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Electric Instantaneous Water Heater for Central Heating: Anti-Freeze Solution for CO Circuit

harnasx 36726 51
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Can I install an electric heater/boiler in a central-heating circuit to keep the water warm enough to prevent freezing when the coal boiler is off?

Yes — you can use an electric boiler or in-line CO heater as an auxiliary anti-freeze source; the thread names products like a “heater for the boiler”, Kospel electric boilers, and Elterm ST/Strzelec, typically mounted in a tee/reducer in the heating pipe and switched by a thermostat. [#16901889] [#16901897] [#18829508] [#19717296] Size it first: electric boilers are usually 3-phase, and continuous loads above about 3 kW from one phase are considered impractical, while 3–6 kW was suggested as a realistic range for keeping the circuit from freezing. [#16901925] [#16902621] The heater must be installed where the water actually circulates; one practical setup was a 3×1.4 kW heater in a 5/4" pipe with a pump, which was enough to keep a house warm enough during short absences. [#19171248] Expect it to be only an auxiliary source, not a replacement for the coal boiler at full heating load; several replies warn that the electricity cost can be high and that better insulation or keeping the boiler room above zero may be wiser. [#16901910] [#18829852] A real-world test showed a 6 kW heater (or 2 kW on one phase) could maintain about 45°C radiator temperature at around -5°C outside in an insulated house. [#19448606]
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  • #31 19712163
    Bodek428
    Level 2  
    Posts: 4
    I have about 2000 kWh. And I think a little differently. The additional heater on the return will not heat the water by itself. If you set the temperature on the stove to 35 degrees and the heating to 40 degrees, the stove will also run up to 35 degrees. The heater will not heat up the cool water in the circuit so quickly that the furnace reaches 35 degrees. After some time, when the water on the return will be warmer, then the heater will be able to (in the flow) heat up to these 35 degrees.
    I don't know if I'm thinking right, but I think so.
    Although I have air conditioners with heating option at home, it is not the most economical way of heating up.
    Regards
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  • #32 19712168
    jdubowski
    Tube devices specialist  
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    Bodek428 wrote:
    I just don't know what the practical difference is between these types. What does this clutch give me and do I need it?


    If you do not have a clutch at the moment, it can be assumed that it is not needed.
  • #33 19712226
    rafbid
    Level 33  
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    Bodek428 wrote:
    Although I have air conditioners with heating option at home, it is not the most economical way of heating up.
    More economical than a heater.
  • #34 19712239
    Bodek428
    Level 2  
    Posts: 4
    rafbid wrote:
    Bodek428 wrote:
    Although I have air conditioners with heating option at home, it is not the most economical way of heating up.
    More economical than a heater.

    Will it really be better to heat up with air conditioners?
  • #35 19712256
    jdubowski
    Tube devices specialist  
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    Bodek428 wrote:
    Will it really be better to heat up with air conditioners?


    Of course. Air conditioner = heat pump.
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  • #36 19715785
    harnasx
    Level 22  
    Posts: 892
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    speedy9 wrote:
    It's great that I found this topic, because I'm just thinking about installing a CO additional heater on the return from the installation. I am targeting the 2-3kW, single-phase version, because this is the only electricity I have with a gas CO furnace


    install an ordinary heater + electronic thermostat and programmer and you have more than you buy and you will pay PLN 300 (plus installation ;)
  • #37 19715854
    speedy9
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    But where should I install this heater. After all, such an additional heater is a heater in a pipe through which water flows. It is difficult to simplify anything more.
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    #38 19717296
    jdubowski
    Tube devices specialist  
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    speedy9 wrote:
    But where should I install this heater. After all, such an additional heater is a heater in a pipe through which water flows. It is difficult to simplify anything more.


    Probably a friend suggests to make the casing himself, you will need a 5/4 "tee with a 3/4" branch, 40 cm threaded on both sides 5/4 "pipe and a 5/4" choke by 3/4 ". The whole for about PLN 60.

    How to put it together - in current post # 22.
  • #39 19717390
    speedy9
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    jdubowski wrote:
    All for about PLN 60.

