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.

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

These questions & answers about tankless coils or hot water coils were posted originally

at TANKLESS COILS - home - so you'll also want to review the installation and troubleshooting information organized there.

On 2017-12-05 by (mod) - pressure relief valve and expansion tank ON a hot water tankless coil in a steam boiler?

Not normally found on that system, but in SOME jurisdictions the local code may require a pressure relief valve on the hot water line at or near the tankless coil.

And of course the steam boiler has its own TPR valve - keep in mind that residential steam operates at very low pressure, like 0.5 psi while the domestic hot water might be anywhere between 20 and 70 psi in a single family residence.

On 2017-12-05 by clambx

Are a pressure relief valve and an expansion tank needed for a hot water tankless coil in a steam boiler?

On 2017-12-01 by (mod) - ratings for actual hot water delivery from a tankless coil

Lynn

Noting that a finger test of hot water is not a quantitative nor objective measurement, it's subjective.

The word "intermittent draw" in the company's description is essential to understand the hot water provided by a tankless coil on a heating boiler. That phrase means "not-continuous" - that is saying that the coil manufacturer does not claim that the system can give endless hot water.

No home heating boiler oil burner, a device sized and fired to provide home heating, can ever provide a high enough BTU rate to keep a tankless coil flowing hot water endlessly.

On 2017-08-24 by Lynn Harry

Thank you for your comments, Here are the boiler Specs, Utica Boilers, Starfire III Model SFH 3100, Tankless heater #S15,. 5 gpm Intermittent draw. It is noted that both hot & cold water flows are identical,

Our City water pressure is about 90psi.. as for the finger test, index finger used with hot water spigot fully open for approx. 5 minutes.

note that this water is hot from the get go and is regulated with a mixing valve.. Thank you for the input.. I will challenge any negative reports regarding this tankless heater..

On 2017-08-24 by (mod) -

Adding:

Boiler manufacturers like Dunkirk provide tankless coil GPM water heater flow rates for boilers that include a tankless coil. That data reminded me that I should have included in my list of determinants of tankless coil water heating capacity, the firing rate of the boiler, such as an oil burner rated in some number of gallons per hour, like 1 gph.

Eg: A Dunkirk Model 4EW1.25T with a nozzle and firing rate of 1.25 gph of oil, and an input rate of 175MBTU will deliver 3.50 gph of hot water raised from 40 degree incoming temperature to 140 degF output temperature, with the boiler water at (a maximum setting) of 200 degF and **intermittent** hot water draw.

The manufacturers are being very honest that no tankless coil gives infinite hot water at any temperature.

On 2017-08-24 by (mod) - "finger test" for hot water?

Lynn

I'd sure like to know the details of the "finger test" - doesn't sound too objective.

Most tankless coils are rated as capable of providing a specific temperature degree rise across the coil, from input to output, at a specified boiler temperature and at a specified water flow rate in gallons per minute (often 5 gpm). If the water flow rate is faster than specified the coil will not heat water to as high a temperature.

There is no ASTM or industry standard single tankless coil temperature rise number, but a typical figure might be like this (from Lindy Tankless Coils)

With boiler water at 180 degF the tankless coil can raise incoming water from 40 degF to 140 degF at a flow rate of 5 gpm.
But
Watch out: any and all tankless coils can suck heat out of the boiler faster than the burner can put heat into the boiler. So you never ever will have endless hot water.

The temperature increase as water runs through a coil is indeed specified or at least limited by the manufacturer, but the actual temperature increase will depend on at least:

- the temperature in the boiler water itself

- the temperature of the incoming street water

- the flow rate of water through the system

- the presence of and setting of a mixing valve or tempering valve at the boiler.

Watch out: In my OPINION the finger-for-too-cool-temperature test would be not the most useful test for a tankless coil since that's not really the worry nor the issue.
The worries are

1. the coil is making water too hot, scalding someone - fixed by installing or adjusting a mixing valve

2. the coil is clogged with mineral deposits - detected by noticing that hot water flow rate is significantly slower/weaker than cold water flow rate at the same tap or fixture - fixed by cleaning or replacing the coil.

I wouldn't waste time nor upset the sale of your home over this. I'd instead discuss the concern with your realtor or your buyer if there's no realtor involved, and ask how the finger test could explain a need for a new coil.

If communication was poor and on clarification there is actual evidence there's actually a new coil needed, such as a leaky coil or one that is so jammed with mineral deposits that it can't be repaired, I'd offer an allowance in consideration that if that's the issue on sale of a home it certainly isn't even 0.1 percent of a typical home sale price.

On 2017-08-24 by Lynn Harry

Does a standard exist for the relationship of temperature drop over a specified time. I just received a home inspection for the sale of my home. The inspector commented that I may need a new tankless coil for my hot water boiler.

