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Convert existing vented cylinder to thermal store and PHE

Discussion in 'Plumbing and Central Heating' started by 1028741, 4 Apr 2013.

  1. 1028741

    1028741

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    Hi, I'm not a plumber, but I'm fairly knowledgeable and confident with domestic plumbing. Currently, I have a 24kw worcester greenstar conventional boiler that feeds the existing radiators, underfloor heating and old vented copper 900x450 cylinder which is fully insulated.

    I want to get rid of the cold storage tank in the loft, and the shower pump that supplies DHW to the showers, kitchen spray tap e.t.c. and was wondering if it would be possible to use the existing cylinder as a thermal store, and install a plate heat exchanger like this: http://heatweb.com/solar/schemat.GIF )

    This would be the cheapest option The house is big and we have 2 bathrooms. My question is, can you simply turn up the cylinder stat to 80 degrees, fit a flow switch and use the existing tank as a thermal store loop with the heat exchanger? Do you think the capacity would be high enough, and is the coil smaller than a modern thermal store (recovery time fast enough?--at the moment, the hot water runs out just as the bath is almost full, but the cylinder is only set to 55 degrees)

    The radiators/underfloor heating are not pressurised (there is a small expansion tank in the loft)-Can I leave it this way, and use the same expansion tank as a filler for the cylinder exchanger loop also?
    Thanks, I simplified this, apologies for the wordiness, would really appreciate any thoughts
     
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  3. hazetimesfive

    hazetimesfive

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    could you start a new line occasionally.

    no one is going to read that.
     
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  4. SimonH2

    SimonH2

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    OK, lots of questions.

    First off, yes you can convert the existing cylinder to a store - and it's often cited as a cheap way of getting one. Stores using an external PHE are often called heat banks so as to differentiate them from the type with a DHW coil inside.

    Personally I'd suggest feeding the rads from the store. It decouples the boiler from the rads so you can optimise the two loops which have differing requirements.

    The rads can have a variable flow rate (fit a modulating pump and TRVs on all the rads, do away with a room stat, and just run the pump from the programmer). This will give you a nice quiet radiator system where the flow is just what the rads need, regardless of what the boiler needs.
    Then we turn to the boiler. Some will say to keep it indirect, but heating direct is more efficient - just take the coolest water from the bottom of the store, feed it through the boiler, put it back into the store near (but not at, the top) - boiler runs only on the store thermostat(s). This way, the boiler (almost) always runs at max efficiency.
    If you run the boiler indirect, you'll need to range it down to suit the coil capacity. If you run the boiler at too high a power, you'll simply pump hot water round the coil fairly quickly - and you'll get your boiler return hot enough top prevent condensing most of the time (= more gas consumed).

    You should be able to just use the F&E tank for the store, but you may be better with a slightly larger one as there'll be a lot more water in the system. You can use the one F&E tank for the store and boiler if you keep it indirect - but then you won't keep the separation of fluids the going indirect is designed to provide.

    This is a schematic of the system I put in the flat - thread about it here, and the current tenant is quite happy with it. Ignore the TMV on the boiler loop, and substitute the PHE loop for the internal coil. That should give you some idea what I'm talking about.
    [​IMG]

    And I think this is nearer what you are looking at - again ignore the TMV on the boiler loop. I think I drew that for an old thread here where some ideas were being tossed around.

    Ideally you want the flow rate in the PHE loop to be a low as is required to give you the DHW flow/temperature required. Any excess simply pumps hot water into the bottom of the store - do too much of that and you increase your boiler return temp. Best case is filling the bath etc, worst case is running a tap at a dribble - you may have to think about that for a few minutes.


    In terms of modifying the cylinder, if the immersion heater can be removed then you can get inside to fit "full" flanges. If not then you'll have to use Essex(?) flanges which can be fitted from the outside. Keep the immersion heater, it can be a useful backup if the boiler breaks down - probably can't heat the house and run baths, but it's better than nothing !
     
  5. 1028741

    1028741

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    Thanks SimonH2, that's given me a lot to think about. However, I was intending on keeping the boiler and radiator configuration as it is, and have the boiler loop running through the internal coil in the cylinder as it currently does. Then, rather than fitting flanges or removing the immersion, I was thinking of simply using the current cold water inlet at the bottom with filling supply from the header tank, and hot water outlet at the top with expansion vent to the the header tank to create the separate pumped loop around the PHE just to heat DHW (see diagram). This would be simplest, as we have a LOT of radiators aswell as towel rails and underfloor heating on a separate zone valve and controller that probably wouldn't benefit from the thermal store. I was then thinking of using two three-port valves either side of the PHE so that if the temp. of the water in the store got too low, the PHE store loop would be bypassed and switched over to the boiler loop directly (e.g. like a combi-boiler). That way, if we're using a lot of hot water and the tank goes cold, we switch over to instantaneous heating from the boiler loop instead to temporarily maintain the supply. What you think?
     
