Storage vs Boiler Sizing.

[grainfedprairieboy]

One thing the boys at Hearth.com have convinced me of is a wood gasification boiler and that in spite of some manufacturer claims, thermal storage is necessary. But I am having some difficulty determining my unit size and storage capacity.

I have done a heat loss calculation for the house at 250,000 BTU and if I choose to heat any ancillary outbuildings I would need another 200,000. The outbuildings are not necessary but would be nice. The house is being heated by a combination of infloor and radiators in northern Alberta where the temperature can swing 30 degrees Celsius in 12 hours. I am also putting in a natural gas back up boiler for when I am away.

My question: Does anyone know how small of a boiler and how large a tank I need just to supply the needs of the house? It seems that everyone wants to sell an oversized unit (300-350K BTU) and ignore the potential in the thermal mass of the water or the ability of the gas boiler to supplement the heat for the odd exceptionally snaps.

 

[Nofossil]

250,000 BTU Heat load? That's amazing, unless you're heating a school or something. My 3500 square foot house in Vermont is about 30,000 BTU/hr at -20f.

Do you have any figures for your current annual fuel consumption?

 

[SolarAndWood]

It seems that a smaller boiler with more storage makes a lot of sense as long as you don't mind feeding the fire and/or adjusting the temperature of certain zones during cold periods. Nofossil, does your EKO 25/880 gals of storage ever not keep up with demand and force you to run the oil boiler simultaneously? I would prefer to spend the additional upfront cost of a bigger boiler, piping, heat exchanger(s), etc on more storage, electric backup boiler and solar water for the off (no freeze threat) season.

 

[stee6043]

My worst case heat load (-15 degrees outside, 70 degrees inside) is roughly 50-60,000BTU/Hr for 3200 square feet. My home is 2003 built with lots of windows. I measured this during the past winter via my tank temps. I was shocked by my consumption...

I'm running an EKO 40 with 1000 gallons of storage. I'm extremely happy with the setup but if I had to do it again I'd strongly consider the EKO 60. A larger boiler will charge your storage more quickly. This makes for less fire tending time. I can currently charge my storage after a day of heating with about 8 hours of fire. For me, the extra cost of the EKO 60 would be justified if I could charge my storage in 5-6 hours, no question...

 

[Nofossil]

Nofossil, does your EKO 25/880 gals of storage ever not keep up with demand and force you to run the oil boiler simultaneously? I would prefer to spend the additional upfront cost of a bigger boiler, piping, heat exchanger(s), etc on more storage, electric backup boiler and solar water for the off (no freeze threat) season.

I've never had to run the oil unless I'm away for a few days.

My philosophy is that you want the SMALLEST boiler that still meets your peak heat load. Here's my reasoning:

1) The goal of all of this is to make life simple and convenient.

2) Once your fire goes out, you have a certain amount of time (based on storage) before you need to build another.

3) You want to run at full output, charging storage with whatever excess heat is produced beyond what the house needs.

4) The smaller the boiler, the longer it will take to heat your house and storage.

5) That means it will be longer until you have to build another fire.

For example, assume that I can store 300,000 BTU of usable heat and my heat load is 15,000 BTU/hr. With my little EKO 80 putting out 55,000 BTU/hr, it will take me about 7.5 hours to charge the tank (I'll have about 40,000 BTU/hr left over after meeting the house's heat load). Storage is good for 20 hours, so If I start the fire at 5:00 in the afternoon then it will be done a little after midnight. I'll need to build another fire by 8:30 the next evening - nice flexibility. This is very close to an average winter day for me, by the way.

Same scenario with a boiler that puts out 215,000 BTU/hr: My tank is charged in 1.5 hours. If I start the fire at 5:00 in the afternoon, I'll be done at 6:30. I'll need to start the next one 20 hours later, by 2:30 in the afternoon. NOT convenient.

This only makes sense if you're around to tend the beast, but I think it's realistic.

 

[grainfedprairieboy]

250,000 BTU Heat load? That's amazing, unless you're heating a school or something. My 3500 square foot house in Vermont is about 30,000 BTU/hr at -20f.

Do you have any figures for your current annual fuel consumption?

Total area heated is about 6000 sq ft. 1/2 of house built in 1931. Northern Alberta also gets very cold.

 

[Robby]

I am also from Alberta (southern), 4400 sq.ft. 10 year old house. Harman SF360 on coal or wood no problem running 8+ hours in the coldest weather. 250,000 per hour @ 70% effiecency would require 44 lbs of coal or 60 lbs of Alberta wood per hour. Seems maybe there could be an error in your figures.

