More heating boiler aquastat control diagnostic questions and answers.

These aquastat FAQs discuss how to set an aquastat's HI, LO, DIFF dials & how to troubleshoot & repair heating boiler aquastat controls.

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.

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.

On 2016-11-20 by bill

Wood fired boiler we are using auquastat to run a selinoid to open a damper.

Its a Honeywell 7006a. Problem is its a 3 wire setup and we need only two. Running off a switch for main power how do we wire it to work properly

On 2016-11-18 by Glenn

I have an oil fired boiler. I'm having this issue: I have to press the red reset button to start the burner. It then runs fine until it reaches the high limit temperature.

Then the boiler shuts off and trips the red reset button. The red light illuminates. If I press it once the burner will start again until it reaches temperature then it trips the circuit once again.

I've replaced the CAD sensor, electrodes and set the electrodes with the Beckett gauge, replaced the fuel nozzle, all the filters and the Honeywell R8184 really. The aqua stat is a very old Honeywell. Not sure of the model. Could it be the aqua stat causing the issue?

On 2016-11-10 by (mod) - what is the difference is between the L4148e 1257, L4148E 1265 and the L4148e 1299 model aquastats?

Honeywell offers aquastat controllers for both oil and gas fired heating equipment such as hydronic (hot water) heating boilers and water heaters. Both single function (high limit or low limit are separate controls) and multi-function aquastats are provided.
The contemporary Honeywell aquastat line includes the

• L6081 HI LO & Circulator controller
• L7103 HI LO limits & circulator controller,
• L7248 HI LO & Circulator controller & LEDs & EnviroCom communications
• L7224U a universal replacemennt controller with an LED diagnostic display
• 7224 aquastat with electronic temperature sensor, UL listed, controlling burner & circulator + LEDs
• L4081 HI/LO circulator controllers for oil and gas boilers,
• L8100 single limit control

I have not found an L4148 series such as in your question. Guessing you meant L8148 I note:

• The Honeywell L8148 series aquastats are used on hydronic heating equipment and use an immersion-well sensor to inform the aquastast of the boiler temperature.
• Honeywell L8148A aquastat switches line voltage to the burner, mounts horizontally, contains a HI limit only, uses a 24VAC thermostat
• The Honeywell L8148E-1299 is a vertical-mount aquastat with an 15deg DIFF control and a 50VAC transformer. The limit control can switch both burner and circulator. This unit is designed for a low-voltage burner circuit.
• The Honeywell L8148E-1265 is a vertical-mount aquastat with a 15deg DIFF control using a 24VAC transformer. Its high limit opens the burner circuit only. A remote low limit controller can be added.
• The Honeywell L8148A-1017 is a horizontal-mount aquastat with a 8deg DIFF control. The controller switches both burner and circulators.
• The Honeywell L8148J is a vertical or horizontal-mount aquastat switching a millivolt circuit (such as found on gas control valves)

Often an alternative case is available to convert vertical to horizontal mount for these controllers.

A link to the Installation and Operating guide and cross reference manual from Honeywell is give in the article above.

On 2016-11-09 by Gary

Would like to know what the difference is between the L4148e 1257, L4148E 1265 and the L4148e 1299 model aqua stats. Thanks

On 2016-10-26 by richard

hi there, looking fro some wiring advice on hooking up a dual aquastat honeywell L6062a for a wood boiler only with one circulator and dc draft controller with one thermostat
any thing you could provide would be great.

On 2016-09-07 by (mod)

Ryan:

First you can rule out both a bad thermostat AND bad thermostat wires by disconnecting the thermostat at the boiler rather than at the thermostat

If heat continues to circulate then you could have a bad circulator relay that is stuck ON (that's unusual)
OR
heat may be circulating by convection even if the relay isn't trying to run the circulator and even if the circulator itself is not running. In that case a check valve at the boiler hot water exit piping may be stuck open.

