Honeywell L8124 Triple Acting Aquastat Question

Rizz861

I’m having some difficulty understanding the differential and low limit settings on this control. Before I get to that, to me this control doesn’t seem correct for a boiler that doesn’t make DHW. Is that correct? 

So my basic understanding of this is the high limit turns the burner circuit off. That’s straight forward. I know the low limit and differential setting are what’s controlling the circulator. So let’s say I go the standard 180 setting for high limit. What would the ideal setting be for the low limit and differential be on a boiler without DHW? What would be the preferred setting be on a boiler with DHW? Lastly, can you explain how the settings make and break the R-B circuit and the R-W circuit? 

leonz

There is nothing wrong with the use of this double aquastat for boiler with a domestic hot water coil, you need to make use of a tempering valve to mix cold water in with the hot water to prevent scalding.

I have the same aquastat on my coal stoker boiler; they are actually a double aquastat.

180 Degrees Fahrenheit high limit and 160 Degrees Fahrenheit low limit would be fine to run the boiler with a 10 degree differential.

You can always lower the high limit and low limit in the warmer months.

I have to run but someone else will tell you how the make and break contacts work for this.

EdTheHeaterMan

The L8124 is designed to offer DHW priority on boilers equipped with a tankless coil.

If you do not have a tankless coil for DHW then you have the wrong control... but wait... you can convert the L8124 to work like the L8148 by removing a wire and moving another.

If you want those instructions just ask.

The low limit has 2 functions. There is a common connected to L1 on the control. The common on the low limit (R) will power the burner by way of a break on temperature rise contacts (B), thru the high limit until it reaches the low limit setting. This low limit setting is to maintain enough temperature in the boiler for DHW operation. When the low limit is satisfied, the burner stops and the power from L1 to the Common (R) of the low limit will switch to the Make on temperature rise Contact (W) to power up the circulator thru the relay contacts.

If there is no call for heat the circulator will not operate. If there is a call for heat A. the burner will operate and if the boiler temperature is high enough for DHW operation, B. the circulator will operate. If the boiler temperature drops below the minimum to maintain DHW, from cold return water, the Low Limit will switch back to the burner only operation break on temperature risecontacts. The circ pump will stop operating until the boiler temperature rises above the low limit setting at which time the circ pump will resume heating the radiators.

I hope this helps your understanding of the L8124 triple Aquastat
@leonz Triple = High limit, Low limit, and relay. three functions. Double aquastat plus a relay

@Rizz861 let me know if you need that wiring adjustment for your control. It may save you $$$ in fuel

Rizz861

EdTheHeaterMan that was very helpful. I’m a newer tech and I saw this on a heat call and thought something was wrong because I had never encountered this Honeywell control and was correctly under the impression it was for a DHW setup on a boiler. There is no tankless coil on the boiler, and they have a separate water heater. This control was pre existing. The call was for something else. I will be going back there at some point to service the boiler/clean it. I’d like to know the work around you speak of. Thanks in advance. 

EdTheHeaterMan

This is the internal wiring of the L81824 A control for oil heat. You can see the logic by following the path of the wiring. The off white block that is the Aquastat part has a high limit marked B and R. These are break on temperature rise contacts.
The Low Limit side has W, B and R. R is common and R to B is break on temp rise while R to W is make on temperature rise.

As long as the return water temperature back to the boiler will maintain at least 130° then you can convert this to a cold start boiler. Be careful that this control was not selected on purpose to reduce or eliminate flue gas condensation on a large water quantity boiler system like an old gravity system.

Assuming that you have a small water content system and the hot water will return quickly to the boiler you can remove the R and the W wires on the low limit side and wire nut them together. This eliminates the low limit from the system and you will have a control that operates exactly like the L8148 control for a cold start boiler.
There is one more option if you want cold start and minimum boiler temperature for circulator operation. I have uses this on old gravity systems to keep the boiler from operating at low temperature for too long in order to reduce flue gas condensation.

Rizz861

EdTheHeaterMan thanks again. Now when you say be careful of a large quantity water heating system, would a steam system that was retrofitted to hydronic fall in that category? To elaborate, the distribution piping is all old steam lines with original steam radiators all tied into a hydronic boiler. Given the much larger capacity of steam lines vs traditional hydronic piping I would think this would be what you are talking about. With my basic knowledge of standard efficiency systems, I know that flue gases condensing on these systems is a no no, so would this control then be correct for this application? In my opinion the circulator is too small too. They have two zones, one with normal baseboard elements for an addition (007 and all dedicated copper pulled from supply and return of boiler) and the other one is two floors powered by a 007 (this circulator is the one for all the steam piping). Just doesn’t seem to be enough. 