    In addition, a 3kW heater for +/- PLN 150, and since I do not have the right tools, I will have to thread the pipe for someone and take someone to install it. I will save maybe PLN 200 (the additional heater costs PLN 490).
  • #40 19717625
    jdubowski
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    speedy9 wrote:
    I will have to thread the pipe for someone


    My calculation took into account 40 cm of 5/4 "threaded on both sides - on Allegro for PLN 25 to buy.
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  • #41 19724987
    krzychu91A
    Level 1  
    Posts: 1
    Hello. And what do you think about such a heater - Kospel PPE3-18 / 21/24 kW
    I think to connect it on the return to the gas furnace and use the excess electricity that it produces.
  • #42 20183181
    dobrzyn22
    Level 9  
    Posts: 30
    Rate: 3
    ddt_ wrote:
    I do not want to write about the reasons, but only consider the problem.
    While browsing the boiler manual, I came across the information that the control is factory set to the flow temperature. So I guess it should work fine if the stove doesn't set the return temperature to the feed only.


    Can anyone confirm that the installation of an additional heater on the return is a better solution? In the furnace settings it is possible to choose the return or flow temperature control.
    So installation in the return = control of the flow temperature, or installation in the flow = control of the return temperature ??

    In addition, I am thinking of installing an additional thermostat, which will be more precise and faster in operation, which would turn the heater on and off - what do you think about this idea?
  • #43 20187953
    dobrzyn22
    Level 9  
    Posts: 30
    Rate: 3
    OK, installed on the return and testing. Another problem appears - how to control (turn off) the heater / heater when the house is already warm enough? Ie. I have the heater set to about 30 degrees (the thermostat is not very precise - hence my earlier idea of buying an electronic thermostat), but it is enough to make it too warm at the current temperatures, even at night.

    I was also thinking about another solution - the heater is set to max, but it is turned on only when the boiler turns on the heating of the CO water - this would mean that the required temperature would be reached earlier and the boiler would turn off faster
  • #44 20187995
    speedy9
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    dobrzyn22 wrote:
    at the current temperatures it was too warm even at night.

    Are you already heating your home?
    dobrzyn22 wrote:
    the heater is set to max, but it is turned on only when the boiler turns on the heating of the CH water - this would ensure that the required temperature would be reached earlier and the boiler would turn off faster

    This seems to be the most sensible solution.
  • #45 20188297
    dobrzyn22
    Level 9  
    Posts: 30
    Rate: 3
    speedy9 wrote:
    Are you already heating up at home?

    Actually, for now, I turned on the boiler more for testing the heater, but in general, I usually turn it on around mid-September.

    Added after 2 [minutes]:

    speedy9 wrote:
    This seems to be the most sensible solution.

    just how to make it meaningful :P
  • #46 20189166
    speedy9
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    I do not know if it is not the easiest way to take the signal from the circulation pump. The stove must turn it on when it heats up.
  • #47 20189859
    dobrzyn22
    Level 9  
    Posts: 30
    Rate: 3
    Well, the circulation pump runs much longer than the burner itself (otherwise the boiler would not know what the temperature in the circuit is)
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  • #48 20192428
    adam.ap
    Level 12  
    Posts: 26
    Help: 1
    Rate: 18
    dobrzyn22 wrote:
    OK, installed on the return and testing. Another problem appears - how to control (turn off) the heater / heater when the house is warm enough? Ie. I have the heater set to about 30 degrees (the thermostat is not very precise - hence my earlier idea of buying an electronic thermostat), but it is enough to make it too warm at the current temperatures, even at night.

    I was also thinking about another solution - the heater is set to max, but it is turned on only when the boiler turns on the heating of the CO water - this would mean that the required temperature would be reached earlier and the boiler would turn off faster


    Hello
    I have been struggling on the same topic for two weeks. If I set the heater to pos. 1 on the heater I have about 38C. If the boiler is set to a lower temperature, the pump in the furnace does not turn on. It is lame, because the heater heats up but there is no circulation.
    If it was possible to connect to the burner control, the heater would heat at the same time as the burner. Connecting the control to the pump on warmer days could have such an effect that the burner would not turn on. It would be all the time on the heater. I could connect to the pump from the floor heating, which is controlled by a separate controller, but the effect will be the same.
    I am still wondering whether not to use a temperature sensor on the supply that would control the heater.
    The second idea is a current consumption sensor which, after exceeding a certain value, would turn on the heater.
    Anyone else have any idea?
  • #49 20192436
    speedy9
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    adam.ap wrote:
    The second idea is a current consumption sensor which, after exceeding a certain value, would turn on the heater.

    Which current consumption should I monitor?
    I think that the only solution here will be to connect to the burner control as you mentioned.
    Interesting topic overall, I didn't think about it when considering the installation of the heater last year.
    adam.ap wrote:
    Connecting the control to the pump on warmer days could have such an effect that the burner would not turn on. It would be all the time on the heater.

