Aquastat with high and low limit? L8148 with high limit only

HandyFS

Is there a reason to set my low temperature limit lower than 170F? Efficiency and oil usage? Make sense or no?

I currently have a Honeywell L8148 on my Weil Mclain oil fired boiler. All is well and running as expected overall. Right now its running up to about 185F-190F but I'm going to turn that back a bit and see how it does when its max high limit is set to 180F. Almost certain this aquastat is defaulted to turn on at 170F and has no low-limit set option, and then shuts off on the set high limit.

I had 2 people tell me they have a different controller and bring their low limit down to 150F-160F but I see that's not possible with the L8148, however the L8124A may allow both a low limit, and high limit to be set.

This house is newer to me so I don't know its common oil usage, and steady heating abilities. Does it typically make sense to set a lower limit lower than 170F, and would it make sense to upgrade the aquastat to a L8124A, or an electric model? Would oil savings be that much for a 10 degree difference, or will it be lost in the extra heating to 180F for the high limit? Or is a 170F low limit just fine and leave as is?

EDIT: I meant for this to go in the hot water boiler category, not steam.

EBEBRATT-Ed

Where the high limit is set it doesn't really matter. I recommend 190. Remember on a call for heat the thermostat runs the boiler. It will only get to 190 if the boiler has more output than the radiation. Setting it to 190 does not mean the boiler maintains 190degrees. The HL is just that a HIGH LIMIT. it's not an operating control. Turning down the HL will only make the boiler short cycle on a heat call and that is just inefficient.

As far as a low limit goes you only need that if you have a tankless water heater. In that case you set the LL as low as you can and still get enough domestic how water. That should be adjusted in coordination with the mixing valve on the tankless.

If you don't have a tankless you don't needa low limit.

Mustangman

The L8148A is normally used on a cold start boiler. Right now, on a call for heat, the relay will turn on the burner and circulator. It will run up to the limit setting of typically 180. At limit of 180, if the stat is not satisfied, The L8148 will drop the burner out and let the circulator run until it hits the differential setting, which will bring the burner back on. This cycle continues until the thermostat is satisfied.
The Other aquastat you are talking about is more for boilers with a domestic hot water coil. They have an actual low limit setting, typically set at 140. The boiler will maintain that 140 365 days a year. On a call for heat, it will run up to the high limit and shut off until the water temp hits the differential of 10 degrees. The circulator will not come on until the low limit is reached. This gets guys confused as there is still a call for heat, yet the circ is off. You must hit that 140 for the circ to come back on. You can see it on your Temp gauge. On a call for heat, you may have 140 in the boiler but your system water may be 70. When the circulator comes on and brings back that 70 degree water, the temperature in the boiler drops like a stone and will shut the circ off until the boiler water temp gets above 140 or low limit setting.
This type of aquastat would cost you more money to operate than what you have now. Keeping a boiler at 140 everyday of the year is expensive. In this day and age, I am surprised they still make boilers with a domestic coil. Its horribly inefficient way to make HW. The other negative is, the coils build up with minerals which acts as an insulator slowing down the heat transfer and becoming even more inefficient.
My advice, leave well enough alone.
Good luck

HandyFS

EBEBRATT-Ed said:

Where the high limit is set it doesn't really matter. I recommend 190. Remember on a call for heat the thermostat runs the boiler. It will only get to 190 if the boiler has more output than the radiation. Setting it to 190 does not mean the boiler maintains 190degrees. The HL is just that a HIGH LIMIT. it's not an operating control. Turning down the HL will only make the boiler short cycle on a heat call and that is just inefficient.

As far as a low limit goes you only need that if you have a tankless water heater. In that case you set the LL as low as you can and still get enough domestic how water. That should be adjusted in coordination with the mixing valve on the tankless.

If you don't have a tankless you don't needa low limit.

Great info, thanks for sharing this. I always see people say 180F is optimal and where they set systems high-limit so I figured that would be more ideal on the dial. Right now the system seems to cycle between right around 170F and climbs up to about 190F before it shuts down. No short cycling or at least not what I can tell. This old system is new to me keeping it heated at a steady temperature, so I'm learning it now.

