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[Solved] Correct installation of the expansion vessel in the DHW installation

alienone 113919 31
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How should a domestic hot water expansion vessel be installed, and what precharge and size should it have?

Yes, it can be installed, but on the cold-water side with a check valve between the boiler inlet valve and the tee/safety group so hot water cannot flow back into the domestic installation [#17767530] With a pressure reducer set to 4 bar, set the vessel precharge about 0.2 bar lower, i.e. around 3.8 bar; DHW vessels are normally factory-filled around 3.5–4 bar, while 1.5 bar vessels are for heating and are not suitable here [#17768846] [#17768275] For an 80 l boiler at these pressures, an 8 l vessel is too small; a 12 l vessel is the closer suitable size [#17781033] [#17781229] If you use a non-flow vessel, the simplified diagram is correct, but it should be mounted vertically and on a quick-release connection so the precharge can be checked [#17781515] If you buy a flow-through vessel, use the proper DHW type (for example Refix DD / Airfix D) and do not install it in a way that isolates it from the boiler [#17781033] [#17781251]
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  • #1 17767439
    alienone
    Level 11  
    Correct installation of the expansion vessel in the DHW installation

    Is this diagram correct?
    I have a similar system, only behind the water meter there is a reducer set at 4 Bars, which indicates 6 Bar while the water is heating.
    I do not have an expansion vessel, so I am asking if it can be installed this way?
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  • #2 17767530
    stanislaw1954
    Level 43  
    alienone wrote:
    which indicates 6 Bar while the water is being heated.
    It is natural, when the water is heated, its volume increases, and since there is no outlet, the pressure increases.
    alienone wrote:
    I do not have an expansion vessel, so I am asking if it can be installed this way?
    There can only be one correction: between the valve on the cold water inlet to the boiler and the tee for the safety group, a check valve should be installed so that the water does not flow back from the boiler to the entire domestic installation, but only discharges its excess in the aforementioned safety group.
  • #3 17767564
    alienone
    Level 11  
    A few more questions
    1.supply of the safety group with what minimum cross-section
    2. Does the location of the vessel bottom - top in relation to the bolier have any significance?
    3. vessel with initial pressure with what parameters
  • #4 17767910
    stanislaw1954
    Level 43  
    1- even 1/2 "is enough, because it only releases pressure.
    2-there is no, and it makes sense to return, as in the picture.
    3-approx. 1.5 bar is enough.
  • #5 17767923
    alienone
    Level 11  
    I do not understand on the return, that is, not on the cold water supply
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  • #6 17767954
    stanislaw1954
    Level 43  
    alienone wrote:
    I do not understand on the return, that is, not on the cold water supply
    This postage was unintentional. Of course, on the cold water supply. Include a check valve.
  • #7 17768001
    alienone
    Level 11  
    Thank you for the clarification, this week I will want to rebuild. After the modernization I will let you know.
    I do not really understand the dependence of the pressure from the network, e.g. 4Bar and the expansion vessel, e.g. 1.5 Bar or 4 Bar, which one to choose?
  • #8 17768050
    stanislaw1954
    Level 43  
    Since you always have 4 bars in the network (I do not take into account the increase caused by water heating), 4 bars in the expansion tank is too much. Search the internet, but I assume 1.5-2 bar is enough.
  • #9 17768275
    ls_77
    Level 38  
    DHW expansion vessels are factory filled with ~ 3.5-4bar.
    Those with a filling pressure of ~ 1.5 bar are most often for CO and are not suitable for DHW!
    Look for hot water vessels.

    If you have ~ 4 bar at the input from the mains, then as much as at the input I would set in the vessel.
  • #10 17768596
    DiGital_Butcher
    Level 11  
    ls_77 wrote:
    DHW expansion vessels are factory filled with ~ 3.5-4bar.
    Those with a filling pressure of ~ 1.5 bar are most often for CO and are not suitable for DHW!
    Look for hot water vessels.

    If you have ~ 4 bar at the input from the mains, then as much as at the input I would set in the vessel.