Search InspectApedia.com for HEAT WON'T TURN OFF to see how to diagnose and fix this problem in more detail

On 2016-09-07 by Ryan

Could a defective aquastat cause the heat in one zone to keep pumping even if the thermostat is not calling for heat. Note thermostat has been ruled out. purchased a new one and also swapped with other zone.

Thanks

On 2015-10-29 by Anonymous

i do not get hot water my settings are low please help

On 2015-10-26 by kathy coleman

i need to know hi and lo settings on honeywell triple aquastat relay l8124ac and dif

On 2015-10-17 2 by steve

my control is not calibrated right. My low is at 180 and my hi is at 150 but it works perfect. hot water is 200 and kicks on at 160 for lo.

what gives?

Question: What are the Optimal LO & DIFF settings for moderate hot water use in summer?

What would be the optimal setting for the LO & DIFF settings for fuel savings with low and with moderate hot water usage in the summer months when the boiler functioning for heat does not come into play. If you would please identify the low usage from the moderate usage settings it would be greatly appreciated. - Mark

This is an excellent article on the DIFF setting as it pertains to using a Tankless Coil. During the summer in North Carolina when outside temperatures are in the 90's and my system never calls for heat, I am only running my Burnham boiler to provide hot water. It is not unusual to burn 1 gal of oil/day (30 gals / month) just for residential hot water. With oil prices well above $3.50/gal, $100/ month for hot water is a little extreme.

I am curious if some combination of LO Limit / DIFF might cause the burner to run less while still maintaining adequate heat to provide hot water. For example, if my current settings are LO 160/ HI 180/ DIFF 10, might adjusting the LO/DIFF settings either up or down lower oil consumption? I suppose another option could be to use an electric hot water heater with a tank during the summer months. - Tony

I have a VERY old system. Have two separate aquastats, one used for LO and one used for High. No Diff. I have a separate hot water heater. Because of the old system the oil company technician feels that I should keep the furnace on during the summer so that there isn't a "shock" to the system when it is turned on in the fall. Question: What are the best HI-LO settings in the summer and in the heating season.? - Steve

Reply: Summer Aquastat Setting Recommendations

Mark and Tony and Steve: For summer use, if you set the LO to its lowest temperature setting (typically 102 F) you will be keeping minimal heat in the boiler to support the tankless coil in summer and will lower your heating fuel consumption by as much as 30%.

For summer use, if you set the DIFF to its lowest setting you will also lower your fuel consumption.

The result of these settings will also be a reduction in the quantity and also the maximum temperature of hot water supplied to the home - which may be OK for cases when hot water use is only moderate.

If you have an automatic tempering valve or mixing valve installed on your hot water supply that will increase the total hot water quantity available by drawing heated hot water from the boiler only at the rate needed.

Tony:

You can leave the HI setting on your Aquastat alone in summer; since the system will never be calling for heat, the HI will be asleep all summer.

If you set the LO down to a lower number in summer you will be keeping less heat in the boiler, and in that sense burning less oil; but if your mixing valve is a MANUAL one you may need to adjust it so that water at the tap is hot enough.

If you keep the DIFF to its lowest number in summer you're also minimizing the oil burner on-time. Or conversely, if you set the DIFF to a higher number (say in summer), quoting from the article above:

The effect of setting the DIFF up from 10 to 25 is that when the burner is re-heating the boiler (for example while the tankless coil is in use and you're in the shower), the burner heats the boiler temperature up to a higher level before the burner is turned off.

Steve,

Given that you are not using your heating boiler for making domestic hot water, the reason a service tech would recommend keeping it on in summer is most likely because some older cast iron boilers can develop leaks between the boiler sections if the boiler is allowed to cool off to stone cold. Such a leak may not "heal" on its own when heat is restored, and it can lead to worse boiler damage.

For summer use on such a system, keeping just about any heat at all in the system, say 100 °F should be sufficient. If your LO is currently set above that number, try setting it down for the summer - but don't forget to restore it at the start of the heating season.

