I also think the distinction between a buffer and a storage tank is kind of hazy. I think we need a clearer definition of what's what. Please excuse me if this sounds too basic this late in the thread. Buffer tanks are use on many fossil fuel systems and are specifically sized to the system to limit short cycling of the appliance. It keeps efficiency up. Most manufacturers recommend some kind of cycling limit per day and a buffer tank is used for this application. The boiler would typically heat the buffer directly and the loads cycle through it. The size is based on the mass of the load, the appliance, heat loss, etc. In this case I don't see where stratification is as much a concern. Typically they would be no larger then a moderate water heater. I have a short cycling problem with my oil boiler, especially with burning wood and solar so I have to see about solving that problem. To me a thermal storage tank is a buffer tank on steroids and is way pass the cycling limit for other reasons. In this case it is a wood boiler that we are limitng the cycle of since it makes no sense to convert chemical energy (fuel) and store the thermal energy with the resulting standby losses. A storage tank is perfectly suited to keep a wood boiler operating efficiently. Many on here have the boiler set up to heat the zones first and then the storage tank. A quick way to get heat to a zone while maybe adding some complexity. Stratification is typically wanted in a storage tank as it increases the "effective" size of the thermal storage. A hydraulic separator is another concept to keep pump flows from interfering with each other in a system. A buffer or storage tank can be used like this. Obviously they all can have some overlap between them. The only reason I made these points is that I have a concept that I am considering where I may use both a buffer and a storage tank separately in a kind of hybrid system. Mainly it would be to keep high temp loads (wood/oil/baseboard) separate from the low temp solar/radiant system. The high temp would be a smaller pressurized tank whereas the low temp may be a large unpressurized. I want to keep things simple as possible so it is a work in progress.
That's my take on it. Anyone else?
A buffer tank/ thermal storage can and does have several functions. Knowing that it is impossible to always match the boiler output EXACTLY to the everchanging building load. So one function would be to prevent short cycling when the loads from the building are low. This is whay manufactures add modulating fans to try as best they can adjust the boiler output without harming the appliance efficiency or creating creasote issues. It's not easily done with cord wood boilers. Easier with bio mass or pellet, still nowhere near what can be done with gas or LP fired boilers.
The other function of a buffer tank is a "parking space" It allows you to run a hot efficient fire and store energy above and beyond what the load requires. The size of the buffer would then be based on how many hours of heat would you want without needing to stoke the fire. Again this is an everchanging number. Most calculate that "coast" number based on design load. Or the load the building has on the coldest expected day.
There is no perfect size tank for any application. Ever job presents a specfic set of qualifications, space for equipment, loads, and largely budget.
The total load of the building would possibly be the biggest determining factor when sizing for drawndown time. Smaller homes with smaller loads, perhaps 500 gallons will suit. I think a 40K or smaller boiler works well with 500 gallons of buffer. I shoot for a 12 hour no burn time buffer capacity at design conditions.
Keep in mind if the boiler is sized EXACTLY to the design load there will be no extra capacity to charge the buffer tank! So only less than design conditions will allow tank charging.
Your location has a lot to do with sizing also. if you winters get and stay cold consistently the system may look different than one in Missouri for example wher in one month January, we see sub zero to 60plus temperatures. hard to design a perfect system around that target.
Oversized boilers are another reason to consider storage. Remember energy, regardless of insulation value will slip away. You will never get 100% of the energy loaded into a buffer back out. The bigger the buffer the more loss.
Buffer size depends on what YOU are trying to accomplish, don't look for an industry standard.
Piping is the other critical component. A great insulated buffer with a poorly designed piping is usless. I feel with any buffer over a couple hundred gallons, you should pipe a means to provide all of the boiler output directly to the load, bypassing the tank. Especially if the boiler is sized accurately to the design load.
Allowing for a proper bypass of the buffer does require more components ($$) and controls to do it properly and by automatic means, no manual valve operation
Ever solid fuel boiler customer needs to sit down and write a list of what and how they prefer their system to operate. Compare that to the money available for the project and adjust until you are content.
The good news is ALL of the tools and formulas to do this along with many personal experiances have crossed this great site. Search around for the ideas you are comfortable with. There will never be a "canned" plug and play solid fueled system IMO. But nobody should have to "guesstimate" anything to get a safe, comfortable, reliable system, based on your expectitations.
hr
Mike
