Hot water heating circulators or circulator pumps & relay FAQs:
Questions & answers about how to install, troubleshoot, & repair hydronic heating system (hot water boiler) circulator pumps and circulator relays or controls.
This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
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Heating circulator pumps or "zone circulators" are used to force hot water from the heating boiler through radianting devices such as hot water baseboards or radiators. The circulator is switched on as needed or in some designs may be wired to run continuously.
Our photo at left of a red B&G heating circulator pump shows equipment more than twenty years old and still spinning along nicely.
[Click to enlarge any image]
After a brief introduction we describe what goes wrong (or how to get things to go right with heating zone circulator pumps. We also link to related articles for circulator choices, installation, troubleshooting, repair or replacement.
On 2017-01-14 18:39:53.656444 by (mod)Dom
On 2017-01-14 14:48:18.166922 by DomHi I have a forced hot water baseboard syst. One of my zones is not getting heat. the circulator seems to be working because its humming (problem?) there is no air in the system.
On 2016-12-20 00:29:32.439402 by (mod)John,
On 2016-12-18 20:48:46.771093 by John WilsonLooking for an alternative to Anti-Freeze. Wife (who claims she's much smarter than I) keeps the house cold on winter days (64) and cranks it down to 56 at night. We've argued about cranking the heat up on the really bitter cold days (less than 10 degrees) because some of the piping runs up the outside walls to the 2nd floor but she won't give in. I came up with an idea to throw a bypass timer on the circulator pump for that upstairs zone to just turn the pump on for let's say 5 minutes per hour. I'm thinking that by occasionally circulating room temp water through the pipes at a regular interval, it would prevent the pipes from freezing. Any thoughts on whether this might be a viable alternative or am I just spinning my wheels?
On 2016-12-18 16:58:00.376867 by DelfordMy daughter home is hot water radiator but the pump is connected to run though the water heater
On 2016-12-16 19:25:46.087298 by Anonymousthe pump is new
On 2016-12-11 03:43:51.666035 by (mod)System could be air bound. Search inspectApedia.com for AIR BOUND HEATING BASEBOARD OR RADIATOR for details.
On 2016-12-10 21:23:42.648589 by merediththe lines on the outside wood furance was getting hot and then it stopped what could be wrong the pump is running but the water from pump is not hot in the
On 2016-12-02 13:59:11.513607 by CecilCan a bad capacitor stop a circulator pump from working?
On 2016-11-29 19:53:01.427846 by RosieDuring my routine maintenance I was told I needed a circulation pump replaced on my furnace. He said he could tell by the temperature of the unit. It's hotter than the pipes. Is this a valid test.
On 2016-11-13 22:49:02.540529 by (mod)Jeff
On 2016-11-13 21:33:32.447611 by jeffMy boiler, Burnham p206w, comes on for about 10 seconds then stops. No heat from the radiators, and no water from the radiators when I try to bleed them.
On 2016-10-11 15:08:35.207247 by DonMy fingers are too fat. All should read 120v NOT 129
On 2016-10-11 15:06:34.406954 by DonA friend asked me to look at his heating system. It's a hot water system that has been changed a number of times. It was originally a three zone system with three thermostats. It was changed to a three zone , one thermostat system. This caused huge temperature variances from room to room. He wants it back to the way it used to be. The box has three thermostats, four relays, 120V power, 120V/24V transformer, and the two wire 129v leads to the pump. I wired 120v power to the relays, 24v thru the relays to the thermostats and from the relays to the pump with 129v. Problem: when I tested without the pump, all worked fine. When I connected the pump, all three zone valves turned on. I'm obviously getting feedback power. I tried two or three different wiring ideas with the same problem . I'm baffled . I either need a different wiring idea and suggestion ,or, perhaps a fool proof solid state controller. Any suggestions would be great
On 2016-08-04 15:35:47.481502 by (mod)Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies,
On 2016-08-04 15:35:07.868760 by (mod)Balamurugan I'm sorry but I don't understand what you are doing. Hyronic (hot water) heating systems circulate hot water through heating piping and radiating devices, powered by one or more circulator pumps. Circulator pumps don't have much lift capacity and depend on the system being full of water - else the system will be air-bound and hot water won't circulate.
