it and that half has only the blanket R6 stuff they use on steel buildings...but I'm heating both and my 5036 isn't exactly burning all the time...I was getting 8 to sometimes 10+ hr burn times with crappy past seasoned deadfallen wood during this past cold snap...10+ hrs easy once I got into some good ash I had...
I keep hearing this theory of more water storage for less wood usage also...I don't understand that either....all that water is doing is storing the BTU's from burning the wood....more water will strech out the time between burn cycles but will just take a longer bigger burn cycle to bring all that storage back up to temp....how does that save wood? In these things BTU's only come from the wood you put in, how does more storage equal less wood, I don't get it....
I really thought about a 6048 and it certainly would have been justified with my shop on line but price became and issue for me....now that I'm burning I think I'm OK with my 5036, we got a tad warmer this past week and it sat dormant for so long waiting to bring down the temp I was half worried it wouldn't light again next burn cycle, can't imagine how long it would sit dormant with a bigger unit and how much charcoal would be in there..in my short time running this it seems to run better when it can fire once in a while, smells better too

 

[mtfallsmikey]

Here's where I get it from..please prove me wrong!
The overall efficiency of most OWB's (not including gasifiers) is around 30-50%...if a boiler is "rated" by it's manufacturer at 250,000 BTU, then 100-125,000 BTU would be realistic. Are they rated by ASME, or any other standard?...no! The larger sizes have more water capacity, higher amount of drawdown before the water cools off. If our 5036's hold 190 gal. of water @ 180 deg., how much can be drawn down before the temp. will drop? Maybe half, or roughly 100 gal.? What does that do for your HX's when the water cools down?.. ...It de-rates them. An HX rated at 50,000 BTU @ 180 deg.@ 4 GPM may only do 35,000 BTU @ 160 deg. 4 GPM flow for, let's say...a 10 min. run cycle=40 gal. Multiply that times other HX's in the system...that will take the water temp.in the boiler down real fast. Compare that to an oil/gas boiler, which is using fuel that has a finite BTU content per gal./therm, at 80% or better efficiency..........oh what the heck...sorry for the over-engineered explaination, but that is the reality..not the "how many square feet 'ya heatin'" routine from many OWB dealers.
:deadhorse::deadhorse:

 

[Scootermsp]

Mt falls

I respectfully disagree, I heat a 4800 sq/ft home and DHW. My 5036 easily keeps up and gets 12 burn times below zero. The reason we are all different is that the OWB may be the same BUT the number of other variables is infinite: size, insulation, windows, doors, ceiling height, wood quality, etc. all come into play in a major way. I have R-30 in my attic and plan to add unfaced R-25 over that.....increase efficiency of heat AND A/C. When I purchased my CB, these were all questions my dealer went through with me to achieve proper sizing.opcorn:

 

[mtfallsmikey]

That's good, scooter, and from my 30 = yrs. in the HVAC biz you are a lucky and educated man...I've been taking care of a few "problem" children-type installs due to poor planning, sizing,installation. I do a complete heat loss calculation, size HX's, BB, radiant,piping,pumps,head loss with time-tested, established methods used by any reputable contractor, which factor in construction types, window/door construction, and insulation. My total heat loss in my house was 46,000 BTU, installed a Heil cased coil rated at 56,000 BTU @ 180 deg. I can heat the house with this coil down to 130 deg. water. I stand by what I stated.

 

[mtfallsmikey]

That's good, scooter, and from my 30 = yrs. in the HVAC biz you are a lucky and educated man...I've been taking care of a few "problem" children-type installs due to poor planning, sizing,installation. I do a complete heat loss calculation, size HX's, BB, radiant,piping,pumps,head loss with time-tested, established methods used by any reputable contractor, which factor in construction types, window/door construction, and insulation. My total heat loss in my house was 46,000 BTU, installed a Heil cased coil rated at 56,000 BTU @ 180 deg. I can heat the house with this coil down to 130 deg. water. I stand by what I stated.

