How does a heating boiler work - what are the steps in its operating sequence?
This article describes how a hot water heating system (hydronic heat) actually works, step by step, to heat a building.
An understanding of the sequence of steps in the operation of a heating system, from the moment that a thermostat calls for heat until the moment that the thermostat stops calling for heat, can help us diagnose and fix many heating system problems.
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Training in proper operation sequence of heating system equipment and in the function of its controls is a step towards technical correctness. If you do not understand how a mechanical system works, you cannot
reliably expect to observe missing or defective components.
This discussion is an exercise using sequence of operation to work for completeness. It is not technically exhaustive, it focuses on a specific example: oil-fired hot water, zoned, heating system.
Examine the accessible parts of the heating system. Let your eye travel from component to component in the sequence of operation.
Apply the inspection logic discussed
at HEATING INSPECTION CONCEPTS
at each step. Consider the implications should any component be missing, damaged, inoperative, leaky, noisy, sooty, repaired by an amateur, etc.
Think through the operating sequence as you examine each component in that order. The following are the steps in one common set-up. This list is lengthy and detailed. The actual visual examination may take only a few minutes.
What follows is a detailed, step by step description of how a heating boiler works. We name each heating system component and what it does, in the order that heating system components operate during the heating cycle.
Items shown in [brackets] are ones which may not be present on some heating systems. We include links to technical articles that explain the operation of various heating system components and parts.
The following steps in a heating boiler operating sequence are discussed as part of a complete heating system inspection procedure for hydronic or hot water heat beginning
at HEATING BOILER INSPECTION GUIDE.
For steam heating systems, details are
Similar information is provided for warm air heating systems
On a typical oil fired heating boiler in the U.S., the wall thermostat is controlling the hot water circulator, turning it on or off. It is the temperature of the water circulating through the system (sensed at the primary control on the heating boiler) that actually turns the oil burner on or off to reheat the water.
That's why the wall thermostat is not an "accelerator" and that's why, if the thermostat has been set to 60 °F., and the room temperature is at 60 °F., and we want to warm up to 68 deg.F., we just need to set the wall thermostat up to 68 deg.F.
Setting the thermostat higher than that will not warm the room faster.
On a typical oil fired heating boiler in Canada, where temperatures are cooler for more of the year, the circulator pump may be wired to run continuously all during the heating season, whenever power is turned on at the boiler. On these systems, the wall thermostat turns the oil burner on or off directly in response to room temperature.
This design tends to produce more even temperatures in the home, and it has an advantage which should be considered by anyone who owns an older home where drafts or poor insulation mean that there is a high risk of freezing heating pipes (freezing can occur in a heating distribution pipe, baseboard, or radiator when heat temperatures are set low and some corner or elbow or location of piping is exposed to very low temperatures).
If heating pipes freeze, the result is loss of heat even if the boiler and circulator try to turn on, which in turn means there is risk of burst piping, water damage, mold contamination, or other costly problems. By forcing the water in the heating system to circulate continuously, the risk of this freeze-up is greatly reduced.
This heating boiler operation article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs.
We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.
The articles at this website also describe the components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs.
We include product safety recall and other heating system hazards.
The limitations of visual inspection of heating systems are described. We continue to add to and update this text as new details are provided.
Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
...
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
On 2022-09-16 by InspectApedia-911 (mod) - Weil Mclain WTGO-4 tankless oil boiler settings
@David,
Remember that a short oil burner firing cycle can be a very inefficient way to use your heating equipment.
That's because on a typical oil fired heating system it can take as much as 5 minutes for it to reach normal operating temperature.
If you're frequently running it in short on-cycles the oil burner efficiency drops and heating cost increases. That's the reason people downfire the nozzle size.
On 2022-09-16 by David
@InspectApedia , thanks again for replying. If the digital thermostat i used is accurate, then that would mean my stack temperature was around 442 F when considering the room temperature at the time of measurement
I could also downsize the nozzle firing rate but i’d rather not have to play with the air band on the Carlin EZ1 burner & just use what the last tech i ever paid for had set.
The unit was installed back in 1999 so i‘m just keeping it clean & running as best i can with the tools & DIY skills i’ve learned.
On 2022-09-15 by InspectApedia (mod)
@David,
Those numbers sound reasonable although the stack temperature might be slightly high. I'm not familiar with the specific brand soot removing flare that you used- I used bushes and a vacuum.
On 2022-09-15 by David
@InspectApedia-911, thanks for your inquiry. I finished running tests & here are my stats. Using the Bacharach Draft-Rite, i measured the draft over the fire at about -0.02 WC & the draft in the breech at about -0.04 WC. I measured the stack temperature at 520 F with room temperature at 78 F since the boiler was running over 10 minutes (2 zones circulating) by the time i took the reading.
Note that the heat exchanger was already cleaned using a soot saber before i replaced the combustion chamber.
Please let me know your thoughts & suggestions as time allows. Thanks muchly for your time.
On 2022-09-14 by InspectApedia-911 (mod)
@David,
What are the draft over the fire and in the breech?
