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[intensedrive]

When I look around at specs for wood boilers I see different models offering to heat the same square footage with different water capacities.  Some models almost have double the water capacity for the same heating areas.  For a novice like myself it would make more sense to have larger water reserves for the coldest of days and when your drafty house demands more heating and feels like the blower never kicks off.  Its apparent from my research the only difference for boilers to heat a higher square footage is burn chamber size along with water capacity.  It makes perfect sense that more water capacity is, it's going to demand more wood to heat the extra water reserves, yet if your blower never kicks off and you use the extra water reserves are you not back to square one? Or even worse left with such a large amount of water to heat you will never recover that loss?  This water capacity has me very confused... Are New units trying to skimp by to make their product more affordable and advertise a higher square foot heating range?  I understand not one size fits all when it comes to housing.  You can have David on 9th street with 3000SQFT House with 2x6 walls newly constructed home, and Joe Blow with a 3000SQFT farm house drafty as you crap.  I welcome a open discussion... I'm curious as hell I have tried reading back through old threads never got a good answer.

Thanks

[slimjim]

Extra water capacity does not create more heat period, on older conventional units the extra water volume helps with buffering the temperture delivered to the home, they were slow to heat back up with a call for heat because the fire is so slow to recover and therefor a large volume of water was needed. Today's units, especially gassers are a much hotter fire by burning the gasses themselves and therefor need less water, the newer models also rely on proper flow inside the boiler to extract more heat faster meaning less fuel and higher efficiency. In some cases where a huge demand all of a sudden is needed, an extra storage tank for a buffering tank can be added, extra water does not create extra heat but does allow the homeowner to bank that heat for those sudden huge heat loads like a greenhouse that requires little to no heat during the day and as soon as the sun goes down a full heat load is used.

[shepherd boy]

Sq. ft. on sizing an outdoor stove doesn't make much sense to me unless you qualify it. Like r19 walls, r30 ceiling, good windows, in Florida or Maine? You can bet a company selling cheap furnaces and claiming they are the best will also over rate them. Their best tool to sales is a website that sucks you in and big sq. ft. ratings helps. There are different reasons for large water capacities. Some units are made with high recovery rates and tight draft controls which operate more like an oil or gas furnace which use very little water. Others need to store heat or need longer burn times like gassers. Some units were built with manuel draft which needed lots of water to even out btu draws in a given period of time. Some have solar added which needs to store lots of energy. Constant run lower amp pumps with larger piping create a more even temp in the boiler, less hot-cold spots, and less need for high water volumes.

while I was typing slim as reading my mind . Good post Slim.

[slimjim]

Another thing that we as proponents of gassers should be promoting is the fact that gassers typically store lots of energy / BTUs in the ceramic / refractory base therefor we need less water for the BTU storage.

[yotehunter66]

Wish I could afford one.

[thompsbren]

My Heatmor has 377 gallons of water storage with a small fire box and a 30 degree differential. I think the large water jacket makes the stove more efficient (longer burns and fewer cycles per day). The downside in mild weather like this with a newer house is the fan might not cycle for 6 hours or more. But, it allows you to light one small fire a day and use the thermal storage once the wood is gone. This is handy in the shoulder seasons.

[dukethebeagle]

i know slim has alot more experience with this but i would say i would have to agree with thompsbren.
i think with larger capacities and fewer cycles means more energy sucked from the wood instead of spending half  the time
consuming itself smoldering,european wood boilers work this way.and even a conventional can burn with alot less smoke if
allowed to burn hot with less cycles.i agree you can`t store btu`s but u can buffer the boiler this way.so in turn u cycle less.
who knows!!!!!just my opinion

[cartod]

You can store btu's in an insulated slab in your basement.