I have an indirect water heater, but it has a holding tank and is just a separate circulator zone on the system, so I guess that low level doesn't apply.

HandyFS

Mustangman said:

The L8148A is normally used on a cold start boiler. Right now, on a call for heat, the relay will turn on the burner and circulator. It will run up to the limit setting of typically 180. At limit of 180, if the stat is not satisfied, The L8148 will drop the burner out and let the circulator run until it hits the differential setting, which will bring the burner back on. This cycle continues until the thermostat is satisfied.
The Other aquastat you are talking about is more for boilers with a domestic hot water coil. They have an actual low limit setting, typically set at 140. The boiler will maintain that 140 365 days a year. On a call for heat, it will run up to the high limit and shut off until the water temp hits the differential of 10 degrees. The circulator will not come on until the low limit is reached. This gets guys confused as there is still a call for heat, yet the circ is off. You must hit that 140 for the circ to come back on. You can see it on your Temp gauge. On a call for heat, you may have 140 in the boiler but your system water may be 70. When the circulator comes on and brings back that 70 degree water, the temperature in the boiler drops like a stone and will shut the circ off until the boiler water temp gets above 140 or low limit setting.
This type of aquastat would cost you more money to operate than what you have now. Keeping a boiler at 140 everyday of the year is expensive. In this day and age, I am surprised they still make boilers with a domestic coil. Its horribly inefficient way to make HW. The other negative is, the coils build up with minerals which acts as an insulator slowing down the heat transfer and becoming even more inefficient.
My advice, leave well enough alone.
Good luck

Wow, this helps me understand the different systems so well, thanks for this. This system is a Weil Mclain oil fired boiler, which has 2 zones, 1 for each floor of the home, and a separate zone for the indirect water heater. Definitely a cold start boiler then the way you explain it. Either of the 2 zone radiator thermostats call for heat and trigger the boiler to fire until they are satisfied, or, the indirect water heater.

Looks like the machine will be running all year long for the hot water heater only for the warm months. The indirect water heater is a Weil Mclain Gold 60, which I've heard is a rebadged Triange Tube indirect. This is not considered a domestic hot water coil, right? I'm guessing this is NOT that, and this boiler doesn't have this separate domestic coil. At this time, the machine will fire to heat the water separately as needed, when needed from dead cold, to hot enough to satisfy the water heater thermostat. From the way it sounds, seems like its just a cold start boiler and it drops all the way down until 1 of the 3 zones requires heat.

I may drop that high limit down to 180F and see how it does.

So, you guys make a great point and have educated me, no low limit required. THANK YOU!

Mustangman

No problem. At 65 with 47 years in this field, I love sharing what I have learned. No you do not have a domestic coil, you have an indirect water heater. I am a fan of WM products. You have a good set up. 180 high limit is what is recommended. All or most of the heat emitters are based on 180 degree supply water.
Be well

HandyFS

Mustangman said:

No problem. At 65 with 47 years in this field, I love sharing what I have learned. No you do not have a domestic coil, you have an indirect water heater. I am a fan of WM products. You have a good set up. 180 high limit is what is recommended. All or most of the heat emitters are based on 180 degree supply water.
Be well

Thank you! This setup has taught me so very much. I've lived with forced air nearly my entire life through different homes and rentals. I've always had oil fired burners though, so its nice to see have that old knowledge that translated right over to this oil boiler setup. But the rest of the system was completely new to me. After changing out/replacing nearly all main components on the system, I have a great understanding now, thanks to many of you here who are willing to share your knowledge and time.

On the 180F temp setting, do you find a boiler will typically creep up a few degrees beyond the limit in normal cycles? Guessing on a day that isn't so cold, it may creep up 5F or so, but I will study the system once I adjust to see what it does.

I'm just solving an issue with the indirect water heater that I was almost ready to throw the towel in on, but I'm almost sure I have that figured out, and this system gets to live on and keep on working.

