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convert cold water tank/vented cylinder installation into vented heat store with HIU for DHW

Discussion in 'Plumbing and Central Heating' started by mafj, 19 Sep 2020.

  1. mafj

    mafj

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    Hoping to solicit opinions before venturing into this project.

    The motivation is simple - shower/kitchen pressure (mind this is bungalow with 1.2m head ove r shower).

    I have been toying with the idea of connecting PHE to cylinder directly with pump and bunch of necessary components. But this would require time, effort, skill and expensive components easy to mishandle. Now that Heat Interface Unit are readily available this should be way easier, less space, less time effort and skill required to do this DIY.

    My primary idea is to use HIU like this:
    https://wattswater.eu/catalog/pump-groups/domestic-compact-water-units/friwa-easy/
    The 25l/m can be had for £400.

    The job in my mind requires:
    * rewiring all appliances to take mains CW (it comes at 3bar)
    * rewiring all appliances to take the HW from HIU
    * wiring supply of HIU to HW tank top (leaving vent connection intact)
    * wiring return from HIU to HW tank bottom (leaving connection to cold water tank intact).
    * leaving boiler connection and heating intact for now (isolated with cylinder coil)
    I know I can later bypass coil to get boiler directly interfacing hot water tank
    * isolating valves with strainers next to the unit.

    This all should be fairly cheap and easy as `rewiring` really means reconnecting the pipes, which come together into one place anyway.

    Leaving the most of the cost to be the HIU, with which I want to minimize the amount of work and risk and save space with compact a unit.

    Some Questions:
    * what might have I missed
    * any other recommendations for the HIU?
    * what would be good to think of now as well (low hanging fruit)
    * is flow of 25l/m enough (2 bathrooms - 1 bath 1 shower, 2 basin faucets, 1 kitchen faucet)
    My shower does 6l/m currently - would I get greedy with pressurized water?
     
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  3. oldbutnotdead

    oldbutnotdead

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    Rewiring, silly and wrong word to use.
    What capacity is your existing dhw cylinder?
    You can leave the boiler heating the cylinder via the coil, low odds you have enough taps on the cylinder to go direct.
    Shop around for the heat exchanger-£400 is quite pricey
     
  4. bernardgreen

    bernardgreen

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    For what it is worth,

    This works well in my cottage, boiler is a 16 kW which is ample for heating the cottage via radiators

    water shower.jpg

    The passive coil requires no pump so no electrical power is required ( showers during a power cut ) and is a lot cheaper than £400

    The cylinder with the second coil cost £ 207 + VAT £ 41.40
     
    Last edited: 19 Sep 2020
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  5. oldbutnotdead

    oldbutnotdead

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    One big bonus of @bernardgreen s setup is it reduces any variations in shower rate from kitchen taps being turned on. Another bonus is no pump required.
     
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  6. mafj

    mafj

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    This is heat exchanger plus pump plus the tmv and thermostats, regulating valves and control unit. New price is 1400euro.

    Second coil seems like a simple solution i thought problem with those is low rate.


    My tank is 200L.

    With the 2nd coil :
    I could try to get like for like tank with just extra coil.
    And one by one move appliances to mains pressure cold and hot water.
    ... And when I'm done with the above,then
    * switch boiler feed direct to the tank.
    * Retire huge cold water tank.

    Does that sound sensible
     
  7. oldbutnotdead

    oldbutnotdead

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    Nope. If you go @bernardgreen 's route (with high pressure shower and low pressure kitchen & basin supplies) you'll still need the large header tank.
    If (still don't understand why you want to do this) you connect cylinder as direct you lose the above flexibility, if your heating system is currently unvented/sealed then you're creating a rather large potential bomb. If your system is vented then you'll need a bigger primary header tank to cope with increased system volume so whats the point?
     
  8. mafj

    mafj

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    I'll clarify.

    I'd intend to eventually move all the appliances (taps and shower) to mains pressure.
    And then the hot water tank would feed nothing (two coils would go through it - that is it),
    so this would be stagnant water.
    And the cold water tank`s outstanding function would be left only to resupply evaporation of hot water tank.
    Meaning I could remove cold water tank completely.
    And to allow faster heating of the hot water tank, I could switch to feeding it directly from boiler.

    The why is - it would be a bit simpler and I could salvage plenty of space in the loft. And my wife would love aerated water at taps and being able to flush bath with extra pressure.

