Question about BTU rating

[PNWguy]

I'm looking at EKO boilers (25kw/85kBTU) and (40/137) and wondering how to choose a size.

My building is odd - small house (900 sq ft) with a large attached shop (2,500 sq ft). Slab on grade, and very well insulated because it's new construction. I'm in the PWN with cold winters (but not cold like you midwest and NE guys).

I'm trying to size the boiler and storage tank to reasonable size. The smaller unit is much less expensive, which would be great.

Is the BTU rating BTU/hour? If so, what duration would you guess I'd get from a full load of dry softwood? How many total BTU/load is what I'd like to determine. And of course that should help plan the storage tank size.

Thanks

 

[Woodman1]

A load of hardwood in my Eko60 seems to go like this. First hour getting the fire going, coal bed is building works out to less than half of rating. The next 2 hours are ripping at or a little above rating. The last couple of hours things are slowing down and back to half of rating. Basically think of a bell curve over 4 or 5 hours

 

[PNWguy]

Woodman1 - thanks for the details. So over 4 to 5 hours you might get a total (from your 200kBTU unit) of 600K - 700k BTU?

That's awesome. I'm hoping to get an analysis (guess) of what my heating requirements will be, based on the blueprints. Once I have that, I'm hoping to be able to size the boiler & storage to a rational size.

 

[jebatty]

You can get an approximate BTU output capacity of a gasification boiler by weighing the wood you load and making some basic assumptions: moisture content of the wood and flue temperature. Assume MC at 20% and average flue temperature at 400F. Then the available BTUs from the wood are 6050/lb. 100 pounds of wood deliver 6050 x 100 = 605,000 BTUs to your heating system over the course of a burn.

You also can measure the BTUs delivered to your home/shop by data logging the delta T between the cold water return and the hot water output, and also by knowing the gpm flowing through the boiler. BTU output = delta T x gpm x 500. Example: cold water return = 140, hot water out = 160, gpm = 10. Btus then are 20 x 10 x 500 = 100,000.

As Woodman1 suggests, BTU output varies much like he describes in his post.

 

[maple1]

I would size storage based on how long you want to go between making fires.

I would size the boiler by how often or how many times I want to load it during a burn.

So it comes down mainly to your heat loss numbers (have you done any calcs?), and your schedule & preferences.

I have 660 gallons, and a 40kw rated boiler. Average burning time over the winter is around 6 hours/day. I really have to stretch it to get my storage to last between burns, on colder days. Tricks used are also using the house as storage (heating a bit warmer when burning, using setbacks during the day) and timing burns so I am burning when heat demand is greatest. I could get by with a smaller boiler, and just load it an extra time or two a bit more frequently, but I would not want any less storage - wouldn't mind more, actually.

You should be able to ballpark anticipated burning practices with heat loss calcs, and firebox size specs, generalized lb/cu.ft. & BTU/lb numbers. Although, I think my usable firebox size actually measures a bit less than it is spec'd at.

 

[WaltBranch]

85k BTU is the max. output of that boiler, a figure you may never achieve unless you have perfect wood, offhand I would recommend you move up to the Eko 40 to heat 3,400 sq. ft. If your state requires EPA certification choose a different brand.

 

[maple1]

85k BTU is the max. output of that boiler, a figure you may never achieve unless you have perfect wood, offhand I would recommend you move up to the Eko 40 to heat 3,400 sq. ft. If your state requires EPA certification choose a different brand.

That would come down to how much time you have or want to spend on tending the boiler. But yes I would likely also recommend a 40.

Our house is 2700 sq.ft. (plus another 1500 unfinished basement). Used to have a very very inefficient boiler, that I could never find any kind of BTU ratings for, but after a few years with my new one & comparing how it does under the same conditions would estimate was putting no more than 40k BTU/hr max into my heating system all those 17 years I had it. That was max, which was not often.

I wouldn't wish those 17 years I was a slave to that thing on anybody.

 

[PNWguy]

Lots of great replies - thanks everybody.

Once I get the heat loss calcs back, I'll make a decision. I'll definitely go for lots of storage.

 

[Bad LP]

I'm in the bigger is better camp. I spend a lot of time screwing around with the boiler getting my tanks up to temp. Much more time than I thought.

 

[jebatty]

Once I get the heat loss calcs back, I'll make a decision. I'll definitely go for lots of storage.

It's easy to calculate the amount of btus in storage. When you have done that, divide by the btuh heat loss calculation, and you will have an estimate of how long storage alone, without burning, will provide needed heat at the heat loss parameters.

 

[Highbeam]

It's easy to calculate the amount of btus in storage. When you have done that, divide by the btuh heat loss calculation, and you will have an estimate of how long storage alone, without burning, will provide needed heat at the heat loss parameters.

And make sure that length of time is 24 hours if you want to burn once per day right?

 

[maple1]

And make sure that length of time is 24 hours if you want to burn once per day right?

Not quite. It would be more like, 24 hours, less your burn time. Storage gets you from end of burn to the start of the next one. About 18 hours here, give or take.

 

[Highbeam]

Not quite. It would be more like, 24 hours, less your burn time. Storage gets you from end of burn to the start of the next one. About 18 hours here, give or take.

The theory being that while you’re burning you’re not drawing from storage? Makes sense but then doesn’t that subtract from the energy available to charge storage during the burn time?

 

[maple1]

The theory being that while you’re burning you’re not drawing from storage? Makes sense but then doesn’t that subtract from the energy available to charge storage during the burn time?

Yes on both counts. But these boilers can pump out a lot of heat in a short time - so when burning, heat is usually going both to the house & storage. When I start my burn, not only is my storage depleted but usually also parts of my house are starting recovery from a degree or two of setback. Plus by the time I finish my burn, some parts of the house are finishing a degree or two of 'set ahead' - heated warmer than otherwise typical room temp. So the house also acts a bit like storage too, and the boiler has max load on it at minimum return temps for as long as possible. Some tricks to extending storage mileage & boiler efficiency.