Photograph of a multi function combination control on a heating boiler Heating System Boiler Control & Switch FAQs

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Heating system control FAQs:

Questions and answers about any and all controls used on heating boilers used for providing heat or hot water in buildings.

This article series describes the operating and safety controls on a Heating System Boiler - central heating systems. The photo at page top shows a modern cad cell relay on an oil fired heating boiler - one of the safety controls which we discuss in this article.

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Heating Boiler Control FAQs

Photograph of an oil fired heater stack relayRecent questions & answers about heating boiler controls

These FAQs and this article series describes the controls used on hot water type heating systems, also known as hydronic heating systems.

Hot water heat is provided to a building by a heating boiler that heats hot water that in turn circulates through heat radiating devices in the building.

On 2017-01-21 18:15:26.084519 by (mod) - Troubleshooting a Honeywell R4184 Cad Cell Relay that Keeps Tripping


When a thermostatically-operated heating system relay clicks to start the oil burner but the burner doesn't start that could be ok (not the case you describe) or bad.

First the OK case: the relay clicks to call for heat a circulator pump turns on - the boiler's burner won't turn on unless or until the water temperature in the boiler is at or below that set on the boiler's aquastat. Eventually cooler water returning from the building cools the boiler and the burner starts.

Second the bad case: the relay clicks and the burner should be starting but it doesn't and after some seconds the safety lockout trips turning off the burner rather than pouring unburned oil into a combustion chamber of an oil fired burner.

Pressing the reset limps past whatever operating problem (probably a dirty nozzle or bad flame, maybe a bad cad cell sensor or even a loose wire) may be and the burner runs normally for a time - that's good.

Watch out: don't keep relying onpressing that reset button on the primary control. See RESET SWITCH, CAD CELL RELAY - repeated pressing of the rest button risks a PUFFBACKS, OIL BURNER of unburned oil - use the link or search Inspectapedia for the capitalized phrase to read details.

If cleaning or replacing the cad cell, checking fixing wiring connections, or cleaning/repairing the oil burner assembly (nozzle, oil filter, etc) doesn't fix the problem AND IF the service tech decides it's the control itself that's defective, then one ither buys another Honeywell R4184D (some are still sold) or a replacement unit like the Honeywell R8184M1051 Protectorelay Oil-Burner Control.

The cleaning and replacement steps you took sound right but unless you're trained and have the right test equipment I wouldn't be certain that there's not a burner operating problem still going on.

Just so you don't think I'm being too tough on you (watch out a mistake can cause fire, explosion, death) - I confess that early in my oil burner career I over-tightened the nozzle adapter on the burner gun oil line, stripping its threads. I couldn't figure out why the flame was so messy and smoky until I found that leak as it was hard to see when the burner was running.

I had to replace the nozzle line and adapter along with installing a new nozzle.

Please see CAD CELL RELAY SWITCH for details about the Honeywell R4194 Primary Control cad cell relay.

On 2017-01-21 18:10:44.584979 by Ted

any one have this problem or have any suggestions? what is necessary to change to an 8184 cad relay since the voltage is 24 instead of 120 on the 4184.

On 2017-01-21 17:25:35.705223 by Ted

sometimes when thermostat calls for heat I will hear a click in the cad cell relay and a few seconds later the reset button will pop up.

No other sounds the burner does not even attempt to start, the blower doesn't come on.

If I press the reset after 30 seconds or so the burner will fire normally and may work fine for hours or even a day or 2.

After this problem started a week ago I cleaned boiler and replaced oil filter and nozzle but this had no effect. the control is a 4184D which Honywell doesn't make any more.

On 2016-11-12 13:48:42.326660 by Anonymous

My new gas furnace keeps boiler temp, how go I set it or can I set it to just fire up by thermostat temperature, not constantly run to keep boiler temp?

On 2016-11-04 10:36:47.363837 by Rich Conkling

I live in an apartment that has water heating with copper pipe and fins. The problem is even though I have a new T-stat (digital heat, heat pump, heat and AC) with it set at 40 degrees it still is heating at 85 to 90 degrees.

The Maintenance person says it just has to be that way but it makes no sense. Why have a T-stat if it's just going to heat anyway. I'm running my AC a lot. It takes 3 hours to drop 1 degree. Sleeping is next to impossible.

Any suggestions?

On 2016-04-02 00:36:25.278232 by (mod)

Is this a tankless coil on a heating boiler? Someone needs to check out the aquastat, including its temperature sensor and the quality of contact of the sensor to the sides of the sensor well.

On 2016-04-01 23:39:26.633491 by Anonymous

what would cause hot water to come and go, have had service 4 times now and no one can find problem.

On 2016-03-17 23:17:12.826427 by (mod)

I'd look for a problem outside the thermostat itself such as its wiring or the low voltage transformer.

On 2016-03-17 16:07:42.274289 by Dan

Thermostat only works about one month then have to replace

On 2016-02-29 13:56:11.040578 by (mod)

I would consider checking or replacing the boiler gauge or making an independent test to confirm what you think is wrong with the temp.

It may also be that in your home a shot of cold water from radiators or baseboards is dropping the boiler temp quickly so that a normal boiler-on is occurring.

OR the boiler controls may be improperly wired. In some countries such as the U.S. installers wire the boiler so that on a call for heat the circulator runs first and the boiler only runs when temp drops. In Canada, for example, installers wire the boiler with circulator always-on.

On a call for heat the boiler will run because by definition, as boiler water was already running through the piping system, the boiler temp is too low and more heat is needed.