    Now, I ve mentioned bath... would that coil be good enough for bath?
     
    Last edited: 19 Sep 2020
  9. oldbutnotdead

    oldbutnotdead

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    Please be clear about the devices you are using.
    You're planning to use your hot water cylinder as a thermal store with an external heat exchanger to supply mains pressure hot water- cylinder heated indirectly via coil by the boiler.
    Or you're going to buy a twin coil cylinder- bottom coil for indirect connection to boiler, top coil for mains pressure hot water.
    Either way the cylinder will still be in use. You will still need a feed and expansion tank connected to it- needs to be at least 10% of system volume (so 50 litres or so).
    If you go for a mix of mains pressure and gravity hot water (quite sensible for stable shower pressure) you'll still need your full size hot water F & E tank.
    If you have a conventional (vented) heating system that also requires F & E tank, if you have sealed heating system then do not even think about direct heatin hig of the cylinder yourself.
    Space in the loft- what are you going to put up there instead in the whole cubic metre you'll recover?
    Bath- dunno, check with @bernardgreen (I've gone the pumped hex route with direct cylinder cos its what i had, woodburner is primary heat source)
     
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  11. bernardgreen

    bernardgreen

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    Not a good idea. Mains pressure water into a wash basin can result is water plashing out of the basin.

    Probably not but why use the coil for the bath. Use the hot water in the tank to supply bath(s) and basins.

    it will not be stagnant if you feed bath(s) and basin(s) from it at the pressure from the tank in the loft.

    This is my cylinder, cold water tank and header tank. ( during installation )
    Surface of cold water cistern ( tank ) is only 2.2 metres above the level of the bath taps. Plenty of flow, bath fills quickly.
    hot water cylinder bath room.jpg
     
  12. JimCrow

    JimCrow

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    The cold feed and the hot water draw off on your cylinder are both piped wrong Bernard.
     
  13. Madrab

    Madrab

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    Have you got your HW cylinder almost setup like some type of open vent/thermal hybrid @bernardgreen? On one side it's acts as a normal vented cylinder with gravity HW out to the normal outlets but the top coil is used to provide mains HW to the shower?

    How do you balance usage? If you've used up the hot water via the outlets then you need to wait until the cylinder recovers and vice versa? I take it it's a solar/HP cylinder with the 2nd coil like that?
     
  14. bernardgreen

    bernardgreen

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    I ordered the cylinder direct from the manufacturers as a standard cylinder with an extra coil at the top. The original plan was to use a pump
    and plate exchanger.

    hot_water_mains_ shower.JPG

    I actually bought the plate exchanger before discovering the cost of the tank with the second coil was low enough to change the plan and avoid the problems that a pump and plate exchanger would create.

    There is very little usage as the cylinder only supplies the bathroom and separate toilet wash basin.

    It is a second cylinder in the kitchen that supplies hot water to kitchen, utility and downstairs toilet. It is fed from the cold water cistern in the bathroom.
    hot water cylinder kitchen.jpg

    This "crazy" installation was necessary due to the layout of the cottage and it's kitchen "annex" making very long pipe runs between bathroom and kitchen.
     
  15. mafj

    mafj

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    Not an issue - nice modern mixer faucets with aerators give that soft and light but plentiful water.

    Looking at your tank - this is not a big tank at all, I wonder how big is mine but hundreds and hundreds of liters for sure.
    Obviously I can ensure good supply and I really would not need the tank to be big anyway.
    Bath draws 18l/m, basins would draw perhaps 2l/m each, and I am pretty certain I could deliver 40 from the mains.


    I have got PHE and TMV... years ago... but then amount of other components and piping and the space and time and disruption that it would take put me off.
     
  16. mafj

    mafj

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    So this is my setup and planned changes in pictures, no part of the system is sealed.

    I wish I could just go for an integrated combi ideally with small tank... but that could only replace existing boiler in the kitchen where space is at premium.


    upload_2020-9-19_18-49-20.png
     
  17. mafj

    mafj

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    Just posted drawings above that include the existing setup and planned changes with coil cylinder.
    No part is vented. 10% F&E sounds reasonable.
    My current cold water tank occupies middle spot in my loft with best headroom. it is raised on some timbers across trusses to be a bit higher and spread the load. All this takes like 10 sqm including inaccessible space behind.
     
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