Calculating number of burns needed

[Bluelagoon888]

Hi

I am curious how to calculate the number of times I would have to load and fire a wood boiler to supply primary heat and DHW to the house.

My house is 3500 square feet, heating by in-floor radiant heat connected to the DHW system which is currently heated via propane. I have calculated that I roughly use about 42,000 BTU/hour for DHW and house heat.

I think I can calculate how many hours the stove would need to burn to meet my heating needs. For example:

EKO 25
Rated at 85,000 BTU/hour
Rated at 91% efficiency = 77,350 BTU/hour
De-rate that efficiency by additional 25% to create a more real-world number
This comes to a real world figure of 56,100 BTU / hour.
I use about 1,000,000 BTU / day, so dividing 1M BTU / 56,100 BTU means I would need to run the stove for 18 hours to meet all of my needs.

Does that look right?

So 18 hours is an awful lot of burning, but I probably care less how long the stove will burn for and care more about how many times I have to go outside and fill it. This is where I am stuck. How do you figure this out?

By the way...is a 25% de-rate a reasonable number to use to approximate real world performance?

Thanks!

 

[Clarkbug]

I think you are starting off with a double hit to efficiency. If the output is rated at 85,000, thats where you start. The efficiency side will be on the other end, and your input will be 9% higher. But if you derate another 25% after that, I think thats fairly reasonable.

Im assuming you are looking into installing storage? (since you are already calculating an output bigger than your load...). If you are, this calc doesnt really tell you what you need, since the boiler will dump to storage as much as it can, and then your house will be fed from there. You could upsize the boiler to be able to output to storage faster, and only have to load twice a day.

We need more info to help steer you where you want to go.

 

[stee6043]

I'm not 100% sure on the EKO 25 but I believe you're going to be looking at 3-4 hours of burn time on a full load at peak burn. But since you look to be expecting a bit of idling with your rig perhaps you could plan for 4-5 hours per load. So my wild, ballpark guess would be that you're loading between 4 and 6 times per day on a 42k btu/hr kind of day. And these loads would be quality, fully seasoned hardwood.

That's a lot of trips to the boiler no matter how you look at it. If you throw in a start cycle every day it's possible you'll be a slave to this thing. If you're plan is to install the boiler only, no storage, and if efficiency isn't high on your list of priorities you may want to consider upsizing the boiler (significantly). You will idle more with a larger boiler, which is a waste of wood, but you can also jam a lot more in there each time. You may also at that point want to consider letting her burn 24/7 to avoid the restarts. Not the most desirable setup but neither is feeding a smallish boiler 6 times a day.

 

[maple1]

Is that 3500 sq.ft. including a basement? And how old is the construction?

I have 2700 sq.ft. in two stories, plus the unfinished basement (another 1500), 'standard' 20 year old construction, exposed hilltop. 'Typical' winter days I burn no more than 6 hours, with my setup. Two trips to the boiler. On the very coldest of our winter days, it can get to about 12 hours a day. That's absolute max, and despite how cold a winter it's been this year, it didn't happen any more than a handful of times. You do mention boiler outside though - so not sure exactly what kind of setup you're talking. I have everything in my basement though - not doing that will no doubt give an efficiency hit of some sort.

 

[Bluelagoon888]

Thanks for the comments. I plan to post a larger post with my setup and what I'm thinking for a system. I just couldn't figure out how long the stove will burn for each time I load it, and couldn't find anything online to help me explain how to calculate it.

 

[Bluelagoon888]

I'm not 100% sure on the EKO 25 but I believe you're going to be looking at 3-4 hours of burn time on a full load at peak burn. But since you look to be expecting a bit of idling with your rig perhaps you could plan for 4-5 hours per load. So my wild, ballpark guess would be that you're loading between 4 and 6 times per day on a 42k btu/hr kind of day. And these loads would be quality, fully seasoned hardwood.

That's a lot of trips to the boiler no matter how you look at it. If you throw in a start cycle every day it's possible you'll be a slave to this thing. If you're plan is to install the boiler only, no storage, and if efficiency isn't high on your list of priorities you may want to consider upsizing the boiler (significantly). You will idle more with a larger boiler, which is a waste of wood, but you can also jam a lot more in there each time. You may also at that point want to consider letting her burn 24/7 to avoid the restarts. Not the most desirable setup but neither is feeding a smallish boiler 6 times a day.

Thanks stee. That is super helpful info.

I definitely want to avoid being a slave to this thing. I'm trying to figure out a design that has me loading only twice per day in winter....

I'm actually looking at the EKO 40 as one possibility. Since you also have that stove, can I ask how long it burns with a load of wood if it is dumping to storage and can go full tilt?

