I have an outdoor wood boiler... with a heat exchanger in the plenum of my oil furnace. I use the furnace blower to blow through the exchanger... water temps going in are 178 and coming out is 142... but the air temp outta my registers is only 91... is this enough drop thru the exchanger and shouldn't register temps be about 110?.. not sure what's happening.. earlier this year I was getting 110 temps.. but now it not getting my house to 72... it runs all day trying to reach it and is stuck at 70
OWB Heat exchanger issues
- Thread starter Beegee0131
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maple1
Minister of Fire
Sounds like a reasonable temp drop thru the HX. Are those temps accurate? How far between the HX and where you're measuring register temps? And where does the duct work travel through in between? Sounds like you're losing heat out of the duct work in between - say from it passing thru a cold space or something like that. May need to insulate the ductwork. Can you measure your duct temps just downstream from the HX?
Those measurement are at the heat exchanger in and out lines... even the vent in the plenum right above the HX seems to blow only Luke warm... I took these measurement right after throwing some card board into the OWB so not sure if maybe I was getting false readings.. late last night I was having trouble getting a good fire. I'm assuming this is more my problem.. acts like it's not getting enough air... I tried cleaning off the coals on the grating and under hoping it allow more air flow to the fire but hasn't seemed to help much...
hondaracer2oo4
Feeling the Heat
Way to much of a heat drop across the heat exchanger because of slow flow caused by the pump thst moves water through the water to air hx. Two possible issues thst I can see. One is an air locked pump moving very few gpm, second is a failing pump which I would say is the most likely. Disconnect pump at owb return and see how many gpm you get out of it. You should be seeing no more than 20 degree heat drop across the heat exchanger.
This kinda makes sense. I had a pump fail about a week ago and had this one on the shelf. I changed it out same day. Maybe air is in the lines?
hondaracer2oo4
Feeling the Heat
Yes. I would first close off the isolation valve above the owb pump and then loosen the bolts below the pump. Open the top valve back up and let water flow out of the bottom of the pump. Then while it’s still draining water tighten the bolts back up. Then plug the pump back in. Recheck delta t across heat exchanger. Also close your valve downstream from the pump for 2-3 seconds and then quickly open it. Repeat this until you hear no more air bubbling back to the owb in the return line.
Did as you described w the pump and the purging of the air. Def seems to be doing a lot better.. how should I be measuring the vents temps? I'm getting 88 across the top just measuring the air... 105 measuring the metal register itself and 110 inside the register
hondaracer2oo4
Feeling the Heat
I am more concerned with temp drop across the heat exchanger from supply to return ports first. After we know thst we can move on. What size pump do you have on this thing?
maple1
Minister of Fire
Still pretty light on some details here. What do you have for underground piping? Like, size & length? Is this the first winter for this setup? If not how was it last year? Where is your cold air return drawing from?
Have the 1" thermaplex 50'from the house w about 15' of uninsulated pex on the inside of my home. This is my third winter. The previous have been a struggle. I've had three ppl look at it.. with three diff opinions...one did say.. the OWB was rated 120kbtu and my heat exchanger 145kbtu and that was the problem but the others agreed that didn't make since... air return is on the first floor directly above the furnace.
hondaracer2oo4
Feeling the Heat
Temp drop across the heat exchanger is influenced by two things, Gpm flowing through the hx and cfm blowing through the heat exchanger. Cfm is constant so the only way for us to improve the temp drop by decreasing the drop is by putting a larger pump on to move more gpm through the hx to give you less of a drop in temp thus delivering more btus to the heat exchanger which will increase your output temps from the hx.
Second thing to look at is how is your hx plumbed? The supply water should be entering the output side of the heat exchanger. That is so the air touches the hottest water right before it leaves the hx. Instead of having the return water being the last thing thst the air hits before leaving which would cause the air to cool back down. Check how yours is plumbed.
Second thing to look at is how is your hx plumbed? The supply water should be entering the output side of the heat exchanger. That is so the air touches the hottest water right before it leaves the hx. Instead of having the return water being the last thing thst the air hits before leaving which would cause the air to cool back down. Check how yours is plumbed.
maple1
Minister of Fire
I'm sort of over my head a bit since I don't have any W-A HX experience.
But...
My first thought was not enough pump for your piping. But to get a handle on that, we'd need to know more about your heat exchanger, mainly how much pressure drop or head it presents. That would be rated at so much flow - so you'd need to know how much flow you would need to get your BTUs to the house. Which leads on to other stuff like heat loss calcs etc.. But assuming some things from some charts I found real quick and using round numbers, your 1" should deliver around 100,000 btu/hr at around 10gpm with 5fps water speed. I think 5 is considered about as high as you should go, or thereabouts - so I used that for a constraint. 10gpm through 130' of 1" would be about 16' of head. Looking at the pump chart for your pump, at 16' of head, it won't move any water at all - it goes to zero at just under 10' of head. Which would be around 6gpm. So there is a pretty short window on the bottom end for that pump, between not pumping at all, and moving maybe half of that 100,000 btu/hr. And I didn't throw anything in for head loss across the HX - an unknown. Or anything else that is presenting flow resistance.
That was kind of circular and I think I have myself a bit confused now - but the moral of it is that it may be underpumped. The chart shows a 008 being a lot more capable - but not sure how much one of those would run you.
