[Greenbuilding] Wood Heating Experience & Exterior Wood Boiler charging Thermal Tank for Radiant Heat

elitalking elitalking at rockbridge.net
Sat Dec 24 09:12:58 PST 2011

I can relate to Wood heating discussion.  Discussion on thermal tank is 
below my summary of  my 30 years of experience of heating with wood inside 
the house, though my knowledge is still evolving.  If you have a drafty 
house, you need to keep the fire going to maintain somewhat even 
temperature.  However, with a tight house, a hot intermittent fire heats the 
house and the house holds the heat.    I am in the process of upgrading my 
thermal envelope to qualify for the latter.  However, starting and ending of 
fires are the most polluting. The best way to avoid creosote is a hot fire. 
I have air injection stove that works great.  It has a glass door which is 
lovely.  However the front loading often spills ashes that I vacuum up with 
shop vac in crawlspace with hose and switch indoors near stove.  This is 
another hole in my thermal envelope.  Avoiding down drafts requires a hot 
chimney.  I have removed my clay liner in a conventional cmu flu block and 
replaced with 6" stainless steel stovepipe.   I filled the space between 
stovepipe and block with vermiculite to insulate the stovepipe.  As a result 
of the low mass of stovepipe that is insulated, the chimney warms up fast, 
facilitating the famous chimney affect.  I also use a wood stove with 
outside air feed.  For the most part, this avoids the depressurization of 
sucking air from the conditioned space and the occasional back draft.  In my 
aspirations for a tight house, I am in pursuit of plugging holes and thermal 
bridges through the thermal envelope.  It occurs to me that the access door 
to ash tray is not tight.  It has no weather stripping.  It is held shut 
with magnet.  Likely there is plenty of air leakage from ash box access that 
is all the time.  Also the chimney is a large thermal bridge of masonry that 
goes from interior to exterior.


I am contemplating using a thermal tank to store heat from exterior wood 
stove.  The idea is to have large fires intermittently with bigger pieces of 
wood, burned as hot and efficient as possible to heat a boiler that feeds 
hot water to the thermal tank.  I can track the temperature of the tank to 
know when I have enough stored heat to last up to a week.  Then the heat 
from the wood stove can come through a hydronic system which can be 
continuously delivered and modulated in a very clean, efficient and 
automated way.  Suggestion are appreciated.


The space I have available for the thermal tank for water after 10" of xps 
is installed to achieve R50 is 7.25'x8.5'x6.25' which results in  385 cf 
which when filled with water weighs 23,380#'s.  When temperature is raised 
35F it stores 35x23,380= 835,789 btu's.  That is the equivalent of 8.8 gal 
propane.  If house performs as calculated, I need 120,000 btu's on an 
average Virginia January day.   Therefore I have about 6 days stored when 
35F above the temp needed to get the output.


I like these outdoor wood boilers that have a water jacket surrounding the 
firebox.  Typically they are meant to feed a significant supply of hot water 
to a large heat load while the fire is burning.  I like the heated air 
feeding the fire and the automated forced draft.  I think they are a problem 
when heat demand is low and the controls dampen to keep fire going such that 
it could be stoked up when the heat demand returns.  However, smoldering is 
more polluting, more creosote depositing and less efficient.  My intentions 
are to only burn at maximum heat to charge thermal tank up.


The volume of water in their main models are only 48, 59 and 76 gallons 
respectfully.  This system does not store much energy, requiring continual 
burning if not connected to thermal tank.


Gives btu outputs.  The smallest one puts out 107,000 btu's/hr.  It would 
take about 7 hours to charge the tank tank to give me 6 days of heat stored 
in tank.   However, the cost $9000 is quite high.  I would be interested 
infinding the same system, but smaller.


The next issue is setting up a hydronic heat transfer assembly to release 
heat to room.  Since I have a built and occupied house, I am limited on my 
options.  However, I read in JLC guide to Energy Efficiency page 244 about 
radiant wall.  That author describes a radiant wall installation underneath 
drywall with pex pipes about 8" apart. He claims their results have 
determined that the output is 1.4btu/sf/hr x temperature difference between 
tank and room.  Therefore, Tank temp 140F and room temp 70F results in 
140F-70F=70F, 70F x 1.4btu/sf=98btu/hr.  For design temp of 16F, I need 
8500btu/hr output to maintain 70F.  To get the required output of 8400btu at 
the design temp, I divide 8400btu/98btu/sf*hr=85sf of system.  The area I 
have to work with is only 8x8=96sf.  8,400btu/96sf=87.5btu/sf required 
output.  87.5btu/sf / 1.4btu/F=62.5F difference, or room temp70F+63F=133F 
water temp needed to heat at design capacity.  If we see a particularly cold 
snap coming, we can boost the temp of the tank.  However, this is peak 
output required.  Most of the time, much less output is needed.  Therefore a 
lower temperature can still satisfy the heat requirements.  Are there ways 
to boost heat output by increasing the circulation of water at the lower 
temperature or installing the radiant pipes closer together to get larger 
heat output for the same temperature difference?

A traditional radiator might have a greater output.  Can people on list 
suggest radiators. Some of the literature specifies a heat output, but fails 
to mention the water temp required.  Where a water temp and heated room temp 
is given, when the water temp is half the difference, I presume the heat 
output is half or proportional.

If I pull this off, I will insulate over my chimney (thermal bridge, seal 
the hole going through the floor to feed air to my current stove, and 
consider removing my shop vac and sealing that hole through floor.  The 
resulting system will deliver even temperature in an efficient, clean and 
convenient manor.  I will miss the image of the fire burning through the 
glass door to indoor wood stove.

Your thoughts will be appreciated.


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