On 2016-08-04 06:15:55.101781 by smaruthabalaThe vacuum distillation column, fired heater (re-boiler) circulation (centrifugal) pump pressure goes to zero, when applying vacuum. so i cant circulate the (lubrication) oil for heating. pls give solution for getting pressure.
(Dec 28, 2012) Mary Tilma said:
What is the average price of a circulator pump?
Mary, Grundfoss & Bell & Gossett heating zone circulator pumps are typically in the range of $100. to $300. depending on the pump model.
Those heating zone circulator prices do not include the costs of additional relays, wiring, plumbing, or other installation needs. In other words we're giving you the heating circulator assembly cost, not the installed-cost.
(Feb 8, 2012) Kelly said:
Can you continue to run the boiler if the circulating pump is not working properly? The house is heating but I'm concerned about how safe this is.
Reply: Yes, as we explain in detail at BYPASS a BAD CIRCULATOR. But this is also a stop-gap or backup method of keeping a building heated that will not work in every situation.
(Mar 2, 2012) Ken said:
I'm seeing pictures of products installed incorrectly with no mention that they are wrong.
Thanks Ken, Please check out these articles where we address circulator pump mounting in detail. Further comments are of course quite welcome, especially if you can add citations to manufacturers or other sources for clarification of your view.
We moved this discussion to a new article: please see MULTIPLE HEATING ZONE CONTROL
We moved this discussion to a new article: please see CIRCULATOR WONT RUN
We moved this discussion to a new article: please see CIRCULATOR PUMP RUNS INTERMITTENTLY
We have moved this discussion to a new article: please see WEAK HEAT, WEAK CIRCULATOR, TOO COOL
We have moved this discussion to a new article: please see CIRCULATOR NOISES.
Also see HEATING SYSTEM NOISE DIAGNOSIS for a more broad range of heating noise diagnosis & repair procedures.
(Mar 2, 2014) Jared said:
Why does heat come out of zones where thermostats are not calling for heat?
Jared, for space and completeness we moved this discussion to a new article. Please see
(Dec 1, 2014) Anonymous said:
Replaced water heater and now pump does not work.
Those are separte systems; check for power turned off.
Dec 2, 2014) Rob said:
basically I haven't maintained system over the last two years.
- stove not run for the last two winters over concerns with propolyne glycol in system
- want to save on wood, wood boilers eats a lot of wood
- did not cover the stove / chimney pipe therefor water got down in the damper / blower box
- pipes in firebox worn / blown out from where the forced air goes through them
- pump motor got hot but no water going through-- pumps were shot!
- replaced 2 pumps, one in house 1/6hp and one coming out of stove 1/25hp
- seem to be pumping water but pipe coming into house not getting hot
- when I turned on damper switch it popped and electrical smoke came out
- started my system a little late when it was getting real cold out
- water in pipes after pump by stove froze, got the ice out thought it was going to bring heat to house but about an hour after this the pipes felt real cold again.
- could the electrical lines to power the pump be backwards and make the pump go backwards?
You describe equipment that has been sorely water-damaged including electrical components - making the whole shebang unsafe to try to run without a thorough inspection and repair.
If a pump is running backwards I suspect a control or even a start capacitor was damaged. Try first replacing the start/run capacitor for the motor.
Yes some motors can run backwards if a start capacitor has failed; see ELECTRIC MOTOR RUN DIRECTION
(Jan 9, 2015) Ed said:
I have 2 zone upstairs/downstairs baseboard heat all baseboards are cool except for the upstairs bathrooms which are directly above furnace,I noticed the circulator pump appeared to have water leaking from the electrical connection compartment when I opened it up to check for ac it had a bit of water/grease in the compartment. I'm assuming a seal in the pump has ruptured and shorted the motor. The output and return both go into the ceiling so I'm assuming from reading the other questions that it's just convection warming the rooms above. The furnace appears to be working fine and the pump feeds two zone valves above. Am I on the right track?
Sounds as if the leaky circulator needs to be replaced and that perhaps as well air has leaked into the heating system piping. First replace the circulator. Keep me posted.
(Feb 16, 2015) Anonymous said:
I have no power going to the circulator pump. I have rigged a manual power supply to it for now. What would stop power to the pump?
