Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: GCTerpfan on October 30, 2014, 07:29:45 PM
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I installed my first OWB about two months ago and have been heating my DHW with it since. Just last weekend I hooked up temp gauges on my supply and return lines at my furnace. I have a 370' loop of 1 1/4" pex and with no load I was seeing a consistent 4 degree temp difference between supply and return. Tonight I turned on all three zones in my house just to check my return temps, and it was 129. I understand it should not get below 150, so do I need a bigger pump? I currently have a Taco 009.
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Terpfan, it certainly sounds like you need a bigger pump. 370' loop is not a small loop by any means and when you mention that you have 3 independent zones, it brings to mind valves, T's and 90's, all of which create pressure drop and thus a drop in flowrate. There are two items that you did not mention that are important here and that's supply temp and whether or not you have much of a load on your system right now.
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I don't buy into the return temp theory. I'm waiting for mine to spring a leak but I know too many that run less than 180 and have had zero issues.
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Okay, here is some more info on my system. My supply temp is 180. The 370' of 1 1/4" pex runs through two 5x12 plate heat exchangers, a 50 plate that heats my indoor boiler and a 30 plate that heats my DHW. I ran through the Taco worksheet and calculated my head loss at 8.9' (not including the heat exchangers because I don't know how to account for them).
When I tried this experiment it was fairly warm out, the house was at about 68 deg. which is where we keep it. I simply turned all three thermostats up to about 75, so that they would call for heat and walked out to the boiler. The return temp dropped quickly and seemed to hold at about 127 to 129. There was no call for heat from DHW at this time. Does that seem right? The return temp was a lot lower than I expected.
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Yes 370' is quite a loop, and 4 degrees with no load is pretty respectable. Most boilers are recommended to NOT have regular return temps below 140f, as it reduces the possibility of creosote/rust forming. However, if you have a temperary drop of return temps below 140f you should be OK, since I only turn my pump on between OWB and FPHE when there is a call for heat (I get short 5-10 minute windows where return temps to boiler are below 140)
Question, you do have a CB so do you have the thermostatic bypass installed? Because if the supply temps from boiler drop low enough (the by-pass) will simply divert water back to boiler to be heated up to temp, before sending it out to radiant application.
**Good luck, if you do decide to go with a bigger pump (based on a quick check of a pump curve chart from TACO) it looks like a 0011 would be the next step up, from a 009. Just be careful you are not OVER-PUMPING the system, which causes noise and eventually erosion of metal fittings
EDIT: Added word NOT & below
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Yes, I have a CB classic and no I do not have a thermostatic bypass installed. I wasn't aware of them until after I installed my system. I inherited the boiler, so I did the install myself. There are lots of things I would have done differently if I was to do it again.
How does a thermostatic bypass actually work? If it diverts hot water from the supply to the return, wouldn't that just reduce the flow to your heating system, thereby compounding the problem?
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The Thermostatic by-pass works in 3 stages; (all water temps stated are coming off the HOT side of the boiler)
1: Water temps below 140f get returned to be reheated
2: Water temps between 140-165ish, a portion of the water is returned to OWB and some is let out into Radiant application
3: Water temps above 165sh, valve is FULL open and all water from OWB is going to radiant application
Basically if water temps coming off the OWB drops to low, it diverts flow from heating loops back to OWB, so it has a chance to catch up. The thermostatic bypass does NOT take into account the return temperatures being too low, it only tries to keep the supply temperatures above 140F
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Got it. That makes sense.
Any other opinions on whether I should be concerned with the return temps I saw? I just don't want to find out there is a problem the first week the temps hit 20 degrees and have to deal with it then.
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Sorry reread an earlier post and made corrections... :bag: :bag:
...typical return temps shoudl be above 140f, with temporary dips below 140
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Sorry reread an earlier post and made corrections... :bag: :bag:
...typical return temps shoudl be above 140f, with temporary dips below 140
What's the maximum length of time would be acceptable for a temporary dip below 140? Boiler temp set @175 on/180 off. Thanks
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To GCT
I don't know if low return temps will hurt your boiler. It hasn't for me. This is now year 6 and my system has spent a good part of it's life running at lower than recommended temps. Anyways...to me, your 50 degree temp difference sounds awfully high. I question the 9 foot head estimate. Does that take into account elevation, pipe length and every possible flow obstruction? What I do to confirm flows is pull the return line off at the boiler and measure with a pail and a stop watch. Crude but effective and it eliminates guess work.
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Oaky, I can not say a definately what kind of time frames are acceptable or not; but it is generally accepted that return temperature should be above 140f. As temps below that are more likely to create condesation on interior of reaction chamber, create creosote, which leads to rust, which leads to LEAKS.
**I am not saying, that you can't run with return temps below 140F, but IMO I woudl try to chance some stuff so my return temperatures where a bit higher.
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I would agree with you as well LittleJohn
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I would agree with you as well LittleJohn
Hey Slim want to tell that to my WIFE, that I am somewhat knowledgable ;D
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If she's anything like mine, she won't listen anyway!
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I question the 9 foot head estimate. Does that take into account elevation, pipe length and every possible flow obstruction?
I did screw up the head calc. I have a 370' loop of 1 1/4" pex with 10 elbows, 4 ball valves, 2-5x12 plate Hx with 1 1/4" ports, and a 6' elevation difference. Disregarding the Hx the 'correct' head number is 15.6' (unless I screwed up again).
With the Taco 009 this should mean I have a flow of about 7 gpm. (http://s3.supplyhouse.com/manuals/1350986824086/84486_PROD_FILE.pdf (http://s3.supplyhouse.com/manuals/1350986824086/84486_PROD_FILE.pdf))
So, I had supply temps of 180 and return temps of 127 for a difference of 53 degrees, lets take away the 4 degree drop I see with no load and that's a difference of 49 degrees. Assuming I am getting a flow of 7gpm that means by turning all three thermostats up from 70 degrees to 75 degrees at the same time on a warm day, my system required 171,500 BTU?
Am I missing something or doing something wrong? These numbers just don't make sense based on what I know (or think I know).
Am I over thinking it? We had temps in the 30's this weekend and when I fixed the fire I never saw return temps below 165.
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I would assume since you did not account for the HX, your flow rate is a bit lower, so BTUs are also a bit lower.
What is you application, cause 171k BTU/hr might not be out of the question, but without know what your load is or how big your structure is its hard to say?
...I do think a delta T of 50f seems a bit high, but my longest run is 100' one-way
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What is you application, cause 171k BTU/hr might not be out of the question, but without know what your load is or how big your structure is its hard to say?
...I do think a delta T of 50f seems a bit high, but my longest run is 100' one-way
It's a well insulated house with an 1,100 sq. ft. floorplan and a partially finished basement for a total of about 1,800 sq. ft. The previous winter which we had very low temps including a two week spell of subzero temperatures I used less than 500 gallons of fuel oil to heat my home.
The three zones in the house feed a total of about 100 linear feet of hydronic baseboard.
So another way would be to assume 600 btu/hr per foot of baseboard and I'm stiil only at 60,000 btu/hr. I just cant figure out where all of the btu's went unless I am getting significantly less flow than I think. But I admit I have no idea what I am talking about either ???