Boiler and buffer tanks

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Retro_Origin
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Post by Retro_Origin » Sat. Jul. 23, 2022 1:02 pm

Anyone have good information/principles on the concept of buffer tank sizes and boiler tank sizes? Last night I read through a dozen pages of Don's "Axeman heats wood boiler" and there was a pretty stark disagreement between some on whether that setup was better or worse than just a standalone boiler.

Am I correct in saying the boiler's BTU rating is really by the heat generation capability (fire or nozzle sizes (for oil)) and the actual water content is really just the manufacturers' figures on how fast of a pickup their fire has?

If running long and hot is BEST for a boiler (of any fuel sort) then why wouldn't they all have large tanks to prevent short cycling? I understand thermal loss but what about firebed ramp up and down time? Wouldn't running 1 hr on 1 hr off on a very cold day be better for a boiler than 15 minutes on 15 minutes off (same time collectively).

I was in my buddy's church where they have two natural gas boilers 220,000BTU each, literally the size of a mini-fridge (is that even considered a boiler at that point?!). The copper mains in the basement were 3" :o :o :o which tells me those slammers could heat that water as fast as it could come in.

My Keystoker and AA are nearly the same BTU rating yet the keystoker tank is almost double the volume of the AA. Is this the manufacturers design because of fire pickup time?

Is large volume and old fashion designed? (still kept by keystoker, and long since disproved by AA?)

Am I missing something, or just thinking about it all backwards?

 
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Post by waytomany?s » Sat. Jul. 23, 2022 10:58 pm

That's a good question for anyone looking to switch to a boiler.

 
lzaharis
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Post by lzaharis » Sun. Jul. 24, 2022 7:02 am

As our dear friend Sting would say;

"It Depends".

The best place for you to learn about buffer tanks and thier benefits is www.heatinghelp.com www.caleffi.com and any solar heating system sites.

Pressure rated steel water storage tanks for hot water storage can be placed on the cool return to have thermal mass to allow the boiler to have warmer feed water entering the "steam chest" to reduce the amount of fuel used by providing a larger source of hotter feed water that requires less energy to be reheated or on the hot water side for greater thermal mass and hot water storage.

The high school I attended has hot water baseboard heat and it hoovered as the construction of the buildings was from 1960-61 and heat loss was not an issue that was considered as a problem as they used so much natural gas for hydronic heating and pool water heating they purchase transportation gas at a lower price from the local pipeline and still do.

The heating plant for the numerous buildings and the pool depended on the gas boilers and also used 10,000 gallon hot water storage tanks in the boiler building to store hot water for heating and domestic hot water and the circulators for hydronic heating and delivering domestic hot water to all the buildings are huge.

Top fed or bottom fed Coal fired Gravity Hot Water Heat is a heating method developed in Canada that became popular in the lower 48 because steam heating boilers at the time had a bad habit of exploding, knocking down buildings and killing people.

Top fed gravity hot water heating is still the easiest and simplest way to heat a building as the water reaches 170 degrees Fahrenheit and rises by convection to the top of the home or building and goes wherever it is directed by the heating systems proper piping design. The water in the Top Fed Gravity Hot Water heating system does not require bleeding of trapped air of piping and radiators because there is none in the water causing havoc with slugs of air as the pushed out of the system through the open to air expansion tank in the ceiling of the top floor or the attic.

Bottom Fed Gravity Hot Water heating systems require that each and every radiator is bled of air in order to work correctly to allow the cooler water to sink to the basement where the boiler is and enter the boiler sump to be reheated again.

There are original coal fired converted to natural gas gravity hot water heating systems still in use that are over 100+ years old and still in use because (1) they were designed well, (2) sized correctly, (3) installed correctly, (3) have been properly maintained and (4) provide slow even heat by convection with 170-180 degree water that keeps these homes and building warm no matter the weather.

Old steam coal fired boilers were sized based on the size of the building or home where the homes windows and doors were wide open in Zero degrees and had to heat the home or building because they had little or no insulation if I remember my history right.

Buffer tanks or hot water storage tanks are a simple way to store heat energy, The Garn wood boilers are an excellent example of this as they have large thermal mass and also have the provision for using electric resistance heating of the hot water in the storage tank with hot water heating elements supplied by Garn.

Hot water buffer tanks are used to reduce the cycling of heat pumps, hot water storage tanks are used to store hot water for heating and making domestic hot water through copper coils.

The Caleffi company has a great deal of useful information about hot water storage and buffer tank systems

Solar hot water heating systems use buffer tanks to store the hot water made by the collectors to heat homes and buildings storing up to 5,000+ gallons of solar heated hot water also known as money in the heat bank in insulated non pressurized tanks at night by using huge copper coils and circulators to heat the water to store the heat energy in the off peak periods, some of them require supplemental heat from resistance heating elements exactly like electric hot water heaters.

This image shows a basic low pressure hot water storage tank and heating system.
buffer tank 1.png
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The twin gas boilers in your friends church that you mentioned are what are referred to as cold start boilers where the fuel whether it gas or oil is fired and then shuts off when the boiler high limit temperature is reached or the thermostats are satisfied; coal or wood boilers cannot do that as they continue to have a fire after the demand is satisfied.

