Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: higgins11 on December 17, 2011, 04:36:47 PM
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1)Everyone always talks about measuring return heat why is it important for the temp returning not to be to low?
2) why use a heat exchanger in domestic hot water........why not just use the water out of the OWB tank and plumb the local well up to refill the OWB when water is used out of the OWB tank? My domestic hot water tank is only 80 gallons ........the OWB is 750 gallons
Thank for entertaining the silly questions of a newbie
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1. So you can see what your heat load is.
2. Really? Don't you treat your OWB? How would you control your ph and nitrates if your drawing water off your OWB? Do you really want to use treated water in your house? I don't think so.
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It depends on the boiler if a low return temperature is bad or not. Most it doesn't matter on. Especially if they have a pipe inside the boiler for the return that runs to the front.
There are several reasons to use a heat exchanger for domestic water. Most outdoor boilers are not pressurized so it would just run over the top constantly.
If you have a pressurized system you wouldn't want to run to run the pressure that high either.
The water usually has chemicals in it to stop corrosion.
Even if you did have a pressurized boiler that cold run at the well pressure and didn't use chemicals it is still going to be dirty compared to normal domestic water.
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What some of the manufacturers are finding out is that when water in the boiler gets below a certain temperature, the inside of the burn chamber forms condensation, or starts "sweating". This condensate in and of itself is corrosive, but when it mixes with ash, it becomes even more corrosive. I believe that that magic number is around the 140* mark. That's why a few of the bigger manufacturers are even including a tempering(mixing) valve with the OWB purchase. The valve helps maintain at least 150*. I think there have been many boilers that corroded through prematurely because of too low temperatures.
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What some of the manufacturers are finding out is that when water in the boiler gets below a certain temperature, the inside of the burn chamber forms condensation, or starts "sweating". This condensate in and of itself is corrosive, but when it mixes with ash, it becomes even more corrosive. I believe that that magic number is around the 140* mark. That's why a few of the bigger manufacturers are even including a tempering(mixing) valve with the OWB purchase. The valve helps maintain at least 150*. I think there have been many boilers that corroded through prematurely because of too low temperatures.
That is the water temperature in the boiler you want to keep up. If the return is colder than then it is pretty much instantly heated as it mixes with the hot water and won't cause any condensation. (as I said above this depends on the boiler design)
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His question is "why is it important for the temp returning not to be to low". The reason is to prevent condensation in the event that your heat load is too large and your boiler temp gets too low. I am not sure about the different types of stainless out there, as their metallurgy is much different, but cold formed and hot rolled carbon steel are effected by low boiler temps, as well as the so called "boilerplate". They will fail prematurely, no matter what OWB brand, if boiler temps get below 140* on a regular basis. That is why it is important to make sure your return temperature is not too low. There are many posts on this forum about too high of heat load vs too low boiler temps.
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Yeah we had a post on this somewhere. Once you get above 180* it cooks out the oxygen in the water. Fall below 140* you have to start all over again to cook out the oxygen.
Marty is right too about the condensation. That's why us cb owners have thermostatic valves.
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You don't drink the the water that is in your oil furnace.. My hot water 40 gal holding tank has a heat exchanger inside the tank..Def don't want to drink the water that is in your owb or in your existing oil furnace..
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His question is "why is it important for the temp returning not to be to low". The reason is to prevent condensation in the event that your heat load is too large and your boiler temp gets too low. I am not sure about the different types of stainless out there, as their metallurgy is much different, but cold formed and hot rolled carbon steel are effected by low boiler temps, as well as the so called "boilerplate". They will fail prematurely, no matter what OWB brand, if boiler temps get below 140* on a regular basis. That is why it is important to make sure your return temperature is not too low. There are many posts on this forum about too high of heat load vs too low boiler temps.
I think we are talking about two different things.
If you are pumping say 2 gpm you will have the return water going back to the boiler at a very low temperature. Because it is such a small amount of water it will mix with the hot water and not cause any condensation. If you are pumping say 20 gpm and have the same heat load you will have a much higher return temperature. The higher gpm will allow for a much larger heat load / boiler temperature loss.
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I believe the valves are used with mild steel furnaces that have no grates and use natural draft to fire the furnace. This system has a slower recovery time and so proper furnace sizing is very important. If a heat demand is too great the furnace may drop below 140 unless they use a valve to not allow the heat to leave. This isn't a problem with furnaces with a quick recovery time so you don't see valves sold with furnaces brands that have grates and fan drafts.
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not to get off suject but would,nt be great if you put a valve to fill your coffee cup in the morning ;D ;D ;D
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There is some correlation to efficiency if the return heat is not lower than 20 degrees from the supply heat. This is an observation I have read about concerning industrial applications. I think each heat exchanger will drop your return heat by 2+ degrees, I doubt in a residential application that you would have more than 3 heat exchangers, so I doubt you would be out of that 20 degree range. I know there are more factors that effect return heat temp but that should give you a rough estimate with which to stay in range. Monitoring return heat could help indicate if there was a problem with your lines too.
