Help OWB losing too much heat at exchanger

[Maddieg7]

We have a Central Boiler CL 5036 and we think we are losing too much heat at return and need help troubleshooting. We have temp gauges at the boiler that measure outgoing and incoming water temp and we have noticed that when the furnace is running we are at a 40-50 degree difference (loss). I.e. Water from the boiler going out at 184 and coming back 140 or so. We have bled the lines to check for air pockets. We also know that we are not losing the heat on the way to furnace as we only have a 20-30ft run to house and have thermopex 4ft down. And we know it is not pump because we put in a brand new one yesterday in hopes that was the problem. I would guess heat loss from boiler to the furnace is no more than a couple of degrees. The other reason we can assume this is because when not under load water temp at return is around 10 (or much less) degree loss. So we have concluded the problem lies within our plenum/heat exchanger. Please correct if you feel that might be wrong.

Our theory is this: heat exchanger is not large enough for our large furnace that pumps A TON of air. It cools exchanger down faster than the water can flow through. Is that plausible? Our exchanger fills our entire plenum so my next question is this: do they make different "sizes" of hx, as in thickness or water flow speed? Or is it just the dimension that can change in them (which isn't our problem). Please help!! We would appreciate any advice/insight any of you have!!

 

[NSMaple1]

You need to also accurately measure your temps in & out of the HX - and whatever else it goes in & out of.

(A good IR temp gun helps - spray shiney measured surfaces with flat black paint first).

And a big heat difference doesn't really mean the heat was 'lost' - if it ended up in your house, it just got transferred from one place to the other. If there was a big temp diff from one end of your underground pipe to the other - then that would be a loss, to the ground.

Is your house warm enough? Can you slow the fan down? Usually a 20°dt thru the HX is a good ballpark - that could be narrowed by either slowing down the fan, or speeding up the pump.

 

[muddstopper]

I dont have a owb and dont really know how they are supposed to work. I cant visualize your problem because it would seem to me the HX is doing what it is supposed to do. If the temps coming out of you boiler was the same as the temps going back into the boiler, then I would think that no heat was being exchanged at the HX to provide heat for the home. If I had a big change in temps between out of the boiler and into the exchanger, I would get worried. checking the temps going into the HX and coming out of the HX would tell you how much heat was made available to the house. In your example you are getting 44* of heat out of your exchanger. I would think the closer the two temps where, the less heat you would actually be getting from your boiler. The greater the temp difference, then the more heat that would have been exchanged to provide warm air for the house. Am I looking at this all wrong??

I had a buddy install a geothermal system to provide a little AC for his house. His Exchanger was put in the duct work to cool the air before it was circulated thru his house. Checking temps going into and out of the exchanger showed only a 2degree difference and did little for cooling his house. It wasnt worth the money to trench in all that pipe. He later added another heat exchanger to the system and was able to get a few more degrees difference in cooling. My take from that is the bigger the difference between the temps in and out, the more warm/cool air that is being tranferred from the system, and if the temp difference isnt really great, then the cost of installing the system isnt worth the effort.

 

[Maddieg7]

You need to also accurately measure your temps in & out of the HX - and whatever else it goes in & out of.

(A good IR temp gun helps - spray shiney measured surfaces with flat black paint first).

And a big heat difference doesn't really mean the heat was 'lost' - if it ended up in your house, it just got transferred from one place to the other. If there was a big temp diff from one end of your underground pipe to the other - then that would be a loss, to the ground.

Is your house warm enough? Can you slow the fan down? Usually a 20°dt thru the HX is a good ballpark - that could be narrowed by either slowing down the fan, or speeding up the pump.

Right. It wouldn't bug us so much if it was keeping up to temp in the house but it seems like the fan runs long and often on somewhat mild days (20-30 degrees). We set thermostat at 74-75 and it seems to struggle to get there. Air coming out of vents feels lukewarm. We are trying to find a way of measuring temps accurately at hx but I can tell you it is a significant difference you can feel by grabbing incoming and outgoing pex lines. One side I'm able to briefly touch but extremely hot, outgoing pex line I can hold for several seconds. Husband was out at boiler during last furnace cycling and he said that it was 182 out and 175 in...and then he could tell when furnace kicked on because within seconds return temp dropped to 130-135. In the meantime we will try to get ahold of thermometer.

 

[Maddieg7]

I dont have a owb and dont really know how they are supposed to work. I cant visualize your problem because it would seem to me the HX is doing what it is supposed to do. If the temps coming out of you boiler was the same as the temps going back into the boiler, then I would think that no heat was being exchanged at the HX to provide heat for the home. If I had a big change in temps between out of the boiler and into the exchanger, I would get worried. checking the temps going into the HX and coming out of the HX would tell you how much heat was made available to the house. In your example you are getting 44* of heat out of your exchanger. I would think the closer the two temps where, the less heat you would actually be getting from your boiler. The greater the temp difference, then the more heat that would have been exchanged to provide warm air for the house. Am I looking at this all wrong??