    Exactly as a colleague writes, you need a DHW vessel (preferably buy a reflex DD) and the pre-pressure in the vessel is 4 bar at home, set 3.5-3.7 on the reducer and you'll be fine. If you set a lower pressure in the vessel than you have in the installation, you will fill it with water immediately after unscrewing the valve, and the point is that the vessel will expand the excess pressure that increases when water is heated.
  • #11 17768846
    emigrant
    Level 29  
    Correct installation of an expansion vessel for DHW is a flow vessel and its gross capacity is calculated taking into account several variables. As the installation has a pressure reducer, the initial pressure in the vessel should be set by 0.2 bar lower than the pressure downstream of the reducer. In your case, the precharge pressure in the vessel should be set to 3.8 bar. To work properly, the vessel must always be partially filled with water. Must be visible for installation. Enter how many liters you have a boiler, I will tell you what capacity you should buy an expansion vessel. Remember about a good branded check valve that should be installed in front of the expansion vessel.
  • #12 17781010
    alienone
    Level 11  
    I have 80 liters of Boljer. Tomorrow's assembling the chip. I was a bit surprised by the white Flamco expansion vessel, the manufacturer recommends installation with the valve facing down, so I have to change the safety group as in the photo with the appropriate diameter for the vessel. I think it will be correct.


    Correct installation of the expansion vessel in the DHW installation
  • #13 17781033
    emigrant
    Level 29  
    Remember that Flamco Airfix A and Airfix D are flow-through vessels and must not be installed as a non-flow vessel. In general, non-flow vessels for domestic hot water should not be installed ...
    With the parameters you have and the pre-pressure setting of 3.8 bar, you should have a vessel with a gross capacity of 8.6 liters, i.e. the closest you can buy is a 12-liter vessel. The redundancy of the vessel does not matter.

    BTW, this safety group in this photo doesn't meet any drinking water requirements. There, only rust will be produced from the inside, and it is not known what else. If you want, some safety groups are entirely brass, not steel, painted with paint. Apart from the fact that in this photo there is a non-flow vessel attached. In fact, it is not known for what purposes the water from this installation is in the photo.
  • #14 17781041
    alienone
    Level 11  
    This is Airfix R, so I can not mount it as in the photo?
    I have a safety group Diamond Brass CWU 6Bar. The photo I put in is borrowed from the network
    Replace the 12 liter tank with the blue Diamond too and supply it directly from the tee keeping the valve facing down, okay?
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  • #15 17781099
    emigrant
    Level 29  
    The photo rather shows how not to do. Airfix R is a non-flow vessel and you can install it as in the photo ... BTW, the R series is probably a slightly different application than DHW ... Even there is no PZH approval like the A and D series. Water can smell a bit of EPDM rubber from this R vessel I see they used a diaphragm pouch as it is in the non-flowing blue Reflex Refix DE.
  • #16 17781119
    alienone
    Level 11  
    I edited the above post, will it be okay if I mount the vessel with Diamond?
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  • #17 17781166
    emigrant
    Level 29  
    No, it won't be okay. Return this Airfix R and simply buy the flow-through Refix DD and a Flowjet for it. Connect the cold water supply by flowing through the vessel (tee) to the boiler. Google Refix DD (green or white) and Flowjet google. No security groups are needed. The ZB 6 bar should be as close to the boiler as possible, the maximum distance from the boiler 100 cm (at least this is what SYR recommends for its membrane ZB). Preferably right next to the boiler connection. Not installed in some safety groups ...

    That it wasn't. If you install a non-flow vessel, it will work properly as it should. Only for DHW in homes, such dishes should not be installed. Such a vessel can be the habitat and multiplication of legionella. It's really just about sanitation. Because whether it will be a flow-through vessel or a non-flow vessel, both will work properly and fulfill their function. There is always fresh water in the diaphragm at the flow-through vessel ...
  • #18 17781184
    alienone
    Level 11  
    Now I have a problem. Today I bought accessories, they don't have them in my shop
    Reflex Refix DE.