I'm curious about those that do monitor the return heat, what differential are you measuring between supply heat and return heat and how many heat exchangers on your circuit? What kind of drop do you see with a radiant heat setup?
Michael
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I get a 2* drop just running through the exchangers. I get 15* drops on each exchanger when they are in use.
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drops in temp through heat exchanges are because (mostly) the size (rating of the echanger) and GPM. if you are moving only (for example and not true figures) 2 GPM through an exchanger rated at 50,000 btu per hour and you drop the temp from 180 in and 140 then you increase the GPM to say 10 your actual drop might now only be 5 degrees. also the low heat return could indicate that when theoutside temps are severly low that the 2GPM may not be able to keep up with the heat demand due to heat loss of the home in the extreme cold.
you must remember that though your furnace may be rated at 500,000 btu per hour it is in fact your GPM and how big your exchanger is rated for. it is said that 1GPM can deliver 10,000 btu per hour to your home so to in fact get the 500,000 btu that your furnace may be capable of producing you would need to deliver the water to the home at 180 degrees and moving at about 50 GPM and of course have enough exchangers to extract that heat. on average with the small pumps that we use on these stoves we may move anywhere from say 8 to 12 GPM (in most cases) so a low return temp on an average winter day is a warningn that perhaps on the few coldest days of the year that you may not be able to keep your house at your desired temp. this on top of all the info allready provided about condensate and such.
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For my clarification regarding sweating.
Are you talking about actual condensation being caused by such water temp diiferentials, cold water returning to a warm/hot surface and not the condensation of the moisture being released from the wood during burns? I have been getting that type of condition in the burn chamber, even worst during this mild weather, longer idle times. I have my temp set @140 with 5 degree on/off. Would your recommendations be to raise temp or widen on/off to help prevent this? This temp setting has been heating the home ok. I haven't taken a return water temp reading so I do not know what it is. The temp gauge on supply @ hx stays within that range. I have a open system boiler. I can understand the water jacket sweating due colder temp exposure.
Thanks,
Church
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Mine was set on a 5 deq. diff when I got it and the fan shut off at 160.... After reading some here I changed it to a 15 deg. Diff and set the blower to shut off at 165. I think it does alot better job now.
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Church,
I'm surprised that you don't have water running out of your loading door all the time. 140* is way too low of temp to run an OWB at. There are those that will argue that is fine to run that low and that no adverse effects will occur where boiler longevity is concerned, and I suppose there isn't anything that can be said on this forum that will change their minds. I won't argue with them on this forum. I will simply state facts that engineers and technicians have relayed to me.
1. Boiler temps under 140 will cause premature failure of burn chambers(corrosion).
2. Boiler temps under 140 will cause premature failure of areas jacketing water(oxidation)
There are 4 doors on my P&M Optimizer 250. 3 of the 4 will leak water(condensation) until the boiler temp is above 150*. My Empyre Pro 100 did the exact same thing, as well as the home built ones I've put together over the years. You will lose a little efficiency with higher temps, but you will gain longevity.
My advise to everyone would be to call your manufacturers and see what they recommend, and follow what they say and keep accurate records along the way.
There are at least 3 manufacturers now that I know of that recommend installing a mixing valve if your heat load is more than your boilers capability at times. The trend in factory preset temps appears to be higher than what it was even 1 year ago.
I have over 10k into my boiler. I'm going to do everything in my power to make sure it lasts as long as possible.
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Thanks Martyinmi,
I have mentioned to others ( not here on the forum ) but those who ODW as well, ( networking ). And they were surprised at my temp setting as well. I just never readjusted it because it has been maintaining the house temp. I contacted the manufacturer as well and inquired their recommendation, they said they had one in use and it was running @ 130. Didn't mention anything in regards to raising it, just that if I had difficulty maintaining house temp, then to raise it to meet demands as needed. That it could be lowered in more moderate temps. Are you suggesting that I then should raise it at least 150* ? Still learning wth unit, didn't fire it after installed until Feb., this year. Ran it for couple of months using their stat setting @ 140*. Fired it this year around the first of Nov. It has really depended a lot on the weather as to how much h2o was on the door shelf. I have been playing with the blower, tryng to get it turned in if you will. I believe they have to much cfm for the unit, The stock blower was less in regards to their upgrade I went with. Like I said still following the leads found here applying what I feel what might be of advantage to efficiently operating it.
Thanks again for your feed back. Any other info would be appreciated.