He used the finger test? he ran the hot water for a period of time and said that the coil may need replaced as it was not to his satisfaction, The boiler setting HI 200 degf, lo 180 degf, dif. 20 degf. so is a astm standard available for this type of test. I need to challenge his method of testing!!

On 2017-05-15 by (mod) - using a hot water recirculating pump on a tankless coil

Thanks for an interesting question about using a hot water recirculating pump on a tankless coil hot water heating system, PB

I certainly respect the quality of components made by both Watts and Grundfos and have used both of them. My impression is that Grundfos offers more models (over 20) of hot water circulator pumps so might have more experience in alterenative designs.

But NEITHER of these pumps is designed for a tankless coil hot water heating system: circulating hot water through the tankless coil in the boiler will cause the boiler to run intermittently 24 hours a day even when you are not using hot water, as you're now keeping the water in the piping loop hot.

Because the amount of heat stored in a modern high efficiency home heating boiler is probably less than that stored in a 40 or 50 gallon hot water heating tank (cylinder, calorifier, geyser - depending on where you live), short cycling may be an issue for some systms.

It MIGHT be possible to use a circulator pump anyway in YOUR tankless coil hot water system design, arguing that the volume of water in the hot water piping is so small that it won't cause the boiler to run too frequently. But I would NOT install either of these without first discussing the idea with the manufacturers (Watts and Grundfos).

The Grundfos Comfort PM Auto ($175 - $225. U.S.D.) is a low-noise, low-power-consumption hot water circulating pump (5 to 8.5 Watts) using a control that runs the pump intermittently when needed to keep hot water in the loop sufficiently hot. It also has a 100%-on or "always-on" mode that can be selected if needed. The company's sales literature describes it as

The COMFORT pump with AUTOADAPT features three operation modes.

The AUTOADAPT mode learns, stores and adapts operation time to the consumption pattern of the home owner. The temperature mode keeps the water temperature within an automatically detected
range in the individual system. And finally, the 100% mode lets the pump run constantly at full speed. Regardless of the operation mode, the COMFORT range provides silent operation and hot water in an instant.

The Grundfos pump uses two temperature sensors, one within the pump and the other connected to the pump by a cable installed in the hot-water flow pipe. The pump is installed in the hot-water return pipe.

This pump also has a vacation mode I don't see on the Watts model:

If no tappings are detected in a 24-hour period, the pump will (self-modulate) automatically switch to standby mode (except for disinfection) and reactivate to the same consumptive pattern when 2 tappings occur within a 20-minute time period.

As with all pumps of all types, the actual flow rate capacity of the pump depends on the lift or head height of the water system - something that may not be much of a worry in a residential hot water heating loop system.

The GPM ranges from 2.2 gpm at 1.5 ft. head, to about .2 gpm at about a 3.75 ft. head. For a typical home my OPINION is that unless your hot water piping loop is long and uninsulated and running through a cold space, even a low flow rate will still keep the water hot enough.

The Watts 50800 hot water recirculating pump ($120. to $220. USD) uses a programmable 24-hour automatic timer to control pump operation. The company's specification sheet describes their pump and includes a warning that might come up with the Grundfos too:

A pump with a built-in programmable timer (push-pull pins or tabs on a clock dial) is installed on the hot water line from the water heater (Fig. 1). A patented sensor valve (Fig. 2) opens when the water on the hot water side cools and pushes the cool water back to the water heater. As the temperature in the hot water line hits 98º, the valve closes.

Some homes are designed with multiple hot water loops, one per floor, etc. If one section of the house does not receive hot water, you will need to purchase a Watts Sensor Valve Kit (Fig. 3) for each loop. For best results, the valve should be located at the faucet furthest from the water heater in each loop.

As standard, both of these circulators mount at the water heater (gas or electric, possibly oil for some models), and both need a nearby electrical receptacle for powering the pump.

In my OPINION either will work perfectly well with a tank type water heater; both sell in a similar price range. The Grundfos has a more automated self-programming control that may appeal to some users.

Contact

Grundfos: Grundfos North America
2001 Butterfield Rd, Suite 1700
Downers Grove, IL 60515-1011

Watts
Tel: 800-752-5582
Website: www.watts.com
Watts Water Technologies, Inc.
Watts USA
815 Chestnut Street
North Andover, MA 01845-6098 USA
P: +1 978 688 1811

If you are not in North America, both companies have services and sell products world wide.

