25KW or 40KW?

[velvetfoot]

I am leaning towards a 40 KW unit, because, while a 25 KW unit would work, a 40 KW boiler would have increased future flexibility for more heat zones, and also make up for shorter wood.

Are there downsides to oversizing some? I envision 1000 gallons of storage and modulating my wood load to shoot for a complete burn and storage charge.

 

[AK13]

I don't understand what you are asking. What type of "units" are you talking about? Electric boilers as back-up to a wood fired boiler with storage? Does the boiler heat the 1000 gallon tank as well?

 

[velvetfoot]

Sorry, I was talking about which size wood gasification boiler to purchase. No electric boiler.

 

[Clarkbug]

I am leaning towards a 40 KW unit, because, while a 25 KW unit would work, a 40 KW boiler would have increased future flexibility for more heat zones, and also make up for shorter wood.

Are there downsides to oversizing some? I envision 1000 gallons of storage and modulating my wood load to shoot for a complete burn and storage charge.

With 1000 gallons of storage go with the 40, especially if you are on the fence. Will let you heat your zones and storage at the same time, and if you don't want full output, just don't fill as full. You might spend more time babysitting a 25 kw unit to fully charge storage.

Just seems like the larger unit gives you more flexibility.

 

[velvetfoot]

That's what I am thinking.

 

[__dan]

I am leaning towards a 40 KW unit, because, while a 25 KW unit would work, a 40 KW boiler would have increased future flexibility for more heat zones, and also make up for shorter wood.

Are there downsides to oversizing some? I envision 1000 gallons of storage and modulating my wood load to shoot for a complete burn and storage charge.

Unfortunately there is the common perception, by jungle rules, that bigger is always better. In engineered systems, it is more common that bigger causes problems that can be fixed simply by going smaller.

If you don't need the output of a 40 kW unit, the faster burn rate and higher standby losses will add up to much less efficiency. If you need a 40 kW unit in a regular residential application, the heat load may be more easily met by insulating, windows and siding upgrade, uprgading the building envelope. Doing that first could put you back in the range of the smaller boiler at less total cost over time.

You can increase the output of the smaller boiler tremendously just by going from a woodpile to a woodshed with pallets underneath.

With the smaller boiler, you are storing heat in the wood prior to burning, where the standby and idle losses (for the unburned wood) are near zero. Burning slower but matching the load in the smaller boiler, transfer and idling losses are at minimum.

The opposite is true for the bigger boiler that is oversized for the load. If you burn the wood faster but have to store the heat instead of use it immediately, fuel consumption, idle and standby losses, are a lot. Going oversized on the boiler would seem to me to be all downside unless you really need the bigger output to keep up with demand.

 

[velvetfoot]

Storage is involved.

 

[Fred61]

In my opinion the 25 would take too many re-loads to charge a 1000 tank. My 25 is a perfect match for my 500 gallon unpressurized tank. If you want to batch burn, very few homes have a light enough heat load that can be satisfied with 500 gallons of storage. I'm talking about running your storage down to 125 to 130 or so.

 

[Clarkbug]

The opposite is true for the bigger boiler that is oversized for the load. If you burn the wood faster but have to store the heat instead of use it immediately, fuel consumption, idle and standby losses, are a lot. Going oversized on the boiler would seem to me to be all downside unless you really need the bigger output to keep up with demand.

I agree when dealing with a conventional boiler. In this case, I believe the intent is for batch burning into storage, so no idling or cycling losses. There will be standby losses from the tanks, but they are to the envelope of the home. Well insulated storage will have minimal losses.

I think that the storage here allows for flexibility in the varying heat load, while also maintaining efficiency. The larger output of the boiler allows for a faster storage charge and should be a detraction.

 

[AK13]

Unfortunately there is the common perception, by jungle rules, that bigger is always better. In engineered systems, it is more common that bigger causes problems that can be fixed simply by going smaller.

Certainly there is such a thing as "too big". And bigger is not always better. But a little too big is MUCH better than a little too small. I am not an expert in wood gassification boilers. But I have designed several commercial and residential pellet fired systems and it seems to me that as long as your storage is adequate that there would be no harm with having a boiler that is a bit larger than your design heating load. And if the storage tank is located within the envelope of the home then I don't see an issue with standby losses.

I don't know what kind of heat output you get from a "batch burn" in each boiler. But if you know that it would be easy to figure out if it will match up with your 1000 gal of storage.

 

[velvetfoot]

Maybe I'll be joining the "weigh your wood club".

 

[JP11]

I personally went with a boiler which can output almost exactly double what my house needs per hour. With my storage, it's a nice match. I can let the storage run down to next to my low limit (around 140) and very quickly 'catch up' supplying both the house with heat, and some heat to storage.

JP

 

[maple1]

Unfortunately there is the common perception, by jungle rules, that bigger is always better. In engineered systems, it is more common that bigger causes problems that can be fixed simply by going smaller.