[Mr. Maple]

Intense drive,my 2 cents worth,or 1/2 cent, - water capacity is a BIG part of choosing the right furnace,as well as how efficiently it heats that water.My Father's house has a Taco 007 pump,I have a Grundfos pump,between the two pumps we are circulating 36 gpm,let's say for easy figuring 30 gpm.Our first furnace held 300 gallons of water,so we totally circulated the water jacket in 10 minutes,not that you are using all of the heat,but certainly on a real cold day the furnace will be cycling quite often,using up the fuel( wood ) quickly. The second furnace held 160 gallons of water,so water jacket was totally circulated in 5.3 minutes, didn't have the reserve to keep up,took twice as much wood when it got real cold.Present furnace holds 555 gallons of water,so water jacket is totally circulated in 18.5 minutes,lots of reserve to keep up.
    Can you get too big with your water jacket?Yes I believe so.Am I oversized with this furnace?Quite possibly,but I have to handle a lot less wood a lot less often,which suits me and my bad back just fine.The coldest it has gotten here so far is 12 above zero,so it hasn't really been tested yet.But in that" cold" snap,3 ten inch blocks held for over 12 hours,certainly the best of any furnace we have worked with,so we are using a lot less wood,putting wood in much less often,and never having the houses becoming cool during the night or early morning.

[intensedrive]

Seems everyone has great points.  Everything stated here in response make sense in theory, and some have posted real world results which helps a bunch.  Real world blows theory away in my opinion.   For the current discussion can we eliminate gassers as most members do not own one.  Going back to my topic discussion can anyone explain water capacity down the the science? hah I'm still a bit confused. 

[fryedaddy]

My take is a little different than the rest.

I have a somewhat modified conventional. My setup has a firebox then (6) 4" tubes running 4' to a vertical flue.
I have around 650 gallons of water and batch burn. Think of the water as a battery. During our coldest part
of the season I burn (2) fires per day and get my water to 180-190 then use the energy until its down
to 150 degrees. Some people say you have to stay with the fire when you batchburn but thats not
true. I usually check my fire three times per day.

My setup is made to go inside a shop and you get the benefit of a heated shop as well.

[mlappin]

Some of this is strictly manufacturers preference as well. Like some swear by mild steel and others use stainless of one variety or another.

One thing I do notice are the ones that use a forced or induced draft can have less water as the recovery time is so much faster.

Sometimes my G400 can raise the water from 160 to 180 in less time than my old natural draft stove took just to get burning good.

Both hold the same amount (250 gallons), but the G400 has probably at least twice the heating capacity.

[dukethebeagle]

my setup is about the same as fryedaddy
the whole system holds about 500 gallons
i also batch burn
twice a day
have a 300 gallon insulated tank in my basement
we have a pallet mill so i get alot of scrap pallets
when up to temp i fill with hardwood and slow burn
takes less wood then when i had the boiler only
and u don`t get all the smoke of the idle and burn with regular usage
u can also make usage  of poor quality wood and make alot of heat
mine is forced air but i control the air intake depending on the wood

[Smokeless]

My research and only a opinion. 
    The low water volume gassers do require a thermal storage tank. Especially when in the shoulder season. If not the creosote inside is extrem because they can not get into a full burn. They are always short cycling. On n off  remember (btu = 1pound of water raised 1*degree ) 250gal x8=2000pounds x30*deg differential = 60,000btu. Most houses eat that up in 30 minutes on cold days. Not really good for a 210,000 btu boiler. System needs at least 500 gal to get it into the mix range. 1000gallons is better but that is a lot of weight. I have two people that r running two units with no thermal storage and very dry wood Having bad, real bad results. It will get better for them when cold weather gets here and the furnace can burn longer. Also don't make the mistake of flowing massive volume threw the low vol boiler it can cause low pressure areas creating the cavitation.
    That is why I didn't go that route. The G-200 has 200 gallons in the stove like slimjim says it helps to buffer and even out the temp that is going to the demand. The 200 gallons gives it enough time to get into a hot burn n dry the furnace out and prevents the short cycle problem. Also the G-200 purges out the firebox every hour removing the condensation👍👍👍. I also batch burned in August for my domestic hot water.  I load it n forget it. It does it all by itself. Very satisfied.


         

[Smokeless]

In addition to your original question.
(Are New units trying to skimp by to make their product more affordable and advertise a higher square foot heating range? ). Yes it is definitely cheaper for the manufacturer to build the smaller vol furnace. Then add in your thermal tank and the price just went up above a higher vol boiler. You need to consider all the extra plumbing extra circulators and the area to put one. Best in the basement. Any heat loss is going to help the house.  I hope this helps you out.   Best regards.