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What's the pros and cons of adjust your water temp

Started by mrwood, November 02, 2011, 08:22:19 AM

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mrwood

I have a CB 5036 and Im curious what could be pros and cons to adjusting your water temp down. I know alot of you guys on here have eclassics so I know that is a totally different ball game. I know CB says it has to be at 185, but I have a neighbor who said he turned his down and he has plenty of heat and uses less wood. Any thoughts?

beenthere

That is a great question mrwood. I've wondered along why the high temp push.
Is the idea to keep the range tight so the swing between high and low temp doesn't last long, therefore not letting the fire be closed off from air too long and go out?

Often my water temp is down to 100 deg and I try to keep it from hitting 180 as that is when the damper closes and the fire dampens down (no air). Then again, mine is not the high tech but when the wood is burning with air there is little to no smoke and no creosote.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

MTJAG

Yes, I asked my dealer about lowering the temperature to save on wood.  He basically said the same as this thread...less burn with more cresote buildup, the potential for the fire going out and if you have an aquastat, your propane boiler may run more since the recovery would be longer.
CB CL5036 OWB, Husky 450, Montana 4WD

mrwood

Ok because I am just beginning I am still learning all this burning business. Why would lowering the temp 10-15* cause more cresote build up?

MTJAG

My understanding is the fire is not as hot, so a slower burn results in water in the wood evaporating at a slower rate and condensing on the walls of the chamber...does that make sense or has someone zoomed me...again?
CB CL5036 OWB, Husky 450, Montana 4WD

muckamuck

I think you have to consider that everything is a trade off of sorts.  First, if your boiler is not hot enough then not enough heat gets moved to the house at the rate your pipe and stove circulator (and heat exchanger) can move it, and as a result your house circulators will be running with lower temp water in the pipes and they will have to run longer to heat the air in your house but at some point on a cold day they won't keep your house warm enough--so there is sort of the minimum temp for your boiler on a cold day.  On the other hand the colder your boiler water the more efficient the heat transfer from the fire gases to the water of the boiler so you burn less wood,  and less hot exhaust smoke goes up the chimney it is just warm smoke at that point.  So in general you want to keep temp down in the boiler and have just warmish smoke going up chimney (most heat from wood is going to the house rather than up the flue).  But, as temp of water is down and smoke temp is down then the creosote oils and acid water in the smoke want to condense on those cold surfaces like the inside of your firebox and inside of your chimney and the acids start corroding your metal.  I think CB wants your water to be 185 or better to avoid that problem and so your smoke is relatively hot with some waste heat up the chimney.  To make things most efficient it is best to have very dry wood, good gassification (less creosote and water in smoke) and to clean your boiler heat exchanger surfaces to maximize heat transfer.

doctorb

I heat with hot water baseboard (HWB).  My plumber wants me to have the temp in my basement oil furnace of 180 degrees to provide optimal HWB heat.  I have no verification that this is correct or optimal, it's just his reccommendation.  To obtain that I need my OWB set at 195 degrees.  This allows for loss of 5-7degree heat on the 300' transfer to the basement from my shed.  It also permits the temps in the oil furnace jacket to be kept at a minimum of 180 degrees when my OWB is at the lower levels of the water temperature bracket.

I have a question about the fuel usage, though.  Obviously it will take more fuel (energy) to raise a boiler temp to 185 degrees than 160 degrees.  If one assumes that the OWB cycles through a 15 degree bracket (turning off at the set high point and turning back on 15 degrees below that point), does it take more energy (fuel) to raise the temp of a given volume of water from 170-185 than from 145-160?  I don't think it does.  So I am not so sure about fuel usage being vastly improved at lower temps, assuming that the demand for heat is the same in both situations, and the size of the temperature bracket is the same.  Somebody smarter than me correct me!
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

Dean186

Quote from: doctorb on November 02, 2011, 12:19:51 PM
I heat with hot water baseboard (HWB).  My plumber wants me to have the temp in my basement oil furnace of 180 degrees to provide optimal HWB heat. 

Yes, I agree with this, you want the temperature that most efficiently heats the house.