    The purchase of the group was made to facilitate the assembly and to have an overview of the pressure. Currently, I have a safety valve directly on the bolt head and 5 liters of water is dripping a week
  • #19 17781193
    emigrant
    Level 29  
    Water is dripping because you don't have an expansion vessel. Due to the fact that each connection should have an anti-pollution check valve, the internal installation is made in a closed system. Therefore, an expansion vessel is required. The ZB 6 bar itself is a total last resort and is supposed to work only in an emergency, e.g. if the diaphragm has broken, or the thermostat in the heater has broken down and heats the water until it boils.

    Install a thermo-pressure gauge on the hot water outlet of the boiler. I have one installed in the range up to 10 bar and 120 degrees Celsius. You can view pressure and temperature.
  • #20 17781199
    alienone
    Level 11  
    Tomorrow I will try to replace the vessel and recharge it according to the manufacturer's instructions
  • #21 17781209
    emigrant
    Level 29  
    What capacity did you buy the vessel with that you intend to replace? I understand you will replace non-flow ones with non-flow ones?
  • #22 17781224
    alienone
    Level 11  
    I bought 8 liters but from what you wrote earlier it should be 12 liters.
    this is the diagram and is it a flow vessel or not?



    Correct installation of the expansion vessel in the DHW installation
  • #23 17781229
    emigrant
    Level 29  
    An 8 liter vessel would even be fine for a 100 liter boiler, but not with this downstream pressure setting. If you want to use a pressure of 4 bar downstream of the regulator, the initial pressure in the vessel should be 3.8 bar. The 8 liter container is too small.

    I do not see the diagram exactly, because the photo is too small. Flow-through vessels on the Polish market, you probably have only 3. From Flamco Airfix A and D and REFLEX Refix DD. I guess that's it. I don't know the others.
  • #24 17781242
    alienone
    Level 11  
    I should mount the flow-through how to recognize it
  • #25 17781251
    emigrant
    Level 29  
    After all, I wrote like ... Eh ... From the company REFLEX, the flow units are marked as Refix DD, they are green or white. Blue Refix DE are non-flow vessels. From Flamco it is white with the designation Airfix A (not full flow) or Airfix D (full flow). I don't know the others.
    Don't think that you give a tee to the non-flow underneath and make it automagically flow-through.
  • #26 17781255
    alienone
    Level 11  
    It was a boyish boy right away. I understood a lot, thank you for your help.
    greetings
  • #27 17781257
    emigrant
    Level 29  
    I think you already know what you will do and what you will buy is your business. Good luck.
  • #28 17781505
    alienone
    Level 11  
    I simplified it to such a solution. They don't have flow-through vessels in the store,
    cold water supply - check valve - triangle with 12 l vessel - 6Bar safety valve - 80 L bolster plus PEX 20 connection


    It'll be all right ? Correct installation of the expansion vessel in the DHW installation
  • #29 17781515
    emigrant
    Level 29  
    The diagram is correct for a non-flow vessel. Remember that the ZB should be in a vertical position (they are not through). Pass-through are the so-called combined with the non-return valve, but you cannot use it in this situation as shown in the diagram, because you will cut off the expansion vessel from the boiler. Also remember that the vessel is not mounted "spicy". It must be on the so-called quick-release coupler, so that the vessel can be disconnected from the system at any time, e.g. to periodically check the initial pressure. The initial pressure is checked with the vessel disconnected from the system. No closing of valves and no depressurization of the boiler through the hot water tap.
  • #30 17781539
    alienone
    Level 11  
    You deserve a good bottle
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Topic summary

✨ The discussion revolves around the correct installation of an expansion vessel in a domestic hot water (DHW) system. The user inquires about the accuracy of a provided diagram and the feasibility of installing an expansion vessel in their system, which includes a pressure reducer set at 4 bars. Responses emphasize the necessity of a check valve to prevent backflow and the importance of selecting the appropriate pre-charge pressure for the expansion vessel, which should be set lower than the downstream pressure. Recommendations include using a flow-through expansion vessel, specifically models from Flamco and Reflex, and ensuring proper installation to avoid issues such as water dripping from the safety valve. The correct pre-charge pressure for the vessel is discussed, with suggestions ranging from 1.5 to 4 bars depending on the system's pressure. The conversation concludes with the user planning to implement the suggested changes and confirming the installation details.
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FAQ

TL;DR: A 50 °C temperature rise expands water by ~2 %, which can push 4 bar mains up to 6 bar without a correctly-sized DHW expansion vessel. “Install the check valve before the tee” [Elektroda, stanislaw1954, post #17767530]

Why it matters: Correct placement and pressure settings stop nuisance valve dripping, save water and reduce Legionella risk.