What is the stacked temperature after 5 minutes or longer?
On 2022-09-14 by David
@InspectApedia, thanks for your thoughts. I wanted to give an update for future search users. My DHW (Domestic Hot Water) cycle only (off-season; not winter) seems to have stabilized & it is now taking on average 2 minutes to heat the boiler from 160 (LO) to 180 (HI) (New gauge installed too).
I believe the new combustion chamber has now “cured”. Hope this helps future DIY’ers who are mechanically inclined & well researched & don’t want to spend excessive fees for ‘Pro’ jobs that can be done (at their own risk of course).
On 2022-09-12 by InspectApedia (mod)
@David,
It's possible that you're running the burner too hot which in turn can damage the heater. While I appreciate do it yourselfers and have been one myself, it's the unfortunate truth that it's pretty much impossible to correctly set and tune an oil burner without instruments.
On 2022-09-12 by David
Hi, love this site as a DIY'er. I have a question that i have yet to find asked & answered directly, so here it is along with background info. I have a Weil Mclain WTGO-4 tankless oil boiler in which i recently replaced the combustion chamber lining & nozzle.
Through the peer hole the flame looks good, no flame-end sparks, no smoke & i can see the new chamber glowing hot during & after the cycle. With the new chamber, i noticed that my cycle for the domestic hot water only (DHW; tankless coil in the boiler) is fairly quick now, less than a minute, until it reaches set HI temp & cycles off.
Is this okay & efficient? I am assuming the new chamber is contributing to this quick DHW heating cycle but just want to make sure. Thanks for your feedback!
Question for search engines: How long should a oil boiler's domestic hot water (tankless heater) cycle be?
On 2022-02-16 by Inspectapedia Com Moderator (mod) - hot water heating systems are cold
@Mark,
Typically in Canada, hot water heating systems are set up so that the circulator pump(s) run continuously, 24/7 throughout the heating season. In that set-up, when the thermostat calls for heat, all it does is turn on the burner to reheat the boiler.
The fact that all 3 of your zones are cold argues against a bad zone valve or bad individual circulator pump.
Once your heating service tech has assured herself that there are not obvious embarrassments like a loose electrical connection at the primary control, circulator(s), or oil or gas burner, then she'll probably look for a burner that's turning off on re-set and being re-set by cycling its power.
The tech might also check that the boiler water temperature sensor that communicates to the aquastat is properly mounted and has good thermal contact at the boiler.
A more thorough step by step series of diagnoses is given at
NO HEAT - BOILER
If that's the case your system may simply need cleaning and adjustment of the oil or gas burner.
On 2022-02-16 by Mark
I recently bought a large older home with a hot water base board (slant fin / 3 zone) system. Twice we have a had a problem where we wake up in the morning (both times it was very cold outside) and the house was cold. The gauge showed the temp at 100 degrees, PSI about 7.
Both times I turned the breakers on the unit off (none were blown) and on and the system immediately began heating the water and warming the 3 zones. Do you have any idea what the problem might be and a solution? Thanks very much for any help you can provide. This is in Canada by the way
On 2021-12-02 by Inspectapedia Com Moderator
@April,
No, it's also common for the control to turn off the burner when the thermostat is satisfied.
On 2021-12-02 by April
@Inspectapedia Com Moderator,
That is very good to know, thank you! It never does it in succession, just randomly about 3-4 times scattered throughout day (that I notice). I was under the impression that reaching the HI-LIMIT was the only means of ending the cycle, so the whole thing seems odd to me. Boiler works flawlessly, otherwise.
On 2021-12-02 by Inspectapedia Com Moderator - boiler will shut off mid-cycle when the call for heat ends
@April,
As long as your boiler isn't short-cycling on and off repeatedly during the heating cycle,
then a short on-cycle at the last "ON" cycle before the thermostat is satisfied isn't likely to be a problem. The system is hot and we don't expect to accumulate unburned heating oil in the combustion chamber.
But in other conditions, repeated short ON cycles that don't run the burner long enough to get the combustion chamber up to full operating temperature can lead to trouble. By that I mean that there are many short 5-10 second ON cycles in succession.
If the total "on" time for an oil burner is quite brief in any cycle AND if the combustion chamber isn't quite hot from prior burner operation, there's concern that we leave some un-burned or incompletely-burned oil in the combustion chamber. In extreme cases that can accumulate as soot by causing rough starts at the next burner on cycle. If that happens too much we could have an oil burner puffback.
On 2021-12-01 by April
Hello! During the heating season, my boiler will shut off mid-cycle when the call for heat ends (for example, only running 5-10 seconds). This started when we replaced our aquastat with a different model. Is this going to be problematic in the long run?
Is it a setting that can be adjusted? The previous aquastat would allow it to complete its cycle and reach the HI limit (180), even if there's no longer a call for heat.
On 2021-11-27 by Inspectapedia Com Moderator - circulator stays off unless the red lever is pushed
@TJ,
The aquastat may be wired and set to run the burner and keep the boiler hot for a tankless coil on the boiler.
