Diagnostic questions for aquastat controls on heating equipment:

These questions & answers help you to understand how to set an aquastat's HI, LO, DIFF dials & how to troubleshoot & repair this heating boiler control.

This article series explains how aquastats work and what the different aquastat controls are, what they do, and how they are set. We define the HI LO and DIFF controls on heating boiler aquastats and explains what they do and how they work.

We explain the location and use of the heating boiler reset button found on aquastats.

Aquastat Diagnostic & Setting Questions & Answers

Questions and answers about aquastats posted here or at AQUASTAT CONTROLS or at AQUASTAT TROUBLESHOOTING.

Be sure to see the aquastat diagnosis and repair advice in either of those two articles.

Question: retrofitting a 15 year old boiler with a new Honeywell Electronic Aquastat

I am thinking about retrofitting my 15 years old oil burner with new Honeywell electronic aquastat which includes Outside Temperature Reset module. Has anybody done it ?

This would allow me to adjust HI water temperature as a function of outside air temperature. They say this would result in 10-20% fuel savings. Has anybody done it ? Is it worth doing it ? Thank you. - Pavel 2/13/12

Reply:

The retrofit aquastat control you describe will function properly if properly connected. the actual heating fuel savings ... well that depends on quite a few variables. "Up to" claims, in advertising law, need be true only about 10% of the time.

Question: trying to understand how aquastats work on heating boilers or water heaters

(Feb 10, 2014) Tom said:

Hi - I am trying to better understand the operation of an Aquastat. If have hot water coming out of my gas water heater (actually cold for a while cause it is a long run) does it hold off delivering water to the output connection (to sink) until the water is of the correct temperature?

While that is happening, is it correct that the cold water in the feed pipe from the water heater to the Aquastat goes back around into the home's cold water supply?

(Nov 24, 2012) Radhames said:

I have a room that does not get hot enough, the radiators in this room are the furthest in line...so by the time these start to get warm the boiler shutts off. Should I raise the temperature on the Hi and Lo settings, raise the temperature on the thermostat or relocate the thermostat. Thanks

(Jan 3, 2013) Brian said:

I have a unique setup with a boiler and aquastat that heats (3) zones - one is the coil air exchanger to heat the indoor pool room, the next a line for the hot tub and lastly a line for heating the pool. Just this week we noticed the hot tub was to the point of overflowing a few days in a row, and then this morning was pretty much drained, and the pool level higher. Is this a failure of the aquastat or the boiler itself?! Thanks in advance.

(Jan 9, 2013) Cathy said:

I have a 3 zone boiler heating (hydronic)system. Upstair work fine but lower level only works when we bleed the valves.after closing the valves downstairs the system there gets cold again even though the circulating pump is still running. The aquastat is set at 180 degrees and shuts off at exactly 180 degrees when thermostat temperature is reached but on lower level which is on a separate thermostat,it doe not shut off like upper level does, the temperature goes up to 230 degrees.We have been bleeding the system continuously and have had 4 plumbers come and no one can find the problem.

(Jan 15, 2013) PHILIP said:

I juat replaced the aquastat, the furnace is lit NO HOT WATER circulating... OUCH

(Jan 23, 2013) Greg Macpherson said:

Ihave a heat exchanger hooked up to a coal boiler. Iwant an aqua stat to turn the circulating pump on and off, which one do i need?

(Feb 26, 2013) Joe B said:

I am running a Honeywell triple aquastat L8151a with

(1) circulator pump and

(3) zones. My thermostat is calling for heat, and the zone valve is open.

The circ pump doesn't come on line and therefore the boiler does not go online either. I have replaced the pump and have been able to make it run by forcing the circ pump relay closed.

Of course as the cooler water enters the boiler, the boiler lights off. I have checked all Hi and Low and Diff settings. All look within the parameters described above. Is it safe to assume that the aquastat needs to be replaced? Is this aquastat suitable for this kind of (3) zone, (1) circ pump operation? This boiler does include the domestic hot water service.

Reply:

Tom, the aquastats used on a heating boiler, because they have more jobs to do, are the devices shown on this page;

a domestic hot water for washing and bathing, if made in a dedicated water heater, perhaps in your case a hot water tank heated by a gas burner, will also have a limit control, but a simpler one - search InspectApedia for Water Heaters to see those controls.

In any case, no aquastat holds off on delivering water - these are temperature controls that turn a burner (oil or gas) on and off.

