Connect Vented/Unvented water tank to combi boiler hot water

Discussion in 'Plumbers' Talk' started by Nik83, Jun 6, 2021.

  1. Nik83

    Nik83 New Member

    I have Worcester 25i combi boiler. The hot water supplied by it is hard to use mostly because it's min water flow requirement is too high and when you adjust the hot water tap so it decreases the hot water flow below requirement the boiler stops producing hot water at all.

    I considered an electric water heater (I thought of Ariston Oversink Water Heater 3kW 15Ltr (5106G) from ScrewFix) to be connected just after the output of the boiler's hot water, so, it will not consume much electricity, but still provide hot water with no minimal water flaw requirements and without gaps of cold water if you switch water off and back on.

    I asked a plumber to install it and he said it cannot be done. He claimed it was not allowed legally, but even if it were, it would not work like that because of valves that stop reverse water flow. He would have no problems If I wanted to connect the heater to main cold water supply. He is a certivied unvented hot water tanks installer.

    So, I wonder, can it be done at all? It would be the cheapest solution to my problem. If not, what other possible options I have? An expensive one I know myself already - replace the boiler and install unvented hot water tank. I am really considering it as hot water suppy from my Worcester 25i is unusable, but I am still paying for my current boiler. I don't want to install "instant" electric water heater as they are not instant and I will also need to do special wiring to it and it will cost me probably more than replacing the boiler.

    I called a Worcester engineer and he measured everything and said the boiler operated within its limits as specified in the manual.

  2. andy48

    andy48 Screwfix Select

    1. You don't need to replace your boiler to install an unvented hot water cylinder. Just zone the central heating output of the boiler to give space heating in one zone and water heating in the other.
    2. However, to successfully install an unvented hot water system you need:
    2a. Somewhere to put the cylinder. Generally a little bit bigger than a vented cylinder.
    2b. A means of running the pressure relief pipework to outside or to a suitable (metal or appropriate plastic) drain.
    2c. At least 2 bar dynamic pressure on incoming water main.
    2d. A flow rate (cold main) of at least 20 litres / minute.
    2e. Possibly need to run cold main to cylinder in 22 mm pipe rather than 15.
    3. A common approach where the boiler is in or near the kitchen is to use the boiler's hot water output for the kitchen taps, with all other hot fed by the unvented cylinder.
    4. Has the advantage that you can fit an immersion heater to have hot water if the boiler breaks down. Would require a dedicated radial circuit from the consumer unit.
    Nik83 likes this.
  3. Nik83

    Nik83 New Member

    Hi Andy, thanks a lot, very clear variant and looks like my water supply satisfies the specs above. It also should be reasonably priced, as I considered updating to smart thermostats on the radiators, so, I will need to budget for zone valves, cylinder and work only and the controller and temperature sensors are part of the smart update budget.
  4. Nik83

    Nik83 New Member

    I still wonder why I cannot connect a 15l water heater just after the boiler, but now, when there are another very viable option, this is just curiosity...
  5. kiaora

    kiaora Screwfix Select

    if I understand your original plan, installing a 15lt water tank off the hot water flow, the water in the tank being kept up to temperature by the electric element, would waste a lot of energy, you pay for the hot water to fill the tank, then pay to keep it hot, and when the tap is used, the hot water in the pipe goes cold, and needs reheating?

    the unvented cylinder, is better and you get a good performance in the home,
  6. Jimbo

    Jimbo Screwfix Select

    Maybe it would be cheaper to replace the boiler for one that will serve a lower minimum flow? I don’t remember ever having such a problem with the Vaillant EcoTech I had for about 12 years
  7. Nik83

    Nik83 New Member

    I see it like this: I configure the temperature in the 15l water tank to be slihghtly below the hot water temperature that comes out of the boiler. In this case I will only use electricity to keep the temperature, not to heat the water from cold. The water will be heated by the boiler and when it enters the 15l tank it is already hot. Electricity is also used when I use the water with a flow that is below the minimum requirement. So, when a water is consumed in volume - like having a shower - I do not use electricity to heat water from cold.

    Unvented cylinder seems a solution here. However, I would not say it is better - it consumes much more space, a bit harder to install as it will require zoning, is restricted in volume, so, for example, 120l one would allow 1 shower and 1 bath in 1 hour roughly and looses more heat as the surfice volume is higher. More insulation will slightly help with the latest, but I can easily insulate the 15l tank as well.

    About keeping it hot - everybody who has a hot water tank or cylinder or even additional hot water reservoir in their gas boiler have to pay for it to keep it hot.

    There is 1 very big advantage I see with unvented: the water to use for heating is going to be treated and part of closed cycle, so, potentially, no limescale treatment is going to be needed for the boiler.

