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How Much Water is Generated with a RMH?

 
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Hi guys, I've just finished my 10 inch batch box rocket heater and it's working fine and burning clean, but it seems to be generating a lot of water that ends up dripping from the base. At first I thought this may be water being purged from the refractory castings, but now I'm not so sure. I've done about 10 burn loads over the last 3 or 4 days and it doesn't seem to be easing off. If I had to guess, I'd say I'm collecting about half a litre per burn cycle.
Any advice on this would be most appreciated. Thanks.
 
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Wood is primarily cellulose, which has the repeating formula (C6H10O5)n...The hydrogen and oxygen make up about 55% of the total mass of cellulose, so for every two kilograms of wood burned, you are generating 1kilogram of water.
 
Eric Kendall
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Thanks Michael, so I imagine this H2O that is generated would normally exit the system via the flue. I wonder why then that I'm having this problem? Any suggestions on how to overcome this?
 
Rocket Scientist
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Wow Eric;  
A ten inch batch! That's one big dragon!

The moisture content of your wood  is what comes to mind .
Cob is also a common place for excessive water, but  I suspect your build might be all metal and brick.

What kind of exhaust stack temperatures are you getting?   Over 140F ( 60C ) ?    

Check the moisture content of your wood, it must be 15-20%  or you will generate excessive water.

Pictures of your build PLEASE!!!  
 
Eric Kendall
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Hello Thomas, thanks for the reply. The wood that I'm currently using is off cuts of kiln dried pine framing timber, so I'd imagine that the moisture content would be on the low level.
You are correct in that there is no cob in the build. It's all steel and refractory construction. I'm just guessing that the exhaust gas temperature is around 60C. I have no way of measuring it as yet. I'll have to buy one of those infrared thermometers. The bottom of the bell sits at around 140C (as in the image). So from there it's about a 9 metre flue run to the exit point through the roof.
I'm still hoping it may be from the refractory castings that make up the riser segments. But it just seems a lot.
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thomas rubino
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How Sweet it is!!!   Outstanding work Sir!   Hats off to you!
I have seen your build over at Donkeys.

I'm wondering about having to heat all that mass.
From your picture you are getting quite a bit of water dripping out.
Framing cutoffs should be low water content but if your air is humid they might pick it back up again.
Need to know your gas temp in the flue.  Surface temp with an infrared gun can be misleading.
 
pollinator
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Eric, that is one beautiful beast! Fantastic!
 
Eric Kendall
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Thanks Guys, maybe I'll just stick a mercury thermometer in the flue at the outlet from the roof. I'll do that tomorrow if the weather's OK and I'll report back.
 
Eric Kendall
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I'm thinking if this is going to be an ongoing problem and not the refractory drying out, I can pump some type of high temperature liquid sealant into the base plate so that I get a seal between the base plate and the pipe bell. I can then attach a fitting with a flexible hose and just run it straight out through the floor. This would be a fairly easy solution if it persists.
The only question is what type of sealant would be best? Maybe  a high temperature silicone based sealant. The viscosity would have to allow it self level but be thick enough so as not to run out like the water currently does. Silicone would have the flexibility to cope with the thermal movement.
I don't think a self leveling grout type compound would provide the flexibility need, nor would it seal the edges too well.
Any ideas?
 
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That really does look cool,I would love to see a walk around video?
Kiln dried wood only has a low moisture content for as long as it is not exposed to moisture, perhaps try a different batch of wood, hardwood maybe?
I would assume the water is condensation caused by moisture inside the stove, so the moisture has to come from somewhere.
I know that fire bricks can hold a lot of water but after 5 burns it should at least be diminishing.
 
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Eric, it might be that the temperature of the exhaust gases is not high enough. As Michael explained, the process of wood combustion generates quite a lot of water. In case the exhaust gas temperature is too low, condensation will occur inside the heater or chimney. There are two ways to counter this effect: implementing a drain or raising the end temperature. Keep in mind these condensation fluid is slightly acidic, not every material is able to cope with that for years on end.