I must say, the heat from old cast iron radiators feels much more comfortable when compared to forced air, and the boiler function so far has been pretty bullet proof up to this point, even though its no spring chicken.

Thanks again.

HVACNUT

How many zones (thermostats) are connected to the boiler?
If there's more than one thermostat, is TT jumped in the L8148A? If it is, then the boiler is constantly maintaining temperature when it shouldn't. 
 
I believe you want to set the limit to the lowest possible temperature needed for a design day. No reason to bang away at 190° when 160° gets the job done. Unless you need it for domestic hot water. 

EBEBRATT-Ed

@Mustangman

Your right about the radiation being rated at 180. That's the issue 190 out of the boiler and a 170 return with the normal 20 deg TD gives you 180 average water temp in the radiation that is why I go that way.

I don't understand why everyone but me prefers a lower limit setting of 170 or 180.


The radiation determines what the boiler needs to run at. The boiler output and the radiation output will balance at some temp whether that is 100 deg or 200 deg depends on how much radiation is installed in relation to the boiler size.

To me the thermostat will run the boiler and the boiler and radiation temp will try to balance. The boiler temp will only get to 190 if the thermostat drives the boiler in that direction.

JMHO

LRCCBJ

HVACNUT said:

I believe you want to set the limit to the lowest possible temperature needed for a design day. No reason to bang away at 190° when 160° gets the job done. Unless you need it for domestic hot water. 

This!

The lower the temperature of the boiler, the lower the loss to the basement. In similar manner to outdoor reset which lowers the boiler temperature depending on building load, manually lowering it will accomplish the same thing...............save fuel.

In the OP's situation, there will be a limit...........probably near 155F...........where the recovery of the indirect will start to become a problem and the boiler will cycle endlessly because the energy transfer to the indirect is severely compromised due to the narrowed DT between the coil and the HW stored in the indirect.

It will take a bit of experimentation to determine if 155F can be achieved. Of course, 155F likely cannot be maintained when there is a very high CH load. The boiler will cycle endlessly trying to maintain temperature in the building and the emitters are insufficient at 155F.

I am also of the opinion that a control such as the L7224 with has the capability of a 20F differential will provide savings to the OP due to, effectively, doubling the time of a cycle (and thereby halving the number of cycles). I have used this control extensively and I am impressed with its accuracy and features. In the case of the OP, the LL would remain off.

EBEBRATT-Ed

I don't agree. Shutting the boiler down with the limit set low when there still is a call for heat is crazy. The baseboard is asking for more heat and so is the thermostat. Sure running the boiler at a lower temp saves money. What are you loosing by constant short cycling and ware and tera on the boiler and it's components and the standby loss going up the flue.

Let the boiler run up to the limit set it higher and and at the end of the run purge that heat up to the house with an aqaustat to keep the circ running

LRCCBJ

EBEBRATT-Ed said:

I don't agree. Shutting the boiler down with the limit set low when there still is a call for heat is crazy. The baseboard is asking for more heat and so is the thermostat. Sure running the boiler at a lower temp saves money. What are you loosing by constant short cycling and ware and tera on the boiler and it's components and the standby loss going up the flue.

Let the boiler run up to the limit set it higher and and at the end of the run purge that heat up to the house with an aqaustat to keep the circ running

It is a certainty that it will save fuel to run at a lower temperature. It is not a certainty that the boiler will necessarily short cycle. Of course, if the boiler is significantly oversized (typical of oil) this will likely occur. The 10F differential on the 8148 certainly causes a much shorter cycle. If you accept the shorter cycles due to the 10F differential, then switching the control to the 7224 and cutting the cycles in half will absolutely give you some margin to reduce HL. It all depends on the specific installation.

EdTheHeaterMan

EBEBRATT-Ed said:

I don't agree. Shutting the boiler down with the limit set low when there still is a call for heat is crazy. The baseboard is asking for more heat and so is the thermostat. Sure running the boiler at a lower temp saves money. What are you loosing by constant short cycling and ware and tera on the boiler and it's components and the standby loss going up the flue.

