Limit switch on a steam boiler tankless coilTankless Coils on Heating Boilers: FAQs

  • TANKLESS COIL FAQs - CONTENTS: diagnostic questions & answers about tankless Coils on heating boilers:
  • POST a QUESTION or READ FAQs about tankless coils used to produce domestic hot water: clogging, leaks, water quantity, water temperature, scale formation and cleaning

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Tankless coil FAQs:

Questions & answers about tankless coils for domestic hot water supply: capacity, safety, clogging, adjustment, cleaning, & Repair Guide. How much water can we get out of a tankless coil before running out?

These diagnostic questions & answers help explain the function, use, capacity, inspection, and repair of Tankless Coils on heating boilers.

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This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

FAQs about Tankless Coils for Domestic Hot Water Production

Photograph of a multi function combination control on a heating boiler

Question: should I have my tankless coil cleaned?

(Feb 24, 2014) Gail said:
Could you tell if I get the coil cleaned would that help.


Gail, I don't have a shred of information about your system or the problem that's a concern.

In general, if the concern is reduced hot water flow through the coil because a tankless coil has become mineral-clogged, indeed cleaning the coil can for a time improve hot water flow rate (more "pressure" at the tap).

If that's confirmed to be the problem you may need to install a water softener.

And coil cleaning is not always possible: if the coil is corroded and perforates it'll have to be replaced; furthermore, some sources warn that the acid cleaning etches the coil internal surfaces, speeding the re-clog rate unless a softener is installed.

All those warnings said, it's worth a try.

Details you should read are at CLOGGED PIPES / TANKLESS COIL DE-SCALE

Question: lower hot water temperature when not at home?

(Mar 4, 2014) Anonymous said:
Can I turn down the hot water temperature during the day when not home?


You could, but if we are talking about a tankless coil hot water supply system and we are talking about winter use during the heating season, lowering the LO or changing the DIFF - the two aquastat controls that have to do with the tankless coil probably won't do a thing since the call for heat in the home is what keeps the boiler hot - that's controlled by the HI limit control on the aquastat.


However in general, lowering the room thermostat settings when you are not at home is indeed a good way to save on heating costs.

Question: boiler cycles on and off even when not calling for heat

(May 26, 2014) JGreen said:
We have an oil fired boiler/ tankless water heater. This is continually cycling on and off even ( not related to home air temperature) so presumably to keep the water hot. I have received conflicting opinions on wether the coil can or should be descaled. Can you tell me whether sediment build up is possible? And if so can cleaning be done to reduce this oil consumption. thank you.

Further to my question ... , we do not have a problem with water flow, is that the only indicator of a coil that may ned servicing or replacing? We have been told by the annual furnace maintenance person that coils cannot have mineral deposits unless we are in a hard water area. That does not make sense to me. it is a Saturn high efficiency boiler ,Model K 26 is on the plate.



The boiler cycling on and off and oil consumption is a separate issue having nothing obvious to do with coil scaling.

The boiler is controlled (most likely) by an aquastat. Check your aquastat settings - see


Coils do indeed become blocked with lime or scale; minerals settle out of water fastest at the hottest place in the system - the coil.

Your boiler company is right that scale formation only occurs at problem levels where the water supply is high in mineral content.

Yes a tankless coil can be de-scaled, See CLOGGED TANKLESS COIL or PIPES, LIME SCALE at for details.

But if the hot water flow is not diminished then the scale problem is probably not yet severe on your system.

Watch out: de-scaling can leave the coil interior etched so that scale forms faster than before.
If scaling is found to be a significant problem the better fix is to install a water softener.
Or see

Question: limit switch for water to air heating control as well as separate tankless coil control

(Jan 2, 2015) John Dee said:

Correction to the post below: It's actually a Johnson Controls limit switch.

I have an oil furnace that includes a tankless coil for hot water, which has its own anti-scald valve. The furnace also sends hot water to a forced air handling system. This air handling system has one of the "Single Function Boiler Temperature Limit switches" pictured above, connected to the hot water going in to the air handling system. I can't find any information as to what the temperature should be set to! Any ideas?



We need to know what the switch is supposed to be doing. I would expect that located at a water-to-air heat exchanger the limit switch could be in use to call for heat OR to keep the blower from running if the water temperature is too low - to avoid blowing cool air on occupants; but I don't know your system or what other controls are present. Give us the switch model number and the air handler brand and model number and we might find the manufacturer's recommendations.

What is the present limit switch setting? Who set it? When?

Meanwhile a safe setting is probably 120F

Also see


Question: long time for hot water to arrive from tankless coil to the bath fixture - debugging a hot water problem with a tankless coil

27 January 2015 Dick shouted (caps deleted)

Have summer/winter (tank-less domestic h2o)oil furnace. Must run hot h2o for 5-10 min to get hot water. Once it gets hot have plenty of hot h20 for the rest of the shower. What's up with this?