In short: maybe you built the extracting side of this heater too effective?

As an aside: is there any particular reason why you made the port less wide and full firebox height? In the past I found out this isn't the best configuration, efficiency- and pollution-wise.
 
Eric Kendall
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Scots John wrote:That really does look cool,I would love to see a walk around video?
Kiln dried wood only has a low moisture content for as long as it is not exposed to moisture, perhaps try a different batch of wood, hardwood maybe?
I would assume the water is condensation caused by moisture inside the stove, so the moisture has to come from somewhere.
I know that fire bricks can hold a lot of water but after 5 burns it should at least be diminishing.



Thanks Scots, I did 3 burns yesterday and the water pretty much stopped halfway through the last burn,  so hopefully if was the refractory castings. I'll find out over the coming days.
 
Eric Kendall
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Peter van den Berg wrote:Eric, it might be that the temperature of the exhaust gases is not high enough. As Michael explained, the process of wood combustion generates quite a lot of water. In case the exhaust gas temperature is too low, condensation will occur inside the heater or chimney. There are two ways to counter this effect: implementing a drain or raising the end temperature. Keep in mind these condensation fluid is slightly acidic, not every material is able to cope with that for years on end.

In short: maybe you built the extracting side of this heater too effective?

As an aside: is there any particular reason why you made the port less wide and full firebox height? In the past I found out this isn't the best configuration, efficiency- and pollution-wise.



Hello Peter, thanks for the advice. I'm hopeful that it's only water from the refractory castings. As I mentioned to Scots, it seems to be abating somewhat, so I'll just keep my fingers crossed.
The only reason that I changed the port configuration was for the ease of manufacture. I kept a similar CSA, but if you advise that using the original dimensions would be preferred, I can easily widen the port and reduce the height with appropriate inserts. I don't have a Testo, but it seems to burn extremely clean. No soot build up in the firebox and the glass stays clear and the bricks stay white. The output from the flue seems to show signs of water vapor, but no other coloring.
I'm also pretty happy with the fact that I don't need to use the top by-pass valve for cold starts. This is really the only reason that I included it. I'm quite surprised that it draws down 4 metres and makes the exit through the lower valve from a cold start. The actual flue outlet is around 7.5 metres above the heater outlet. It has a small horizontal run of a couple of metres.
Thanks again Peter. Let me know what you think.
 
Peter van den Berg
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Eric Kendall wrote:Hello Peter, thanks for the advice. I'm hopeful that it's only water from the refractory castings. As I mentioned to Scots, it seems to be abating somewhat, so I'll just keep my fingers crossed.


In that case, just wait and bide your time. Every mass heater shows the same sort of teething problem, normally it will go away in about a month's time. I didn't expect it to happen with your steam punk dream heater since it's mainly built out of steel. But then, it's a biggie so the cast parts are voluminous as well.

Eric Kendall wrote:The only reason that I changed the port configuration was for the ease of manufacture. I kept a similar CSA, but if you advise that using the original dimensions would be preferred, I can easily widen the port and reduce the height with appropriate inserts. I don't have a Testo, but it seems to burn extremely clean. No soot build up in the firebox and the glass stays clear and the bricks stay white. The output from the flue seems to show signs of water vapor, but no other coloring.


I'd recommend to do nothing apart from firing it and see how it develops, maybe view this entire winter as maidentrip. The development model happened to be much, much smaller and later on the scaling system was worked out. Having seen larger systems through the years one fact appeared to be crystal clear: the larger versions tend to be less finicky. Yours is at the top of what has been built so far, keep an eye on the chimney output and check for soot. The chimney's inside temperature would be a tell-tale sign as well, above 60 ºC is OK and 120 ºC in normal operation I'd regard as good, given the size of this apparatus.