 

[Bluelagoon888]

I think you are starting off with a double hit to efficiency. If the output is rated at 85,000, thats where you start. The efficiency side will be on the other end, and your input will be 9% higher. But if you derate another 25% after that, I think thats fairly reasonable.

OK thanks. That makes sense.

I had spoken to one person who had a Vedolux 37 for sale (used) and he said that he was only getting 50-60K BTU out of a stove rated for almost 130K. That seemed like a crazy de-rate number to me. He didn't say how seasoned the wood he was using it, but he was basically selling it because he wasn't getting enough output. I was hoping that 50% de-rate factor is not a common experience.

Im assuming you are looking into installing storage? (since you are already calculating an output bigger than your load...). If you are, this calc doesnt really tell you what you need, since the boiler will dump to storage as much as it can, and then your house will be fed from there. You could upsize the boiler to be able to output to storage faster, and only have to load twice a day.

We need more info to help steer you where you want to go.

So that's what I'm trying to figure out. Let's say I will dump to storage, so my boiler is not idling. I can upsize the boiler to dump to storage faster, as you say. What I need to figure out is how big of a stove do I need to upsize to in order to be able to load it only twice per day? Essentially, if I need a million BTU per day and I only want to run it twice per day, then I need a stove that can put out 500K BTU per load of wood. Stove manufacturers publish their BTU/hour rating, but I haven't found anywhere that tells me how many hours the stove will run when full.

I guess do I need to look at the size of the stove's wood box, look at the type of wood I will load in, and somehow use that information to calculate roughly how many BTU of wood will be in there? That sounds cumbersome and likely inaccurate....That's where I am stuck.

 

[Bluelagoon888]

Is that 3500 sq.ft. including a basement? And how old is the construction?

I have 2700 sq.ft. in two stories, plus the unfinished basement (another 1500), 'standard' 20 year old construction, exposed hilltop. 'Typical' winter days I burn no more than 6 hours, with my setup. Two trips to the boiler. On the very coldest of our winter days, it can get to about 12 hours a day. That's absolute max, and despite how cold a winter it's been this year, it didn't happen any more than a handful of times. You do mention boiler outside though - so not sure exactly what kind of setup you're talking. I have everything in my basement though - not doing that will no doubt give an efficiency hit of some sort.

Hi Maple,

Thanks. Yeah....I find the number pretty high as well. I have a pretty new house, built in 2006, and I live in between Vancouver and Victoria, BC so it's not like it's a cold climate. But I do have vaulted ceilings basically everywhere and lots of windows. I came to 42K BTU/hour by doing two separate calculations, both of them rough and sort of inaccurate:

1) I looked at my propane consumption, which is a bit inaccurate because :

• I also use propane to generate electricity in winter time,
• I am not home all the time so the house goes cold when I am away and then it takes lots of BTUs to heat it back up when I get home,
• I also heat the house with the wood stove.

But I kind of built some assumptions into a spreadsheet and came out with roughly 42 K BTU.

2) I did a heat load calculation using an online tool and actually used dimensions of windows, walls, doors, etc.

The heat load calc and my propane consumption gut check came out roughly similar.

This summer I will definitely do some work on air infiltration, try to figure out how I can get that 42K BTU number down. On the other hand, we do plan an expansion to the house that will add another 1/3 floor space for a nanny suite, so I figure even if I can get the number down it will climb back up again later on if we expand the house, so I might as well design based on roughly 45KBTU....

 

[brant2000]

I also use propane to generate electricity in winter time....

Really? What are you using to do this? I apologize if this is a stupid question, but what are the reasons/benefits to generating your own electricity?

 

[maple1]

OK thanks. That makes sense.

I had spoken to one person who had a Vedolux 37 for sale (used) and he said that he was only getting 50-60K BTU out of a stove rated for almost 130K. That seemed like a crazy de-rate number to me. He didn't say how seasoned the wood he was using it, but he was basically selling it because he wasn't getting enough output. I was hoping that 50% de-rate factor is not a common experience.



So that's what I'm trying to figure out. Let's say I will dump to storage, so my boiler is not idling. I can upsize the boiler to dump to storage faster, as you say. What I need to figure out is how big of a stove do I need to upsize to in order to be able to load it only twice per day? Essentially, if I need a million BTU per day and I only want to run it twice per day, then I need a stove that can put out 500K BTU per load of wood. Stove manufacturers publish their BTU/hour rating, but I haven't found anywhere that tells me how many hours the stove will run when full.

I guess do I need to look at the size of the stove's wood box, look at the type of wood I will load in, and somehow use that information to calculate roughly how many BTU of wood will be in there? That sounds cumbersome and likely inaccurate....That's where I am stuck.