Church
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Between my sidearm for my dhw and my water/air exchanger in my furnace, I pull 20*-21* of heat out of my boiler water. Knowing that 20* are pulled out, I would never lower my temp off point to less than 154*. I don't ever want my water returning to the boiler under 150*. I have my boiler set points at 195* off and 187* on and it works great for me. My furnace fan doesn't run near as much as it did when I kept my temps lower(165* off 154*on). The only thing I ever change anymore is the differential. Mine is a gasifier, and in order for proper gasification to be maintained, the refractory material has to stay hot, so I force it to cycle more frequently when the weather is warmer. If I know that I am going to encounter a few 50* days, I will go to 195* off/ 191 on. This ensures that gasification will take place almost instantly, and it insures a clean, smoke free burn. I have no way of knowing for sure, but my wood consumption doesn't appear to go up with higher set points, and my electricity usage definitely goes down. The higher temps also fool my water heater in to thinking that it's bigger than it is.
I think you need to follow your manufacturers recommendations for your boiler. It's not my place, nor anyone else, to give you advise that is contrary to what they tell you either in print or by phone. The only advise I might throw out there would be to have them put in writing that running your set points that low won't void your warranty.
Marty
What brand and size is your boiler?
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1)Everyone always talks about measuring return heat why is it important for the temp returning not to be to low?
2) why use a heat exchanger in domestic hot water........why not just use the water out of the OWB tank and plumb the local well up to refill the OWB when water is used out of the OWB tank? My domestic hot water tank is only 80 gallons ........the OWB is 750 gallons
Thank for entertaining the silly questions of a newbie
Higgins,
Don't worry about the "silly questions" everyone has to learn about this stuff when they first get started.
1) I see two reasons why you want to be concerned with return water temperature. 1st and most important to me is wood usage. The way I see it is that my goal is to keep my water at 180 degrees. If I do that then my house is warm and my domestic hot water is hot. So the way I keep my water hot is by burning wood, when the water temperature drops, my draft fans kick on and burn my wood hotter (faster) to get my water back to 180. The less that happens the less wood I burn. The 2nd reason would be the condensation issue that everyone else is talking about, but really that is secondary because if you set your aquastat temp high enough and you keep wood in your stove you won't drop that far in temperature too often.
2) As far as I know no one does this. The main reason is just about everybody treats their boiler water with chemicals to remove corrosive elements from the water. Some people put anti-freeze chemicals in their water to reduce the chance of their pipes freezing due to a malfunction. Bottom line is that the water that is cycling through your boiler would not be considered safe for human consumption. Also it would cause you to burn more wood due to what I talked about in my answer to your first question.
Hope that helps,
Andy
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I would suggest you turn the temperature up to 180 and then lower it over time. This will allow you to see how much difference it makes on wood and how clean it runs.
You don't want to get the boiler water temperature under 140 but i am not sure why the return temperature is that big of a deal. If you have 180 degree water in the boiler and the return is 135 it means you have a very low flow rate and the water will mix in the boiler.
Can anyone confirm that the bypass valves do actually go off the return temperature? I would have guessed they see the supply and cut it off when that gets too low.
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temps in the return are a big deal (or can be as i stated before) 180 in and 130 out suggests low flow..so say this is happening when the outside temps are nice winter days (20 or so degrees) and your house is staying at your set temps...when the outside air gets to one of those cold -5 or -10 degrees i would be willing to bet that that low flow that was causing your low return temps will now not be able to meet your homes demand for higher btu. btu delivered to the house are directly related to GPM and water temps, the more GPM the higher the return temps. the way your btus are transfered to the home is by the size of your heat exchangers or feet of rads or feet of lines in your radiant heat sytstem
lets say your home is needing 50,000 btu per hour to heat on a cold night, that means you would need at least 5 gpm at 180 to deliver that amount, your return temps may be (again not a true amount) lets say 130 and just barley heating your home. that same water at 180 and only going 2 gpm would only deliver 20,000 btu per hour and would not heat your home and the return temps would be a lot lower. on the other hand ,again, if you were delivering 10 gpm you would have available 100,000 btu per hour and only needing 50,000 your return water would be much higher...so i guess one thing you could say is the higher the return water temps give you a "cushion" that you can survive those - 40 nights, they may not be often but they do happen
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It also depends on how often the blower runs. If it is a low return temp but the blower only runs 10% of the time it will just run longer when it is colder.
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Ridgekid, any idea what your rate of flow is in GPM?
Your monitoring system is quite spectacular, I've seen your graphs. It would be neat to take that a step further with a programmable microcontroller and be able to do things like increase/decrease GPM flow based on return temps, etc... Of course that would require a pump that could be set electronically instead of manually.
I still follow what someone posted months ago on another post, "why take something that is simple and make it complex." Or something to that effect. Still, something like that would be cool to compare to a standard install. And compare too, if the added cost would even be worth the benefit.
