L8124A Setpoint / Diff Questions

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Old 10-19-20, 11:01 AM
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Question L8124A Setpoint / Diff Questions

I have one of these (tankless coil) with 2 zones. Suppose for argument sake, one of the thermostats is set to 90 and is always on. Also suppose hi limit is 180, low is 150, diff is 10.

1) The circulator is on and the boiler is running, then someone takes a shower. What temp does the circulator shut off?

2) The shower stops, what temp does the circulator come back on?

3) Leaving the setpoints alone, what does changing only the diff do?

4) If you want the circulators to run more when the boiler is off, what changes do you have to make? Is this even possible without sacrificing too much DHW?

Thanks a lot.
 
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Old 10-19-20, 02:21 PM
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slade

I’m not one of the knowledgeable boiler guys that come here, but I have the tankless coil and 2 zones also and had the 8124A, and I maintain my own system (for whatever that’s worth –lol). But I’m sure the boiler guys will be along.

A few years ago I took one of Honeywell’s diagrams which describes how the 8214A works and I modified it to help me to try and understand it. I couldn’t find my diagram for the life of me for a few years, but recently I found it. I don’t like how I named some things and I should have used colors or something for important names.

Anyway, here it is with the example right at the end. The ADIFF is the adjustable differential and can be set between 10-25. Fixed high limit differential (FHLDIFF) and the fixed low limit differential (FLLDIFF) are fixed at 10 by the control and can’t be changed.

If I remember correctly R-B turns on the burner and R-W turns on the circulator. Maybe this would help a little. I think it’s correct although a little hard to read.



 
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Old 10-19-20, 02:32 PM
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The high limit only comes into effect when the heating is calling for hot water. So if a zone is calling for heating....the boiler will maintain 180°. The burner will shut off at 180° and restart at 170°.

With no call for heat..... the boiler will maintain a water temperature of 150°. It will shut off at 150° and turn back on at 140° with a differential settting of 10°.
 
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Old 10-19-20, 03:40 PM
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Ok, so the answer to my question 1 & 2 is 170?
 
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Old 10-19-20, 04:27 PM
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slade -

I think this is correct, maybe someone else will double check.
1) The circulator is on and the boiler is running, then someone takes a shower. What temp does the circulator shut off?
I think the answer is 140. While taking a shower if the water in the boiler continues to cool, when it drops to 10 below the Low Limit (which you set at 150) R-W breaks and the circulator shuts off (point D on the diagram) but R-B makes and burner comes on( also point D on the diagram).

2) The shower stops, what temp does the circulator come back on?
I think the answer is 150. The burner comes on (I.e. R-B makes, mentioned above)) when the temperature drops to 140, and when the temp climbs up to [Low Limit – 10 + adjustable diff] the circulator comes back on (i.e. R-W makes):

150 – 10 + 10 = 150 = (Low Limit – Fixed low limit diff + adjustable diff) (point C on the diagram)

On the diagram in your case point C would be the same point as the point labeled “Low Limit and Circulator Setting”. It’s a little tricky how the control calculates that point C. It takes the low limit setting and subtracts 10, and then adds back in the adjustable diff setting.
 
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Old 10-19-20, 04:31 PM
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I'm not aware of a fixed differential of 10° on the low side. The low side is settable from 10°-25°.
The high setting has a preset differential of 10°.
 
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Old 10-19-20, 09:00 PM
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I'm not aware of a fixed differential of 10° on the low side. The low side is settable from 10°-25°.
The high setting has a preset differential of 10°.
I believe that's right, but I think the 10° he's referring to is the fixed subtraction part of it.

Would be great if someone could confirm this:

I think this is correct, maybe someone else will double check.
Quote:
1) The circulator is on and the boiler is running, then someone takes a shower. What temp does the circulator shut off?
I think the answer is 140. While taking a shower if the water in the boiler continues to cool, when it drops to 10 below the Low Limit (which you set at 150) R-W breaks and the circulator shuts off (point D on the diagram) but R-B makes and burner comes on( also point D on the diagram).

Quote:
2) The shower stops, what temp does the circulator come back on?
I think the answer is 150. The burner comes on (I.e. R-B makes, mentioned above)) when the temperature drops to 140, and when the temp climbs up to [Low Limit – 10 + adjustable diff] the circulator comes back on (i.e. R-W makes):

150 – 10 + 10 = 150 = (Low Limit – Fixed low limit diff + adjustable diff) (point C on the diagram)

On the diagram in your case point C would be the same point as the point labeled “Low Limit and Circulator Setting”. It’s a little tricky how the control calculates that point C. It takes the low limit setting and subtracts 10, and then adds back in the adjustable diff setting.
 
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Old 10-20-20, 09:22 AM
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hi guys -

Yep poor name.

