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Choosing Furnace & Heat Buffer Size for 114m2 House, Solar Panels & Underfloor Heating

spinkamidi 67860 36
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How do I size a solid-fuel boiler and heat buffer for a 114 m2 house, and how should I connect it with underfloor heating, domestic hot water, and solar collectors?

A sensible starting point is 40–80 l of buffer per kW of boiler power, so a 25 kW solid-fuel boiler points to roughly 1000–2000 l; 1500 l is a reasonable compromise and a larger tank is more comfortable [#13114370] Use a Laddomat or similar mixing valve to keep the boiler return around 65–72°C and the boiler itself at about 80–85°C so the buffer charges in layers instead of mixing, and let a 3-way valve set the underfloor temperature from the buffer [#13153325][#12990250] For DHW, use a separate tank with two coils: one for solar and one for boiler water; typical bivalent tanks are not ideal because the boiler coil only heats the upper part, so if you want the whole tank heated you need a mixing arrangement or a pump from the buffer to the upper DHW coil [#13145286][#13171932] For the buffer itself, one solar coil or a plate heat exchanger with an extra pump is enough [#13145286] In summer solar should mainly make DHW, while in winter it has little value for space heating, so the CO circuit is usually closed and the boiler/valves handle the domestic hot water separately [#13058183][#13172055]
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  • #31 13147309
    William Bonawentura
    Level 34  
    spinkamidi wrote:
    The question is what power should be heated with such a buffer.


    We assumed that you have a peak demand of 5 [kW]. Let's assume that during this -20 frost you want to smoke once a day (24 [h]) for 4 [h]. You need a 5 * 24/4 = 30 [kW] boiler.
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  • #32 13153205
    spinkamidi
    Level 9  
    William Bonawentura
    The stove has been purchased - it has 25KW (Kostrzewa Cermik). Thank you for your help in this regard.

    miki_fx
    Thanks for the diagram.
    An interesting option is the water mixing system in the tank. How do you control the pump at the tank - does it run non-stop?

    Is it a good solution to introduce a coil at the top of the DHW tank to the buffer, which will lead water to the bottom of the DHW tank. Thanks to this solution, while heating in the furnace, we can heat up the entire utility water tank.
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  • #33 13153325
    miki_fx
    Level 12  
    lolson28 wrote:
    Hello

    I will write you briefly what it looks like with me. I smoke with wood. I pump from the boiler to the buffer with one pump, the second pump pumps from the DHW buffer, depending on the needs and the set temperature on the controller, the third pump works in the exchanger - 3D valve system, and the fourth largest pumps to radiators and floor heating. When I burn in the boiler, three pumps work, the fourth one from the DHW is rarely turned on.
    The advantages of such a solution, a buffer as an additional protection of the boiler in the absence of voltage, I burn in the boiler all the time and all heat goes to the buffer and is stored, I do not throw dirty smoke into the atmosphere :-) .
    Minus: long heating up of the buffer with the daily firing up, or something wrong :-) .
    It seems to me that the system consumes a lot of electricity.

    in case you had any questions, I'll be glad to help.

    greetings


    If you do not have a laddomat or even a mixing valve between the buffer and the stove, so that the return temperature is set to at least 65-72 degrees, it does not work properly. The furnace does not load the tank in layers from the top, but mixes the water in the tank (because there are large flows). Having a laddomat installed, after firing up in the furnace at the top of the tank, I have a temperature of 80 degrees and twenty centimeters lower the temperature drops drastically and is 30 degrees. Over time, the buffer is gradually loaded from the top, so the boundary between hot and cold water moves down.





    Added after 29 [minutes]:
    spinkamidi
    If you have a lot of money, the pump is on non-stop, if little is a differential thermostat.

    The coil idea sucks in my opinion. If it is a hot water tank in the buffer.
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  • #34 13168479
    spinkamidi
    Level 9  
    lolson28 wrote:
    Hello

    I will write you briefly what it looks like with me. I smoke with wood. I pump from the boiler to the buffer with one pump, the second pump pumps from the DHW buffer depending on the needs and the set temperature on the controller, the third pump works in the exchanger - 3D valve system, and the fourth largest pumps to the radiators and floor heating. When I burn in the boiler, three pumps work, the fourth one from the DHW is rarely turned on.
    The advantages of such a solution, a buffer as an additional protection of the boiler in the absence of voltage, I burn in the boiler all the time and all heat goes to the buffer and is stored, I do not throw dirty smoke into the atmosphere :-) .
    Minus: long heating up of the buffer with the daily firing up, or something wrong :-) .
    It seems to me that the system consumes a lot of electricity.

    in case you had any questions, I'll be glad to help

    greetings



    How do you heat the DHW from the stove? Do you have heat indicators in the buffer or in the DHW tank?
    I would like to look at your connection diagram :D I wonder how you have the combined buffer and hot water and where is your entrance from the furnace to the buffer. Do you have one?

