Boiler temp not consistant

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Carl Webber

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Sep 8, 2014
122
New Ipswich, NH
I just finished getting my Tarm OT-50 installed. I have noticed that the temp gauge mounted on the front of the boiler and the oil burner aquastat both mounted on the front don't quite match as far as temperature reading. And then both of those are off compared to the overheat aquastat that is mounted in the back of the boiler. Everything is installed according to the boiler manual as far as i can tell. What i've noticed is that the oil aquastat is set at 160 and 180 with the differential set at 20. According to the manual this would kick the burner on when the water temp hits 140 so the boiler doesn't get low enough to create condensation. According to the temp gauge the burner comes on somewhere between 150 and 155. The aquastat and the temp gauge are mounted about a foot apart on the front of the boiler so it seems odd to me that they would be that much different. The other difference i'm noticing that is much bigger is that the overheat aquastat is reading a temp much higher than both of those. The sensing bulb for the overheat aquastat is in a well in the center of the top of the boiler in the back. I have it set to kick on and dump heat when the boiler reaches 200 and the differential is set to 5. According to the gauge on the front of the boiler the overheat aquastat is kicking on and dumping heat at 185. It seems to me like the water in the boiler has hot and cold pockets.

Has anyone run into this before? Does anyone have a solution to this problem? I've tried to turn the circulators off so that colder water is not being introduced into the boiler to see if that made a difference. I didn't see any change. Beyond that I've double checked to see that everything is installed properly so it is reading accurately. I'm out of ideas. I was considering just turning up the temp on the overheat control, but i'm not sure if that is safe or not because as far as i can tell it is reading correctly for that part of the boiler. The last thing i want to do is blow up my boiler.
 
Verify all temps with an IR gun.
 
I have an IR temp gauge and it doesn't seem to be accurate on some surfaces. If i aim it at the bottom door where the ashes and coal bed are, it will tell me the door is 350. That sounds believable to me because this is the same door with the draft. If i aim it at one of the flanges on the back of the boiler it only says 150. That doesn't seem right to me. But then i was thinking maybe it is only reading the surface temp and not the inside temp. I also notice the same issue if i aim it at the copper supply or return line. On those lines it will tell me they are 99. I know that is obviously wrong because the water inside is somewhere around 200 and the lines are too hot to touch. I've also noticed the same issue with the stove pipe. If i aim it at the outside of the pipe it will tell me 86 and i know that is not right, but if i hold open the damper and aim it at the inside wall of the pipe it will tell me 450. It appears to me like an IR thermometer wont' work for this application.
 
What aquastat is on it?

Do you have the manual for the aquastat itself?

Some stats diff settings work a little oddly when the diff is set to more than 10. I'm looking at the manual for a 6081a now, and with a 140/25 setting, it works at 155 & 130.
 
My IR thermometer doesn't work if aimed at shiny things like silver colored stove pipe or copper pipe.
 
velvetfoot said:
My IR thermometer doesn't work if aimed at shiny things like silver colored stove pipe or copper pipe.
Click to expand...
I've found that copper pipe wrapped with black electrical tape gives a much better reading for an IR temp gun
 
I am not so worried about the gauge and the aquastat on the front of the boiler. if they differ a little its not too big of a deal. I'm more worried about the overheat control that is mounted in the back of the top of the boiler. Because it is reading that the boiler is at 200 when the front of the boiler is only at 185 greatly reduces the window that i can burn a fire in. It makes it harder for me to get a good fire going without the overheat kicking in. what I've been doing is let the boiler cool to just about 155 and then restart the fire as fast as i can before the boiler hits 155 and kicks on the burner. Then in about 20 minutes it has warmed up the 30 degrees and it starts to slow down just about the time the overheat kicks in. Then the overheat kicks in for about 5 or 10 minutes to bring it from roughly 190 back down to 180 when it shuts off. If everything was working properly the fire would start slowing down and turning to coals and i wouldn't even be at 200 yet and everything would be good.
 
It sounds like you are getting tuned in with your setup already.

Where are all of the sensors located? Temperatures throughout a wood fired boiler vary quite a bit until later in the burn, then things become more equalized. Knowing your boiler and the location of sensors will give you a better understanding of what is going on. Circulator running will change things, along with the return water temps.

At the top of this boiler, on the 2" boiler out pipe, I have 3 sensors. The readouts of the 3 will vary as much as 7 degrees. One is slower reacting but all 3 get close nearing the end. They are touching each other so one would think they would read the same, but not. The stock location of the controller sensor was crap in my opinion.
 
As long as your pressures are good, going over 190 a bit should be OK.

Do you hear any odd little noises inside when it gets to 190? Both my old one and this one will start making little noises when it sees 190+ at the top. Might be what is called 'kettling', but simply sounds like I would expect the very first stages of boiling to sound like. By 200 it's pretty hard to miss. I also think temps at the top would be a bit hotter if the well is situated close to a hx tube inside, and that that would likely be the hottest spot. If you don't hear any noises you might try going from 190 to 195.
 