I am inferring that the HI on your system is an upper limit on a call for heat and won't be operating in summer. If I'm mistaken, just keep the HI 20 °F above the LO.

Question: What are the best Hi Lo DIFF settings on an aquastat?

Right now I have baseboard heat and I have the boiler set at HI-170 LO-150 and DIFF at 10. Is that a good Setting. I'm trying to save money on oil. thank you - Matt

I'm still not 100% sure on what my settings should be. I have a Weil Mclain boiler that's about 6 years old with a DHW coil installed. I had a 30 gallon electric hot water heater installed after the coil, and during the summer I would shut off the boiler and use only the hot water heater. During heating season the water would be pre-warmed by the coil and the electric heater wouldn't be doing much.

Problem is, we seem to be using a lot of oil. The boiler is constantly running, every time we use the shower, run the dishwasher, wash our hands, etc. It's also constantly kicking on to keep hot water in standby. During the early heating season, it's a waster, since the boiler isn't on very much to provide heat to the house. What should my settings be? Will I save more money by just not using the coil and telling the aquastat there's no coil? - Matthew

I live in Maine and the winter season is here. What would be a good Hi and Low setting for my aquastat? Its is starting to dip below 10 degrees at night. Thanks. - Nick

Reply:

Nick and Matt:

Under AQUASTAT CONTROL FUNCTIONS (the article above on this page) you'll find good aquastat HI LO and DIFF setting recommendations in the article titled Optimal LO/DIFF settings save fuel

Matt: you can significantly save on heating costs by lots of other measures including using a thermostat setback to lower temperatures at night or when the home is unoccupied. See HEATING COST SAVINGS for our suggestions of a variety of ways to save on heating costs, and importantly some priorities: what heat cost savings steps should we address first at a building?

Question: What are Normal Boiler Temperatures When the Aquastat has Turned off the System?

Our aquastat shuts off burner at 190 degrees but old and new temp gauge continues to rise to 215 degrees. is this normal? - Warren G.

Reply: It is normal for boiler temperatures to sometimes rise a bit after the burner has shut down

Warren: it is normal for temperatures inside a heating boiler to continue to rise a bit after the Aquastat has turned off the oil burner at the HI or when the thermostat has stopped calling for heat.

This temperature rise may occur because:

• The latent heat in the heating boiler combustion chamber and in the steel or cast iron body of the boiler itself continues to heat the water inside the boiler even after the oil or gas burner has stopped firing

and

• The circulator pump(s) have simultaneously stopped sending hot boiler water out into the heating zones.

As long as boiler temperature does not rise high enough to cause the pressure/temperature relief valve to spill you should be fine.

Question: When Abandoning a Tankless Coil: Should We Change the Aquastat Primary Boiler Control Settings

I am currently installing a hot water heater to replace a tankless coil in my boiler. Do I need a different aquastat to keep my boiler from trying to heat the coil? Can I replace my present aquastat #l8148a with #l8124a??? - John Haines

We installed an electric hot water heater, do we have to change anything to make the boiler function just for heat and not hot water (for bathing, dishes etc) ? - Ernie

I have replaced the tankless coil in the boiler with a gas water heater thus eliminating the need for the boiler to heat water for bathing,washing etc. question does the aquastat still needs to be connected for my boiler to run to heat the house? - jorge gonzalez

Reply: Disable the LO & DIFF settings OR set them to their lowest setting when abandoning a tankless coil

John:

Take a look at the instructions that come with the aquastat and you'll see a list of devices it can replace. But most likely you do NOT have to replace your aquastat with a different model just because you are ceasing to use the tankless coil for domestic hot water.

On the aquastats discussed here, the instructions (link given below) describe a simple process for disconnecting a wire that will disable the low-limit control, thus eliminating that function when a tankless coil is no longer in use.

