Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

wood boiler storage tank sizing

mburg
mburg Member Posts: 41
I have a customer with a older hs kedler tarm model 502 wood and oil boiler. He currently has 120 gallons of storage but would like more to allow longer time between burns. The fire box holds about 70lb of wood. 25% moisture content wood is 5692 btu/lb and assuming roughly 60% efficient it comes to 240,000 btu per burn. currently the distribution system requires 180-190 degrees on a design day the heat loss is 62,000 btu/h. My first inclination is to suggest new panel radiators and some staple up radiant to reduce distribution temp bellow 150 to increase time storage tank is effective in heating space but even still is this enough storage? The existing distribution system may have to stay. using siegenthalers heating with renewable energy book i followed a formula for sizing storage tanks and came to 2,944 gallons if temperature could only drop 10 degrees obviously this is not pragmatic. but even with allowing tank to cool to 160 it still showed 736 gallons i cant imagine this would be effective with a boiler that only holds 70 lb of wood. It seems to me that if i size the storage tank at a size that would allow for the home to run from it for an appropriate period of time than i feel the boiler may struggle with getting the tank back up to temperature after its been depleted. whats a good compromise? Is this boiler just to small or not effective in running this home with any substantial storage? Any suggestions on sizing the storage with this boiler?

Comments

  • SWEI
    SWEI Member Posts: 7,356
    Is there an ODR-controlled mixing valve on the downstream side of the buffer tank?
  • hot_rod
    hot_rod Member Posts: 15,064
    Isn't that about a 32KW boiler, say around 110,000 BTU/hr at 70% efficiency you have around 77K actual output, running at ideal conditions good, dry hardwood.

    So the boiler is probably close to the load size on design days, not a lot of extra hp to start adding to a buffer tank.

    What is you climate data, how many days at or below design?

    What is your target "boiler off" time, that is where the storage size creeps up. Pretty tough to get more than a 8 hour non burn time, at design without those large gallon buffers. Also heat creeps out of a large tank like that without some really thick and tight insulation, maybe 4" plus.

    You really need to leverage a larger ∆T to make this realistic. Ideally run 180- down to 120, now you have a lot more ∆T on your side.
    Generously sized panel rads might do the trick, radiant floors, ceilings or walls or combinations of those radiant surfaces would allow those lower SWTs.

    Being a long time wood boiler user, you have to be pretty on top of that burn, in a small firebox to get a consistent 180, poking, adding fuel, stirring the coal bed, etc. It is not an easy appliance to load and walk away from.

    Your 180- 150 concept is better, 180- 120 would really help that tank sizing.

    In any case pulling the load off the tank with reset control is a must.

    Run the numbers with a couple different ∆T options to see how the numbers come out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mburg
    mburg Member Posts: 41
    The boiler is 110,000 btu/hr. The job is near Rochester NY.

    The customer would like 8hr off times, but I'm gathering with the existing boiler and distribution system that's not feasible.

    so if we upgrade to panel radiators sized for 120 on design day.
    heating tank from 120-190 we would need about 420 gallons of storage and a firebox that can hold about 130 lbs of wood. "i think" does this seem like a appropertly sized boiler for a house with a 70,000btu/hr heatloss? are wood boilers even made that large?

    He currently has 120 gallons of storage that was installed by previous contractor, they never increased expansion tank size so cant run hot or relief valve opens. would it be better to remove this tank and just say that storage would not be effective unless he reduces distribution temp and increase boiler size, or would simply reducing distribution temp make the 120 gallons of storage acceptable?

    I had allot of fear that even if I put two 120 gallon storage tanks reduced distribution temp to 150 or less with panel radiators, mixing reset, and trvs with constant circ that on colder days when he gets home from work and tanks are depleted that he may spend days trying to increase the temperature in tanks as well as in house if we are near design day.

    The boiler is 20-30 years old so replacing may not be out of question although he is farely convinced that it has plenty more life in it.
  • SWEI
    SWEI Member Posts: 7,356
    What is the building envelope like?
  • hot_rod
    hot_rod Member Posts: 15,064
    There are a few buffer tank simulation tools online, try Boiler Buddy and Lochinvar.

    I like this one in Siggy HDS as it lets you change all the variables right down to tank dimensions and insulation on the tank.

    It also allows you to toggle loads in and out if it is zoned in a way that might allow for a basement or garage to be dropped off at times for example, that would help you drawdown times.

    The main issue is most of the higher efficiency Euro gasification boilers have small fireboxes. If you do not load and monitor carefully you can get bridging where the wood does not fall down near the gasification nozzle. Output plunges. These really are interactive boiler, they like attention.