    Added after 3 [minutes]:

    [quote = "miki_fx"]
    lolson28 wrote:


    spinkamidi
    If you have a lot of money, the pump is on non-stop, if little is a differential thermostat.

    The coil idea sucks in my opinion. If it is, it is a water heater in the buffer.


    I don't really have an idea how to heat DHW from a buffer.
    I would like to achieve such an effect that with the stove turned off and hot water in the buffer, I would have non-stop hot water. If I use a DHW tank with 2 coils (solar panels + stove), only the upper part of the tank will be heated with the stove? The solution is another pump at the DHW tank to the mixing system, but this is to increase the number of all pumps.
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  • #35 13171919
    lolson28
    Level 14  
    miki_fx wrote:

    If you do not have a laddomat or even a mixing valve between the buffer and the stove, so that the return temperature is set to at least 65-72 degrees, it does not work properly. The furnace does not load the tank in layers from the top, but mixes the water in the tank (because there are large flows). Having a laddomat installed, after firing up in the furnace at the top of the tank, I have a temperature of 80 degrees and twenty centimeters lower the temperature drops drastically and is 30 degrees. Over time, the buffer is gradually loaded from the top, so the boundary between hot and cold water moves down.


    I have a mixing valve on the return to the boiler. The pump between the boiler and the buffer pumps to the buffer and additionally back to the boiler to protect against low temperature.
    Overall, I have a very extensive installation and I don't fully understand how it works exactly :cry:
  • #36 13171932
    miki_fx
    Level 12  
    spinkamidi wrote:

    I don't really have an idea how to heat DHW from a buffer.


    Go back to the diagram that I have posted for you and look at it in more detail. Like an ox, there is a pump that pumps water from the buffer to the upper coil in the DHW tank.
  • #37 13172055
    lolson28
    Level 14  
    spinkamidi wrote:

    How do you heat the DHW from the stove? Do you have heat indicators in the buffer or in the DHW tank?
    I would like to look at your connection diagram :D I wonder how you have the combined buffer and hot water and where is your entrance from the furnace to the buffer. Do you have one?


    I heat the DHW through the buffer. So first I heat the buffer and the buffer heats the DHW and CO. The buffer has no coil. The buffer tank is 500 liters and now I think that it is definitely too small (a bigger one would not fit into the boiler room). It plays an accumulative role and protects the boiler against boiling. Holzgas boilers are high temperature boilers and the output must be around 80 degrees. However, when the boiler is heated to 80 degrees and fully loaded with wood and there is a puff and no energy, a large tank connected with a thick pipe to the boiler is useful, which will receive heat, and in extreme situations, a cooling valve that will let cold water into the boiler and hot water will flow into the sewage system. Currently, each boiler of this type should have such protection.



    spinkamidi wrote:
    I don't really have an idea how to heat DHW from a buffer.
    I would like to achieve such an effect that with the stove turned off and hot water in the buffer, I would have non-stop hot water. If I use a DHW tank with 2 coils (solar panels + stove), only the upper part of the tank will be heated with the stove? The solution is another pump at the DHW tank to the mixing system, but this is to increase the number of all pumps.


    For me, it looks like this that when I no longer heat the central heating and would like to heat only the hot utility water, I close the 3D valve and turn off the central heating pump controller. Then I heat the buffer to 70-80 degrees and set the DHW controller to 45 degrees. And depending on the needs, the DHW pump heats the water in the tank. I found this solution not economical enough and I heat the three warmest months of the year with electricity, then I close two valves in front of and behind the DHW tank.
    I considered installing a solar system, but I know that a good installation with assembly costs money, and I do not know if someone will guarantee me 10-15 years of trouble-free use of such an installation.


    greetings
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Topic summary

✨ The discussion revolves around selecting the appropriate size for a furnace and heat buffer for a 114 m² house with underfloor heating and solar panels. The user seeks advice on a solid fuel stove (wood/coal) and a heat buffer, considering a 1000-liter tank. Various responses suggest that a larger buffer (1500-3000 liters) may be more beneficial for efficiency and heat retention. The importance of proper temperature management and the use of a mixing valve or Laddomat for optimal heating is emphasized. Recommendations include considering a gasification furnace for better efficiency and lower maintenance. The integration of solar panels for domestic hot water (DHW) heating and the need for a dual-coil tank for effective heat exchange are also discussed.
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FAQ

TL;DR: A 1000 L heat-storage tank covers ≈14 h of heating gap for a 5 kW peak-load house [Elektroda, William Bonawentura, post #12981739] “The bigger the buffer, the better” [Elektroda, irus.m, post #13114370] Size storage at 40–80 L / kW and pick a 20-30 kW wood/coal boiler for 114 m².