My pressure is only at about 18-20psi when the overheat control kicks in so i'm sure i can go higher. As far as the noises you are talking about, i am familiar. One of the first times i ran the boiler i got the temp on the gauge to read just over 200 and i could hear the noises you are talking about. In my opinion the sensor for the over heat temp is in a spot where there is going to always be a pocket of hotter water. In the center of the top of the boiler in the back is directly over the flu connection. right where that sensor is there is probably only 6 inches of water and that will heat up much faster then a large mass of water. The two temp sensors on the front, the burner aquastat and tridicator are both located in the large mass of water where the DHW coil would be and its about 2 1/2 feet deep and about 30 or 40 gallons of water.

I was considering going up on the overheat control, but i wanted to get some opinions first. I especially wanted to see if there was someone on here with much more experience and knowledge than me that would agree with me before i went ahead and tried it. I think i'm going to go up on the overheat control from 200 to 210. In theory this would mean that the over heat control would kick in when the tridicator reads 195 instead of 185. I'll have to try it tomorrow during the day when i can monitor it to make sure i don't hear any funny noises or anything crazy going on so it doesn't get out of control.


While we are on the subject of boiler temps, is there a way to place one sensor and run all the controls for the boiler based on the reading of that one sensor so that everything is the same and there is no guess work involved. I was thinking along the lines of something with a digital readout that could replace my tidicator and also be used to run all of my temp related controls (burner firing, and over heat control).
 
Personally, 18-20 psi is a bit high for my comfort. I like seeing mine limited to 15. Others may differ there. But I think the blow off on my boiler would start letting some PSIs out at that level.

I am also quite sure there are pockets or areas inside most boilers that don't see all the circ flow through the boiler. Eddies in circ flow in places, I guess. My dump stat is in the top of my boiler, towards the back, left side. I think I have it set to 190. If it kicks in, the tridicator guage in front of the boiler, top right, is usually a few degrees cooler. So I am quite sure there will be some slight temp inconsistencies inside most boilers.

If you are going to try changing things a little - I would pay close attention to PSI, and the noises it is making. What is your pressure when boiler temps drop to where the house would start losing temp (140?) ?
 
my pressures ranges from about 12 psi to 20 psi. I think this may have to do with how i pressurized my boiler. The first time i filled my boiler the water was between 55 and 60. when i heated the boiler the pressure was at about 28 psi when i was around 200 and the blow off valve started leaking around 210. But, i had a problem with the gasket that i replaced for the domestic hot water coil cover. I replaced it and i misunderstood the guy at Tarm and I over tightened it and put a hole in it. So, the first time i heated the boiler i had a blowout. So, i went and got a new gasket. I cooled the boiler down to a level where i felt comfortable working with it and opening it up. I think somewhere about 120 or 130. I released the pressure and i replaced the gasket. Then I turned the water supply back on. So somewhere around 130 it was set at 12psi. When I heated the boiler back up the pressure only went to about 18psi by the time i was at 190. If i shut everything off and completely cooled my boiler down to room temperature i think i would see the higher pressures again because it would reset things to 12psi at 70F. I think my expansion tank is just barely big enough to handle my system. I think at some point i'm going to get a new expansion tank but for right now it seems to work. When the system was set up i told the plumber that i had a budget and asked him if he thought he could do it on my budget and he said he could so he went with the bare minimum that was needed for the system to operate. From what i've been reading, if i get a big enough tank, the pressure won't go above 15psi no matter what unless i see temperatures round 250, and that should never happen.
 
Carl Webber said:
While we are on the subject of boiler temps, is there a way to place one sensor and run all the controls for the boiler based on the reading of that one sensor so that everything is the same and there is no guess work involved.
Click to expand...
For such a simple, common sense idea you would think someone would already produce a controller like this. Never having worked with the Lambda controlled boiler I don't know but maybe that controller does everything off one sensor.
 
Carl Webber said:
my pressures ranges from about 12 psi to 20 psi. I think this may have to do with how i pressurized my boiler. The first time i filled my boiler the water was between 55 and 60. when i heated the boiler the pressure was at about 28 psi when i was around 200 and the blow off valve started leaking around 210. But, i had a problem with the gasket that i replaced for the domestic hot water coil cover. I replaced it and i misunderstood the guy at Tarm and I over tightened it and put a hole in it. So, the first time i heated the boiler i had a blowout. So, i went and got a new gasket. I cooled the boiler down to a level where i felt comfortable working with it and opening it up. I think somewhere about 120 or 130. I released the pressure and i replaced the gasket. Then I turned the water supply back on. So somewhere around 130 it was set at 12psi. When I heated the boiler back up the pressure only went to about 18psi by the time i was at 190. If i shut everything off and completely cooled my boiler down to room temperature i think i would see the higher pressures again because it would reset things to 12psi at 70F. I think my expansion tank is just barely big enough to handle my system. I think at some point i'm going to get a new expansion tank but for right now it seems to work. When the system was set up i told the plumber that i had a budget and asked him if he thought he could do it on my budget and he said he could so he went with the bare minimum that was needed for the system to operate. From what i've been reading, if i get a big enough tank, the pressure won't go above 15psi no matter what unless i see temperatures round 250, and that should never happen.
Click to expand...