The design of our keystokers requires that a small fire be maintained to keep the boiler active to provide heat to the living space.

The same can be said for the Axeman Anderson boilers except that they have a larger fire that does not require the hold fire function to operate as often since it has a much larger fire to take advantage of when the aquastat, anthrastat or hold fire timer calls for heat.

The Axeman Anderson coal stoker uses the size of the fire on its rolling grate to its advantage by heating the water behind the curved inner sheet steel of the firebox that allows more steel to be exposed to water to heat the water in the firebox faster.

A cold start boiler is just like a hot air furnace they are designed to transfer heat from combustion;

(1) to water to make steam

(2) to water to make hot water/hydronic heat

(3) to create hot flue gas combustion air flowing into a heat exchanger while a separate air flow passes through the heat exchanger to provide scorched air heat.


Time for chores.

 
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Post by Rob R. » Sun. Jul. 24, 2022 8:01 pm

Coal boilers have to deal with two things that oil and gas boilers do not.

1. A fire that takes time to reach full output, and also time to return to an idle (it is never "off").
2. Heat generated from timer cycles/sleeping fire.

If your fire takes a long time to respond, having heat stored in the boiler acts like a flywheel and prevents the boiler temperature from crashing when a load is put on it. The boiler also needs to absorb heat from the fire (without overheating) when the heat call stops.

The AA is unique in its ability to quickly go from an idle to full output in a matter of minutes. You don't need a big "flywheel" to cover the load while the fire is waking up. The lack of thermal mass on the AA is apparent when a long heat call ends - the boiler temperature tends to get on the high side of 200F. This can be handled with a properly sized expansion tank and/or a dump zone, but more thermal mass would do it also.

With all of that said, I think adding a buffer tank may help with operational issues (like overheating or taking too long to recover when a big zone calls), but I have hard time seeing how it would improve efficiency. I previously had a 6-section oil boiler piped into my system that acted like a large buffer tank (15 gallons water, 800 lbs cast iron), and now that I have run two winters without the oil boiler in the system I would say my overall efficiency has improved. By removing that oil boiler, I reduced the standby loss in my basement, and the basement is cooler as a result.


 
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Retro_Origin
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Post by Retro_Origin » Mon. Jul. 25, 2022 8:02 pm

Thanks for info everyone! I do think there can be a lot of speculation on our parts, but when it all boils down (no pun intended) we have to credit the manufacturers for knowing what they're doing. While modifications and adding extra systems etc. may be tempting and sound like we can 'hack the system', there is a reason why our boilers are sized the way they are. It's great stuff to think about. One of the things that got me thinking about it was this pretty sweet snippet from the 520 manual
Screenshot from 2022-07-25 19-48-38.png
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Pretty impressive how you can get such a wild difference in output by just adjusting feed an air. I think this is the first time it finally 'clicked' in my head that air is meant to be tuned to match feed...I was thinking about it a little backwards...

 
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Post by StokerDon » Fri. Jul. 29, 2022 11:00 am

Retro_Origin wrote:
Mon. Jul. 25, 2022 8:02 pm
I think this is the first time it finally 'clicked' in my head that air is meant to be tuned to match feed...I was thinking about it a little backwards...
YAY! Glad you figured that out!

With a "coal pusher" like most stokers are, EFM, Van Wert, Keystoker, Yellow Flame, ect... You set the feed rate for the desired BTU output. Then you set the air to burn the amount of coal that is being pushed. I think most people don't get that concept at first.

A note on Buffer Tanks;

The first thing to know is that the moment you put heat into it, you are loosing BTU's. How many BTU's per hour you loose depends on how well the tank is insulated. There is no way to 100% insulate something.

You only use a buffer tank if your system needs it. Burning wood is a good example of where you might need a buffer. A wood fire has a very peaky heat output curve. It starts out very hot, then levels out for a short while, then slowly declines. This is why batch burning into a heat storage tank can help efficiency when burning wood.

A coal fire is very different. The heat output curve has a much smaller initial peak. Then it levels out and stays constant for many hours before it needs tending. This relatively long, flat output curve negates the need for heat storage.

The Axeman/AHS type of boiler is a small, low water volume, high efficiency boiler designed to operate as an "On Demand" system. With this type of system, the best heat storage is to just leave those BTU's in the coal bin until you need them.

-Don

 
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Retro_Origin
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Post by Retro_Origin » Fri. Jul. 29, 2022 1:58 pm

StokerDon wrote:
Fri. Jul. 29, 2022 11:00 am

A coal fire is very different. The heat output curve has a much smaller initial peak. Then it levels out and stays constant for many hours before it needs tending. This relatively long, flat output curve negates the need for heat storage.

The Axeman/AHS type of boiler is a small, low water volume, high efficiency boiler designed to operate as an "On Demand" system. With this type of system, the best heat storage is to just leave those BTU's in the coal bin until you need them.

-Don
Great stuff!

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