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[Red97]

I had some time so I figured I would do an experiment with my wood burner it is a home made unit just installed it in dec. The stove hasn't had any major issues but I can Never leave "Good enough alone".

Against my better judgement I made the stove natural draft, blah. It has worked ok and heated my house 950sf of barly insulated 10 ft ceiling 1900's house to a nice cozy 72 degrees all winter. But, when I turn on the fan to my 24x40 garage I can watch the temp drop and never catch up until I turn the fan off.

As many have had a rough winter, here in south central MI it was mid 40"s the other day the stove went 28 hr on a fill but today it was 0 -1 during my test

ok the "test" my set points are 170 on 179 off natural draft takes 90 min on average to reach 179

with the 0 temp outside I turned the fan on in the garage with the furnace on in the house the stove drops 1 degree every 1.5 min so it cycles quite often,

so I took my shop vac supposed to be 312 cfm not sure if that is the same number positive pressure vs negative, once the damper door opened I shoved the positive pressure side in the opening and turned it on.

with both loads running it took 5 min to raise 1 dgree water temp with both loads off  1 dgree a minute just over 30 minutes from 170 to 179 stove holds just over 300 gallons of water

so with a blower the cycle times are roughly 1/3 the time, what would that mean for wood usage?  I just wanted to share my findings and see if that is some what normal

now some pictures

normal idle


just after damper opened


just after blower installed


10 min after blower installed


mostly coals in the stove except for the lagre round arter 12 hour over night burn except for the large round put that in in the am


wondering what size blower I should install and can I just tap into the solenoid wires and run that off the ranco aquastat?
thank you for the help getting me to this point

[mlappin]

How long your wood will last depends a lot on how much heat your capturing now.

Try your experiment again, but this time get a stack temperature running natural draft, then again using the shop vac.

If their is too large a difference your wood usage will increase as your blowing heat right out the stack before the water can capture it.

Another suggestion might be to even try a hair dryer, try it on different speeds and see what happens with recovery time and stack temperatures. If your getting 312 CFM out of your shop vac that is  LOT more than most forced draft furnaces have. I haven't paid too much attention to standard forced draft OWB's but I'm thinking around 40-50 CFM is more the norm.

[Red97]

mlappin do you mean the actual gas temp or the outside of the flue? my flue is uninsulated and have had to clean it 2 times already it plugs with creosote junk at the last 90. I thought that most blowers were in that range too. thank you for the info.

[Red97]

Ok tried the hair dryer  the stack temps the gasses are

natural draft 160-190

hair dryer 250-285

shop vac 340-380 degrees mostly coals left in the fire box so zero smoke.

natural draft


shop vac

[mlappin]

Those aren't bad stack temps, but where did you get them? From the back of the stove at the elbow or at the top of the stack?

One thing I've found that helped me tremendously on the stack creosoting up, spend the big bucks and get a piece of double walled insulated stainless stack. Used to clean mine out several times a month, this winter have done it twice all winter since installing the SS stack and neither time was it to the point of affecting stove performance.

[Red97]

top of the stack flush with the top aiming down into the pipe. the hair dryer on high was 10 min for 1 degree raise. shop vac is still 1 min for 1 degree. which stack temp range is ideal for a good burn?

[fryedaddy]

I'd suggest a 140cfm fan with a damper to lower the air volume until you get it exact amount figured out.

[mlappin]

I'd suggest a 140cfm fan with a damper to lower the air volume until you get it exact amount figured out.

 

I'd suggest getting a fan around the range of your shop vac or a little smaller, then adding an adjustable baffle to the inlet, then just adjust it till you find the sweet spot between better recovery time, wood consumption and exhaust temp. You could even go a little on the low side for normal weather, then open it back up a little for extremes like what were seen this winter.

[RSI]

How long your wood will last depends a lot on how much heat your capturing now.

Try your experiment again, but this time get a stack temperature running natural draft, then again using the shop vac.

If their is too large a difference your wood usage will increase as your blowing heat right out the stack before the water can capture it.

Another suggestion might be to even try a hair dryer, try it on different speeds and see what happens with recovery time and stack temperatures. If your getting 312 CFM out of your shop vac that is  LOT more than most forced draft furnaces have. I haven't paid too much attention to standard forced draft OWB's but I'm thinking around 40-50 CFM is more the norm.

The hotter stack temperature doesn't necessarily mean more wood usage. More heat is going up the chimney when it is running but it may be burning cleaner and it will be a shorter time. Overall heat up the stack could be less.

[Red97]

That is a good point rsi. I did manage to burn 90% of the creosote off the fire box today. Is their a good range for stack temps? Wouldn't keeping the stack hotter clean up some of the creosote problems in the flue? Does that only apply to a straight flue? thank you for the replies.

[mlappin]

I think your best bet is just to experiment with what works for your stove considering it is homemade and no one will have a identical one to compare to.

[slimjim]

I would start with the insulated smoke pipe mentioned earlier and measure the temp at the breach of the boiler, that will give you a far more accurate number on stack temp!

[Red97]

slimjim, "breach of the boiler"? you mean closest to where the gasses are coming out of the fire box? I can take the flue cleanout cover off and check their. thank you all for the replies.

[slimjim]

Yes, where the boiler transitions to the smoke pipe, you should be able to drill a small hole in the bottom of the vertical pipe and install a candy thermometer or if it's above 400 degrees, your supply house should have a probe type stack temp gauge that will give you an accurate measurement of the exhaust temp itself, measuring the stack itself is not very accurate as the ambient air cools it off way to much, you need to get the exhaust temp to be accurate.