How high is the highest part of your system with respect to your system circulator(s)?

Do you have a fresh feed setup constantly attached/valved in? Not sure what you mean by 'reset to 12psi' - but is that what you mean? The fresh feed won't let things go below 12?

I have about 20'. And my system won't circulate below 120 or so. So I aim for 10psi in the 100-120 area. I don't see anything more than 15 when fully up to temp. You maybe could stand more expansion - but you could also lower your cold pressures some too (i.e. overall system pressure) I think. Depending on how much elevation you have in the system.
 
I'm going to guess and say that the highest point in the system is about 15 feet above the boiler. that probably makes it about 12 or 13 feet above the circulators.

When i said reset to 12psi, i meant that i have fresh feed setup that maintains the boiler at 12psi minimum. So, when i depressurized the system and then pressurized it again when it was around 130, the cold feed set the pressure to 12psi at 130. If i let the boiler temp go doing to 60, the cold feed would still maintain the boiler at 12psi so when it heated up to 200 it would require that much more expansion because the pressure was higher at a lower temp.

If i lower minimum system temp do i risk the water in the boiler turning to steam because it is pressure that keeps the water from boiling?
 
No on the steam thing.

15 feet translates to 6psi, so 10 would give you a good 4psi 'cushion' for proper circ operation. I would aim for 10psi at 120 or so. Do you have a bladder expansion tank? To get the 10, you would have to shut your fresh feed off. Then drain water out until you get to 10. Then check your air psi on the air side of the expansion tank. You could set that to 10 too, but it would all depend on the temps when you're doing this. If cold maybe set both to 8, if 120 maybe set both to 10. That is all assuming you have a bladder tank. If you have a non-bladder cushion tank, you'll likely have to drain water out of it too, in the process.

I leave my fresh-fill line valved off all the time BTW. It's safer to do that when you have storage - but I kept it valved off all the time when I had my old boiler too. But if you're tight on expansion room, maybe you should leave it on all the time just in case you have a blow off - but it will be fighting with your expansion when your pressures get down to the 12 mark. But is it adjustable? Mine is, I think.
 
I could check and see if mine is adjustable. I'm not sure how I feel about shutting off the supply. I was thinking about things like what could possibly happen if the boiler blew a gasket and the supply is on. But on the other hand, i would hate to overheat the boiler and have it blow off some water and then not refill and run when low on water. It kind of a double edged sword. It goes both ways.

I have a bladder type expansion tank. Its an Extrol EX-60.
 
I don't see why you couldn't use one temperature sensor for several controllers. I have several Ranco controllers and so far, so good, but it's only been for a few months. One I have is a two stage model and the other two are single stage.
 
velvetfoot said:
I don't see why you couldn't use one temperature sensor for several controllers. I have several Ranco controllers and so far, so good, but it's only been for a few months. One I have is a two stage model and the other two are single stage.
Click to expand...
Are all 3 Rancos sharing one sensor?
 
BoiledOver said:
Are all 3 Rancos sharing one sensor?
Click to expand...
No. The two stage shares one sensor.
I didn't actually do that, and I just did some internet searching and I found only one reference to connecting more than one controller to one sensor and he failed. So, doesn't look good.
 
I like redundancy - so I like more than one sensor. Even if they might not read quite the same.

Also, things can get controlled better if a flue gas sensor/stat is used for starts & stops with burn starts & stops. So don't think I would strive for a singel sensor solution.
 
I just tried this on an Azel two sensor thermometer that I'm not using. It read one degree F higher with both displays connected to the same sensor. The polarity had to be right though.
 
I think I'm going to check this out. There has to be some sort of plug and play setup you can get that has a temp sensor connected that you can program to do what when and have one control run everything. Its got to be out there somewhere. If i can't find one I think I might try to build one. I think I could build one with a project box and a PLC and some relays. It wouldn't be too hard. Its just simple programming. If this thermostat calls then turn on this circulator. If the temp is below 140 don't let the circulators turn on. If the temp gets to this set point turn on the burner. If the temp gets to this set point shut off the burner. You could probably even set it up so that you could have multiple overheat zones. Dump in this zone first then if that takes more than 15 minutes to bring the boiler under control turn on a 2nd zone. Or by temp, If the boiler reaches 195 dump into this zone, If it reaches 200 dump into this zone too. You could even expand on that and have it monitor stack temps too. If you had it monitor stack temps you could even set it up to graph the data so you could see how you burn historically and how your boiler temp is historically. You would also be able to see how your burn cycle looks and your peak temps and how long it stayed at the peak temps. Its so simple. There has to be something like this already out there. If there isn't, maybe I need to start building and selling some controls.
 
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