[Greenbuilding] Wood Heating Experience & Exterior Wood Boiler charging Thermal Tank for Radiant Heat
elitalking
elitalking at rockbridge.net
Sat Dec 24 11:12:58 CST 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.
WOOD BOILER FEEDING THERMAL TANK
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.
THERMAL TANK
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.
WOOD BOILER
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.
http://www.aqua-therm.com/products_aquatherm_eco_one.htm
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.
http://www.centralboiler.com/e-classic.html
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.
RADIANT TRANSFER TO ROOM
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.
Eli
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