My Honeywell L8124C/R8184G4009 Operation Question

JMGotts

My oil-fired system has this configuration (the diagram is modified for my installation.)
I'm having a hard time getting my head around the term "reverse" in aqua stat talk.
Be gentle, I'm old.

EdTheHeaterMan

A reverse aquastat is a thermostat that measures water temperature and the contacts close on temperature rise

A normal aquastat will open on temperature rise.

Thanks for the detailed drawing. What problem can we solve for you?

EdTheHeaterMan

First question: Does your oil burner still make your hot water by the internal tankless coil?

MikeAmann

@JMGotts
Yes, reading that Honeywell description of how a triple aquastat functions can be quite confusing.
Let me add a few things to help you understand. And @EdTheHeaterMan will be along shortly to provide a great description of the operation.
Okay, HI limit and LO limit is what screws people up. People think that these 2 controls create a temperature window for the appliance to operate within. WRONG. Both are actually HI limits.
Let's assume you do make your hot water via tankless coil in the boiler. It's summertime and you are not asking the boiler to heat the house. You would then be operating off of the LO hi limit - 140 in your case. This control has an adjustable differential. If set at MIN, then it's 10 degrees. So the burner would fire and cutoff at 140. When the water temp dropped to 130, the boiler would fire again to maintain your DHW temp. Let's say that you wanted to widen that range to 20 degrees instead of 10. So you set the DIFF to 20 and think that you're going to get 140/120. WRONG. Adding DIFF actually increases the temperature above your limit setting. The formula is set point + DIFF - 10. Therefore 140 + 20 - 10 = 150 and the the water temp range would now be 150/130. Get it? A side note: this is also the reason for the required 20 degree difference between the HI and LO limit settings. Suppose you maxed out the differential to 25 degrees. Then the math is 140+25-10 = 155. That's pretty close to your HI limit setting of 160. The 20 degrees is necessary to prevent the controls from crossing over each other.
Why is this important? Because the LO limit is also the control for your heating circulator. The CIRC will operate only within this range (or above).
Okay, wintertime now and you need to heat the house. The aquastat receives a call-for-heat CFH from your thermostat. The aquastat will now operate off of the HI limit, which has a built-in, non-adjustable 10 degree differential. This is the boiler cutoff temperature and is as hot as the water should get (aside from temperature creep). So you are set at 160. With a CFH, the burner will run until the water gets to 160 and will refire when the temp drops to 150.

EdTheHeaterMan

Exctly right Mike. With one addition that often gets forgotten when adding circulator zones. The low limit has two functions.

#1 As you mentioned, is to have a lower High limit (or operating limit) to maintain enough heat in the boiler to make DHW when there is no call for heat

#2 Is to place the DHW on priority. It does that with a Reverse limit which is a set of contacts that breaks when the temperature falls (better known as Makes On Temperature Rise). this reverse aquastat will stop the circulator from dumping all the hot water in the boiler into all the radiators in the house on a call for heat. Otherwise, If you were in the shower and the thermostat called for heat and all the hot water went into the radiators you would end up getting a cold shower. No One wants to take a cold shower (except your wife may want you to take one some nights, but that is another story). So the circulator pump will stop heating the home before the boiler gets too cold, then the burner can catch up to that operating limit temperature. Once the boiler is hot again the circulator can turn on to heat the home again.

Now this is a lot to follow so I like to break it down into sections.

Section 1 is the high limit. that will keep the burner from over heating the water in the boiler
section 2 is the low limit. This will maintain a minimum water temperature in the boiler Less 10°. So if you set it to 150° then the lowest temperature is 140° or the burner will come on.
Section 3 is the reverse that is connected to the low limit to prevent the circulator pump from stealing all the hot water
Section 4 is the relay that is connected to the thermostat. That will operate the burner and the circulator to heat the home unless something in section 1, 2 or 3 tells it not to.

That is the simple explanation of that control

Still want a to know why @JMGotts is asking the question. If they have a tankless coil boiler, and if the DHW is still being used. There is a lot of stuff that is not needed if that old coil was disconnected 10 years ago. Can we hear from you @JMGotts ? Enquiring minds want to know!!!

EdTheHeaterMan

@MikeAmann and @JMGotts and others. I just noticed this note in the diagram.

So I am wondering if that is the question in code.
If my High Limit is set at 160°, then how is my boiler temperature at 188° with no call for heat? And that is a fair question!

There is a simple explanation.
Assumptions:
1. The boiler temperature and pressure gauge is not “rocket propulsion laboratory” issue.
2. The dial on the Aquastat increments are not measured in microns.
3. And the two places on/in the boiler where the temperature is being measured are not the same place.

When a call for heat is completed the thermostat will open a set of contacts that starts a chain reaction for events. The pump will stop moving water, the zone valve(s) will close. The residual heat in the combustion chamber will radiate into the water in the boiler near the temperature gauge and may go over the limit setting even with the burner OFF.

What if:
The boiler just reached the high limit temperature when the call for heat ended?
The aquastat is out of calibration by 5% to the low side meaning that 160 reading is actually 168° water temperature.
The thermometer reading is actually 5% high meaning the water temperature at 188° on the gauge is actually 178° in the boiler.
The boiler temperature exceeded the limit because the circulation stopped inside the boiler
You were there to look at the gauge at just the right time.

Then I would say that is normal. Because 5% deviation is within the tolerance of the controls and gauges on a boiler sold in America. And you can get a 10° overheating on shut down.


If that sounds like it is wasting, energy then you should think about the EK boilers because they have post purge feature that puts that wasted heat into the last zone that was calling for heat before shutting down the zone valve and pump. Even if it is the water heater zone. When are you going to need a new boiler?

JMGotts

Gentleman, thank you! This is a tankless Weil MClain boiler, and yes, it was the end of a heat cycle. The red lines within the control are 120V in this condition. I tested it that way because I wasn’t sure what the cold startup/idle positions of the relay points were.
The details you provided were exactly what I was looking for. Thanks ever so much!