I had a buddy install a geothermal system to provide a little AC for his house. His Exchanger was put in the duct work to cool the air before it was circulated thru his house. Checking temps going into and out of the exchanger showed only a 2degree difference and did little for cooling his house. It wasnt worth the money to trench in all that pipe. He later added another heat exchanger to the system and was able to get a few more degrees difference in cooling. My take from that is the bigger the difference between the temps in and out, the more warm/cool air that is being tranferred from the system, and if the temp difference isnt really great, then the cost of installing the system isnt worth the effort.

That would be awesome if we felt like we were getting the whole 40-50 degrees out of our vents...but air just feels lukewarm at best, and often cool. Also, it just seems concerning because I have read so many forums and I can't seem to find anyone who pulls that many degrees off at the hx. I see tons who get 5-20 degrees off but we just think 40-50 seems crazy, especially since we don't run anything else with our OWB.

Another concern would be return temp affecting life of the OWB. I have read return temp should not be below 140 due to risk of cold spots causing creosote and corrosion, and right now we dip below that set at 185 and at some point (spring/fall) we had thought we would probably reduce somewhere between 165-175. So then if we still have those dips we would get down between 115-135. We want to make our stove run as long and efficiently as possible. Thank you so much for your response!

 

[NSMaple1]

Return temp issues can be resolved with the aid of a boiler bypass loop & thermostatic mixing valve on it.

A big temp drop across the HX is not a sign of the HX losing heat. It is a sign of it transferring heat. If there is heat being lost, I would likely suspect it to be a ductwork issue first. We don't know much if anything about your ductwork though, or layout in general. It could also be possible your boiler pump and/or piping is undersized.

 

[absrio]

it sounds like your furnace HX is working properly. you want the largest HX you can fit in your furnace that way the blower runs for shorter times and less often. also a 30-40* drop in water temp is not terrible. mine usually drops that much I'm seeing a 32* drop on my boiler monitor as I'm typing this. do your vents run thru a cold crawl space? did you install the boiler yourself? or did it come with the house? did you have this problem last year?

as ns maple said, you can help by speeding up the pump or slowing down the fan. what is the make and model of your new pump?

 

[NSMaple1]

Air in your boiler lines is another possibility.

 

[Kevin in Ohio]

HX don't multiply or loose heat due to a flaw, they either work or don't do it effciently as far as getting you enough to keep you warm. Also, thinking your run is so short that there can't be a heat loss is not a good way of looking at it. How long is your HX? It only takes a pipe to be surrounded by ground water in one area to loose a lot of heat, fast. As other said, pinpoint it with real numbers with an accurate temp gun and go from there.

Suggestion about open/unisulated duct in attic or crawlspaces is one to look out for as well. Also if your home is loosing heat you may want to look as to why.

Sometimes local fire departments or energy companies will scan your home to show you with their mega dollar cameras. Our local volunteer fire departmentv has one and I can tell you it has been used to check some homes.

 

[GM_Grimmy]

(A good IR temp gun helps - spray shiney measured surfaces with flat black paint first).

Just wrap black electrical tape around it, and shot the temp of the tape, will do the same thing.

 

[NSMaple1]

I think I tried that, but the tape still had some shineyness to it & temps were still off.

Might have been me....

 

[Jakers]

my advise is only from reading on here and a decent understanding of the way things work in general. i do not own an OWB

one thing i would be looking at is the pump. they have a problem with the fins corroding off on some brands and models. or its possible that it may have been slightly undersized to begin with and now its slowing down some due to being overworked. it seems to me the water is staying in the HX too long if its drawing off that much heat. is the boiler burning hard and often or are your burn times about normal? the reason i think "pump" is because your air in your house is luke warm. others have already mentioned testing for temp drop in the pipes. do that and if no major loss is found then look at the pump

 

[Maddieg7]

Update: still trying to get ahold of a thermocouple or temp gun to test. But I'm just so frustrated right now! My husband turned the fan on our furnace unit down and now air out of vent feels warmer. Return temp went up about 10 degrees but still 35-45 degree difference. Looked out window this morning and boiler was steaming like crazy! Went out to check and temp on gauge said 218 or something......damper was closed. Set for 185, and it is mid-30s today. Why is it SO hot? And return gauge was 210ish. Once furnace kicked on it dropped to like 120. Whhhhhhhy? I just don't understand and our brand new pump was of course whistling like crazy. So now it's probably toast. I turned temp down to 170 on OWB and furnace fan on a continuous run...hoping I can cool it down. Went back out about 10 minutes after changing settings and then it was at 210. So still friggen' hot. Pump was back to making "normal" noises. Is there something I'm missing?

I should also mention that the OWB seems to use a normal amount of wood...my husband fills it once a day and doesn't pack it to the brim. It usually stays within 10 degrees of set temp and acts like it's supposed to.