If the boiler itself were not hot you'd not get hot water from a tankless coil since the flow rate through the coil typically uses hot water faster than the burner on your boiler can keep up. (Furnaces, to be technical, are forced warm air heating systems, boilers are hot water heating systems) .

So I suspect that you have long piping runs between the boiler and the point of use and that those pipes are cold. You can improve matters by insulating the accessible portions of your hot water piping.

(Jan 27, 2015) DICK said:
Dan; the furnace is producing plenty of heat for the house, the house is base-board hot h2o. It just takes for ever for the shower to get hot, then everything is ok for the rest of the shower.



Check right at the boiler when you turn on hot water; you should feel hot water quickly in the supply lines near the boiler. If so then the problem and fix are as I suggested in the FAQs above.

(Jan 29, 2015) Dick said:
Dan When the hot water is first turned on at the shower it gets warm (not hot) fairly quickly, after a minute or so. Then it gets a lot cooler to almost fully cold. Then after a long time running full blast (5 to 10 min.) it gets fully warm/hot and stays that way for the rest of the shower.

I looked at the coil plate on the furnace which looks similar to the picture at the top of this page. Except there is no control box, just an electrical cable which runs to a control box on the side of the furnace where the house thermostat is connected. Could that cable be attached to an aqua stat inside the domestic coil, if so could that aqua stat be bad and need replacing?


Dick often warm water that shows up almost immediately at a shower that's not close to the water heater is water that was warmed by proximity to heating piping or some other warm spot. Once that shot of warm water has passed we're going to see cold water that was in the rest of the piping run until warm or hot water from the water heater itself shows up.

Just how quickly that happens depends on

- the length and diameter of the piping run
- the flow rate at the plumbing fixture
- the temperatures of the building through which the piping runs
- how long the piping has been idle - no water running
- the heat loss or gain rate for the piping run, pipe insulation, exposure to drafts etc.
- the temperature of the hot water stored in the water heater (unless you're using a demand system like a tankless water heater or tankless coil).

For a tankless water heater, the heater comes on as soon as water starts to flow and heats water in seconds as it passes through the heater; though you can get a shot of cold if flow rate falls slow enough to turn off the heater via its safety controls.

For a tankless coil the water temperature in the boiler determines how much heat the coil feels and thus how much is transferred into the hot water headed for the fixture. The aquastat maintains heat in the boiler for the tankless coil - as we explain at AQUASTAT CONTROL Functions in the More Reading articles above.

If your water gets plenty hot but takes 5 minutes for the hot water to arrive, and if your water is made by a tankless coil, then the piping run must be long and cold.

I'm quite unclear just what kind of hot water system you've got installed. You can use our email found at CONTACT US to send some photos if you like.

(Jan 30, 2015) Dick said:

I understand what your saying. All the points you raise have not changed for many years. There is good H2O volume and pressure at the shower head as has it been for many years. The H2O temp performance has been very good until now.

This long heat up situation has been getting worse over the last couple of months. Saying again, all performances of the furnace has been good for years until this situation arose. For some reason the furnace seems to take a long time to realize that the tank-less coil needs more heat.

Is your best guess as to what to do would be to change the electronic piece that is screwed into the tank-less coil mount plate or would a new control box be a better guess (even given that it is working fine as related to the house heat function)? If so what is that part called and could I get it at Lowe's?


Dick without seeing your system I'm not sure what's installed so we're flying a bit blind.

Typically the "electronic piece" you describe in your note is a limit control that turns on the boiler to keep it hot so that there will be hot water for heating the coil even when you're not calling for heat. If that control fails the boiler may not be hot when it should be.

Go to the boiler, note its temperature, and then turn on hot water in the home and feel the pipes to see if hot water is leaving at the tankless coil.

Question: basement flood after installing an electric water heater to replace a tankless coil

(June 1, 2015) Steve said:
I have a heater boiler with an internal coil hot water heater that never worked right, so we installed an electric hot water tank. A boiler company cleaned my heating system and said that I do not need the boiler hot water valves open if I have an electric hot water heater.

There are two settings, one for the heat and one for the hot water. He closed the hot water valves from the boiler but never turned the temperature off from the Honeywell thermostat for the hot water. When I turned on my heat, the pressure relief valve blew and my expansion tank flooded.

They came back, my basement flooded, replaced my expansion tank and some valves, charged me $400 taking no blame. Then, comes back after 1 minute after I paid him and opened the valves the other guy closed. Telling me the thermostat needs to be off if the valves are closed.

My question is: "Could leaving the hot water thermostat ON with the valves closed cause the pressure relief valve to open when the heat is turned ON?"


Which hot water thermostat? Normally the boiler's TP valve spills if the boiler is over-pressure. That might happen IF the tankless coil leaked water into the boiler.


Tankless Coil Water Heater Articles


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