Robby

 

[BrownianHeatingTech]

For example, assume that I can store 300,000 BTU of usable heat and my heat load is 15,000 BTU/hr. With my little EKO 80 putting out 55,000 BTU/hr, it will take me about 7.5 hours to charge the tank (I'll have about 40,000 BTU/hr left over after meeting the house's heat load). Storage is good for 20 hours, so If I start the fire at 5:00 in the afternoon then it will be done a little after midnight. I'll need to build another fire by 8:30 the next evening - nice flexibility. This is very close to an average winter day for me, by the way.

Same scenario with a boiler that puts out 215,000 BTU/hr: My tank is charged in 1.5 hours. If I start the fire at 5:00 in the afternoon, I'll be done at 6:30. I'll need to start the next one 20 hours later, by 2:30 in the afternoon. NOT convenient.

You can, of course, only load it halfway, and start it sooner

I am also from Alberta (southern), 4400 sq.ft. 10 year old house. Harman SF360 on coal or wood no problem running 8+ hours in the coldest weather. 250,000 per hour @ 70% effiecency would require 44 lbs of coal or 60 lbs of Alberta wood per hour. Seems maybe there could be an error in your figures.

250kbtuh for a 6000sf house is 41.67 btuh/sf.

That's on the high side, and I would seriously look into ways to improve efficiency of the building envelope, before investing in a heating system (or, at least, I would plan the boiler based upon the future heat load, and just put up with more-frequent feeding until then), but it's not so high a number as to set off any flags.

Joe

 

[Nofossil]

You can, of course, only load it halfway, and start it sooner

If you run it flat out, that only means that you'll run out of wood sooner. You still only need 1.5 hours of burn time. I suspect a lot of the gasifier problems come from people trying to get them to run at half (or less) of their rated output.

I think large boilers make sense if you have LOTS of storage. With 2000 gallons, I could see having a larger boiler.

Per the original poster - do you have any fule consumption history?

 

[grainfedprairieboy]

250,000 BTU Heat load? That's amazing, unless you're heating a school or something. My 3500 square foot house in Vermont is about 30,000 BTU/hr at -20f.

Do you have any figures for your current annual fuel consumption?

House has never had heat since I bought it but the gas FA furnace that was in the original part of the house was 180K BTU. Of course, I've stripped the sawdust from the walls and filled it with insulation and sealed the building envelope with another inch of high density foam.

When I run the heat loss program it calculates 229,885 which for this climate is not unreasonable at all. My problem is the boiler suppliers want me to buy into something in the 300K range to be on the safe side but other then Garn, not much about storage. What I am trying to figure out is the smallest practicle unit I can use against the most efficient amount of thermal mass.

 

[sdrobertson]

I've always wondered about putting in two smaller boilers instead of one large one. The initial cost would be alittle higher, but during the shoulder seasons you could get by with burning one and during the really cold times, fire up both to satisfy your heat load. By rotating between the two boilers, you should extend the boiler life and you would always have a back up if something happens to one in the middle of the winter. Setting the boilers right next to each other wouldn't take up much more footprint and the piping could be tied together so this wouldn't cost much more. The only problem could be the chimney, I don't know if two identical boilers could share the same flue. I may be wrong, but with my EKO 60, sometimes its hard to keep both nozzles firing all the time and with a smaller boiler and only one nozzle, it seems like it would be allot more efficient. I know I'm not getting the numbers that Nofossil gets. Just a thought, and I haven't seen it discussed here before.

 

[Piker]

250,000 BTU Heat load? That's amazing, unless you're heating a school or something. My 3500 square foot house in Vermont is about 30,000 BTU/hr at -20f.

Do you have any figures for your current annual fuel consumption?

House has never had heat since I bought it but the gas FA furnace that was in the original part of the house was 180K BTU. Of course, I've stripped the sawdust from the walls and filled it with insulation and sealed the building envelope with another inch of high density foam.

When I run the heat loss program it calculates 229,885 which for this climate is not unreasonable at all. My problem is the boiler suppliers want me to buy into something in the 300K range to be on the safe side but other then Garn, not much about storage. What I am trying to figure out is the smallest practicle unit I can use against the most efficient amount of thermal mass.

If you're load is actually 230mBtu's, and you can use a delta T of say 70*, a 500 gallon tank will provide about an hour and 15 minutes worth of heat. The next question is how many hours do you want to be able to go on storage alone at design temps? 5 hours would require about 2000 gallons of storage. Next question, how long between firings at design temps? 12 hour cycles would require a boiler with about 396,000 Btu/hr output. This would theoretically give you a 7 hour burn, providing 1,610,000 Btu's to supply the heat load of the house, and 1,162,000 Btu's for storage. 5 hours after the fire goes out, your tanks are depleted and you fire up again. 12 hour cycle using nothing but firewood.

If you just want to supplement, you could just install a boiler of about 200,000 Btu's, and call it day. You will be feeding it about every 4 hours, and probably using some fossil fuel over night.