Also, this being what sounds like the third winter of similar lack of performance (and not a new issue) - also might reinforce that thought.
But...
My first thought was not enough pump for your piping. But to get a handle on that, we'd need to know more about your heat exchanger, mainly how much pressure drop or head it presents. That would be rated at so much flow - so you'd need to know how much flow you would need to get your BTUs to the house. Which leads on to other stuff like heat loss calcs etc.. But assuming some things from some charts I found real quick and using round numbers, your 1" should deliver around 100,000 btu/hr at around 10gpm with 5fps water speed. I think 5 is considered about as high as you should go, or thereabouts - so I used that for a constraint. 10gpm through 130' of 1" would be about 16' of head. Looking at the pump chart for your pump, at 16' of head, it won't move any water at all - it goes to zero at just under 10' of head. Which would be around 6gpm. So there is a pretty short window on the bottom end for that pump, between not pumping at all, and moving maybe half of that 100,000 btu/hr. And I didn't throw anything in for head loss across the HX - an unknown. Or anything else that is presenting flow resistance.
That was kind of circular and I think I have myself a bit confused now - but the moral of it is that it may be underpumped. The chart shows a 008 being a lot more capable - but not sure how much one of those would run you.
Also, this being what sounds like the third winter of similar lack of performance (and not a new issue) - also might reinforce that thought.
hondaracer2oo4
Feeling the Heat
You are in the right ballpark maple. I did the headloss calc on his system and found thst he’s probably moving about 5 gpm with the 007 on his system. He simply needs to move water faster to deliver more btus which will reduce his temp drop across the hx as well. 1 inch pex moves about 10,000 btus per gallon. If he can bump from 5 to 8 gpm he is going to get 30,000 more btus to the house. He should look at going with probably a 15-58 on speed three or a 26-99 on speed 1 or 2. These are grufundos pumps.
I'm willing to try a new pump... but I must say I've already bumped the size of the original pump up one size already.. but I didn't realize I could calculate what size I needed with pipe size and length but it does make since. I like to have one on the shelf anyway so I'll try and research what Honda racer suggested and hopefully it's interchangeable w the taco I have installed without redoing my plumbing...
I'd like to order the 15-58 being that it's 1/4 the price of the other... but comparing it to what I already have.. the specs I read says it moves 17gpm vs what I have 23gpm.. but the max head on the 15-58 is 19 vs what I have being 10. Do I need more head or more gpm or both?
hondaracer2oo4
Feeling the Heat
Yes, Hydronics isn't a guessing game like some "plumbers" think it is. It is a science with real calculations. If you want to learn about how to size a pump Google taco td10. The taco td10 is the manual on how to size circulators.
I calculated your system earlier and shot for 8 gpm with your piping and hx and fittings. I came out with I think it was 11 or 15 feet of head needed to push 8 gpm which will increase your current gpm from 5gpm. You will e sending 80,000 btus to the house instead of 50,000 btus which will increase your heat delivery by over 60 percent!
Once you find your head loss for a gpm you can start googling pump model numbers with the words pump curve. You now look at how many gpm a pump will move at the amount of head you have on your system. You want to find a pump that will move 8 gpm at least at your headloss. You can get a 26-99 grufundos for $166. It has three speeds and can be slowed down to Taylor your needs. Don't throw good money at a pump because it is less money. You need your system to work for what you have.
I calculated your system earlier and shot for 8 gpm with your piping and hx and fittings. I came out with I think it was 11 or 15 feet of head needed to push 8 gpm which will increase your current gpm from 5gpm. You will e sending 80,000 btus to the house instead of 50,000 btus which will increase your heat delivery by over 60 percent!
Once you find your head loss for a gpm you can start googling pump model numbers with the words pump curve. You now look at how many gpm a pump will move at the amount of head you have on your system. You want to find a pump that will move 8 gpm at least at your headloss. You can get a 26-99 grufundos for $166. It has three speeds and can be slowed down to Taylor your needs. Don't throw good money at a pump because it is less money. You need your system to work for what you have.
Ok ordered the 26-99... in theory do you think this will help me burn less wood? With current temps here in the single digits I'm packing the fire box w as much wood as I can put in there...to check it 8 hours later there is nothing left but ash.. it basically needs filled every 6 hours which is inconvenient since I work.. then come home to have to get the water back to temp to heat the house.
maple1
Minister of Fire
I don' think that will help with wood use. In theory it would make it burn more since it ups heat transfer potential.
Did you ever tell us what you have for a boiler?
Did you ever tell us what you have for a boiler?
hondaracer2oo4
Feeling the Heat
It will not change your wood use or efficiency. It just moves your btus faster . The h2 is a pretty small firebox for a conventional. What temps are you seeing outside right now day and night, how big is your house, how well insulated, tell us about your wood type, when was it cut, how big is it?
I have dry split seasoned hard woods..I'd say half from before last season and half from spring..temps are like high of ten low of zero this passed week. 1400sqft old farm house with the second level being remodeled and very well insulated r-29 insulation..and the first floor unremodeled and very little insulation yet.
hondaracer2oo4
Feeling the Heat
what is your differential in the hardy set at? Can you try and time how long it takes to drop from the high set point to the low set point? That will tell us how many btus per hour that your house is using.
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