Eric Kendall wrote:I'm also pretty happy with the fact that I don't need to use the top by-pass valve for cold starts. This is really the only reason that I included it. I'm quite surprised that it draws down 4 metres and makes the exit through the lower valve from a cold start. The actual flue outlet is around 7.5 metres above the heater outlet. It has a small horizontal run of a couple of metres.


So the bypass is there for the odd day there's actually an equilibrium between inside and outside temperature.
Yes, it's quite surprising that a displacement system like a bell/stratification chamber is able to push the gases down and out, isn't it? At the time in 2009/2010 I tried a narrow and 2.2 m high bell with a fairly small rocket at the very bottom and it worked without as much as a hiccup. Several guys who saw the thing doubted it would work at all but nevertheless it did, no problem at all. In retrospect, your heater is using the same principals although upscaled quite a bit.
What's the distance between riser top and bell lid?
Height of the chimney is OK, the horizontal part of a couple(!) of meters is suspicious to me. Could you have it done with 45 degrees? As it is now, fly ash could build up in that flue. Nothing to be worried about in the short term though.
 
Eric Kendall
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Thanks for all that feedback Peter, most helpful.
I'll be sure to check that flue gas exit temperature, probably at the weekend.
Hopefully the horizontal flue run won't present any problems. I've included an access door just in case in does need a clean out.
I've got around a metre of clearance between the top of the riser and the lid of the bell.
I'll report back after a few more burns as to the status of the water.
Thanks again for your expertise and comments.

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Nice craftsmanship, piece of art! Love the fact that it’s built from new materials.

Peter is right, too efficient... exhaust stream not warm enough or more correctly the assembly bottom isn’t getting up to exhaust temp as being exposed to ambient temperatures at the floor setting up a sustained flue gas condensation situation. Peter is right again that the “condensation fluid” is acidic: I assume you used ferrous material for your build that acid likes to eat. Get some litmus paper and check the condensate P.H. to see how aggressive it might be.

You mentioned a by-pass, is it located in the top horizontal flue Tee or the adjoining flange connection, hard to tell as expanding the picture shows some camera shake and can’t make out the fasteners. I assume it’s there and is a flue damper as there is no blast gate? Having to not use the by-pass with a cold startup kind of tells me your by-pass is by-passing in the closed position. Checking your bottom flue connection temp against your final discharge temp at the roof during a sustained firing with indicated if this is happening.  

 
Eric Kendall
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Joe Danielek wrote:Nice craftsmanship, piece of art! Love the fact that it’s built from new materials.

Peter is right, too efficient... exhaust stream not warm enough or more correctly the assembly bottom isn’t getting up to exhaust temp as being exposed to ambient temperatures at the floor setting up a sustained flue gas condensation situation. Peter is right again that the “condensation fluid” is acidic: I assume you used ferrous material for your build that acid likes to eat. Get some litmus paper and check the condensate P.H. to see how aggressive it might be.



Thanks Joe, that's probably a good way to tell if the water is from the castings, or from the combustion. I'm assuming that water from the casting would still be neutral on the litmus paper. I'll get some and give that a try.

Joe Danielek wrote:

You mentioned a by-pass, is it located in the top horizontal flue Tee or the adjoining flange connection, hard to tell as expanding the picture shows some camera shake and can’t make out the fasteners. I assume it’s there and is a flue damper as there is no blast gate? Having to not use the by-pass with a cold startup kind of tells me your by-pass is by-passing in the closed position. Checking your bottom flue connection temp against your final discharge temp at the roof during a sustained firing with indicated if this is happening.  



Yes the by pass is directly at the end of the horizontal run. It has a butterfly control valve that can be opened or closed proportionally from the quadrant plate assembly mounted on the left side of the firebox. As mentioned, I can run from a cold start with this completely closed off and the lower exhaust fully open. The thermometer mounted near the bottom outlet shows around 140C to 150C after a couple of burns. The probe for this is close to the outlet.
I'll be sure to  check the exit temperature when the weather conditions allow. I've yet to do this. Thanks again.
 
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Erik, that firebox calls for cooking inside!

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