Ernie, When you abandon or stop using a tankless coil on a heating boiler, you can save on heating fuel consumption by disabling the LO / DIFF feature on your aquastat. The effect is that the aquastat (primary boiler controller) will stop keeping the boiler hot when there is no call for heating the home itself.

If you leave the LO/DIFF working, the boiler will keep heating itself even in summer when there is no call for building heat - because it thinks it's staying hot to support use of the (now abandoned) tankless coil.

Jorge, you still need the aquastat control, since it has to manage the boiler on and off temperatures when the wall thermostat(s) call for heat in the home itself. But as we explain above, the aquastat controls can be changed to stop keeping heat in the boiler to support the tankless coil that is no longer in use.

The procedure for disabling the tankless coil heating system is in our Aquastat Settings article at DISABLING LO & DIFF On an aquastat.

Question: How Much Will I Save on Heating Cost by Lowering the LO/DIFF Settings on the Aquastat?

Thanks for answering my questions on my recent hot water heater installation. I disconnected the differential wire (see DISABLING LO & DIFF) on my aquastat and now my system works perfectly. - Tony

Thanks for answering my question from June 28, 2011. I did, in fact, reduce the LO limit setting this summer from 160 to 120 and it reduced my oil consumption for heating hot water by about 30 percent. I left the DIFF setting unchanged at 10. To be honest, I was a little unclear from your response about the effect raising/lowering the DIFF setting would have on overall oil consumption during the summer months.

However, adjusting the LO setting definitely had the desired effect. It was not necessary to adjust my manual mixing valve because the shower water was at a comfortable level. Thanks for taking the time to answer my question and for the valuable information contained on this website! - Tony

Reply: Oil Consumption Savings Reported to be 30% When LO set Back

Thanks for the feedback that lowering the LO setting will save heating fuel during summer months. We have edited the HI, LO, and DIFF functions above to make that text more clear.

Basically, by lowering the LO to its lowest point (120) you reduced that amount of heat kept in the boiler during the summer (when the only function of the boiler is to stay hot in order to heat the tankless coil used to make domestic hot water). Like you I'd have also set the DIFF to 10 as well.

If you are continuing to use a tankless coil for your home hot water supply, you might want to set the LO back up to 20 degrees below the HI in winter because in cold weather the tankless coil is receiving colder incoming water from the building public or private water supply.

Reader Comment: heating cost savings from aquastat settings

Hi Dan. ..I just wanted to clear up that the 30% savings you mentioned in the article were those that I reported in my post as a follow up to the June 28, 2011 post. ...

I can confirm the excellent point you made about setting the LO back up to 20 degrees below the HI in winter because of colder incoming water from the public water supply. It is now late October in NC and I still have the LO limit setting from this summer at 120 with the DIFF setting unchanged at 10. I have not begun to use the system for heat yet and am only running the boiler for hot water.

I have noticed a definite decrease in shower water temperature and will shortly need to reset the LO back up to 20 degrees below the HI. Again, thanks for all the valuable information you provide on your website! - Tony [October 2011]

Question: sorting out HI LO and DIFF - what setting does what?

There seems to be some conflicting info on the site. In some articles, you say that the DIFF is the temperature rise over the LO setting. For example, DIFF 15 and LO 120 would mean you'd have a range of 120-135. In this article, it's explained that there always a 10 degree drop below LO and a rise above LO of DIFF-10, meaning DIFF 15 and LO 120 would give you a range of 110-125. Which is true? And what about aquastats that have a 5 degree DIFF setting? - Brian

Reply: summary of HI LO DIFF definitions and functions on an aquastat

Thank you for the clarification request, Brian. We've reviewed our HI LO DIFF setting articles to be sure that the text is now self-consistent.

Summarizing the definitions of HI, LO, & DIFF controls on an aquastat control

HI Limit Definition: The HI is the cut off point on a call for heat. You can think of the HI dial as the "heat control" in this regard: as long as the room thermostat is calling for heat, when the boiler temperature falls 10 degrees below the HI the burner will (normally) turn on. That 10 degrees is hard-wired into the control - it's not adjustable.