On 2017-05-15 16:52:53.555825 by pbmass

I have a tankless coil in my boiler which combined with a Watts LFMMV thermostatic valve produces sufficient hot water. The problem is it takes 1 1/2 minutes for hot water to reach the baths & the kitchen. I would like to install a Watts 500800 hot water recirculation pump and a 3-4 gallon expansion tank. Any real differences between the Watts 500800 and the Grundfos Comfort PM systems? I have been told this will not work on a tankless but I think there is confusion on tankless which can be the coil in the boiler and not the new point of use electric tankless heaters.

On 2017-04-19 by (mod) - What side of coil pipe cold water if no label on coil

Great question.

If the coil is in use and boiler works, feel the pipes when hot water is running. Cold in will be chilly and hot out hot.

If the coil has been used for a Time, cold inlet pipe will usually be dark in color while the hot Outlet pipe will not.

Else we are stuck, unless we remove the coil gor inspection, but the coil will work even if connected backwards.

On 2017-04-19 by Anonymous

What side of coil pipe cold water if no label

On 2017-01-30 21:16:37.054196 by (mod) -

And you'll want a water softener if you don't already have one.

Indeed hard water and tankless coils don't get along. I used to start a hot water performance by stopping by the kitchen sink to compare hot and cold water pressure. If the hot flow rate was notably less than the cold flow rate I'd bet there was a tankless coil installed, and if the area was prone to hard water I'd bet on coil clogging.

On 2017-01-30 by (mod) -

Anon

I think you're quite right. Once the expense starts creeping up I'd look at an indirect-fired water heater - see https://inspectapedia.com/plumbing/Indirect-Water-Heaters.php

On 2017-01-30 by Anonymous

Thanks for the response. I already have a mixing valve which i replaced trying to get the tempature/pressure of the hot water increase but no luck. The 5 GPM coil is 286$ and the 10 GPS is 585$

and at the price point i assume i should just look into a 50 gallon separate indirect heater or electric water heater. Would you agree? We have very hard water and the boiler is already 26 years old so know idea how long it may last.

On 2017-01-30 20:43:04.638443 by (mod) -

That is, as perhaps you know, a flow rate through the coil that the coil claims to support. Faster flow rates of water through a tankless coil will mean that it won't get as hot, perhaps not as hot as you'd like. Naturally the temperature of the boiler water itself is critical in determining the amount of heat transferred from the boiler into the domestic hot water supply produced by the tankless coil.

As many users find that the limited quantity of hot water provided by any tankless coil (in the boiler) type of hot water system is less than they'd wish, I would install the highest capacity coil I could obtain, and I'd combine that installation with an anti-scald valve or mixing valve that will both increase the *apparent* total amount of hot water you'll get and it will avoid scalding burns.

See https://inspectapedia.com/plumbing/Tankless_Coil_Hot_Water_Increase.php TANKLESS COIL HOT WATER INCREASE for more detailed advice.

On 2017-01-30 20:23:21.207760 by Rob

Im looking into replacing my coil on my burnham boiler. But when I search for replacement parts im given the option of selecting Cap GPM from 5-9. Im not sure what i have currently or how to determine what is the correct size.

On 2016-07-01 15:56:24.242104 by (mod) -

Bob
If the tankless coil is the primary or only domestic hot water source, then the boiler has to be hot to produce hot water for washing and bathing.

On 2016-07-01 14:14:06.820546 by Bob

I'm a new homeowner and I have oil furnace. in order to get hot water do you need to fire the furnace? even if it has a coil?

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.

Reply:

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?

Reply:

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.

See AQUASTAT CONTROL FUNCTIONS

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

See THERMOSTAT SET PROCEDURE

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.

Reply:

JG

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

AQUASTAT HI LO DIFF SETTINGS

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

inspectapedia.com/plumbing/Water_Heater_Scale_Prevent.php

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?

Reply:

John

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

LIMIT CONTROL, SINGLE - at inspectapedia.com/heat/Boiler_Heater_Limit_Controls.php

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?

Reply:

Dick

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.

Reply:

Dick

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:

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?

Reply:

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?

Reply:

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?"

Reply:

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.

...

Continue reading  at TANKLESS COILS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these  

Tankless Coil Water Heater Articles

• TANKLESS COILS - home
  • AQUASTAT CONTROL FUNCTIONS
  • CLOGGED PIPES / TANKLESS COIL DE-SCALE
  • MIXING / ANTI-SCALD VALVES
  • TANKLESS COIL CONVERSIONS
  • TANKLESS COIL HOT WATER HEATER TEMPERATURE CONTROL
  • TANKLESS COIL HOT WATER INCREASE
  • TANKLESS COIL INSTALLATION PROCEDURE
  • TANKLESS COIL INTERNAL LEAKS INTO the BOILER
  • TANKLESS COIL / HOT WATER COIL LEAKS
• TANKLESS WATER HEATERS - home

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