For your second question: most residential hot water supply systems (we're not talking about heating water in baseboards, we're talking about hot water at the sink) are one-way systems: cold water input pressure at the water heater pushes hot water out of the water heater on to the building fixtures when someone opens a hot water valve - say at a sink.

There are hot water circulating systems, used e.g. in apartments and other large buildings, that keep the domestic hot water supply circulating through a loop of piping all the time so that there are not long waits for hot water at the fixture; those systems include an additional circulator pump and heater control.

From just your question I can't say what you've got installed. You're welcome to use our CONTACT link to send photos if youy like.

Question: boiler turns on and off too rapidly

(Feb 19, 2014) Mark said:

My boiler is firing on and off rapidly. It seems that the contactor inside the aquastat that controls the burner is opening and closing when heat or hot water is called for. Can this be replaced individually or does the entire aquastat need replacing?

Reply:

Mark I am not sure what's going on, but one would think that the problem is one of proper control wiring and adjustment, not a need to replace the equipment. Take a look at our advice on aquastat control settings to see if there is an obvious mistake with the HI LO and DIFF settings on your aquastat

(Feb 20, 2014) Mark Thomas said:

Thanks for the reply, Dan. I guess I should have been more clear. My boiler is 14 years old and this problem just started.

Reply:

OK Mark. So IF nobody has changed the HI LO DIFF settings and they're at typical numbers, say 180, 160, (we'll excuse the DIFF number) and the control is behaving erratically, it still could be a flame sensor problem, service problem, or indeed there could be a failing relay in the control.

If you hear buzzing or clicking at a relay that'd be a clue.

In a multi zone hot water heating system typically a relay in the primary control operates one circulator and additional circulator controls (and relays) operate each additional circulator. So most likely for an indirect fired water heater there is a separate circulator relay.

One would think that the problem then would lie in the

- the water heater's sensor monitoring hot water heater temperature
- the control wiring for the hot water circulator control
- the circulator control itself
- a loose wire somewhere in that system

BUT if your system is wired as they usually are in the U.S., it is the heating boiler temperature that is sensed by the aquastat on the boiler (not on the indirect fired water heater tank) that turns the burner on and off. If that equipment is working properly the boiler will turn on when boiler temperature (not your indirect water heater temp) drops to the LO or cut-in and the boiler would keep running until boiler temp reaches the HI or cut out temp.

So I'm back to looking for a bad temp sensor on the boiler, bad burner relay, or a loose wire in that circuit. Of course there could be some other issue like a bad relay in the flame sensor safety control.

BUT again, and thinking as typing: if the problem were in the boiler controls it ought to show up on a call for heat not just on a call for hot water.

vs

If the problem is in the hot water control wiring (maybe it's not wired as I'm guessing) then the problem would only show up on a call for hot water.

By turning DOWN the thermostats in the home (so to be sure you're not calling for heat) and running hot water until your water heater is cold and you're calling for hot water you ought to be able to sort out the two.

And this is an example of flying blind: I assume from your comment your "hot water" is an indirect fired water heater; if your system is using a tankless coil then everything is right at the boiler and all the diagnostics are in its aquastat.

 

Question: aquastat changes when eliminate a tankless coil

(Apr 4, 2014) Anonymous said:

if I eliminate tankless coil and put in electric water heater, what happens with aquastat? I have forced hot water for heat

Reply:

You don't have to do anything, Anon, but if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read

AQUASTAT LO & DIFF DISABLED

Question:

(Apr 12, 2014) Jimmy said:

How do I correct the the temputure control with electrical settings

Reply:

AQUASTAT HI LO DIFF SETTINGS

Question: hot water heating circulator won't come on

(Apr 21, 2014) Chanthy said:

My boiler won't circulate the hot water. I have changed the pump and the aqua stat has a broken pin where the switch would click. Does that make a difference? The switch is clicking/pulling in but hot water won't circulate. Any suggestion what could be wrong.

Reply:

Chanthy

First check that the thermostat is calling for heat and the thermostat wires are pulling in the circulator relay.

In addition to checking and repairing the aquastat if its relay is not pulling in to turn on the circulator, check that any zone valves or manual control valves are open (if present).

Question: Peerless WB-3 boiler with a Honeywell L4081b 1047 aquastat. I don't need the boiler for domestic hot water

(Oct 20, 2014) Ray said:

I have a tankless Peerless WB-3 boiler with a Honeywell L4081b 1047 aquastat. I don't need the boiler for domestic hot water because I have a separate electric water tank, so I only use the boiler for heat. Right now the aquastat is set to H 180 L 160 Diff 10. Should I change the settings? Can I disable the aquastat and how do I do it on that specific model?