On 2016-02-27 19:47:33.619672 by keith

my furnace will light every time the thermostat calls for heat even though the boiler temp is at 160/180. Why? what would cause this and how do I fix it? I'm going through tons of oil

On 2016-02-13 17:02:21.247530 by (mod)

Thanks xyz

On 2016-02-13 10:43:31.991517 by xyz

Jer: The article is correct. It is saying that (1) the system is turned on and (2) the room is warmer than the setting on the thermostat. In the event that the room temperature is above that of the thermostat, the thermostat will command the zone valve control to remain shut so no more heat is added to the room. Only when the room temperature drops below that of the setpoint will the system run.

On 2015-10-28 04:44:22.006388 by jer

"If electrical power switch to a hot water heating boiler is in the on positionand the room thermostat is set above room temperature, the heating system (hot water or steam) boiler or (warm air) furnace should not run."

is that right? shouldn't the boiler run in this case?

Question: where to find info about the fan limit switch

I am in HVAC school trying to find info on the fan limit switch and limit switches - Paul Gibson, 5/24/11


Paul you are looking at boiler controls on this page.
Fan limit switches are used on warm air heating furnaces. Take a look at the large amount we have published about the FAN LIMIT SWITCH and how it works, is wired, diagnosed & repaired.

Also see other furnace control links listed at the "More Reading" links at the bottom of this article under FURNACE CONTROLS & SWITCHES. If any of that material is unclear or incomplete let me know by email.


Hi we have a 1950 to 60 old boiler in our home which we want to remove. All gas has been turn off at the meter. This furnace has not been operational for at least 10 years now, and everything is more than old to say the least.

In order to empty the baseboard style radiators in each room where do this.

The release valve is totally broken on the actual boiler itself. Is there a page on your website which deals with questions like this? We plan to have a new gas forced air system installed in our home once this has been removed. Anon; 6/26/2011

Reply: how to drain an old heating boiler & radiators when converting to forced hot air heat

Anon, just to clear up what we're discussing,

a "boiler" uses hot water or steam to heat a home through radiators or convectors or baseboards or radiant floor tubing

a "furnace" uses warm air to heat a home by circulating air (not water) through supply and return air ducts

You are describing a heating boiler - as you mention a broken (what sounds like) pressure-temperature safety relief valve. And you say you are converting to forced warm air - a furnace system.

To drain an old boiler including all of the radiators or in your case what sounds like convector units, you can usually drain everything from the boiler drain; that's because typically the individual convector or radiator valves have been left in the "on" position. Indeed draining will be a LOT faster if you can let air INTO the system.

I'd look for an air bleed valve at the highest convector unit and see if, after you've bled off the initial water pressure in the system you can also open that valve to begin letting air into the system.

Since you are replacing everything, there's nothing lost to cut pipes where necessary to get things to drain, just do your cutting where you won't leak water into the finished structure.

Question: is it legal to bypass a heating boiler control?

Isn't it illegal to bypass a flow sensing device and leave the boiler in operation? - Chris, 8/14/11


Chris I'm not sure what flow sensing device you refer to, but

Watch out: in general it is absolutely unsafe and improper to bypass or subvert a safety control on heating equipment. If at an inspection I found that boiler safety controls had been removed, bypassed, subverted, I would turn off the system, red tag it, and notify the owners/occupants in writing of an unsafe condition.

Question: Can I increase the cold water feed pressure to my heating boiler?

will it damage an operating hot boiler if I increase the pressure on the water feeder valve by turning the top screw clockwise?. - Joey Butter, 12/31/2012


Joey indeed after loosening the lock-nut on many automatic water feeder-water pressure regulators for heating boilers, loosening or tightening the screw adjusts a spring that sets the water feed pressure down or up.

Changing the water pressure to a higher initial setting won't damage a boiler, and that's what we do for some buildings that are more than two stories in height.

Watch Out: if you set the pressure too high you can cause dumping of the pressure relief valve; that in turn can spill water in the house, and ultimately clog the valve - an unsafe condition risking a boiler explosion.

So basically you want to set the COLD water pressure in the boiler to the right number. Standard starting point for the water feed for a hydronic (hot water) heating boiler is 12 psi when the system is cold. The pressure must be set a bit higher for taller buildings since circulator pumps themselves don't have lift capacity - they rely on the heating system water pressure to get heating water to higher levels in the building. .

Under BOILERS, HEATING see WATER FEEDER VALVES, HYDRONIC BOILER for advice on choosing the proper pressure setting for your heating system.

Question: Sudden loss of water pressure in the heating system, clanging pipes

I woke up last night hearing this clinging sound from the pipes. I checked out the heater and the pressure valve used to be at 10 is now at 0. I have oil radiant floor heating in a 1500 sq. ft. 1 story house.

Did a pipe break in my floor and that's why I lost pressure? - Jeremy 1/20/2012


Jeremy, a sudden loss of water pressure sure sounds like a leak to me, but check everywhere. If you're lucky the leak is not in the floor but at an accessible, repairable place - start by checking the pressure/temperature relief valve.


Is your Aquastat works with Solenoid valve ? - Amit Prajapati 6/21/12



The high limit safety switch in aquastats on residential heating equipment don't rely on a solenoid valve, they interrupt power to the oil or gas burner to shut it off.


[Click to enlarge any image]

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:

and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators.

If your building uses warm air heat, also called "forced warm air",

If your building uses steam heat

Index to Boiler Controls and Switches

Limit switch on a steam boiler tankless coil

Here we provide a list of the switches and controls found on heating systems. Each item is defined and we provide links to one or more detailed articles about the inspection, testing, setting, and use of each of these devices.

Article Contents


Continue reading at BOILER CONTROLS & SWITCHES - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see STEAM HEATING SYSTEMS & CONTROLS for information about steam boilers and steam heating system & boiler controls.

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BOILER CONTROL & SWITCH FAQs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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