A thermostat not calling for heat, broken thermostat wires, open switch, failed circuilator relay, power off to the boiler or circulator electrical circuit, bad circulator control on a primary control board.
(Feb 19, 2015) Janet said:
My husband and I turned the main water supply off to the entire house while we repaired a shower valve. When we turned the water back on, our circulator pump no longer works. What could have caused this? Who should we call to have it looked at?
Janet I can't posit an obvious connection between a hot water heating system zone circulator pump operation and the main building water supply being shut off unless your boiler lost water and also includes a low-water-cutoff safety control and also didn't accept make-up water when water pressure to the building was restored.
(Feb 22, 2015) r hernandez said:
boiler runs no water going into radiator for heat can boiler run without water inside without damage to boiler? what do i check for?
If the boiler itself is low or out of water shut it off immediately to aid a serious explosion or boiler damage.
If the circulator is working and the system contains water, bur radiators do not get hot water, for diagnosis and repair see
(Feb 28, 2015) Ben said:
Gas furnace: Only one of the four thermostat will lite up the main burner, not the other three.
What could it be
Bad thermostats, bad thermostat wiring, bad control board.
(Mar 20, 2015) jamie said:
i have a heating system (propane) with 2 zones. up and down as well as my hot water. i dont have a controller system (digital) recently it got very hot in the downstairs. I removed my wires from the thermostat to see if that was causing it constantly request heat. Not the case. even replaced it due to wife thinking it was the problem. only way to stop the heat was to turn off my main valve to that zone. there is two separate valves and i shut them both off. then that night i turn them back on, it heats up the space, (to hot) and i shut the valves off again. my other zone and hot water have no issues so i know its with just that zone and probably a valve stuck open. however at this point my knowledge shrinks and not sure how to proceed. thanks for any help in the matter
Try disconnecting the thermostat wires at the heater end not the thermostat end. If that stops heat then the thermostat wires are shorted.
(Mar 24, 2015) jamie said:
dan, would it be best to disconnect at the zone valve? if heat still continues i assume zone valve issue?
Ah ok so here's the thing:
If TT wires are crossed you may be calling for heat all the time. Disconnecting the TT wires would prove that.
A different problem is a zone valve that doesn't shut. Disconnecting wires won't fix that. If the TT wires are disconnected - you can do it at the zone valve - and the valve stays open, you'll find hot water passing through the valve and the pipes will stay hot on both sides of the valve and heat will show up even on the return point of that zone to the boiler.
Some zone valves can be manually opened (by a lever) but not necessarily forced closed. So if this is the trouble you'll need a new valve.
(Mar 27, 2015) Jamie said:
How easy is it to replace one of these valves. This is out of my comfort zone and wondering if just replacing the valve would be easier then trying to trouble shoot the issue. I am confident it is the valve and not wiring, house is only 5 years old new.
Jamie it's not technicalliy difficult if you know how to drain a heating boiler safely and cut and solder or connect pipes and electrical wires. Chances are the job will be faster and easier and maybe cheaper if you hire a pro.
(May 18, 2015) Lee said:
I noticed my circulator pump running continuously after the boiler shut off. Only after turning of the system power did the circulator pump stop running and it started running again after turning the power back on. After reading the comments here, I used my flashlight to tap the circulator pump in hopes of freeing a stuck relay, and it worked. Would it be reasonably safe to assume the electrical relay on the pump is sticking and can the relay be serviced or replaced without replacing the pump itself? The pump is less than 10 years old. I live in the San Francisco area. Thanks
Lee that sounds reasonable to me. And it's worth making the repair as a relay that sticks today may stick "off" sometime when you're not around and would have wanted heat to be reliably "on" in the building.
If you replace just the relay (which is possible on some controls) I'd like to examine the old one - you can find my mailing address at the CONTACT link at page bottom.
Note that in some boiler installations, particularly as is custom in many areas of Canada, the circulator runs constantly when the heat is "on" and it's the thermostat's job just to turn the burner on and off.