On 2021-11-27 by TJ
@Inspectapedia Com Moderator, Thank you for replying. I reside in the U.S. (North East to be specific) and I still didn't understand why the thermostat would normally cycle the oil burner on and off, but the circulator stays off unless the red lever is pushed beyond 74 degrees Fahrenheit.
Why would the circulator stay off even though the oil burner seems to cycle during a call for heat? I noticed this has occurred with variations in the ambient room temperature which was why I became concerned since oil was being burned without the then heated water being transferred into the radiators.
On 2021-11-26 by Inspectapedia Com Moderator
@TJ,
That's very strange in my view as well.
If you think about it, a room thermostat is not something that works like an accelerator, producing more heat when set to higher temperatures.
Rather it is simply an on-off switch that calls for heat or turns on when the room temperature is below the set temperature on the thermostat
We don't know the country in which you were living and so your heating system may be wired differently.
Although wiring can be different in farther northern countries like Canada, in the US most hydronic heating systems are wired such fit the thermostat turns on the circulator and the aquastat control on the heating boiler turns the burner on when the temperature in the boiler drops.
So you might need to look at the settings on your aquastat to be sure that they are correct.
In the article index on this page you will find an article explaining how to set the aquastat correctly.
On 2021-11-26 by TJ
Hello. I have a hydronic oil fired system with a mechanical thermostat and a circulator. I would have liked to know why the oil burner cycles on and off, but once the red lever is pushed beyond 74 degrees Fahrenheit, both the burner and circulator start.
Otherwise, the burner cycles on and off according to the ambient temperature, but the circulator NEVER starts to deliver the hot water to the radiators. What would cause this problem? Why is 74 degrees the only point where both the circulator and oil burner run simultaneously?
On 2021-02-18 by danjoefriedman (mod)
Mary
I don't understand the question.
A standard oil-fired heating boiler or furnace uses a barometric damper that operates by air movement
discussed at
DAMPERS & DRAFT REGULATOR TYPES
for a complete list that includes draft regulation on wood, coal, oil, and combination wood-oil fueled boiler and furnaces.
Some, but certainly not al,l gas or oil fired heating equipment makes use of power-vents or draft inducer fans.
for example see
DRAFT REGULATOR, DAMPER, BOOSTER
On 2021-02-18 by Mary K
Is there a fan or motor in the damper that leads to the chimney?
On 2020-12-16 - by (mod) -
Rod
presuming that we're talking about a hot water or hydronic heating system, normally the same physical volume of water stays in the boiler and the loop of piping through radiators or baseboards.
That water is heated and circulates through the radiating devices. You don't normally have to add cold water to the system. If your system is frequently adding cold water then it's probably got a leak somewhere that needs to be found and fixed
On 2020-12-15 by Rod
Hello, thank you for info. Does the incoming cold water from the well supply stay on or is this closed once the system is filled with water? I was not sure if the same water would recirculate. Thanks
On 2020-11-22 - by (mod) -
Not exactly, Barbara.
For example two or more heating zones or "loops" of piping might merge somewhere before the boiler and then send a single line from that point back to the boiler.
On 2020-11-21 by barbara
we have a four zone oil boiler. do the return pipes need to match up.with the feed (hot) zone pipes?
On 2020-10-31 - by (mod) -
Great question, Joseph, thank you.
Causes of hot water coming out of cold water faucet:
I see this phenomenon of hot water coming out of the cold water tap initially quite often when the cold water piping in part of it's routing it was close to a heat source. For example at our lab and office on the second floor of a building the water pipes to a small bathroom originate in the boiler room. With no water has been run for a time sheet from the boiler has warmed both hot and cold water pipes and the water contents. Once we have run the water for a minute or less we have flushed out the hot water and the water is again cold as normal.
On 2020-10-31 by Joseph Phillips
I get extremely hot water out of the cold water tap for several minutes, is this normal ?
On 2019-12-18 - by (mod) -
I would look for an air blockage in the heat distribution lines, and I would look at insulating the distribution pipes where they are not running through areas where you want heat to be released such as in a crawlspace and basement.
On 2019-12-18 by David
Oil fired range running constantly, only a pipe stat fitted to regulate the heat. The radiators get warm 35 to 40 degrees but no hotter "pipe stat set to 60 degrees,any ideas on how to get the radiators hotter? The circulation pump is on the return side, i have noticed on previous systems that location of the pump can make a significant difference to the heat from the radiators, im wondering if moving the pump to the feed side of the system would make a difference
On 2019-01-31 - by (mod) -
See the NO HEAT diagnosis and repair steps starting at NO HEAT - BOILER https://inspectapedia.com/heat/Heating-Boiler-Diagnosis.php
On 2019-01-31 by J
I have oil heat and hot water have oil in tank
Have hot water
No heat
...
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Or see BOILER OPERATING STEPS FAQs - questions and answers about the operating steps in hydronic or hot water heating systems & boilers, posted originally at this page.
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