Average boiler output

[kabbott]

I am trying to decide on boiler size for my system and I was wondering if anyone has any 'real world' data on average boiler output over a complete burn cycle
compared to 'rated' boiler output.

I have enough storage to oversize the boiler and would like to be able to bring my storage(650 to 700 kbtu's) up to temp in 5-8 hours.
Something around the size of an eko/biomass 40 seems about the size I would need... thoughts?
Or... should I go for a longer burn with a smaller boiler? I start the fire around 5:00 pm and last fill could be 11:00 pm, so 9-10 hours max if a load gives 3-4 hour burn.

Heat loss around 60,000 on design day(which is rare) Storage will give me 18 hours or more on all but a few of the coldest days.

Kris

 

[stee6043]

Do you mean 60,000 per day or per hour? I assume you mean per hour "design load" otherwise you'd be heating with a space heater. ha.

I have a very similar setup to what you're contemplating. I personally wish (some days) I had gone with the EKO 60. I currently recharge 1000 gallons of storage with roughly 5-7 hours of burning most days just as you describe above. But there are days I wish I could do it in 4 hours...or that I could start burning later....or pile more wood into the boiler on the really cold (design load) type nights. There really is no penalty to oversizing with storage.

My current typical winter day is like this - start a fire at 5-5:30PM with 1/2-3/4 load. Fill firebox full between 7-7:30PM. Fill firebox again at 9:30-10PM and I'm done for the day. The boiler will run itself out of wood sometime after I go to bed. I have a timer on the fan to shut it down at roughly the 4-5 hour burn mark. I heat from storage all night and during the day while I'm at work. Rinse, repeat.

On design days (below or near zero F all day and all night for me) I sometimes have to set an alarm to get up at 1AMish to throw in another load of wood. That happens two or three times per year....

 

[Chris Hoskin]

how many gallons of storage do you have kabbot? If you have a thousand gallons like stee, I would go with a 50 or 60kW boiler. Budget would be the only reason to go with a smaller boiler as far as I can see, right?

 

[kabbott]

Do you mean 60,000 per day or per hour? I assume you mean per hour "design load" otherwise you'd be heating with a space heater. ha.
....

Yes 60 kbtu per hour but that is rare, most of the winter is like half of that load...

 

[kabbott]

how many gallons of storage do you have kabbot? If you have a thousand gallons like stee, I would go with a 50 or 60kW boiler. Budget would be the only reason to go with a smaller boiler as far as I can see, right?

Storage is non-pressurized tank, 1700 gallons and change, 185-140 range and if my math is right comes to about 650,000 btu.
Budget would be the first reason to go as small as possible, next would be my heat exchanger may not handle 140k + btu's. Not a huge problem as I can add to it
and I MAY have a line on some propane tanks, If so I just might switch to pressurized tanks anyway.(Have not ran that by the wife yet.. :zip

Plus replacement parts (refractory) would be more for the larger boilers.
Larger system would be a bit less efficient???

What I am looking for is a ball park number on average btu over the course of the burn...50% of rating...70, 80% ??? or for that matter how many btu's out of a full
load of wood (depends on wood I know) with brand/model x boiler.

 

[EffectaBoilerUser (USA)]

Most European boiler manufactures have a detailed perfromance test report showing the actual btu output during the entire burn cycle. This testing is conducted by an independent, third party test lab and thus the results are non-biased and extremely accurate.

I would ask manufactures for this test report when trying to compare actual btu outputs of boilers as this report shows all the detailed information regarding the entire burn cycle. It's easy to publish BTU output values in literature but its another thing to back up these values with the accurate/non-biased data found in these detailed test reports.

A few European boiler manufactures use the Lambda Sensor technology which monitors the ratio of gas/oxygen in the exit flue of the boiler and then varies the primary and secondary draft controls via. DC servo motors to ensure that the maximum boiler btu output is maintained throughout the entire burn cycle along with the least amout of pollutants.

Regarding water storage, I highly recommend a pressurized storage tank whenever possible as it is more efficient, uses less mechanical components, requires less maintenance and can be plumbed directly into a fossil fuel boiler without using heat exchangers.

Effecta Man

 

[Nofossil]

My EKO 25 is rated at 80,000 BTU/hr. I average about 60,000 BTU/hr over the course of a typical 7 hour fire.

 

[mwk1000]

I have this posted in my kitchen, The family uses it day to day to know when a fire will need to be started. It has proven to be very accurate. I have an eko 60 with ~ 1500 Gal open storage. If you add in the home load during the burn time you can get an idea of what a 60 (205000 rated max) will do with that much storage. I figure a 6 hour burn time 1.5 to 2 loads.

 

[kabbott]

Thanks for the info guys.

Nofossil, Is that 7 hour burn two full loads of wood?

mwk100, My heatloss is a bit lower than yours, around 60,000 at 0 deg. I believe.(can,t find my numbers right now)
I THINK an Eko 40 size boiler will do for me BUT I always second guess myself.

Would either of you wish for a bigger boiler to shorten the burn times even if it means more expense in a larger heat exchanger?

 

[mwk1000]

I think your probably right it would. You want 45 degree rise into 1700 gal. I need a 70 degree rise into 1500 gal. I get it with the 60 and firing at 6:00 pm then refilling 1/2 at 9 and checking one last time at 10 or 11.

However , if you let the tank drop lower like I am than I would be skeptical that a 40 would be a practical size to heat that much water that cold.

Bigger, No -- my unit is pretty large for a single residence, it takes over an hour for it to really start working properly. The "burn time" is nicer on paper than it is in reality. Starts slow and tank is cold = many pump cycles as the cold water drives the eko temp down. Once the unit is hot thru the ceramic it will sustain the tank load and start chugging along. Fire starts to die out but the ceramic is still hot the unit still heats the tank for hours on a slow declining BTU exchange until my sensors show that it has nothing left to give. ( helped by the fact that the tank input temps are now in the 130 range at the tank bottom ) I would guess that it quits when the boiler is at 150 ish and so is the bottom of the tank.

I am spending my time making my house consume fewer BTU's at this point.

Lots of water heated very fast in an open tank sounds like a Garn to me.

 

[Nofossil]

Thanks for the info guys.

Nofossil, Is that 7 hour burn two full loads of wood?

Would either of you wish for a bigger boiler to shorten the burn times even if it means more expense in a larger heat exchanger?

My usual pattern is to start the fire with a small load. I add wood after about 30 minutes. The amount that I add will depend on how long I think I need to burn - typically, I'd add enough for 1 to 3 hours. Finally. I'll add wood one more time, typically before I head off to bed. Again, the amount depends on how long I need to burn. A full load goes around 4 to 5 hours running flat out.

The burn time isn't an issue for me. Actually, since my effective storage is smaller than I'd like, there's an advantage to the smaller boiler. On a really cold day, I'll need to run the EKO for around 12 hours. That means that my storage only needs to cover the remaining 12 hours. If I had a boiler that only needed to burn for 6 hours, then the storage would have to cover 18 hours.