If I were you, I would just move south. BRRRRRRRRRR

cheers

 

[Jim K in PA]

Since the house has a combination of radiant and baseboard convectors (I am guessing that the 1931 half is baseboard, and the new(er) construction is radiant), a 70 deg. F delta T may not be practical, unless we are talking pressurized storage of subtantial volume, and peak storage temps of 220-230 deg F. 145-150 deg. F is about as low as I can practially go down here in balmy northeast PA. I hope you have LOTS of extra convector capacity.

This sounds like a perfect application for a GARN WHS2000 (you knew I would say that . . . ;-) ), + maybe another 500-1000 gallons of storage, if your goal is to go for 8+ hours between burns.

That is quite a heat load during seasonal extremes. How often do you hit those extremes, and for how long?

 

[DaveBP]

I’ve always wondered about putting in two smaller boilers instead of one large one. The initial cost would be alittle higher, but during the shoulder seasons you could get by with burning one and during the really cold times, fire up both to satisfy your heat load. By rotating between the two boilers, you should extend the boiler life and you would always have a back up if something happens to one in the middle of the winter.

I suggested this option once to a poster who had a big uninsulated building (old hotel or something) with a huge load.

Tekmar makes a control that can handle fossil fueled multi-staged boiler setups. Keeps track of hours on each boiler and swaps which is primary and which is secondary to keep wear and tear about equal. Probably others do too. How that would work with a solid fueled boiler that doesn't kick in unless it's loaded with wood, I don't know. But short cycling a monster wood boiler in spring or fall has got to be hard on the equipment, too.

Calculating long term efficiency vs. up front cost seems tricky. Sounds like a job for Spread Sheet Man.

 

[Piker]

Since the house has a combination of radiant and baseboard convectors (I am guessing that the 1931 half is baseboard, and the new(er) construction is radiant), a 70 deg. F delta T may not be practical, unless we are talking pressurized storage of subtantial volume, and peak storage temps of 220-230 deg F. 145-150 deg. F is about as low as I can practially go down here in balmy northeast PA. I hope you have LOTS of extra convector capacity.

This sounds like a perfect application for a GARN WHS2000 (you knew I would say that . . . ;-) ), + maybe another 500-1000 gallons of storage, if your goal is to go for 8+ hours between burns.

That is quite a heat load during seasonal extremes. How often do you hit those extremes, and for how long?

Agreed, concerning the 70* delta T not being practical in baseboard applications. Those were just some numbers to throw out there to see what 230,000 Btu/hr storage might look like under ideal circumstances.

I DID know you were going to say that by the way!! After I wrote that post, I thought to myself, doggonit... I just described what Jim K will point out to be a perfect sized Garn system!! %-P

cheers

 

[SolarAndWood]

Nofossil, does your EKO 25/880 gals of storage ever not keep up with demand and force you to run the oil boiler simultaneously? I would prefer to spend the additional upfront cost of a bigger boiler, piping, heat exchanger(s), etc on more storage, electric backup boiler and solar water for the off (no freeze threat) season.

I've never had to run the oil unless I'm away for a few days.

My philosophy is that you want the SMALLEST boiler that still meets your peak heat load. Here's my reasoning:

1) The goal of all of this is to make life simple and convenient.

2) Once your fire goes out, you have a certain amount of time (based on storage) before you need to build another.

3) You want to run at full output, charging storage with whatever excess heat is produced beyond what the house needs.

4) The smaller the boiler, the longer it will take to heat your house and storage.

5) That means it will be longer until you have to build another fire.

For example, assume that I can store 300,000 BTU of usable heat and my heat load is 15,000 BTU/hr. With my little EKO 80 putting out 55,000 BTU/hr, it will take me about 7.5 hours to charge the tank (I'll have about 40,000 BTU/hr left over after meeting the house's heat load). Storage is good for 20 hours, so If I start the fire at 5:00 in the afternoon then it will be done a little after midnight. I'll need to build another fire by 8:30 the next evening - nice flexibility. This is very close to an average winter day for me, by the way.

Same scenario with a boiler that puts out 215,000 BTU/hr: My tank is charged in 1.5 hours. If I start the fire at 5:00 in the afternoon, I'll be done at 6:30. I'll need to start the next one 20 hours later, by 2:30 in the afternoon. NOT convenient.

This only makes sense if you're around to tend the beast, but I think it's realistic.

Thanks for taking the time to articulate this so well. The smaller boiler/larger storage strategy has always made more sense to me. It seems the biggest downside is throwing a log or two in the stove and being subjected to the flicker of a fire in the great room once in a while.