Why it matters: Correct sizing cuts firing frequency, fuel bills and emissions.

Quick Facts

• Buffer rule-of-thumb: 40–80 L per kW boiler power [Elektroda, irus.m, post #13114370] • 25 kW solid-fuel boiler ⇒ ≈1 000–2 000 L buffer [Elektroda, irus.m, post #13114370] • Heating 1 000 L from 30 °C to 90 °C stores ≈70 kWh (251 MJ) [Elektroda, wowka, post #12993418] • Under-floor supply temp: 30–35 °C; ≥50 mm EPS insulation below slab [Viessmann Guide, 2022] • DIY 1 500 L plain-steel tank cost ≈PLN 2 500 [Elektroda, spinkamidi, post #13114263]

2. How large should the buffer be with a 25 kW solid-fuel boiler?

Design guides suggest 40–80 L per kilowatt. For 25 kW that equals 1 000–2 000 L [Elektroda, irus.m, post #13114370] Below 1 000 L stratification worsens and firing frequency rises. Above 2 000 L gains diminish versus cost and space.

3. How long will a 1 000 L buffer keep the house warm?

Charged to 90 °C and cooled to 30 °C it provides ≈70 kWh. At 5 kW load that lasts about 14 hours [Elektroda, William Bonawentura, post #12981739] Edge-case: at −20 °C with 7 kW load, reserve shrinks to 10 hours.

4. Do I need a stainless-steel tank?

No. The buffer holds closed-loop boiler water with minimal oxygen. Plain steel, well painted and insulated, outlives typical equipment decades [Elektroda, arturccc, post #13118955] Stainless adds cost with little benefit unless the tank runs open-vented or doubles as potable storage.

5. One coil or two when solar collectors are added?

Keep one large solar coil at the buffer’s base and use a separate twin-coil 300 L DHW cylinder: bottom for solar, top for boiler or buffer boost [Elektroda, miki_fx, post #13145286] Two coils inside the buffer complicate hydraulics and seldom improve yield.

6. How do I prioritise DHW over space heating?

Install a dedicated pump from buffer to the DHW coil. Control it with a differential thermostat set to run when buffer ≥ 60 °C and DHW < 50 °C [Elektroda, miki_fx, post #13145286] This sends hottest water to DHW first, then space heating resumes.

7. How do I protect the boiler from low return temperatures?

Fit a loading unit (e.g., Laddomat) or a 3-way thermostatic valve set to 65–72 °C on the return [Elektroda, miki_fx, post #13153325] This recirculates hot supply until the boiler reaches clean-burn temperature, then gradually feeds the buffer. Without it tar and corrosion form—an avoidable failure fact.

8. What pump layout works for buffer, DHW and heating?

Typical four-pump plan:
  1. Boiler → buffer loading pump.
  2. Buffer → DHW coil pump (thermostat-controlled).
  3. Buffer → floor/radiator circuit pump with mixing valve.
  4. Backup pump for solar or emergency cooling [Elektroda, lolson28, post #12992338] A 40 W pump running 24 h uses 0.96 kWh daily (~PLN 0.80) [Elektroda, wowka, post #12993418]

9. Do solar panels help space heating in winter?

No. Forum experience shows winter solar gain is too low for central heating in Poland [Elektroda, wowka, post #13058183] Collectors reliably cover 60–80 % of annual DHW load but <10 % of heating demand [Fraunhofer ISE, 2021].

10. How do I calculate energy stored in any buffer?

Use: kWh = volume (L) × ΔT (°C) × 0.00116. Example: 1 500 L heated 60 °C (30 → 90 °C) stores 104 kWh. Multiply by load to find autonomy. Statistic: every extra 100 L adds 6.9 kWh.

11. What happens during a power outage?

Without pumps, boiler can overheat. Include:
  1. Gravity-fed safety loop to a radiator.
  2. Spring-loaded thermal relief valve that injects cold water when boiler hits 95 °C [CEN EN 303-5]. "A buffer is still additional protection" [Elektroda, lolson28, post #12992338]

13. How thick should the floor screed be to act as thermal mass?

Use 6–8 cm cement screed over pipes to store 50–65 Wh per m² per °C rise [CIBSE Guide A, 2020]. William Bonawentura suggested a “thick, accumulative spout” for extra inertia [Elektroda, 12981739] Ensure insulation below to prevent downward losses.

14. Quick 3-step lighting routine for buffered systems

  1. Ignite full load of dry wood/coal; set air to rated output.
  2. Wait until boiler hits 80 °C; verify laddomat opens.
  3. After burn-out, close air and let buffer cover demand—no refuelling needed until temperature at buffer top drops below 60 °C.
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