    Now the OWF types have huge, and less efficient fireboxes. Doors large enough to drive a Mini Cooper inside, high water content, open systems no worry of relief valve pop.
    I can smell when my neighbors load them with trash, throw in a gallon of diesel, a match and walk away. So they get a long 8 hr. burn, but the neighbors pay a price. Mini Coopers produce a lot of black smoke when combusted:)

    So you can get to where the customer wants to go in your example, just be aware of the tank sizing, piping and control logic required.

    Generally speaking, when looking at this type of storage volume the unpressurized tanks start to look much better $$ wise. I like the tanks Tom builds up in Maine, and the rectangular knock down go into the space nicely. He has spent a lot of time finding the best lining materials.



    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mburg
    mburg Member Posts: 41
    Building is old 18" thick cobblestone walls. All windows are being replaced, attic has new blown in cellulose and basement sills are being spray foamed.
  • hot_rod
    hot_rod Member Posts: 15,064
    mburg said:

    Building is old 18" thick cobblestone walls. All windows are being replaced, attic has new blown in cellulose and basement sills are being spray foamed.

    The more you drop that load by upgrading, the better the picture becomes, a good way to spend money is in shell upgrades first, then size the heating system.



    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    SWEI
  • SWEI
    SWEI Member Posts: 7,356
    Have you done a heat loss calc before and after?

    ODR will really help with those stone walls. That alone might just do it, at least as far as the emitter system goes.
  • mburg
    mburg Member Posts: 41
    I used the tank simulator in siegenthalers software. wow got lost in that for a few hours, so many different variations.

    The heat loss I did was based on the house after all renovations are completed even still the sparse amount of baseboard required 180 degrees granted I am sure it would be less just due to my errors in the heat loss but clearly for any effective storage based system it would require much lower distribution temperatures. I am strongly leaning towards a max of 120 after all the recommendations and seeing how dramatically it effected the burner off times.

    I was looking at the froling FHG boiler it showed a fuel loading chamber capacity of 5 cu ft. Wouldn't this be over 100 lb of wood or am I misunderstanding this? Also it showed a rated output of either 70 or 102 mbtu/hr. If the boiler was loaded full wouldn't it produce more heat than that? or does it just reduce air intake to prolong burn over extended period of time? Isnt that bad? I thought gasification boilers should be burnt as efficiently and quickly as possible starting a new fire when storage tank was depleted?

    I am meeting with customer tomorrow afternoon we will see what direction he wants to go. I imagine whichever direction he goes the system will have many caleffi products :p
  • SWEI
    SWEI Member Posts: 7,356
    mburg said:

    The heat loss I did was based on the house after all renovations are completed even still the sparse amount of baseboard required 180 degrees

    Somehow I was under the impression this was an existing system and that it was (mostly) heating the house before these proposed envelope improvements.
  • mburg
    mburg Member Posts: 41
    This is an existing system that worked without any storage for many many years. Last year 120 gallons of storage was added expansion tank size wasn't increased so they cant get system hot enough or relief valve opens. Also expectations where not met with storage, the customer was expecting longer run times with this amount of storage and existing distribution.

    I based my heat loss on existing renovations and the renovations that will be done in the short term.

    the existing boiler could heat the existing house currently if the relief valve didn't open, but it is far from the type of operation the customer wants, being able to make it over night without loading more wood or go to work and not have to tend to fire or have oil fire.
    This is why I am going to suggest new distribution and possibly new boiler to get longer run time between burns and give the customer what he thought he was buying last year.

    If this is not affordable than there is no reason we cant just make a few minor changes to existing system "increase expansion tank size, put in mixing valve for return water protection, and install something to protect boiler from overheating during power failure or not enough load"
    Doing this as a inexpensive solution may be the customers choice knowing the options now, but he did initially ask for longer run times between burns 6-8hrs which is not feasible with existing working boiler and distribution.
  • hot_rod
    hot_rod Member Posts: 15,064
    It sounds like you have a good handle on the concerns and fixes" Show them some price options for various solutions.

    At the very least boiler protection, and pull the loads from whatever tank capacity via a outdoor reset. Much of the heating season will be below design conditions in most areas. Lowering that SWT will extend the run times and comfort. With baseboard fin tube less noise from expansion, less streaking with lower temperatures in the fin tube also.

    ODR can get you close to constant circulation which makes for a nice even heat.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    SWEICanucker
  • mburg
    mburg Member Posts: 41
    Thanks for all the help!