I have a question about the fuel usage, though.  Obviously it will take more fuel (energy) to raise a boiler temp to 185 degrees than 160 degrees.  If one assumes that the OWB cycles through a 15 degree bracket (turning off at the set high point and turning back on 10 degrees below that point), does it take more energy (fuel) to raise the temp of a given volume of water from 170-185 than from 145-160?  I don't think it does.  So I am not so sure about fuel usage being vastly improved at lower temps, assuming that the demand for heat is the same in both situations, and the size of the temperature bracket is the same.  Somebody smarter than me correct me!

I agree with doctorb on this one too.  The only advantage is, keeping a mass of water stored outside at a lower temperature takes a little less wood. 


muckamuck

Ok DoctorB I will try to correct you.  You are basically correct that the energy needed to raise a pound of  water a degree F is one btu regardless of it being from 160 to 161 vs 190 to 191.  There is also about 7000 btu potential in a pound of wood.  Now when we burn it where does the heat go?  Some goes up the chimney and some btu goes into the water.  The heat has to exchange from air across the cast iron of the wall to the water of the boiler.  The rate of heat exchange across the cast iron is proportional to the difference in temperature.  Consider if your hot smoke is at 200F and you tried to heat 200F water on the other side no heat would move at all because the temperature differential is zero.  To get the most heat out of the smoke and into the water it is best to have cooler water or hotter smoke.  Lets assume your smoke is 600F and your water is 160  you get a difference of 440F (pretty good for heat transfer), but if you are 600F and water at 190F your difference is only 410F so your efficiency is about 7 percent less.  Not much, but over a winter it could save some wood (but your smoke going out the stack is cooler at the lower setting and will condense more water and creosote so the cost savings in the long run is probably negated).

John Mc

Another consideration: the greater the difference between your water temps and the ambient temperature, the faster you will lose heat to the surrounding air/structure. The better insulated things are, the slower this will happen, but higher differential always means higher losses than a lower differential.

John Mc
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

doctorb

Thanks John Mc.  The question then becomes, "When does the temperature gradient between the firebox and the water beome significant enough to create a difference in fuel consumption?"  Your explanation make sense, but, when dealing with the temperatures reached in these OWB's, I am not sure that the wood consumption would be measurably different in the temperature backets hypothesized above.  An interesting thought, though.  Thanks again.
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

stumper

What has been said regarding heat transfer is correct.  Lower water temps means greater heat tranfer from the combustion gasses to the steel and then to the water, it also means less heat loss in the pipe and boiler to the cooler surroundings.  That said removing 1/4 inch of insulating soot will have a greater impact on wood use.

I backed into the temp that works for me. My DHW is set at 145, so I wanted my oil boiler to come on at 165, my boiler is set at 185 shut off, 180 on, 175 high burn on.  This gives me enough room so the oil does not come on when the ODWB is work correctly.  I have tried lower temps but have not been pleased in that the creosote inside seems to be wet, at these temps it seems dry.  Lower when I scrape the roof it globs, at these temp is breaks up into a black powder.  If I try to burn improperly seasoned wood it globs at this temp, so I use that a a clues as if the wood has seasoned (dried) enough.  I'll bump it up to 190 during cold snaps as I think it helps heat the house better but that may be more of a feeling then a true fact.

Gary_C

When you adjust your water temperature down on one of these furnaces, it results in two things happening.

First it reduces the capacity of the furnace. If you have excess capacity, that's not a big deal. If you don't have excess capacity, it a big deal.

Second it reduces the temperature of the heat transfer surfaces inside the firebox and that will result in condensation of undesirables like tar and acids on those colder surfaces. That's never a good thing.

So that's two negatives and thus the best recommendation for upper operating temperatures is the highest temperature that does not cause boiling of the atmospheric pressure water chamber and that recommendation will also reduce tar buildup and corrosion.

So CB is right. Run that stove at the temperature they recommend or as high as you can get it without boiling the water out.

And changing the water temperature does not change the fire temperature.
Never take life seriously. Nobody gets out alive anyway.