EdTheHeaterMan

YES. That old piping and those cast iron radiators hold gallons and gallons of water. That would be a large capacity compared to a house with 2 loops if copper tubing with aluminum fin baseboard radiators. 10 gallons of water content compared to 75 to 100 gallons of water in all those cast iron radiators and big pipes.

As far as theTaco 007 for all that cast iron, That is not unusual. The pump does not need to lift all those gallons of water. As a matter of fact, those old radiators may not even need a circulator. Some of those old radiator system used Gravity to move the water. Hotter water in the boiler was lighter than the colder water in the radiators. Gravity pulled the heavier cold water down to the basement boiler room which forced the lighter Hot water up the the radiators. By adding a Taco 007 to the system, this helped the gravity to move the water a little faster.

The pump just circulates the water, it does not lift it. That is because the weight of the water going up to the radiators is offset by the weight of the water coming down the return pipe. Think of it like a Ferris wheel. If the amusement ride is not busy, then the operator needs to put the riders on the ride evenly. If all the riders were on the first 10 cars and the other 40 cars were empty, the Ferris wheel would be unbalanced and the motor would need to work harder to lift the riders to the top, then the weight of all those riders coming down would make the motor run faster and over speed the ride. That is why the operator needs to balance the ride evenly by skipping some cars.

If there is air in the system, it is like the unbalance Ferris wheel. The pump can't lift the water up, so you get no heat because the system is air locked. Get the air out, and the loop is balanced, the hot water will circulate with a very small pump.

Hope this helps

Rizz861

 EdTheHeaterMan alrighty so the control is what I want for this application and thanks for the info. So it seems that return temperature of under 140F can lead to flue gas condensation which can rot out flue piping and chimney. If I have the high limit set for 180F, it would seem I’d want the low limit to maintain at least 140F if not more to prevent condensation from forming. The directions say a minimum of 20 degrees between high and low, so would having the low at 160F achieve the goal of no condensation? And what would the ideal differential be in that setup? And sorry for asking so many questions, but in the following settings can you explain how burner and circulator kick on and off:
180F High
160F Low
10 Differential

Thanks in advance for all your help!

Tim McElwain

Condensing does not really start until you reach lower than 140 degrees.

EdTheHeaterMan

Rizz861 said:

 EdTheHeaterMan alrighty so the control is what I want for this application and thanks for the info. So it seems that return temperature of under 140F can lead to flue gas condensation which can rot out flue piping and chimney. If I have the high limit set for 180F, it would seem I’d want the low limit to maintain at least 140F if not more to prevent condensation from forming. The directions say a minimum of 20 degrees between high and low, so would having the low at 160F achieve the goal of no condensation? And what would the ideal differential be in that setup? And sorry for asking so many questions, but in the following settings can you explain how burner and circulator kick on and off:
180F High
160F Low
10 Differential

Thanks in advance for all your help!

You can set the low to 140° and 10 °Diff, Remove the Blue wire to make it a cold start. Then the boiler will heat up to 140 before the circulator pump kicks in. when the boiler drops to 130, then the circ will stop and allow the boiler to get up to 140° again before the circ pump stats to put cold water back in the return. This on off of the circulator will continue until the return water starts to come back above 130°

You will have the savings of a cold start and the protection from flue gas condensation.

EdTheHeaterMan

Tim McElwain said:

Condensing does not really start until you reach lower than 140 degrees.

So, @Tim McElwain on this large water volume cast iron boiler with cast iron rads, My experience is that a cold start boiler can take up to 6 to 10 minutes until the return water starts to come back at 130°. Hence the reason for plumbing in new package cast iron boilers with a bypass.. Assuming that our new, eager to learn technician, found a boiler that does not have a bypass, Do you feel cycling the circulator pump off until the boiler reaches 140° is a good way to keep from Flue Gas Condensation?

I always though that 130° was the minimum return temp.. But you learn something new every day

I respect Tim on this topic, He has been a teacher longer that I and has more field experience in Gas heat than I.

vhauk

I rain ran a plant with 2 York Shipley 100hp hot water boilers for almost 30 years. They were equipped with a thermostatic bypass valve, seeing we had 13 acres of floor space to heat. One thing I learned was that flue gas starts to condense at around 140 degrees. Flue gas condensation is VERY corrosive!  Another thing I learned is that many managers are criminally stupid and should make absolutely no decisions about actual boiler operations. 

Tim McElwain

I am a big fan on large boilers of maintaining temperature (140) and if they do not have it then connect up an aquastat to take care of it. I have seen many a rusted up boiler and rotted flues simply because of the long time to reach 140 on systems. It is then long term damage.