 

[Slick]

So your assuming the 250,000 btu rating on the 5036 is the input? So you still have to take the 30-50% effieciency out of it? I assume that's the output...and I would say mine, scooters and plenty of others installs prove it...now way in heck I would be heating even my shop let alone my shop and house if this 5036 was outputing 125K btu.

Those numbers on draw down and heat exchanger effiencies are all fine and dandy but none of it proves why more water storage will let you burn less wood? You said you should have gotten the bigger model to have more storage and burn less wood...I don't get it? The water just stores the BTU's...your house needs a certain amout of heat to stay warm a certain amount of time, say 400K for a night (total guess, no need to calculate it)....you need to burn a certain amount of wood to get that heat...draw down time, hx effiency...none of that matters, if the water temp is low the furnace just runs longer to get those BTUs (mine will heat the house at 120) in the end when figuring how much wood you'll burn for a night or year, you need to burn a certain number of pounds of wood to get that 400K of heat for the night or whatever it comes out to for the year....the size of the firebox will dictate how often you need to fill it which can be a core if it's to small but how on earth does more storage of BTUs mean you need less fuel when looked at for a year?

Here's where I get it from..please prove me wrong!
The overall efficiency of most OWB's (not including gasifiers) is around 30-50%...if a boiler is "rated" by it's manufacturer at 250,000 BTU, then 100-125,000 BTU would be realistic. Are they rated by ASME, or any other standard?...no! The larger sizes have more water capacity, higher amount of drawdown before the water cools off. If our 5036's hold 190 gal. of water @ 180 deg., how much can be drawn down before the temp. will drop? Maybe half, or roughly 100 gal.? What does that do for your HX's when the water cools down?.. ...It de-rates them. An HX rated at 50,000 BTU @ 180 deg.@ 4 GPM may only do 35,000 BTU @ 160 deg. 4 GPM flow for, let's say...a 10 min. run cycle=40 gal. Multiply that times other HX's in the system...that will take the water temp.in the boiler down real fast. Compare that to an oil/gas boiler, which is using fuel that has a finite BTU content per gal./therm, at 80% or better efficiency..........oh what the heck...sorry for the over-engineered explaination, but that is the reality..not the "how many square feet 'ya heatin'" routine from many OWB dealers.
:deadhorse::deadhorse:

 

[Scootermsp]

Cb6048

My dealer and a friend of mine both tell me that I would be using MORE wood had I gone with the 6048. It doesn't make sense to me either but both insisted this is the case (dealer uses one at his farm). I think the bottom line is somewhere in the middle. It may only put out 250K BTUs with optimum conditions but it would not heat my home and water for the 3 women with only 125K BTUs. I would love to see some independant test results on these units.

 

[November Wolf]

My dealer and a friend of mine both tell me that I would be using MORE wood had I gone with the 6048. It doesn't make sense to me either but both insisted this is the case (dealer uses one at his farm). I think the bottom line is somewhere in the middle. It may only put out 250K BTUs with optimum conditions but it would not heat my home and water for the 3 women with only 125K BTUs. I would love to see some independant test results on these units.

This is a question I had asked when I was looking at a 5036 or 6048. I guess I will never know the answer about how much more wood the larger one uses but the biggest advantage is the longer burn times on the 6048.

 

[paladin]

Thanks for the info I guess "it depends" on the wood the temp outside and how efficient the house and shop are.If we buy the house and I add a shop I would have to try it and see if it works good or not.

 

[Laird]

Just as an example, I know someone who uses an OWB to heat his 5,000 sq Ft greenhouse. He has a big OWB, not sure of the make, but its rated @10,000 sq ft. 3-400 gal of water and a big firebox. In Feb when he is starting plants he has to load the thing every 3 hours 24/7. but NO insulation and alot of cubic feet of air space in the greenhouse. Saves himself 6k in propane every year but he never gets more than 2.5 hours of sleep at one time.