 

[Piker]

The smallest boiler strategy works best for efficiency. In essence, you are keeping as much heat stored in the wood as possible, instead of the tanks. There is a balance, however. If your boiler is so small that it requires more than 7 or 8 hours to charge the tanks and heat the house on the coldest day, most people will run into an issue getting a 3rd load of wood into the firbox at night before bed. For folks who work from home or are retired, this is probably not an issue... for people like me, it's not an option. For me, a 12 hour cycle was the goal at design temps (-15,72). A bigger boiler with the output to charge the tanks quickly will require more thermal storage to get that cycle than with a smaller boiler, the difference being that I will only have to fire the boiler for between 4 and 6 hours. I am lucky if I am home 4 hours before bed sometimes. Granted, most of the heating season will give me much better numbers than 12 hour cycles. On sunny days with mild temps, I suspect one fire per day will be fairly easy to attain.

cheers

 

[SolarAndWood]

We have been burning 24/7 6 months a year for 4 years and generally get away with filling the stove when convenient: first one up, first one to leave for the office, second to leave for the office, first home from the office, etc. When it is cold and we are home, more gets thrown in more frequently. We rarely start fires. Does that kind of a fill strategy work with the boilers or does it kill the efficiency of the burn? It seems like the only difference with the small boiler/large storage strategy and where we are today is we would put heat into the tanks where we now dial the stove back.

 

[Nofossil]

We have been burning 24/7 6 months a year for 4 years and generally get away with filling the stove when convenient: first one up, first one to leave for the office, second to leave for the office, first home from the office, etc. When it is cold and we are home, more gets thrown in more frequently. We rarely start fires. Does that kind of a fill strategy work with the boilers or does it kill the efficiency of the burn? It seems like the only difference with the small boiler/large storage strategy and where we are today is we would put heat into the tanks where we now dial the stove back.

For all wood boilers, the most efficient operation is at or near full output. Idling is inefficient, so storage allows you to eliminate idling at the cost of building fires more often.

 

[SolarAndWood]

If you are running at full output and are say half way through a load of wood, does it hurt to open the door and top it off before you leave for the office in the morning? My thought is that if we go with 1500 gallons of storage, we will be running a small boiler approaching 24/7 during the colder periods in the winter. I designed the overhangs/windows of the house so that we get a lot of solar gain in those months which will help with the heat load during the day and allow more BTUs into the tank. The Encore has been our sufficient sole source of heat even with a lot of the house open during the renovation work. It would seem that one of the small boilers should be able to replace it and large storage will give us more flexibility and even heat. The previous owner was going through just under 4000 gallons of propane a year with the old forced air furnace.

 

[DaveBP]

The previous owner was going through just under 4000 gallons of propane a year with the old forced air furnace.

I just got propane delivered this morning. Price is down $.50 from last time... to only $4.30/gallon.

$17000/ year could pay for a pretty fine 'solar and wood' system in very little time.

 

[SolarAndWood]

LOL, that is an understatement. It has bought 50 new windows, structural walls, trusses and roof so far. As my wife likes to say, we bought it for the view, the view with your back to the house.

 

[Nofossil]

If you are running at full output and are say half way through a load of wood, does it hurt to open the door and top it off before you leave for the office in the morning?

In my experience, opening it when it's half full is more likely to get you a face full of smoke. Otherwise, no problem.

With all the money you've put into tightening it up, I assume that your annual heat load is way below the 290 million BTU suggested by your previous propane consumption. Based on the 290 million BTU number, I'd estimate a peak heat load somewhere between 100,000 and 120,000 BTU/hr, with a wintertime average about half that. Scale down based on your new windows etc.

You'll have to figure out the usable storage temperature range. If you assume pressurized storage with a range of 180 to 140, 1500 gallons would give you about 500,000 BTU of usable storage. At a wintertime average load of 50,000 BTU/hr, that's only 10 hours. Adjust the numbers as appropriate. Radiant heat would give you a wider usable temp range, and you may have significantly reduced your hourly heat load by now.

 

[SolarAndWood]

Given the difference the windows, 2x6 walls and 3/4" sheathing has made, that previous usage is largely meaningless. I don't have the interior done yet and I still have no problem heating it with the Encore. I am putting 2" of foam over the sheathing and then 3' of fiberglass in the attic this year as I finish the shell. There will be radiant floors throughout. I am even more confident given your experience that the small boiler is adequate despite what I have been told. Some would argue the cost difference between the small boiler and the 200K boilers is relatively small in the long run. While I agree with this in general, I would rather spend it on PV, solar dhw for the off season and an electric backup boiler.

 

[Nofossil]

Plus, with a smaller boiler you'll have to build fires less often....

Keep in mind, however, that the average output of a boiler over the course of a fire is only going to be 75% of the peak output, which is how they're usually rated. My EKO 25 is rated at 80,000 BTU/hr and actually puts out more like 60,000 BTU/hr average.