(May 18, 2015) Lee said:
Hi Daniel. Thanks for your quick response. Give me a few days to check it out and I'll get back to you. Thanks again. I checked out my pump and found: It's a Grundfos Type UP 43-75BF 216. I took off the side panel where the electrical connections are made and only found a capacitor, no relay that I can see. So, again, I assume it must have an internal relay? At this point, I'm not sure what to do, other than replace the pump, which I would prefer not to do. Any thoughts?? Lee
Lee that pump has a permanent split capacitor type motor with an internal thermal overload circuit - the problem could be a connection or an internal component. Before replaceing the motor I'd double check for a loose splice or wire connection. If it's not working you should be able to replace just the motor component. If your model is a multi-speed version also be sure the speed switch is not loose or damaged.
I should have said first to follow the thermostat wires - those will take you to the circulator relay which will answer the "where is it?" question.
Also check - with power disconnected - that the pump shaft rotates freely. Here is what Grundfos says about that:
When UPS 15-42 and UPS 26 pumps are first started, the shaft may rotate slowly until water has fully penetrated the bearings. If the pump does not run, the shaft can be rotated manually. To accomplish this, switch off the electrical supply, and close the isolation valves on each side of the pump. Remove the indicator plug in the middle of the nameplate. Insert a small flat blade screwdriver into the end of the shaft, and gently turn until the shaft moves freely. Replace and tighten the plug. Open the isolation valves and wait 2 to 3 minutes for the system pressure to equalize before starting the pump. NOTE: After a long shut down multi-speed pumps should be started on speed 3 and then adjusted to the regular setting. The UPS 15-42 has automatic function to assist in restart.
(May 18, 2015) Lee said:
Daniel, thanks for your comments and advice! Much appreciated! I'm fairly sure this is a single speed model. Thank you again!
Yep - I took a look for more diagnostic tips in the Grundfos installation guide, there's not much.
(May 18, 2015) Lee said:
As I mentioned, the pump works fine, it just won't shut off!
Don't forget to check for control wires shorted together and check for a bad circulator relay.
(May 21, 2015) Lee said:
Dan, I wanted to followup on this. After shutting off all power to my system, I checked and retightened all electrical connections. Checking through my AO Smith boiler manual, I located the Relay and High Limit control box, which is mounted on the front of my unit under the front cover panel.
After removing the control box cover, I saw the circulator relay that is clearly marked "circulator". So, it was a little dusty and I could clearly see the relay contacts. So, I took a spray can of electronic contact cleaner, and, following the directions on the can of spray, cleaned the relay. I've turned the power to the system back on and off numerous times and it appears to be operating normally now, ie, the circulator shuts off when the boiler shuts off. So, I think my problem has been solved, but I will monitor it for the next week to insure proper operation. Thanks for your help and a great website!.....Lee
27 Oct 2015 Anonymous said:
I have a one zone baseboard heating system. When I set my wall thermasat to 70 degrees the boiler goes on. The outlet pipe gets hot, you cannot touch it, it has a temp/press. gauge on the outlet pipe on the boiler plus an other gauge set at 180 degrees. The return pipe going into the circulating pump does not get hot until the boiler shuts off. The Temp. of the boiler is 200 degrees @ 15 psi. But I noticed the pump does not go on. When should the pump go on? It is a Taco 110-20 770. Where is the relay located? The pump electrical supply is connected to a box on the boiler. Is the relay in that box? Oct.27,2015 John
The taco relay is in the little box atop the Taco circulator pump;
If the circulator rusn but baseboards don't get hot I suspect the system is air-bound. Search InspectApedia for "air bound heating baseboards" to read details.
(Nov 22, 2015) Mindy said:
Where is the circulator located on a single line hot water boiler furnace?
It could be anywhere but usally close to the boiler on the return side of the hot water heat piping loop
The circulator would work anywhere in the hot water heating loop, but usually it's located close to the heating boiler. Best location is on the return side of the loop close to the boiler where the cooler water temps give longer circulator life.
(Oct 27, 2015) John said:
I have a hot water burner with a circulating pump when does the pump start? does it start at a certain temp.?
would you see the shaft spinning when it starts? it is a taco 110 pump.
John, on most heating boilers in the U.S. the thermostat turns on the circulator pump while independently, the temperature of the boiler, controlled by the aquastat, turns the burner on or off.
When room temperature falls below the set temperature on the thermostat then the circulator pump will start.
In Canada at many hydronic heating system installations the circulator runs continuously and the thermostat will turn the boiler on when room temperatures fall below the set temperature on the thermostat.