    I checked today's Valliant range and the smallest water flow I found was for "ecoTEC exclusive with Green iQ combi". It was 1.5 l per minute which is twice better than my current one but still not ideal and the minimal Signature model output they provide for that model is 35kW. This one is also big and will not get into the current cabbot. Other models require 2l/min which is again better than my current 2.9l/min but not good enough.

    So the question stays: why I cannot install a 15l oversink water heater or similar in between the boiler and all hot water consumers? And if I can - how it can be done, so, I can explain to a certified installer.
    The only answer I had from plumbers I asked was "nobody does it, so, I will not either".
  8. Nik83

    Nik83 New Member

    Hi all, my kitchen is going to be installed on Monday, so, I wanted to ask last time regarding the installation of 15l oversink tank or similar in right after the boiler and before all hot water consumers as I will not have access to plumbing after that without removing the kitchen units. Thank you.
  9. andy48

    andy48 Screwfix Select

    1. While the "nobody does it, so I won't either" may not be very illuminating, it does suggest it is an abnormal practice.
    2. I can't say why it shouldn't be done, because it depends on the heater you choose.
    3. Some water heaters rely on the incoming cold pipework to accommodate the expansion of heated water. One of those after a combi would tend to pressurise the combi in a direction it is not expecting.
    4. The fact that no one on hear has come out with a definitive "yes you can" or "no you can't" would suggest your plan is pushing the boundaries of the possible.
  10. dcox

    dcox Screwfix Select

    I’m coming in late to this conversation, but here’s what I think - probably too late to be useful.

    Simplest solution would be to fix the boiler so it supplies hot water in the way it’s supposed to. If you’ve checked to see if your supply meets the criteria to fit an unvented cylinder then your combi shouldn’t struggle to supply hot water to your kitchen sink.

    If you’re set on getting an under or over sink electric heater don’t complicate things by trying to plumb it into the DHW from the boiler. Connect it to the cold main as it’s designed to be. Don’t forget it will probably need an expansion kit including somewhere to route the PRV discharge.
  11. The Happy Builder

    The Happy Builder Screwfix Select

    Sounds like you need a plumber to sort the pipework out in your house starting with getting rid of any unnecessary isolation valves.
  12. This is exactly what I was thinking, done regularly for solar thermal gains.

    Attached Files:

  13. Nik83

    Nik83 New Member

    Hi Andy, It does make sense as manufacturers implement people's demand and if they do not produce such units - means there is no demand. Same for plumbing work.
    Your initial answer is probably the best way to go - thank you again for the idea, btw - as an alternative would be installing an electric cylinder. I see that replacing the boiler would probably not help as I have not found one with no minimum flow requirements.

    This is what I was trying to avoid as I very recently paid to replace warm air unit + electric tank with the combi boiler and although gas+electric bill did go down a little bit, it became much less comfortable to use. But the money is already paid and my inner self actively resists to the idea of investing a similar amount to redesign it.

    I am not doing it myself - a plumber will be doing the work anyway. I did not get the "getting rid of any unnecessary isolation valves", though. I do have quite a lot of them installed by previous owners during the years, but don't see how they would interferre with minimal water flow reuirements of the boiler.

  14. Jimbo

    Jimbo Screwfix Select

    Just a thought, what hot water temperature is the boiler set to? It doesn't need to be any more than 48°C. If the issue is stemming from mixers, lowering the water temperature will increase the flow through the boiler, everything else being equal.
  15. Nik83

    Nik83 New Member

    Hi, thanks. It is 50C currently. I considered setting it to lower temperature, but the problem is the whole idea of high limit on maximum water flow - when I adjust the tap to be below the limit occasionally - which I don't have any indication of on a tap - the boiler will shut itself off and you will know it because the water became cold and you need to open the tap and wait for 20-30-ish seconds until it become hot again and then you adjust the water again because you opened too much when hot water disappeared, luckily it is instant this time - until you cross the min flow req point again.

    I understand that I can change the usage pattern and not switch water off when I am in the shower and my brain can ultimately get used to the process and I will not notice all these nuisances and will be just using it as many other people who use it every day - in the end, it was Which recommended when I was buying it. I just don't want to. I want the thing to just work - produce as little hot water as I need without shutting itself off completely.

    I don't think there is a point in discussing the topic any furter - until somebody else has same problem. I have the kitchen fitted next Monday and have no access to plumbing after that.
  16. genevaemery

    genevaemery New Member

    Hello, Niki! I also have a plan to install 5l, which I discussed with my plumber, who was unsure whether it was doable. Once your plumber has installed your kitchen with 15l successfully, please let me know so that my plumber may get an idea of how to install it.
  17. Nik83

    Nik83 New Member

    Hi, I decided to not pursue this setup - I found no plumber who would do it and nobody would advise for it online.

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