I don't like quoting already long post strings - but that fellow selling the Vedo 37 was definitely doing something wrong. Maybe the 37 owners here will comment further - but I would definitely buy that if it was close & fit my needs. Mine is rated similarly (40kw) - and it really pumps out the heat. I had to burn 24/7 with my old oil/wood unit, with the oil cutting in when it was real cold out, and the house was colder - now I'm burning 1/4 to 1/2 of the time, using no oil, and the house is way warmer. And I haven't swept my chimney for a year and a half - it was 3-4 times a year before.

And I think generally no matter the size of the gassifier, a load of wood usually lasts about 4 hours or so. The ratings difference comes from the size of the box or how much wood it burns in that 4 hours.

I really like the boiler & storage in my basement, along with the winters wood - but that's quite a debated opinion. I don't think however that it being the most efficient way to burn is up for much debate - it just isn't possible to put as much heat out of a given volume of wood into your building envelope by burning outside that envelope.

 

[Bluelagoon888]

Really? What are you using to do this? I apologize if this is a stupid question, but what are the reasons/benefits to generating your own electricity?

I live off grid and use solar electricity. In the winter when it is cloudy I run a propane-powered generator to create electricity. (Not a stupid question at all.)

 

[Bluelagoon888]

I don't like quoting already long post strings - but that fellow selling the Vedo 37 was definitely doing something wrong. Maybe the 37 owners here will comment further - but I would definitely buy that if it was close & fit my needs. Mine is rated similarly (40kw) - and it really pumps out the heat. I had to burn 24/7 with my old oil/wood unit, with the oil cutting in when it was real cold out, and the house was colder - now I'm burning 1/4 to 1/2 of the time, using no oil, and the house is way warmer. And I haven't swept my chimney for a year and a half - it was 3-4 times a year before.

Great - thanks. I was hoping that was the case.

And I think generally no matter the size of the gassifier, a load of wood usually lasts about 4 hours or so. The ratings difference comes from the size of the box or how much wood it burns in that 4 hours.

OK. Thanks. That is a helpful benchmark to work with. At 4 hours of burning per load, something like an EKO 40 or a Vedolux 37 comes pretty close to my heat needs if I am dumping to storage....

 

[henfruit]

I can get 5 to 6 hours on a full load of wood ,burning to storage. Then I get an other 5 to 6 hours from the tank depending on the outside temps.

 

[maple1]

I can get 5 to 6 hours on a full load of wood ,burning to storage. Then I get an other 5 to 6 hours from the tank depending on the outside temps.

I can only go by the boiler specs, with respect to what I don't have experience with. I think all I've read are in the 4-5 hour range, including yours. That is at nominal output though, so there may likely be additional burn time at much reduced output

 

[stee6043]

Thanks stee. That is super helpful info.

I definitely want to avoid being a slave to this thing. I'm trying to figure out a design that has me loading only twice per day in winter....

I'm actually looking at the EKO 40 as one possibility. Since you also have that stove, can I ask how long it burns with a load of wood if it is dumping to storage and can go full tilt?

If you're looking at the EKO line (great boilers by the way, no frills, nothing fancy, reliable workhorse) I'd actually consider the EKO 60 or even the 80 in your case. My average heat load is something closer to 20,000 btu/hr with my peak loads a little more than double that (3200 square feet in Michigan). I need a starter load, a partial load and then a full load to satisfy my demands most days (with storage). I've often wished I would have gone for the EKO 60 so I could do a starter load and one full load for the average winter day. There is no downside to oversizing the boiler if you have thermal storage. So for your load, I'd personally say EKO 60 at a minimum for maximum happiness. An EKO 80 might even be better depending on whether the 40k btu/hr is your average or peak calc. If it's peak, nab the EKO 60 and live life knowing you've proved that bigger is better...sometimes.

 

[flyingcow]

Adding on for a nanny suite? Now we're talking GARN barn. Put in s garn and have the nanny feed it as part of her job. Genius I am.

 

[Coal Reaper]

My vedolux 37 i get almost half a million btus in one burn of 20" long dense wood. Oak, locust, black birch. It does really well on the birch and that stuff dries fast for me. Anyway, it will bring my 1000 gallons of tanks from 135avg to almost 190 in a 3.5 hour burn. That guys problem may be wet wood, no loading unit, or overpumping through boiler to storage. I have never hear of anybody that wasnt happy with a varm other than one who had the loading unit pump running at full speed.

 

[Clarkbug]

OK thanks. That makes sense.
I had spoken to one person who had a Vedolux 37 for sale (used) and he said that he was only getting 50-60K BTU out of a stove rated for almost 130K. That seemed like a crazy de-rate number to me. He didn't say how seasoned the wood he was using it, but he was basically selling it because he wasn't getting enough output. I was hoping that 50% de-rate factor is not a common experience.