Could barely altering the inside furnace fan have affected it like this?! Or completely unrelated issues?

 

[Kevin in Ohio]

Go out and make sure, if your model has it, that something isn't blocking the damper door. It could be holding it open or could have something around the door seal. Door seals generally don't go out all at once unless you pull out a section and it jams. Generally it a coal or cresote buildup that holds it open. When I got my CB 5648 after 2 years it progressively kept getting hotter. Ended up the damper door was warping and wouldn't sit flat. They replaced it for free once they had it in their possession. Don't think your 2 problems are related.

 

[NSMaple1]

Agreed - unrelated. The boiler should not overheat no matter what is going on in the house. So multiple things now, it seems.

Couple of further questions - how long have you had/used the boiler, are these new problems (worked OK but then didn't?), and does your pump feeding the HX pump all the time or only when the furnace fan kicks on?

And what exactly dropped to 120?

As Kevin is, I am suspecting damper problems re. the new overheat issue (it doesn't take much of an air leak past a damper or door gasket sometimes) - and on the earlier thing am still suspecting you are not getting enough flow to the house, for one reason or the other. Could even be as simple as an airlock or air bubble somewhere.

And another and - the air might have felt warmer, but it probably wasn't. Hard to tell without actually measuring - but air that is moving faster, feels cooler. Usually.

 

[Maddieg7]

My husband got home and went out to check to see what was going on and temp is fine. Everything was right with the boiler. Temp settled. He checked gasket and blower and everything was good. Thinking maybe a piece of wood or ash fell down when blower was running and wedged it open. Possibly it fell loose when I opened the door. Not sure, but all seems fine now. Will keep an eye on it!

We have a continuous loop, so water flows 24/7 through the system regardless of whether furnace fan is on or off. We notice that when not calling for heat (thermostat controlled fan on furnace) the temps in and out at the stove are within 10 degrees typically usually around 7 or so. But almost instantly when the fan kicks on the gauge out at the OWB drops down. It literally happens within seconds while you are watching it.

On the air lock/air bubble note how much water should you be running through it to check? We ran 6 gallons out of it when we first noticed it felt a little sluggish. There was a little air that came out but this was right after new pump install so we expected to see a little air. And like I said we only are maybe 30ft in and then 30 feet back to OWB. Should we try running even more through it?

And 120 was return temp when stove was up at 210, with furnace fan running.

We just installed a Taco 007 pump. It's what the dealer had (I've heard bad things--maybe it's ok). We did have Grundfos UPS 15-58FC prior to this and it was set to low initially so we tried it on its highest (3) setting and it didn't change our heat output inside the house so then we thought the pump was just bad but we probably didn't even need to change it out to begin with.

This is our first winter with the OWB and it just feels like it's heating less than when we first started out.

 

[avason]

Didn't someone on the site have a similar issue?? What's your indoor boiler set at? Should it be set lower than your owb? Doesn't make sense that your owb boiler was at 218 degrees. Again, I think someone here had a similar issue. I'm sure someone will be here to assist.

 

[Kevin in Ohio]

I used all Taco pumps on mine when I installed my own system 13 years ago. At 10 years the main pump, which runs 24/7 during heating season, cartridge (impeller) went bad. Unless they have been bought out, I don't think you'll have issues with it.

When one of the 2 zones in my radiant floor heat cycles in the garage(36' x 50') it will pull the boiler temp down fast as it's such a constant flow of cold water till the slab warms up. generally it only kicks on once a day in cold temps. That is a huge amount of draw compared to a HX I would think. My HX for the house is water to water plate so I have no experience with water to radiator fan. If it's pulling that much it must be REALLY good at heat transfer.

On a side note you should take solace in knowing that for 13 years mine has taken the drastic temp change with no effect to the boiler as you referred to earlier. I've never heard anyone talk about this as an issue before. When ramping up and cold water is being pumped in there has been time when my stove has been boiling water from hot spots in the stove yet overall temp is below 150. Hot fires with full oxygen create hot pots where the water can't take the heat away fast enough. In time in levels out slowly. That's the whole thing with the system. As long as it is a rise and fall and not an instant thing(Like throwing hot water on a frozen windshield) it should be okay in my mind.

How big is the HX? Any pics you can share?

 

[NSMaple1]

What size is your underground piping?

 

[battlefordguy]

Seems to be a few things at play.
Without sq ft details or more info on overall heat distribution methods, some thoughts are.

Losing 30-40 degrees on a large water to air hx is normal. I typically lose 30 degrees on a hx that puts out 133,00 btu.

If you have more zones than just the furnace hx, consider a larger pump. Something like a grundfos 26-99.
Check your aqua stat functions. If issues check the sensors. since the furnace overheated. The sensor dry wells could have blackened & or sensor wires may have been damaged.

With overall circulation, how big is your primary circulating loop?
Do you have a pump controller or zone valve restricting flow to the furnace hx?


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