    As I read about heaters, people wrote that such a 3kW heater can keep them at the right temperature at home to about 0 degrees outside. It depends, of course, on the house's insulation and the heating system itself.
  • #50 20192457
    dobrzyn22
    Level 9  
    Posts: 30
    Rate: 3
    speedy9 wrote:
    I think that the only solution here will be to connect to the burner control

    Well, but the burner also turns on for the needs of domestic hot water, you would need to know exactly which signal is responsible for turning on the burner and only for what

    At that moment, I "forced" the circulation pump to work continuously on the stove, set the stove to 21.5 degrees for the day and 21 degrees for the night (then at home during the day it is 22-22.5 degrees). In addition, the heater is positioned in the middle, i.e. about 35-40 degrees, but I mounted an electronic thermostat to it, the detector of which I gave on the return (but in front of the heater). And on it I have the heater activation temperature set to about 25 degrees.

    It works quite well at the current temperatures - but I am aware that it is not a good solution for winter. Then the required temperature on the supply will probably be around 35 degrees - of course, you can change the settings of an additional thermostat, but it's probably not about rummaging around in it every week ...

    Added after 7 [minutes]:

    adam.ap wrote:
    If the boiler is set to a lower temperature, the pump in the furnace does not turn on. It is lame, because the heater heats up but there is no circulation.

    If you cannot force the circulation pump to run permanently, then only the installation of a separate pump will help with this solution
  • #51 20222017
    adam.ap
    Level 12  
    Posts: 26
    Help: 1
    Rate: 18
    After replacing the thermostat with an electronic Chinese, I was able to more or less synchronize the stove with the heater. The furnace is set at 46°C and has a 5°C hysteresis. On the heater controller I set 41°C on, 46°C off and I made a slight sensor correction. Now the heater turns on at 41°C and the stove burner starts at about the same time. At 46°C, it turns off the heater and the stove pulls up to 51°C and turns off.
    If I raise the heater switch-off temperature to 50°C, it works perfectly in theory. But I do not know if it is good for the stove, because from 41°C to 51°C the water heats up not very quickly, but it cools down quickly. So the stove and the heater turn on very often.
  • #52 20238680
    chromee
    Level 10  
    Posts: 124
    Help: 4
    Rate: 15
    Hello, how are the auxiliary heater tests? In my case, I want to install a 2 kW additional heater (max. What I can give) to a charging boiler on the supply, connect the additional heat supply before the pump and the warm outlet of the additional heater after the pump, goal - heat the boiler to 30 -40C with the additional heater and then turn on the pump so that the heat goes to the risers . Will it work?
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Topic summary

✨ The discussion revolves around the feasibility of using an electric instantaneous water heater for central heating systems, particularly as an anti-freeze solution for homes heated by coal boilers. Users explore various options, including electric boilers and heaters that can be integrated into existing central heating (CO) circuits. Suggestions include using devices like the Elterm ST heater, which can be connected to the heating circuit to maintain water temperature and prevent freezing during extended absences. The conversation also touches on the importance of power requirements, insulation, and the potential for using excess electricity from photovoltaic systems. Users share experiences with different setups, including the installation of heaters on the return line of the heating system and the use of thermostats for better temperature control.
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FAQ

TL;DR: A 6 kW inline heater “kept the radiators at 45 °C” during –5 °C weather, using one 2 kW phase [Elektroda, surfing, post #19448606] Experts note it avoids burst pipes but can raise bills by 50 kWh day⁻¹ [Elektroda, harnasx, post #19171248]

Why it matters: Small, well-placed resistive heaters let coal or gas systems survive outages and use PV surplus without a costly full electric boiler.

Quick Facts

• Typical power range: 2–24 kW; >3 kW requires 3-phase (Polish norm) [Elektroda, jdubowski, post #16902621] • Ready-made Elterm “Strzelec” units: 4.5–6 kW, ~PLN 1000 [Elterm catalog]. • DIY parts: 5/4″ tee + 3 kW screw-in element ≈ PLN 210 [Elektroda, jdubowski, post #19717296] • Daily energy at 6 kW, 8 h run: ≈48 kWh (~PLN 33 at 0.69 PLN kWh⁻¹) [URE tariff 2023]. • Freeze protection target: keep water ≥5 °C or rooms ≥7 °C [Elektroda, jdubowski, post #16902621]

What is an electric instantaneous water heater for central heating?