The 8124A manual does not have a name for the fixed quantity: 10° F (6°C) below the Low Limit set point that is used in the calculation. It just refers to it as “the “10° F (6°C) below the set point”. So I gave it a name for the diagram as “Fixed LL Diff” which is a misleading name – but the diagram is correct and describes how it works if you can tolerate the name.

Maybe it could be called “Fixed Component of Differential” or “Fixed Minimum Differential” or something like that.

I just thought of something, you can verify the operation by observing the boiler gauge. I changed from the 8124A to the L7224U and after I set the limits and diff etc. on the L7224U I watched the boiler gauge and the burner and verified the proper operation of burner ON-OFF with respect to temperature.

If you can also tell when the circulator runs by noise or some other way, you can observe when the burner/circulator turns ON/OFF and match that to the boiler gauge readings and the expected operation according to the 8124A settings.
 

Last edited by zoesdad; 10-20-20 at 09:38 AM.
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Old 10-20-20, 02:13 PM
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S,
To answer your 1st question with your aquastat set at 150 & 180 with a 10 deg. dif and a tankless coil and a tripple action aquastat which is what you have.

To answer your 1st question your circulator would shut off at 140 deg. to make sure your domestic water took preference over your heat.
To answer your 2nd question your pump would turn back on at 150 deg. and start delivering hot water to your baseboards and if it reached 180 before the stat was satisfied it would shut the boiler down but the pump would stay running unti the stat was satisfied. I f the boiler dropped to 170 and the stat still was calling the boiler would come back on to make sure the water stayed hot until the stat was satisfied.

The main thing to remember with a system like yours wil a tankless coil and a tripple action aquastsat is domestic water which is faucet and shower water alwaways takes preference over heat.

If you find your hot water running cool or your boiler taking too long to come on you can raise your LL up to 160 and your boiler would come on sooner. Just remember your dials are adjustable but they must be kept at least 20 deg apart.

Hope this helps a little.
 
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Old 10-21-20, 06:29 AM
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Helps a lot. Tx a lot guys.
 
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Old 10-21-20, 08:32 AM
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i might be beating a dead horse here but just to add my two cents (ps, I believe I got it right here but its been about 7 months since I thought but my boiler).

Low 160, Low dif 10, High 180, High diff 10.

Call for heat, boiler fires up to 180 then shut off. Boiler drops to 170, burner engages and fires to 180 (if still calling for heat).

No call for heat: Boiler maintains 160. If it drops to 150 (160 - 10F differential), boiler fires back to 160. It can be ambient temp loss or hot water use that drop it to 150F, it will always fire back up to 160F.

Call for heat and using hot water: Boiler fires to and then shuts off at 180F. Heat and hot water call cause temperature to drop to 170F (180F - 10F high differential), the boiler fires. If the call for heat/hot water is demanding enough that the boiler temp continues to drop beyond 170F, the circulator will disengage if drops as far as the low temperature (160F). At this point, the boiler can continue to drop a bit but will have a lessened load (since no circ running) and temperature will begin to rise. The circulator will be allowed to reengage when the Low setting is satisfied (ie 160F). The idea here is you set the aquastat to say I NEED 160F FOR HOT WATER. so, if the boiler is at or below160F, then circulator will not be allowed to run. Once temperature is above 160F, circulators can reengage.

where things get a little tricky, but VERY useful, is when you adjust the lower limit differential. A higher lower limit differential will give you more heat for hot water but at the expense of locking out your circulator even longer.

For example, Lo 160F, lo Diff 10F. Boiler targets 160F all the time, once it drops to 150F (160-diff), it turns on and fires back to 160F. Circulator cannot run until 160F is reached. Now, if you increase lo diff to 25F, Your boiler will still target 160F all the time and, once it drops to 150F, it will fire to heat back up. However, here it will fire until the boiler hits 175F (150F (the turn on point) + 25F differential). In this scenario, the circulator will not turn back on until 175F is reached. Circs will not disengage again until 160F is hit, then if 150F is hit, it will refire to 175F, where the crics can then engage.

for my tankless coil, i like a diff of 15F. Just make sure you always keep your Hi temp enough above the low limit plus differential. The safe rule is High limit should always be 20F greater than your low.

man, I hope I wasnt just rambling.

 
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Old 10-21-20, 01:50 PM
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IMHO doesn’t sound like rambling to me. You also tied the circulator into the discussion and thus I think you answered slades fourth question. Seems like a very good description to me.

(btw-
Low 160, Low dif 10, High 180, High diff 10
that’s exactly what my settings are. )
 
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Old 10-22-20, 03:27 PM
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No rambling... Damn good explanation thanks.
 