If you don't need the output of a 40 kW unit, the faster burn rate and higher standby losses will add up to much less efficiency. If you need a 40 kW unit in a regular residential application, the heat load may be more easily met by insulating, windows and siding upgrade, uprgading the building envelope. Doing that first could put you back in the range of the smaller boiler at less total cost over time.

You can increase the output of the smaller boiler tremendously just by going from a woodpile to a woodshed with pallets underneath.

With the smaller boiler, you are storing heat in the wood prior to burning, where the standby and idle losses (for the unburned wood) are near zero. Burning slower but matching the load in the smaller boiler, transfer and idling losses are at minimum.

The opposite is true for the bigger boiler that is oversized for the load. If you burn the wood faster but have to store the heat instead of use it immediately, fuel consumption, idle and standby losses, are a lot. Going oversized on the boiler would seem to me to be all downside unless you really need the bigger output to keep up with demand.

All pretty much irrelevant when tied to 1000 gallons of storage.

 

[__dan]

All pretty much irrelevant when tied to 1000 gallons of storage.

It's only irrelevant if you're not concerned about burning the extra wood.

It's very relevant on the many mild days of the year above 40 deg F when you only need a little heat. To achieve the 40 kw rating, it has to burn faster at a hotter flue temp, losing heat up the stack. If the 40 had turndown capacity, it could burn slower and the larger steel would have more heat transfer, but that's not what is proposed. With no turndown, it will burn as intended at double the 20's rate regardless of demand. The big efficiency gainer is in keeping the boiler off when it is not needed. The 40 would be burning into excess a lot if the 20 will also do the job. Efficiency is less, especially at part load, and I am not convinced it is more convenient. My boiler is turned down as much as possible and I can only load half way on a cold start or two or three splits on a coal bed with the boiler hot.

My primary concern in putting my system together was burning less wood if wood became difficult to obtain. I can routinely make 80 gallons of DHW from a cold start with a measured 2 gallon bucket of twigs and branches.

If the 20 kw boiler will meet the instantaneous demand as expected year round, it is obvious the 20 kw will do the same work as the 40 kw, only requiring longer burn times. During the times the 40 kw is not needed, which is many, the 20 kw is also not needed, or needed less.

The only advantage to the 40 kw as stated is cold start performance, bringing up a cold house and cold storage, which can be addressed in the plumbing by prioritizing the house load before storage. A lot depends on the user's routine and if they can make two fires a day in cold weather or cannot attend to the boiler, repeatedly bringing up house and storage from cool.

 

[ewdudley]

It's only irrelevant if you're not concerned about burning the extra wood.

It's very relevant on the many mild days of the year above 40 deg F when you only need a little heat. To achieve the 40 kw rating, it has to burn faster at a hotter flue temp, losing heat up the stack. If the 40 had turndown capacity, it could burn slower and the larger steel would have more heat transfer, but that's not what is proposed. With no turndown, it will burn as intended at double the 20's rate regardless of demand. The big efficiency gainer is in keeping the boiler off when it is not needed. The 40 would be burning into excess a lot if the 20 will also do the job. Efficiency is less, especially at part load, and I am not convinced it is more convenient. My boiler is turned down as much as possible and I can only load half way on a cold start or two or three splits on a coal bed with the boiler hot.

My primary concern in putting my system together was burning less wood if wood became difficult to obtain. I can routinely make 80 gallons of DHW from a cold start with a measured 2 gallon bucket of twigs and branches.

If the 20 kw boiler will meet the instantaneous demand as expected year round, it is obvious the 20 kw will do the same work as the 40 kw, only requiring longer burn times. During the times the 40 kw is not needed, which is many, the 20 kw is also not needed, or needed less.

The only advantage to the 40 kw as stated is cold start performance, bringing up a cold house and cold storage, which can be addressed in the plumbing by prioritizing the house load before storage. A lot depends on the user's routine and if they can make two fires a day in cold weather or cannot attend to the boiler, repeatedly bringing up house and storage from cool.

All pretty much irrelevant when tied to 1000 gallons of storage.

Nonetheless, if the heat needed to bring a bigger boiler up to temperature when starting up is lost when the boiler cools off between burn cycles, then there is some loss there.

Also a smaller boiler can take better advantage of storage because it can burn longer after the last filling of the firebox while you're gone because a larger proportion of the output is being used to satisfy demand and storage is topped off more slowly. But this means a small boiler will have to run more hours per day and may require more refueling.

 

[Fred61]

The 40 is engineered to absorb the output of the fire much the same way the 20 is designed. One example being the longer firetubes.

 

[mr.fixit]

I am leaning towards a 40 KW unit, because, while a 25 KW unit would work, a 40 KW boiler would have increased future flexibility for more heat zones, and also make up for shorter wood.

Are there downsides to oversizing some? I envision 1000 gallons of storage and modulating my wood load to shoot for a complete burn and storage charge.

What is going to fit your daily schedule better?
Knowing what my 40 will do,seems like a 25 would take a good part of the day to charge 1000 gal. with any other loads on also.