Let the boiler run up to the limit set it higher and and at the end of the run purge that heat up to the house with an aqaustat to keep the circ running

I believe you may have a different idea about how this high limit thing actually works. Based on your idea of letting the burner continue to heat the water to say 190°F will ultimately cost less with short cycling and the like.
I AM NOT RECOMMENDING THIS. IT IS DANGEROUS TO EVEN THINK ABOUT IT. Then why not set the limit to 230°F. As long as the pressure in the system stays above 12 PSI the water will not boil. Then the thermostat will be satisfied somewhat sooner. Then you can do the system purge after the call for heat from the thermostat is satisfied. But who is adding that control so you do not waste all the high temperature in the boiler. Does this actually make sense Ed? Maybe we can go even further and increase the boiler pressure closer to 20 PSI. and have the boiler water get to 250°. That will make even more heat in those radiators. They will satisfy the thermostat even sooner. Then perhaps add the purge idea to get rid of all that extra heat. I believe the L8148 can go that high. And as long as the pressure does not get to 30 PSI, the relief valve should not discharge.
I AM NOT RECOMMENDING THIS. IT IS DANGEROUS TO EVEN THINK ABOUT IT.

Then we can look at a proven technology known as outdoor reset. That has proven to reduce operating cost by lowering the operating limit to 150° or 140° or even 130°; based on the outdoor temperature. The circulator pump may have much longer cycles (not short cycles), and the burner may have shorter operating times ( that saves $$$). You can even set the burner differential to be larger than 10° between the on and off, so that the average boiler water temperature of say 140° will shut the burner off at 150° and not allow the burner to come back on until 130°.

Also, your scenario of short cycling at say 160° with the limit set there has little merit. Based on the type of radiator system, who is to say that the burner will not short cycle at 190° once that limit setting is reached? It just takes a few more minutes for the water temperature in the boiler to reach that additional 30°. I have personally worked on systems where the burner limit is set at 200° and the limit temperature is reached but the thermostat was not satisfied so the burner "Short Cycled '' for over 45 minutes at 200° to 190° to 200° to 190° and so on... (I would not design a system like that on purpose, but they are out there)

I believe that finding the lowest possible limit setting that will heat the home at design temperature is more efficient. I don't believe that the ultimate short cycles at say 160° will be any different than 190°. It will only take a few more minutes to reach that higher temperature.

Your thoughts Ed?

HVACNUT

@EBEBRATT-Ed, If it was a modulating boiler, then a high setpoint isn't a big deal, but with the L8148A, if the OP's boiler can maintain 68° in the heated space with 160° supply water temperature on the coldest day of the year, with running boiler temperature hovering around 159°, then 160° is where the limit should be set. Because every other day, even 160° is too high.
During the shoulder months, a lower limit setting will of course make limit quicker than if it was set higher, but sending 190° water through emitters that can't disperse it will raise the return temperature quicker than a lower SWT will.
The higher water temperature during the shoulder months will also short cycle the boiler through the thermostat. I'd think you'd need to set the anticipator to a higher CPH, than if using a lower High limit setting because the thermostat will satisfy a lot quicker with a higher output and minimal load.
Again, this is this particular situation. 

EBEBRATT-Ed

Maybe I am stupid or forgot something over the last 50 years.

If you set the boiler at 190, 200, 220 whatever does that mean the boiler will maintain that temperature.?

No, the thermostat tells the boiler to run when the space temp is low. The burner fires. If there is enough radiation (more than or = to the boiler output) most of the time the call for heat will be satisfied before the limit temp is reached.... correct? In that condition the limit control will seldom ever operate...correct.

But that is not always the case. If the home is under radiated or the boiler is oversized in comparison to each other the boiler output in btus and the radiation output in btus will try and find a balance with each other. This could happen at 160, 170, 190 or any other temp in an extreme cases.

So if the boiler is oversized in comparison to the radiation output what is the harm in running the boiler to 190 and then purging the heat upstairs?