See AQUASTAT CONTROLS - home
See THERMOSTATS, HEATING / COOLING - home
(Nov 16, 2015) Badmojomn said:
American Classic. Temp and water level good on outdoor boiler. All valves open. Feel water circulating somewhat in hoses coming into the house but the hoses do not feel warm. Any idea why? Temp dropped in the house and woke us up.
If cooler water from the heating zones is returning to the boiler that tells us that the thermostat is calling for heat, and the circulator is running.
If the boiler never turns on there is a NO START problem: out of fuel, dirty filter, etc.
You can push the red reset button on the primary safety control JUST ONE TIME. If the system starts and runs and keeps running then you'll have some heat while you wait for emergency repair service.
See NO HEAT - BOILER for diagnostic steps.
(Nov 21, 2015) George said:
How do you choose the proper circulating pump for a baseboard hot water system?
Good question, George.
Technical selection of a circulator pump for a hydronic (hot water) heating system considers the following:
1. do we want a single speed or variable speed pump; variable speed gives finer control over heat delivery but is usually not required for a standard residential installation
2. What is the design flow in gallons per minute of the hot water heating system
3. Minimum efficiency of the pump
4. We can also specify the water operating temperature range, motor RPM choices and other details.
Cirulator pump manufacturers like B&G and Taco offer and online pump sizing table or calculator; others offer a printed or PDF file of pump size choices.
Typical residential circulator pumps range from about 0.5 hp to 1.5 hp.
What this last line (above) means to some of us is that while an HVAC engineer can, should, and will perform calculations or use a circulator sizing tool following the well-established procedures and rules of the manufacturers and industry, life is simpler for us little guys. Talk with your plumbing or HVAC supplier about the height of your home, the number of feet of baseboard, and the heat loss rate of the building, and choose a pump from among the models in the 0.5 to 1.5 hp.
Put another way, using electrical panel sizing as an example: we can and code instructs how to calculate the required electrical service ampacity for a residential building, when we stop by the local electrical supplier we'll see that there is a range of panel (and ampacity sizes) typically with a minimum of 100A (all are 240V) and a maximum for typical residential of 200A, with choices of 125A and 150A in the middle, and a range of choices of number of circuits the panels hold. Given modest price differences I'll put in 150A or 200A. That's that.
(Dec 4, 2015) Daniel said:
My pump will circulate water when first floor calls for heat but does not circulate when second floor calls for heat, what could the issue be?
improper thermostat settings or switch positions
a bad thermostat wire
a bad thermostat
a bad circulator relay or zone valve
See NO HEAT - BOILER for diagnostic steps.
(Jan 28, 2016) Allen said:
Is it reasonable to install 2 circulator pumps on both the supply and return lines of a hydronic heating system ?
Allen I'm aware of no reason whatsoever to install two circulator pumps on individual zone lines. One pump, properly sized and working should be sufficient.
(Feb 12, 2016) Grant C. R. said:
My housemates and I are renting a house and we've been having furnace troubles, in October a man came in and bled the pipes but most of the radiators still didn't work fully, with two not working at all, eventually the Circulator pump started to make a grinding noise so a man came in and oiled the seal, after this the noise became much worse and the man came back and replaced the entire pump, he bled the lines again and told us that they should be working better.
The radiators which had only been half working now began to work better, but two in the extension part of the house have been ice cold the entire time. I've been watching the furnace and it seems to start the boiler every 20 minutes, once the temperature reaches 150 it starts the circulator pump which runs on average for 45 seconds before shutting off. Is this normal and if so what could be a possible issue as my landlord has been unresponsive to the issue. for context the Thermostat is set to 21c and the outside temperature is on average -10c. It's also a consensus that for the most part the house is not being kept at 21c despite the thermostat being set there.
Grant it sounds as if the air has not been sufficiently bled from the system. See
(Mar 28, 2016) Chris said:
I have an old boiler that was once oil fired, but converted to gas sometime in the past. The circulating pump seized and burned the seals. Under this circumstance, can I even think to repair this pump, or should I replace it. Please respond to firstname.lastname@example.org
Converting the fuel source on a hydronic boiler would not itself have anything to do with the circulator pump operation, though a snafu in restoring the boiler to service such as airbound heat piping, wiring errors, or poor maintenance could certainly cause circulator troubles.
