Multiple Zoned Water Storage Tanks

[DKerley]

Just an idea.

My Greenwood 200 boiler is oversized when the temps start getting above about 0 deg.F requiring me to dump heat into my shop. Would the following idea work to reduce wood consumption and to run my boiler more efficiently?

Install 2 or 3 - 120 gallon storage tanks that were individually zoned and controlled to dump the excess heat in to them as opposed to wasting it in the shop. I could then allow the fire to go out if the outside temps were getting too warm and if there wasn't much demand. Additionally, it would be nice to add additional control to the intake damper to have a minimum open time to allow the fire to get really hot and prevent short cycling. This could somehow be achieved by opening the zones to the storage tanks one at a time. Any thoughts?

 

[Nofossil]

Just an idea.

My Greenwood 200 boiler is oversized when the temps start getting above about 0 deg.F requiring me to dump heat into my shop. Would the following idea work to reduce wood consumption and to run my boiler more efficiently?

Install 2 or 3 - 120 gallon storage tanks that were individually zoned and controlled to dump the excess heat in to them as opposed to wasting it in the shop. I could then allow the fire to go out if the outside temps were getting too warm and if there wasn't much demand. Additionally, it would be nice to add additional control to the intake damper to have a minimum open time to allow the fire to get really hot and prevent short cycling. This could somehow be achieved by opening the zones to the storage tanks one at a time. Any thoughts?

I don't know the output of a Greenwood 200, but even three 120 gallon tanks don't represent much heat storage capacity. If you take them all the way from 80 to 180, that's 300,000 BTU. Unless you can do perfect stratification, you'll be hard pressed to dump much heat into them after you get to 160 or so, and you won't get them down to 80 unless you're using radiant zones.

A more realistic heat storage capacity would be about 200,000 BTU. If you need to dump 100,000 BTU/hr, you're done after two hours.

 

[DKerley]

Hello No Fossil,

If there was anyone that I was hoping for a reply from it definitely would have been you. The Greenwood 200 is rated at 200,000 btuh. The company says that I need to demand at least 25% or 50K BTUH to have it burn efficiently. I have not done a heating analysis of the house yet but soon will. For the most part, the GW is working well except for in the warmer temperatures. The reason for considering storage as an option is to have somewhere to send the heat when the demand is lower than usual and/or in the evening prior to going to bed when the thermostat is lowered and the firebox is filled. In the last example, I experience short cycling for the first 3-5 hours before the outside temps drop closer to the forecast low temperature of the early morning hours. If I was able to initially send the heat to storage ie; create a demand, the fire would soon stabilize and hopefully prevent the short cycling in addition to creating a buffer for future cycles so that the open damper time is extended. I also thought that with storage,when the oat gets too warm to burn during the day, I would allow the fire to go out and draw from the storage until the next evening. The idea of having multiple storage tanks was so that I did not utilize the storage capacity unless needed. Any further thoughts would be most appreciated.

 

[Bob Rohr]

Is the goal to just dump some excess until the boiler simmers down? If so dumping into the shop is a simple over heat protection method. At least it gets used quickly and efficiently, for little distribution cost.

Now if storage to help from keeping it fired daily, that would be better done with insulated storage. If you know the load of the building you can easily calculate how much capacity would buy you how much "un-fired" heating time.

You will never get back 100% of what you send to storage, and pumping costs, however small, add to this energy transfer cost. Heat storage is kind of a necessary evil, or maybe catch 22 of solid fuel heating. How much to build, how to build and insulate, control logic to get it coming and going when needed, space, etc are all details to work through. As you see from surfing this list, there are many, many ways to accomplish this. I don't know any of us have the "perfect" system. Yet! "Best" is in the eye of the beholder.

hr

 

[DKerley]

MOS, The goal of storage is twofold, the first is to avoid having the shop heat turned up to 70 deg to keep the GW 200 operating at a minimum efficiency level when the OAT is above 0 deg F, and secondly, to try and buffer the output so that the unit doesn't short cycle during low demand. I don't have that much volume in my system side and therefore the GW200 can recover it very quickly. I was hoping to make the boiler work a little harder resulting in a cleaner more efficient burn. Thanks for your interest.

 

[Bob Rohr]

We are all kind of jumping into this in the middle.

Really the most important number to help you would be the heat load at design day. How many BTUs does it take to provide 70F inside at 0 or whatever the design is for your area? How often are you at or below design day? ASHRAE actually has a table to show that data.

My initial thought is you are oversized by quite a bit. 200,000 is that actual output at say a 75% efficiency? Or 200K x 75%= 150,000 actual output?

Most modern wood burners will regulate to some extent. A bit of operator influence helps. If the weather calls for above average or mild weather, don't stoke the unit so full. It should then reach setpoint and idle down. My EKO has a variable speed inducer and revs down nicely, without losing the fire. This takes the buffering down to a minmun. You'll never adjust the burn exactly to the ever changing load conditions. Fossil fueled appliances are best for that.

Really buffering for temperature control is a band aid approch. Treating the symptom not the problem. Which may be a largely over sized boiler?

Once you have combusted the wood, released the chemical energy and turned it to thermal energy, storage and shuffling becomes a losing game. Insulation and careful planning will keep the loss to a minum, but assume the losses will take place. It's the basic laws of thermodynamics. It can't be cheated.

If in fact the boiler is over sized, and the temperature control is less than desired, and you want some storage, you really have only one choice.

I have installation manuals from 6 or so different Euro brand gasification boilers. Rarely do the recommend going over 1000L "accumulator" as they call buffer tanks.

That being said, they offer 15, 18, 24, 30, 32, 40, 60 and 80 KW versions in many of the brands. I suspect they size much closer to the load over there. Seems 2 or 3 sizes fits all over here And you can see where their typical loads run. They build smaller and more efficiently over there. A 200K boiler would be laughable for a residence across the pond.

They have better controls on Euro versions, VS blowers, outdoor reset, some even have lamba sensors to watch and adjust to emissions. We don't see those models over here due to listings and various regs required. Also the voltage mis match and lack of market to make it worth importing those models.

Actually pellet boilers seem to be more the norm in the countries I have visited. That or a basic wood burning parlor boiler.

So run some numbers, decide how much storage time you want, how much $$ you plan on tossing at the challange, and we can show you options.

hr

 

[DKerley]

MOS,

Thanks for the reply. I will try and get back to you with some numbers.