LO Limit Definition: the LO is a control function added to keep heat in the boiler when the room thermostat is not calling for heat. This feature is important on heating boilers that use a tankless coil immersed in the boiler to produce domestic hot water.

The DIFF Definition: the DIFF is an added control that defines the cut-off temperature (above the LO) during a burner-on cycle initiated by the LO control - that is, we are not calling for heat, and we're keeping heat in the boiler to make domestic hot water. It works as you described.

Circulator lockout: There is an interaction between the LO and DIFF settings and the delivery of heat to the building however. That is that the LO operating range on an aquastat also will lock out the heating circulator pump - stopping delivery of hot water out of the boiler and into baseboards or radiators - if the boiler temperature is too low. (That's why aquastat instructions specify that the LO should always be at least 20 °F below the HI setting).

The intent is to give priority to the tankless coil and thus the person in the shower. For heating boilers at homes that do not use a tankless coil to make domestic hot water, it is sometimes (not always) reasonable (and a savings on fuel during summer months) to disable the LO/DIFF controls - as we explain at Disabling LO & DIFF

See Aquastat control HI LO settings for details about how the HI and LO settings work.

See Aquastat control DIFF settings for details about the DIFF setting and how it works

See Optimal LO/DIFF settings save fuel for some recommended settings to save on heating fuel cost

Question/Comment: Rusty crud found on the bottom of the heating boiler firebox

As Tony said in his comment above, I also was using about 1 gal of oil per day during the non-heating months. In Connecticut, we are paying close to $4.00 per gal now. I installed an electric water heater in parallel (NOT series) with the boiler's tankless heater. Either the tankless or the electric can be used since the water does not go through one and then the other.

(Glad I still had the tankless piped up since I could still have hot water during the recent 10 day power outage in Connecticut. The 4000 watt generator could run the boiler but not the 4500 watt electric heater.) I found that we are using about 11 kwh/day for domestic hot water by timing how long the 4500 watt heater elements were on each day - a 240 volt relay activating a battery powered clock.

The cost at $0.17/kwh is about $1.90/day - better than 1 gal/day at $4.00. I turned off the boiler when I turned on the electric water heater.

My concern is that when I went to tune-up the boiler for the 2012 heating season, I found about one cup of rust (some of the material was magnetic) on the bottom of the firebox below the cast iron boiler sections. I open the firebox for cleaning each year and have not seen this much rust before. I have brush cleaned the flue passageways twice since new and don't recall seeing this type of material.

The boiler is a Dunkirk that was new in 2001. Water leakage does not appear to be a problem. Should I be worried about damaging the boiler (corrosion) by letting it go stone cold during the non-heating season? - Buddy 1/12/12

Reply:

Buddy, thanks for the important comments & question. Indeed on many boilers, if you leave the unit shut down for months there can be problems with rust and also with solidification of soot and crud deposits that make seasonal cleaning more difficult (and perhaps costly).

If the boiler and tankless coil is left "on" but not in use, it will run only very little, as it runs just to keep itself hot - it's never cooled down to a longer on-cycle by having moved its hot water out into heating zones.

Alternatively at least it's a good idea to have the annual service/cleanout done in the spring at the end of heating season - don't let it sit over the summer.

...

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-2 - more-recently-posted questions and answers about diagnosing an repairing heating boiler aquastat problems

Or see these

Recommended Articles

• AQUASTAT CONTROLS - home
  • AQUASTAT HI LO DIFF SETTINGS
  • AQUASTAT LO & DIFF DISABLED
  • AQUASTAT RESET BUTTON
• MANUALS for HEATING & A/C SYSTEM CONTROLS

Suggested citation for this web page

AQUASTAT DIAGNOSTIC FAQs-1 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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