Quick Facts

• Typical DHW safety valve setting: 6 bar (EN 1487:2007). • Expansion vessel pre-charge = mains pressure − 0.2 bar (e.g., 3.8 bar for 4 bar supply) [Elektroda, emigrant, post #17768846] • Sizing rule: 0.1 L vessel per 1 L boiler at 4 bar; 80 L boiler → ≥ 8 L vessel [REFLEX Datasheet]. • Minimum connection pipe: ½" (15 mm) [Elektroda, stanislaw1954, post #17767910] • Flow-through models (Refix DD, Airfix D/A) meet potable-water hygiene codes [Flamco Manual, 2023].

1. Why does pressure rise when my DHW cylinder heats up?

Water volume grows ~2 % from 10 °C to 60 °C. In a closed system the extra volume has nowhere to go, so pressure climbs—often from 4 bar cold to 6 bar hot—until a vessel absorbs it or the safety valve opens [Elektroda, stanislaw1954, post #17767530]

2. Where should I install the expansion vessel?

Install it on the cold-water feed, after the check valve/reducer and before the tee to the boiler. Keep the safety valve within 1 m of the boiler outlet for code compliance [Elektroda, emigrant, post #17781515]

3. Flow-through vs non-flow: which is safer?

Flow-through vessels (Refix DD, Airfix D/A) rinse with fresh water each draw-off, limiting stagnation. Non-flow units (Refix DE, Airfix R) hold static water and can harbour Legionella if rarely used [WHO, 2007]. "For homes, choose flow-through for sanitation" [Elektroda, emigrant, post #17781166]

4. How big should my vessel be for an 80 L cylinder at 4 bar?

Use the 0.1 L per 1 L rule. 80 L × 0.1 = 8 L gross volume. Manufacturers round up, so a 12 L vessel is recommended [Elektroda, emigrant, post #17781033]

5. What pre-charge pressure do I set?

Set nitrogen/air pre-charge 0.2 bar below downstream mains. With a 4 bar reducer, charge to 3.8 bar cold [Elektroda, emigrant, post #17768846] Too low and the vessel water-logs; too high and it cannot accept expansion.

6. Does mounting orientation matter?

Yes. Follow the label: some Flamco models require valve down to keep the membrane wet. Wrong orientation shortens membrane life and traps air pockets [Flamco Manual, 2023].

8. Do I still need a 6 bar safety valve?

Yes. The safety valve is an emergency device. If the vessel bladder ruptures or the thermostat sticks, it prevents catastrophic pressure—up to 12 bar—by venting [EN 1487:2007].

9. How do I recharge or check the vessel?

  1. Close the quick-release coupling and isolate the vessel.
  2. Drain vessel water via schrader until pressure reads zero.
  3. Pump to target pre-charge (e.g., 3.8 bar), reopen coupling. Check annually [Elektroda, emigrant, post #17781515]

10. Can I reuse a central-heating (blue) vessel for DHW?

Not recommended. CH vessels ship at 1.5 bar and lack potable-water certification. "Water can smell of EPDM rubber" in such units [Elektroda, emigrant, post #17781099]

11. What happens if the vessel is undersized or empty?

Safety valve will drip 3–5 L per week, wasting energy and scaling outlets—exactly what the thread starter reported [Elektroda, alienone, post #17781184] Long term, the valve seat pits and may fail completely.

12. Minimum components in a compliant set-up?

Reducer → check valve → tee → flow-through expansion vessel → boiler cold inlet; branch off tee to 6 bar safety valve + tundish. This meets EN 806 and keeps the vessel in circuit during draw-off [Elektroda, emigrant, post #17781515]
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