Yeah, I gotta guess wet wood, or something seriously wrong. You could buy that and re-sell here on the forums I think pretty easily They like nice dry wood. Im burning some stuff that was for this year, but I left uncovered outside, and I can definitely tell the difference compared to my regular stash. Takes more wood, heat output is way down.

I light my boiler twice a day, and make four to five visits. In the morning I light it, fill it full, and check an hour and a half later. I top it up if needed (depends on the weather the night before), and its good until I get home from work that night. I try to top the tanks up pretty close at night, so there is an extra visit that probably isnt needed.

Burns will be much shorter if you are burning pine instead of black locust or oak like CoalReaper.

As much as I like my setup, I would definitely look at a Garn if I had the space. They follow the KISS principle, let you put in big splits, and are nice and efficient.

 

[stee6043]

My vedolux 37 i get almost half a million btus in one burn of 20" long dense wood. Oak, locust, black birch. It does really well on the birch and that stuff dries fast for me. Anyway, it will bring my 1000 gallons of tanks from 135avg to almost 190 in a 3.5 hour burn.

Your energy output seems reasonable assuming you're putting about 100 pounds of wood in your boiler for your half million btu burn. I think most similarly sized gassers will achieve similar results. But I'd be surprised if you were doing that in 3.5 hours! Especially if you're heating a load while charging the tank. You'd have to be pegging the meter on your 37 to push your tanks 55 degrees in 3.5 hours without any other load. Averaging roughly 93% of the rated output of your boiler for the full burn. I'm not saying this is impossible but it's not "probable". Too many variables with even high quality hardwood for most of us to be able to average these kinds of numbers...

 

[Coal Reaper]

almost half million btus equals about 450,000 if i remember correctly. a packed load is 70-75 pounds when i play puzzle. full length splits of 20-21" are key. a load >20%MC may approach 100 pounds, but the heat output is greatly diminished. at 25% i will get smoke billowing out of the stack. those numbers are during a period of no load. and boiler has already been brought up to operating temp and THEN fully loaded. it might be slightly more than 3.5 hours but not much. i have the fan set to shut off at about 140*C. this leaves some coals to start then next burn. i do not get 55* rise through boiler, its more like 25ish. i have 1" insulation on plumbing and ~5" spray foam on my tanks that hold the heat in very well. the varms are also well insulated. i can hold my hand on any surface of my boiler besides the flu and it only feels slightly warm. sorry if i skewed anything a certain way. i will have more accurate numbers once i get the data logging set up.

calrkburg, do you see similar results?

to the OP, when it is less than 10*F outside and cloudy with a wind i get 9hrs after loading out of my boiler/storage before the house temp starts to fall, about a degree per hour depending on the wind/sun. 2x6, 2500sqft, cathedral cieling on all of second floor and a lotta windows.

 

[Fred61]

Lots of discussion here and I don't remember everything that has been posted but I think most will agree that you should go with a much larger boiler with a storage capacity matched to your load and will be recharged in a reasonable time by the boiler. With adequate storage capacity any small error in boiler sizing will be soaked up by the flexibility offered by the storage

 

[Clarkbug]

almost half million btus equals about 450,000 if i remember correctly. a packed load is 70-75 pounds when i play puzzle. full length splits of 20-21" are key. a load >20%MC may approach 100 pounds, but the heat output is greatly diminished. at 25% i will get smoke billowing out of the stack. those numbers are during a period of no load. and boiler has already been brought up to operating temp and THEN fully loaded. it might be slightly more than 3.5 hours but not much. i have the fan set to shut off at about 140*C. this leaves some coals to start then next burn. i do not get 55* rise through boiler, its more like 25ish. i have 1" insulation on plumbing and ~5" spray foam on my tanks that hold the heat in very well. the varms are also well insulated. i can hold my hand on any surface of my boiler besides the flu and it only feels slightly warm. sorry if i skewed anything a certain way. i will have more accurate numbers once i get the data logging set up.
calrkburg, do you see similar results?

It honestly varies a lot with my wood species and moisture. I have had some issues with bridging when I got into less than ideal wood, but when things are humming along I usually can get a 4 hour burn. That would top my storage off if I wasnt pulling from it, but my house is still in the process of getting sealed up, so its a pretty steady draw. Plus my tanks arent nearly as well insulated as yours. Something I hope to remedy in the future...

 

[stee6043]

Agreed. It's all very case by case with these things. About the only thing we have 100% in common is that we like to burn wood.

 

[Clarkbug]

About the only thing we have 100% in common is that we like to burn wood

Amen brother.