Holmes

   Just so you know,  for every 3 degrees you lower the water temperature you have the potential for 1% savings in fuel. Lowering the water temp in a gasifier 10 to 15 degrees should not create more creosote. Lowering the water temp in an old style non gassing boiler will probably create more creosote.
   The 185 degree temp the boiler makers want is the temp that baseboard heating systems are designed to, to put out enough heat on the Coldest days of the heating season. 150 degree water will heat most homes except on the coldest days ,but you will be about 40,000 btu's short on storage capacity   The best time to save energy is during mild weather, things don't freeze and break in mild weather.  Holmes  
Think like a farmer.

petefrom bearswamp

Where are these opinions coming from?
Any basis from experts in the physics of boiler temps, heat transfer, btu content of the wood, burning temps etc?
We all have our personal ways of stoking, temp setting and all but I still am experimenting after 6 years with my classic 4436 heating 3200 sq ft and domestic HW using  mostly white ash seasoned 2 years. Haven't hit on the optimum yet.
I run 175 max sometimes it boils in warm weather.

Kubota 8540 tractor, FEL bucket and forks, Farmi winch
Kubota 900 RTV
Polaris 570 Sportsman ATV
3 Huskies 1 gas Echo 1 cordless Echo vintage Homelite super xl12
57 acres of woodland

Gary_C

Quote from: petefrombearswamp on November 02, 2011, 06:37:40 PM
Where are these opinions coming from?

Can't speak for the others, but from me, sometimes I get them from thin air, sometimes from my head, and sometimes from other dark places.  :D

Quote from: petefrombearswamp on November 02, 2011, 06:37:40 PM
Any basis from experts in the physics of boiler temps, heat transfer, btu content of the wood, burning temps etc?

Does it really make any difference? I think some are making this question too complicated as it isn't rocket science. Just set the temp as high as you can and not boil water. Then throw wood in it and hopefully it will not get all tarred up or corroded away. You don't need a degree in Thermodynamics or Hear Transfer.  ::)
Never take life seriously. Nobody gets out alive anyway.

doctorb

Welcome back - DarkBlack!   Haven't heard from you in a while!  Your six months away has earned you a clean slate.  So again, welcome back.
My father once said, "This is my son who wanted to grow up and become a doctor.  So far, he's only become a doctor."

martyinmi

Gary_c, DackBlack,doctorb, and Dean186 are all right for these reasons-
1.They agree with me!
2. The agree with the engineers and technicians from two different OWB manufacturers that I have spoken to(Pro Fab & Portage and Main).
No God, No Peace
Know God, Know Peace!

boilerman101

I also agree with keeping water set points high. Over 10 years of heating with a Classic and now EClassic 2400 I have tried operating furnaces at 190 degree high set point and as low as 160 degree set point and could discern no difference in amount of wood used. Most outdoor furnaces operate at either full burn or idle, so it is just a matter of recovering the 10 degree water differential. Should not matter if going from 160-170 or 180-190. In my Classic I did not notice any difference in creosote build up inside firebox. However the 2400 firebox definitely seems to burn drier with less creosote operating at 190 high point. I would expect any gasser should stay cleaner at higher temp. My 2400 also came with and required me to install their thermostatic valve that requires water temps to run higher than 170 for full water flow through it, so if you have this valve you must keep set point higher than 180 to make room for the 10 degree differential drop. Central Boiler says they now include and require the use of this thermostatic valve on all their models, Classics, Eclassic and Maxim to extend life of their products. They said higher temps are better for metal life, expansion and contraction as posted by another member earlier in this string. Also said that return water temps below 150 can shock the boiler causing condensation moisture that can mix with the ash in the firebox making an acid causing rusting problems. Bottom line, I am a believer in keeping water temps high. So far I've never had a pitting or corrosion problem in any of my furnaces. The blower fan on my house furnace also seems to run for a shorter period of time when operating the outdoor furnace at the higher temperatures.

Dean186


martyinmi

Add Boilerman101 to my list.

Hey dean, thanks for helping the guys on the OWB information site. Your 3 years with the 1400 gives you a boat load of knowledge, and you are able to communicate it in a way that we all can comprehend.

   Marty
No God, No Peace
Know God, Know Peace!

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