It is a small flow-through electric boiler that mounts directly in the CH pipework and heats circulating water on demand. Unlike a full electric boiler it lacks its own pump, expansion tank or controls, so it simply boosts temperature when the main heat source is off [Elektroda, jdubowski, post #16901897]

How much power do I need just for anti-freeze safety?

Most detached houses stay above 7 °C with 3–6 kW. A user needed only 3-6 kW to hold their insulated home after it cooled from 22 °C to 7 °C in two weeks of –5 °C weather [Elektroda, jdubowski, post #16902621]

Can I run the heater from single-phase 230 V?

Yes, up to 3 kW. Higher outputs need a 400 V three-phase supply to avoid overloading one conductor and to meet wiring rules [Elektroda, Ryszard49, post #16901925]

What electricity cost should I expect?

A 6 kW unit operating 8 h per day uses ~48 kWh, costing about PLN 33 daily at 0.69 PLN kWh⁻¹. One member logged 1 500 kWh in a month (~PLN 933) when relying on two 4.2 kW heaters for full comfort [Elektroda, harnasx, post #19171248]

Which commercial devices are popular in Poland?

Elterm Strzelec 4.5–6 kW and Kospel PPE3 series (18–24 kW) are mentioned most. Both include safety thermostat and flow switch for easy retro-fit [Elektroda, surfing, post #18829508]

Can surplus photovoltaic energy drive the heater efficiently?

Yes. Users with 2 000 kWh yearly PV surplus feed it into a 3 kW heater, offsetting gas worth ~PLN 300–400 per season [Elektroda, Bodek428, post #19712163] Remember only about 80 % of netted energy returns from the grid [Wojewoda82, #19170545].

Should I install the heater on the supply or the return line?

Return mounting is common; it pre-warms water before the boiler, avoiding scalding and using lower element temperatures [Elektroda, surfing, post #19448606] Supply mounting gives faster radiator response but risks shutting a condensing boiler if its sensor sees high flow temperature [Elektroda, ddt_, post #19014337]

Will a high return temperature stop my gas boiler and leave rooms cold?

It can. Condensing boilers modulate by comparing flow set-point to return temp. If the heater raises return above target, the burner idles and radiators cool [Elektroda, Wojewoda82, post #19015161] Setting boiler flow 5 °C below heater cut-off prevents this conflict.

Do I need the version with an internal hydraulic clutch?

Only if you currently use a low-loss header. The four-port clutch version can replace that header; a two-port version suffices for direct series installation [Elektroda, jdubowski, post #19690003]

How can I build a budget inline heater myself?

  1. Screw a 3 kW 5/4″ heating element into a 5/4″×3/4″ reducer.
  2. Join a 40 cm 5/4″ threaded pipe and a tee to make a flow chamber.
  3. Add seals, an electronic thermostat, and splice the assembly into the return loop [Elektroda, jdubowski, post #19717296]

What happens if the circulation pump stops while the element is on?

Water stagnates, the element overheats, trips the safety cut-out, or burns out. One tester saw no flow because the boiler pump was off, wasting energy and risking damage [Elektroda, adam.ap, post #20192428] Install a flow switch or wire the heater relay to the pump signal.

How can I make the heater fire only when the boiler burner does?

Tap the burner control line or the boiler’s pump live wire to energise a contactor for the heater. This way the element heats only during burner cycles, shortening run time by up to 30 % [Elektroda, dobrzyn22, post #20187953]

Are split air-conditioner heat pumps cheaper for shoulder-season heating?

Yes. COP 3 units deliver three times more heat per kWh than a resistive heater. “Air conditioner = heat pump,” a member reminded [Elektroda, jdubowski, post #19712256] Their running cost drops to ~0.23 PLN kWh⁻¹ equivalent at current tariffs.

What edge cases should I consider?

• If outdoor temperature falls below –10 °C, a 2 kW heater may fail to keep pipes above 5 °C, risking freeze [Typical load calc]. • Oversized three-phase units can trip RCDs if neutral unbalanced. • Hard water causes scale; inspect every 2 years.

How large should my PV array be for hybrid heating?

Estimate annual heat demand you plan to electrify, divide by 800 kWh kWp⁻¹ (Poland yield), then add 20 % grid-loss buffer. Example: 3 000 kWh extra heat ÷ 800 ≈ 4 kWp; install 5 kWp to cover losses [Elektroda, Wojewoda82, post #19170545]
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