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I have a TACO 009-F5. According to my pump flow chart it states:
Flow Rates at common equivalent Lengths:
(Assuming they mean one way-Someone can correct me)
Using 1" PEX
N/A GPM at 50' (Overloaded)
7.79 GPM @ 100'
7.34 GPM @ 150'
6.96 GPM @ 200'
6.63 GPM @ 250'
6.34 GPM @ 300'
I'm at 115' at furthest point, so I assume I'm running around 7.6 GPM?
I scan where I got the info and email it in PDF. Send your request through Ridgekid@msn.com
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Where did you get that chart? You most likely have to use the entire length round trip. You also need to make sure it is for pex and not copper.
The heat exchangers and elbows will slow it down also.
Do you have sensors on all 4 ports of the plate heat exchanger? If you do, you could get a pretty good estimate of your flow rate by running a hot faucet and measure the gallons per minute that it is putting out. Then using the temperature differences can calculate the flow rate.
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lower water return temps at wood furnace is not such a concern,the big concern is if you have a boiler in your house that your wood furnace is maintaining hot water for. undersized hx and incorrect pump flow can cause your indoor boiler too be shocked and crack its water jacket. this is caused when multiple zones are in use and boiler supply of hot water is low returning cold water back to boiler.
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RSI. Got that info out of book supplied by CB. Hydronic component sizing.
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lower water return temps at wood furnace is not such a concern,the big concern is if you have a boiler in your house that your wood furnace is maintaining hot water for. undersized hx and incorrect pump flow can cause your indoor boiler too be shocked and crack its water jacket. this is caused when multiple zones are in use and boiler supply of hot water is low returning cold water back to boiler.
You touched on a subject that I wasn't going to comment on when you mentioned the word "shocked", but, hey, you sorta' brought it up, so here's my comment. The guys that I've spoken to about boiler longevity also talked about what differentials should be to get the longest life possible from your OWB. In a nutshell, the shorter, the better. Metal that has more frequent small expansions/ contractions will last longer than metal that has fewer large expansions/ contractions. In other words, keeping your boiler at a more steady temperature promotes longevity. Shocking your boiler by allowing it to cool down substantially and heat back up is harder on it, as the metal expands and contracts further. I used to run one of my homemade ones with a 20* differential, and when it finished cycling, it would make a few creaking noises nearly every time. I've never tried it on my Empyre or my P&M, so I don't know if they would like it or not. One guy said never run more than a 15* diff, the other said 20*, but they both said that less was more in the long run. I'm not sure I buy into their thinking altogether, but it does make some sense. I think that a 10* diff will be the most I run in the future.
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Where did you get that chart? You most likely have to use the entire length round trip. You also need to make sure it is for pex and not copper.
The heat exchangers and elbows will slow it down also.
Do you have sensors on all 4 ports of the plate heat exchanger? If you do, you could get a pretty good estimate of your flow rate by running a hot faucet and measure the gallons per minute that it is putting out. Then using the temperature differences can calculate the flow rate.
Yes pex lines not copper.
Yes on both hx'ers.
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pex will be less gpm than pex al a pex also pex is slightly smaller i believe and the pex al a pex has a smoother inside meaning less friction but i am not sure how much different it will be
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I was wrong. Thought I had a TACO 007. Went over and looked this morning and it's a TACO 009-F5.
I thought I should think about a spare pump/Solenoid before anything happens. My son is going to check his and we may have one spare set between us if they match.
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i have the same problem and i think my gpm is pretty low. I have the taco 009 and i think i may need a second pump because i run my house and garage on one loop. I go from the boiler to my house to a 20 plate dhw plate exchanger to my heat exchanger on the furnace then it goes back to the boiler here i have it hooked to the supply line for the garage which flows thru one heat exchanger then back to the boiler. So im assuming adding a pump just for the garage would give me more flow thru my system. I just did a test for my temps. my boiler said 186. the temp on the suply line out said 174 and with no load i have a 5 deg drop on the return line. with only the house furnace operating i have a 150 deg return temp. Then i fired up the garage heater at the same time and got a reading of 120 on the return. Lastly i ran my shower and was maybe a deg or 2 drop. Any suggestions??
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All I know is that these "silly questions" sure are a great source of good information. For myself, I'm going to go out and turn up my Grundfos Alpha pump the stage II to get it up to 7gpm instead of letting it automatically select energy savings and pump at 3gpm. I have noticed that on nights when the temp is in the single digits, my OWB undershoots the preset 175 minimum down to the low 160's. My high side temp is 185.
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Sounds like I might be able to do some testing. I just purchased a Grundfos alpha. I'm going to install it when it arrives and keep my taco 009 as a spare for my unit and My sons cl4030.
Since I have enough temp probes, and will know the flow rate from the display on the pump, we should be able to solve some of the mysteries of gpm!!
I'll let you know when I have it installed.