 

[henfruit]

If the 40 had turndown capacity, it could burn slower and the larger steel would have more heat transfer

The 40 does have turn down capacity. All the Vigas boilers have seven fan speeds that you can set and start with, and then the fan will modulate from there. Foe instance yesterday when I started my fire the fan started at 100 % after 3 hours into the burn the fan had modulated down to 39% and the boiler was putting out 184 degree water. The Vigas 40 will run from 27 kw to 140 kw.

 

[__dan]

I know there are professionals responding to this thread, so show by the numbers what the numbers say. Then you have an arguement.

We don't know the heat loss, the load, but the 20 was said to work. Essentially you're going from a 6 cu ft firebox to a 10 cu ft firebox and saying the bigger firebox will save you load cycles.

If the btus in the 6 cu ft firebox will meet the 24 hr load, and two load cycles/day are convenient for the user compared to one bigger fire, the smaller boiler will burn less at part load, and burning longer time is better because while burning, boiler output goes more directly into the load (using storage less).

The numbers are application and site specific, mostly, how much ride through time is necessary from storage when the boiler is not firing. A boiler with turndown will be sitting there with a hot coal bed, probably meeting or exceeding demand at low fire at the steadystate, when the home has been brought up from cold start. That's when the smaller boiler will be preferred. If the steadystate maintenance btu's, which is most of the operating time, are less than 10 kw, you will need the smaller boiler to turn down that low. Most of the boilers offered except for the Froling do not have that kind of advanced turndown capacity. Does it also modulate the priimary air damper?

It's a job that has to be done by the numbers.

As said, you can increase the smaller boiler's output a lot just by having dryer, better prepared cordwood (a woodshed).

 

[Floydian]

The Vigas 40 will run from 27 kw to 140 kw.

140 Kw-Now were talking! You could charge 1000 gallons in one hour. Sorry, I know you meant KBtu's not Kw.

Velvetfoot,

I agree with the general consensus here-Go with the 40Kw with 1000 gallons. Keep in mind the avg output of a 40Kw boiler might be about 80% of the rated output over the course of a burn.

Personally I really like NOT having a fire going in my house while I am sleeping or I am away. Having a boiler that can recharge storage while taking care of the heat load offers a big convenience for me. A big part of this working for me is that I routinely run my storage down below 100 and usually I fire every other day unless it's below avg winter temps, than It's one shorter fire every day.
I have a 37Kw boiler with 1000 gallons of storage. I am very happy with this setup but I wouldn't want a smaller boiler, for sure.

Your lowest usable supply temps will really determine how much convenience you will get with a given amount of storage. Do you know how low you can go?

Noah

 

[henfruit]

capacity. Does it also modulate the priimary air damper?

. Most of the boilers offered except for the Froling do not have that kind of advanced turndown

Dan what do you mean by turn down capacity? If I am not Mistaken you are using a Froling with out storage? I thought that was a big NO NO with that boiler.

 

[velvetfoot]

Keep in mind the avg output of a 40Kw boiler might be about 80% of the rated output over the course of a burn.

Plus, I plan on sticking with 16" firewood, so that derates (my term) it even more; fuel chamber is 22" deep, and no idea how output capacity was calculated, but even if they used 20" wood, it'd be another 80% reduction.

Firebox sizes on the unit I'm considering: 25KW 4.23ft3 40KW 6.53ft3.

 

[Fred61]

Dan, although your posts sound professional and impressive, they give me the impression that you don't actually operate a gasser.

Do you have the real world experience of running one? Just curious.

 

[__dan]

Dan what do you mean by turn down capacity? If I am not Mistaken you are using a Froling with out storage? I thought that was a big NO NO with that boiler.

I posted before what I am doing. 1200 sf radiant basement slab, circ is slaved to the boiler circ, comes on when boiler is at the release circ temp. Two upper floors are pex in thin slab radiant. Everything mixed down with a Tekmar OAT reset injection pump controller. I use the basement slab, constant flow, for storage and am plumbed with taps and valves for a future pressurized tank. Circs are off when the boiler is off, heat comes out of the slab passively.

I maintain constant load and constant flow on the boiler with the basement slab. Everything runs in auto, storage tank is prepared for and in the future but the system works perfectly now.

The Froling is way turned down in the controls to a 20 kw unit and because of the load. I can only load to half in the morning from a cold start or a few splis on a coal bed. The boiler does not slumber because of the restricted fuel loading. I would say the boiler spends 80% of its operating time at max turndown which is 20% primary air, 35% fan speed, and is mostly sitting there with a charcoal bed unless I have added fresh splits.

My fuel consumption is not up a lot from last year and I load it as much as I want for heat. In the range of 3 cord for the entire year. heat and DHW, not even burning the best wood, which I have in the yard and am saving for future years.

Note that the 20 kw boiler even way turned down is too big for my load, 3300 sf total, and I cut it back much more by limiting fuel loading to a few pieces. The boiler is right sized only for the first hour or two in the morning when the slabs and DHW are coming up from being off for 7 or 8 hours. I do not load up to burn overnight, no need to.

 

[maple1]

So are you going to operate the boiler the exact same way after you add storage?