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Old 10-26-20, 10:06 AM
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So I think I was off on one detail. On my hydrolevel 3200 controller, I just observed that the circulator is cut off at the "fire temperature" not the "low temperature". My low is set to 160F and my low diff is 10F. So my circs disengaged at 150F, when the boiler went to fire (not 160F as a previously described). I may have been wrong with that little detail or it is a difference between my old honeywell stat and this hydrolevel. I am confident all else is correct.
 
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Old 10-26-20, 03:25 PM
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Oh one more thing... If I had a variable high diff... I could increase it to allow circulators to run in a greater range without the boiler firing, right?
 
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Old 10-26-20, 05:03 PM
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S,
To be clear your pump only runs when your t-stat is calling. If your set your boiler temp higher or adjust your diff. if your stat is satisfied your pump will not run. Your boiler may run to a higher temp but with no call for heat your pump will not run.

On the LO side of your aquastat your domestic water takes priority so if you set your LO setting at 160 and your diff at 10 your pump will shut off at 150 so your boiler will keep up with your hot water demand. Once your boiler temp reaches 160 again and your stat is calling for heat your pump will come back on and will run until your stat is satisfied. If your boiler manages to reach its HI LIMIT setting and your stat is still calling your pump will continue to run until your t-stat is satisfied. When the boiler drops to 10 deg below your high limit setting and your stat is still calling the boiler will come back on.

This will continue to cycle until your t-stat is satisfied then the boiler will go to hi limit and everything will shut down until another call for heat or hot water is needed.

Hope this helps a little.
 
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Old 10-26-20, 07:24 PM
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A variable high diff will primarily serve the purpose of preventing short cycling.

Increasing your high dif will not have the impact you are thinking about. Low diff and Low temp decide when your circs turn off and can turn back on. High diff will only change the temperature your burner re-fires at when there is a call for heat and its trying to reach the high temp.

For example; High = 180. High diff = 10. Low = 160. Low Diff = 10F. On call for heat, boiler will always fire to target the high temp, 180F. As heat is expended, boiler temp drops and burner ignites at 170F (High - High def) to return the system back to 180F. If you increase your high diff from 10 to 20; the boiler will still try to target 180F upon call for heat. As heat is released into the house, the boiler temp will drop, however, now, with the 20F diff, the burner will reignite at 160F (not 170F) and try to heat the boiler to 180F. Circs will run through this whole process as long as the temperature stays above [Low - 10F]. If the boiler hits that temperature, then circs will lock out until [Low - 10F + Low Diff] is reach. So you can see, high diff plays no part in controlling the circ.

Where you have to be careful is... DO NOT SET.... [high - high diff] to a number lower than [low - 10F + low diff]. This will result in your boiler frequently hitting a temperature where the circulators will be locked out. In some instances, depending on the settings and your boiler cycle time, the circs can be locked out entirely. That's why its recommended to keep High temp at least 20F greater than Low temp. If you set them too close you run the risk of your boiler very frequently locking out your circ and giving you poor and inefficient heating.

One main reason to increase your High diff is to decrease short cycles. During a short cycle (typically when a boiler is oversized for the load), the burner will fire and reach the target temp of the boiler very quickly (ie the load is not drawing heat out of the boiler very quickly). It happens so quickly that flue gasses don't get hot enough and exhaust will condense back into the boiler and eventually damage the system. When you increase the high diff, you allow the boiler temp to drop lower during a call for heat before trying to re-fire back to the high temp. This means it will take a bit longer to reach the high temp and thus, decrease chances of short cycling. Also, it will cause the system to fire less frequently, which saves a bit because you go through the start up and shutdown purges etc less often. In an ideal world, your boiler will burn through the entire heating cycle. In that way, you have a perfect balance of heat product and release into the house.

The general rule for a triple aquastat is 180F, 10F, 160F, 10F for high, diff, low, diff. However, not all systems are the same. For example, In the summer, 155F low is enough for my hot water. However, in the winter, to get fairly steady hot water I need to do at least 165F. As for the high temp, depending on the weather, 175-185 is sufficient. Now, I do get fluctuations in my hotwater during a long shower or tub fill. There's several reasons for this but #1 is my coil really wants to be 180F. I would rather get some slight dips in hotwater temp than pay to keep my boiler at 180F all winter.

I suggest watching your system and try to get a feel for what your house needs. I have four heating zones, two hydroair and two hotwater baseboards. I elect to keep my system for heat at at least 175F; I found that if all zones happen to run, any colder than that will send too cold of return water back to my boiler, which can lead to thermal shock and condensation at the exchanger. As for picking the Lo and Low diff; thats going to be a bit of trial and error in finding what gives you acceptable temperatures; factors at play include your flow at fixtures, desired temp, run time length, among others..

 
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