I would replace the circulator pump assembly entirely. A seized motor that burned the pump seals is likely to be at or close to total failure even if you got the motor going again. I'd prefer not to have to repair the circulator twice.
(May 27, 2016) sid said:
the water to my house was turned off for a sewer line repair. I'm supposed to 'prime/purge' the circulator pump before turning the radiant heat back on - don't know how. the system is open and the heat source is a tank water heater.
See these procedures to remove the air
(June 19, 2016) Steve said:
I have a strange issue. I have a 3 zone system (basement, 1st, and 2nd floor). I had heat in all zone all winter. When trying to replace a 2nd floor baseboard heater I first shutoff the water into the boiler. Then I isolated the 2nd floor return from the system (shutoff) and opened the drain. No water drained. There are no leaks in the system (main shows no water draw - spinning arrow still, no piping is below grade, and no water stains/leaks, or mold is present). If Taco was stuck open where did the water go? Im lost here. Any thoughts would be appreciated.
Good question; possibly the zone was already air-bound - search InspectApedia for AIR BOUND HEATING BASEBOARDS & RADIATORS to read more about that.
If a boiler lost water from a leak anywhere or by having been partly drained at a boiler drain valve, water from upper levels of piping might indeed run back into the boiler.
Or also, it can be difficult to drain water out of a closed system when you open just one valve, varying depending on where that valve is located. For example a closed zone valve will prevent water from draining past that valve.
To try to drain the second floor baseboards again,
- manually latch "OPEN" the zone valve for that zone
- open the drain on that zone, presumably below or close to the zone valve
- open an air bleeder located anywhere on the baseboards upstairs on that second floor
June 26, 2016) Steve said:
Just a followup to closeout this problem. The Taco Zone valve for the 2nd floor was a model 552-3D install dated 3/95. This zone valve had no auto/manual lever to operate and had a circular base. I had to cut and drain the 2nd floor supply-side anyway so I could ensure no water was in the line to install a baseboard heater. When hooking up the shop vac to clear the line I did get suction from the hose on the drain side so it had to have been air-bound. Finally, should the 571-2 vast valve body be disassembled prior to soldering? This is unclear but I believe so, especially if the cast body has any rubber in it. Thanks!
(June 26, 2016) (mod) said:
Thank you for the helpful follow-up;
Typically the zone operating motor is detached, the zone valve is put into the fully-open position, and the valve body is otherwise left assembled during soldering in place. Some plumbers wrap a wet strip of cloth around the valve side of the solder job to minimize heat transfer into the valve body.
Don't push more solder into the joint than needed.
Here's an excerpt from a Taco EBV operation instruction sheet
WARNING: Operator must be removed
from the valve body before soldering.
Ball valve must be in the full open position
before soldering. Valve shipped in
the closed position.
The valve you are installing is widely used and sports a twist-off head and a manual open lever.
Older instructions say the valve can be installed without disassembling but warn about using 50:50 or 40:60 solder and a fine pointed torch flame;
Watch out: if youre using low lead or no lead high silver solder that requires a hotter temperature you should absolutely disassemble the valve.
2016/08/04 smaruthabala said:
The vacuum distillation column, fired heater (re-boiler) circulation (centrifugal) pump pressure goes to zero, when applying vacuum. so i cant circulate the (lubrication) oil for heating. pls give solution for getting pressure.
Note: Before applying vacuum the pump pressure is up to 5 bar.
Balamurugan I'm sorry but I don't understand what you are doing. Hyronic (hot water) heating systems circulate hot water through heating piping and radiating devices, powered by one or more circulator pumps. Circulator pumps don't have much lift capacity and depend on the system being full of water - else the system will be air-bound and hot water won't circulate.
Search InspectApedia for AIR-BOUND HEATING SYSTEMS - home to read details
We do not use vacuum pumps, we do not use distillation columns. And we do not circulate lubricating oil through the heating system pipes.
I presume you're repairing some other type of equipment, perhaps in an industrial process.
Continue reading at CIRCULATOR PUMPS & RELAYS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see BYPASS a BAD CIRCULATOR
Or see CIRCULATOR NOISES
Or see CIRCULATOR OPERATION CHECKS
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