Sounds better to me that starting and stopping the burner at 160. The less start and stops the burner sees the better. Short run times don't give better combustion and smoke and soot come from starts and stops more than from a running burner (that is properly tuned)

If you set the limit high enough with the correct radiation and boiler output the boiler water should never reach limit anyhow.

LRCCBJ

EBEBRATT-Ed said:

Maybe I am stupid or forgot something over the last 50 years.

If you set the boiler at 190, 200, 220 whatever does that mean the boiler will maintain that temperature.?

No, the thermostat tells the boiler to run when the space temp is low. The burner fires. If there is enough radiation (more than or = to the boiler output) most of the time the call for heat will be satisfied before the limit temp is reached.... correct? In that condition the limit control will seldom ever operate...correct.

This condition is ideal. If it occurs in a building, we need not discuss this further.

However, I have never seen it in 35 years in residential OIL. The boiler is a minimum of 85K input and the radiation PER ZONE is typically a maximum of 30K. One typically does not get the benefit of multiple zones calling unless the building is close to design.

What typically occurs is the boiler coming up to HL and cycling on the differential. If you set a 200 HL, you are correct in that the boiler will have less cycling due to the zone reaching the 'stat setting faster. If you have a proper purge so the boiler can return back to 130F, this setup MAY be reasonably efficient. I have never experimented with it.

With most oil equipment WITHOUT purge, it is grossly inefficient to run the boiler up to 200F and then have the 'stat shut it down on limit. Just set the boiler at 160F and it might cycle a few more times before the 'stat satisfies.

Where this approach fails miserably is when setback is utilized. If the HL is set lower, the boiler will definitely cycle for a much longer period (possibly several hours) due to the inability of the emitters to deliver anything close to the boiler output.

EBEBRATT-Ed said:

But that is not always the case. If the home is under radiated or the boiler is oversized in comparison to each other the boiler output in btus and the radiation output in btus will try and find a balance with each other. This could happen at 160, 170, 190 or any other temp in an extreme cases.

The boiler is typically oversized with a fixed output. The radiation has a variable output depending on SWT but it is rarely equal or greater than the size of the boiler (certain CI systems excepted).

There is no balance that can be achieved in this case. Whatever SWT is chosen, the boiler must cycle to maintain it, unless, as you noted, the 'stat satisfies during the climb of the boiler to the HL setting.

EBEBRATT-Ed said:

So if the boiler is oversized in comparison to the radiation output what is the harm in running the boiler to 190 and then purging the heat upstairs?

Generally, the higher the temperature of the boiler, the greater the overall system efficiency losses. However, you make an interesting point. IF you can go to 190F and satisfy the 'stat quickly and return the boiler to 130F in a relatively short period of time, the efficiency losses due to the 190F setting may be largely offset by the short time the boiler remains there.

I do believe that even with this approach, you can gain efficiency when the heatloss is less than half of design by reducing the HL. Why use 190F in April when you can get, effectively, the same result if you use 160F? One additional issue is comfort...........the overshoot on the radiation (especially CI) will be substantial in April if you use the 190F approach.

There are a lot of variables in the decision to reduce HL and these variables change with the season and the choice of setback. You most certainly can cause a loss of efficiency if the HL is set to 175F with a 20F differential, at a 20F overnight ambient, AND the tenant utilizes an aggressive setback because nobody occupies the building at night! The boiler will cycle for six hours in the morning before the 'stat will satisfy!! The boiler is losing energy constantly to the basement for six hours and has 48 stops and starts during that period!

This is one topic where YMMV! No specific approach will ever work for every building.

EdTheHeaterMan

All I'm saying is that for this particular discussion I might select a 160° limit setting for the lower cost of operation. If possible, set the burner limit differential as high as possible (more than 10° if possible). Now if the desired indoor temperature can not be achieved during extreme cold outdoor temperatures, would I increase the limit temperature to allow the boiler to reach 170° or 180°.