Reply:

You don't have to do anything, Ray, but as we noted earlier in this FAQs series, if you want the boiler to stop maintaining temperature when there is no call for heat (which it will do for the tankless coil), then you want to read

AQUASTAT LO & DIFF DISABLED

Question:

(Nov 9, 2014) Blacklab said:

What Honeywell control could be used with a coal fired ,hot water system to CUTOFF power in the event of a No-Fire or Fire -Went Out condition?

Reply:

Ask your heating service tech about cad cell relays.

Question: symptoms / causes of a bad aquastat

(Nov 21, 2014) Don said:

what happens to a l4081b aqua stat when it gos bad

Reply:

Don there are several failure modes in aquastats:

- a failed relay no longer switches on or off as it should

- circuit board components can burn-up

- a temperature sensor may fail

- wiring connections to the device may be loose or broken or shorted

Question: boiler won't turn on

(Nov 28, 2014) Anonymous said:

my boiler will only go on when i put thermostat to 72 degrees or higher boiler and circulator turn on together

Reply:

Anon the thermostat has to be set above room temperature for heat to run.

Also, check for dust clogging in the thermostat.

Question: zone valve won't run - incompatible?

(Nov 29, 2014) Ken keller - kellerwelding@embarqmail.com said:

I have a 3- zone hot water baseboard system. Oil fired boiler, separate electric hot water heater. The other day I had to rplace two old and leaking zone valves.

The Plumber and the electrician wired up the new zone valves and said they won't be able to turn on the furnace with the thermostat because the new zone valves don't work like the old valves. The only way the furnace will heat the water is for the upstairs zone to be set on high. ?? Please help me !!

Reply:

Ken

A separate electric hot water heater - providing domestic hot water for washing and bathing - has nothing to do with an oil fired heating boiler nor its controls. It is a completely separate appliance.

In typical heating boiler operation (for heating your hot water baseboards) when there are three zones, each thermostat is wired to a zone valve. When the thermostat calls for heat the zone valve opens. When the zone valve is fully open, an end-switch in the valve closes a second switch that connects by wires to the boiler's primary control to turn on the boiler.

Traditionally in a 3 zone system there is a slight difference from what I just said: one zone, probably your upstairs zone, is wired directly to thermostat contacts on the boiler's primary control - that thermostat turns on a circulator and when needed, the boiler. The second two zones work as I described above.

So I suspect that the new zones installed are either improperly wired or were not properly chosen. It makes no sense to me that you'd have to call for heat where you may not want it (upstairs) just to get heat where you do want it in a separate zone (say downstairs).

Call the heating service manager at your service company and ask for help from an experienced heating service tech.

Keep us posted.

Question:

kris.century21@gmail.com said:

what would be a good range for the two sensors (Dials) on the honey well L4010B aquastat oil fired boiler w/ coil and circulator 1 zone heat Thanks

forgot to mention ..a lot of info for a layman to absorb

Reply:

Kris you can see

AQUASTAT HI LO DIFF SETTINGS

but indeed a layman should not have to understand heating system controls - you should be able to ask your heating service company for this help at the time of annual service or any time your heating system is not working.

Question:

(Jan 26, 2015) Joe Dlouhy said:

I have a home boiler used for radiator heat only. I have a Honeywell aquastat controller for the boiler(I am not at home so I cant look at any model numbers or brands) which is about 3 yrs old and the boiler and pump about 7 years old. My boiler is currently functioning like this; the thermostat is on and calling for heat, the boiler turns on and starts to heat and the electronic flue opens.

The boiler runs for 30 sec or a min. with no circulation pump turn on and the boiler shuts down and the flue closes.

Last time I had a problem with the pump it was just loose wires under the protective cap where the aquastat wires tied into the pump wires but I checked the wires and they are tight. I think the aquatstat is not telling the pump to turn on (I think the pump is spinning free but I will have to double check as there is not much to grip or spin to be able to check).

I tried turning on and offthe main power switch to see if it would reset but no dice. I have my heating guy coming over later when I get home. Any suggestions?

Reply:

Joe if the circulator does not run the boiler should still keep firing until it has reached its upper limit as set on the aquastat. Start by confirming that.

Then feel pipes at the outlet and inlet of the circulator for heat.

Question:

(Feb 19, 2015) ken said:

oil burner runs for a day and stops. replaced filter, electrodes, bleed system. Runs all night stops in morning. Bleed system again, runs another night and stops again. What is it?>

Question: honeywell r8182f aquastat, not sure if the relay switch is bad, but boiler won't fire up.