I believe this is actually more efficient than @EBEBRATT-Ed's approach. That is why I suggested the ultra high temperature limit settings to illustrate how higher temperature limit settings are ridiculous. If Higher is better then why do we not use 300° temperature water. I mean if Higher is better, then that would also be true for steam systems. Where we might use higher pressure steam to get the radiators to be much hotter. Not only is it dangerous, it also costs more to make that high temperature or high pressure. Hotter is not better.

“Hotter is better.” If that were true then heat pumps would not ever be efficient. A compressor can never make the temperatures that a fire can make. No heat pump would have ever been invented if “Hotter is better”

EBEBRATT-Ed

@EdTheHeaterMan I have Taco design book "guide for Hydronic Engineers" that we used in school in the early 70s. They had no problem recommending temps of 200. 210.220 at that time to reduce the amount of radiation needed.

Times have changed and we know operating at lower temp saves fuel.

Look, this ends this discussion as far as I am concerned.

Setting the HL at 190 does not mean the boiler maintains that temp. The thermostat drives the burner. If the house is underrated it may reach 190 occasionally.

Short cycling is inefficient.

To me short cycling causes more burner starts and stops and with oil that is wear and tear smoke and soot and shorter burn times which all cause Poore combustion. I know, with today's burners not so much of an issue.

I cant see shutting the burner off at 160ish when the stat is still calling. The only time it will reach 190 is if it needs to to heat the house (like after a set back or under radiated.)

HVACNUT

EBEBRATT-Ed said:

@EdTheHeaterMan I have Taco design book "guide for Hydronic Engineers" that we used in school in the early 70s. They had no problem recommending temps of 200. 210.220 at that time to reduce the amount of radiation needed. 

Weird how it isn't the early 70's anymore. 

A correctly sized Hydronic boiler with a high limit only aquastat will make limit almost every time there's a heat demand, except on a design day. Unless it's a gravity conversion or something with huge water content. 

When I do maintenance on a boiler, the first thing I do is run the burner and put it into safety (pull cad cell eye, thermocouple, flame sensor), and run all zones to drop the water temperature as low as I can get it. This is so when I do the combustion test, the boiler is cold enough that with all zones running, I'll be able to run the boiler long enough to reach steady state. In the shoulder months it is extremely hard to keep the boiler from making limit. Even if I raise it to 200°. When there's no load, the BTU's come back to the boiler. Unused BTU's that make their way back to the boiler to be reheated again, because a 9° Delta T is fine.
In a mod con, there is a high limit, but the zone stations use temperature setpoint, not limit. And those setpoints are variable based on supply temperature, return temperature, and outdoor temperature. 
I'm looking at the weather where my boy is at RIT (GO TIGERS!), and it's 33° today but will be 58° on Monday. Does his apartment need 190° heating water on both days? Nope. It doesn't need 190° until it hits -7°.

LRCCBJ

HVACNUT said:

In the shoulder months it is extremely hard to keep the boiler from making limit. Even if I raise it to 200°. When there's no load, the BTU's come back to the boiler..

This needs to be clarified a bit.

If you set the 'stat to 85F, the radiation will have a reasonably constant output, whether the outdoor is 0F or if it is 45F. The radiation does not know and the boiler does not know the ambient temperature. You will see a slight decline in the capability of the radiation, at a constant SWT, as the interior warms. You can't deliver the same 40K at an indoor of 80F as you can with an indoor at 70F. The heat transfer declines slightly.

However, if your test is limited to one hour, it is doubtful you can climb more than 5F in the conditioned space, even in the shoulder season, making the difference in load between design and shoulder a bit moot.

EdTheHeaterMan

I was never taught that 190° is the magic number.
I never assumed that 190° is thE magic number.
I rarely set a high limit at 190°
190° seems so arbitrary.

Since I am now retired, I doubt that i will aver have the occasion to see a boiler in my future, but If I do, I well be sure to set it to 190° so I can tell that boiler owner that this it the best setting according to @EBEBRATT-Ed. Not because it will cost less, not because is will stop short cycling, not for any other reason that I can think of, except..."ED Says So!"

And that will end all discussion because Ed says

Look, this ends this discussion as far as I am concerned.

So glad that we can put this to bed!