(Feb 21, 2015) Tom said:

I have an oil fired boiler, it seems that I am having problems with it starting, thermostat seems ok, power to the aquastat, etc,

but what I am experiencing is when I turn the power on to the unit, all I get is a clicking sound inside the aquastat, take the cover off and look,

I see there is a reset button and next to it a single relay switch, and that is where the clicking sound is happening, turn power on and the switch clicks but will not fire up the boiler, it has a honeywell r8182f aquastat, not sure if the relay switch is bad, but it won't fire up.

Reply:

From your note it sounds like a bad relay. You tried the reset button ONCE, right?
Start diagnosis at

HEAT WON'T TURN ON

Reader follow-up

(Feb 21, 2015) Tom said:

relay is not tripped, when I turn the thermostat to call for heat the relay switch pulls in and makes a clicking sound but the boiler won't fire, does the same if I leave the thermostat up and turn on the power switch to the boiler, its like the switch pulls in and clicks but not firing up the boiler, bad aquastat or maybe the transformer on the boiler itself, any help would be appreciated.

Reply:

If the burner is NOT off on RESET and the electric motor doesn't start up on the burner when the relay pulls in, I'd look for a bad relay or bad wiring connections. Double check that the boiler is not off on safety.

(Feb 21, 2015) Tom said:

well the relay pulls in like I think its supposed to, maybe I'm wrong, but the motor does not turn and it does not fire, so with a call for heat the relay shouldn't pull in or click in, is this correct.

Reply:

If the burner relay on the aquastat does not stay pulled in it may be a bad relay. Check for voltage at the motor terminals.

(Feb 21, 2015) Tom said:

no, it seems to stay in as long as the thermostat calls for heat, it will infact pull in and out with the thermostat being turned up or down simulating a call or cancel for heat, don't think i have voltmeter that is working properly, do have a continuity tester, but probably not the right tester for the job.

(Feb 21, 2015) Tom said:

ok, an update, when I turn the thermostat to call for heat, and leave it on for a couple of minutes, the reset button does pop, not sure if this helps.

Reply:

So the burner is going off on reset. This will happen any time the cad cell relay never sees flame after about 20 seconds. If the motor never even tries to run and if you have voltage at the motor then

• check for a popped thermal overload button on the motor (not all burner motors have this switch).
• check for a seized motor shaft
• check for a seized blower fan shaft or a broken or seized coupling
• check for a seized fuel unit (oil pump)

If you're not seeing voltage at the motor then there is a disconnected wire or a failed aquastat.

In sum:

The reset switch on the cad cell relay or in an aquastat will trip in a short time, perhaps 30 seconds after the aquastqt tries to start the oil burner if no flame is sensed.m

If at startup the motor does not run at all, a thermal protective relay may have tripped on or in the motor, or the motor or any of the devices it tries to rotate may be seized.

Check for voltage to the burner at the attempted start.

(Feb 21, 2015) Tom said:

picking up a volt meter on my home tomorrow, will let you know what I find, thanks for all your help so far..

Question: PEI Aquastast giving me grief

1 March 2015 Raj Mathews said:
The oil fired hot water heater, installed 2011 and serviced annually, in a rental property in distant PEI is giving me grief. In winters, the heater shuts off, and I have to call for service.

Technicians have not been able to find the root cause, but often the issue is resolved by pressing the reset button. It is 60 gallon unit, servicing the six 1 bedroom apartments with regular tubs. What could be the likely cause - faulty aquastat? Does the aquastat have any electrical components that is impacted by moisture or electrical surge? I need some direction to be able to ask the right questions. Thank ypu

Reply:

Raj

Typically if the heater is shutting off on reset it's because of a combustion or fuel or ignition problem at the burner. I don't assume it's the aquastat but it might also be a loose wire at the aquastat.

Ask the heating techs to look for signs of a combustion or ignition or fuel problem that's tripping the safety switch, such as sooting, smells, etc.

...

Continue reading at AQUASTAT TROUBLESHOOTING that explains the best settings to use & how & when to disable the LOW & DIFF controls or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AQUASTAT DIAGNOSTIC FAQs-1 - more-recently-posted questions and answers about diagnosing an repairing heating boiler aquastat problems

Or see these

Recommended Articles

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• MANUALS for HEATING & A/C SYSTEM CONTROLS
• SINGLE FUNCTION LIMIT SWITCH

Suggested citation for this web page

AQUASTAT DIAGNOSTIC FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

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