  6. mogget

    mogget

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    I know very little about thermal stores, but wouldn't that gradually mix the water between the two loops? I suppose it might not be too much of a problem if the cylinder water had inhibitor in it. It would gradually equalise the levels in the two attic tanks though.

    EDIT: I'm sure I didn't see that diagram before, have been having problems with my internet though. Anyhow, I now see you only have one header tank. That'll teach me to post so late at night :confused:
     
  7. Dan Robinson

    Dan Robinson

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    Can't see the point of the three port valves personally. Just put a TMV on the plate to keep the domestic water at a sensible temperature.

    If the cylinder temp drops too low, then switching to the boiler will not help performance.

    If you want to prioritise recovery to the store then wire the heating zone valve through a relay so that the store can shut the heating off if the temp drops too low.

    The store is a buffer to make up for the short fall of the boiler. If the boiler could heat the plate as needed without the store then why bother with the store at all.... just fit a combi.
     
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  8. 1028741

    1028741

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    The problem with combis is, they fire up every time you turn on a tap, and can't very well handle having more than one tap on at the same time. The boiler heats the water to 80 degrees, and the water in the thermal store is also 80 degrees, do does it make any difference? Thought the idea of a thermal store was so you can quickly fill a bath with hot water, and avoid the boiler firing on and off ever time you turn on a tap, am I right?
     
  9. Dan Robinson

    Dan Robinson

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    Yes and no.

    And having a store at 80 degrees is very wasteful. I currently have mine at 60, with the boiler flow hobbled to 70. This means the top section of store is slightly hotter than 60. But more importantly my boiler stays in near enough full condensing mode for much longer.

    I have changed a bit of the plumbing recently, and the bath fill time is now around 3½ minutes. My plate is rated at 100kW.

    You have a condensing boiler so make sure it condenses fully as long as possible. Unfortunately for you though, I don't think your boiler can have two flow temperatures unlike mine.

    Depending on your version of the WB Greenstar though, there are some very advanced features your installer can play with that might make it work better with the store.


    *edited for late night type's and general brain-fart.... relevant info is the same though ;).
     
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  11. Dan Robinson

    Dan Robinson

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    One could argue that is more efficient than holding 150 litres of water at 70 degrees.

    But yes the performance is inadequate for rapid baths and multiple draw offs which is PART of my reason for having one.

    Constant on/off cycles of the boiler aren't great for it I agree, but how often do you really just "blip" a hot water tap? If it is frequently then that is a user issue not a boiler issue. Unless you want a stream of hot water, don't turn the hot tap on.

    Even with ha thermals store, turning taps on and off is still short cycling a pump and flow switch. I grant you less than than the workings of a combi, but still not a component that likes to be on and off a lot.
     
  12. 1028741

    1028741

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    I use my kitchen spray tap in a very off and on fashion lol the shower pump is very annoying. I want to fit a hot water return loop on a timer also, because it takes ages for the hot water to come through (the kitchen is very far away from the airing cupboard). That aside, can you see any major flaws with my diagram? I can't see any myself
     
  13. 1028741

    1028741

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    Problem is, my main concern is that the recovery time is fairy long at the moment, sometimes I switch on the immersion at the same time as the boiler just to make sure there's enough hot water if I'm taking a bath and someone else is taking a shower. That's why I thought I could use a 3-port valve to switch over to the boiler loop directly when the store runs out for temporary instantaneous combi-style water heating
     
  14. Dan Robinson

    Dan Robinson

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    What you are not considering - and we can't tell form the info given so far - is that the coil of the cylinder you have might only be absorbing 4 or so kW of energy from the boiler as well as being scaled up quite badly.

    I really wouldn't bother with the 3-port and heat the cylinder directly as has been suggested and get the full output of the boiler.

    Also, if the kitchen and/or one other sink are the only extremities you are concerned with, and small under sink electric heater would be far more efficient long term i would have thought.

    Secondary loops require expensive pumps. They also waste heat, and need additional controls. You are not supposed to use plastic pipe, so not always easy to retrofit in copper.

    In my house, a secondary loop from the kitchen extension was totally OTT, so instead, I ran a new 10mm supply for the hot.

    The much reduced water volume and mains pressure mean a fraction of the run off is used to get the hot water from store to sink (the tee for the kitchen is within a few inches of the TMV valve of my plate.

    It is also run in plastic, so no joints and very easy to run.
     
  15. Dan Robinson

    Dan Robinson

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    As I said, you shouldn't use it like that - at least with the handle in the mixed/hot only position.

    It will muller your shower pump for a start.

    Mind you - ten years I have been trying to persuade my other half.

    And for ten years I have failed. And we have always had mains fed systems :LOL:
     
  16. 1028741

    1028741

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    okay, thanks, appreciate your advice. We have a water softener so there won't be any scaling, I don't know the rating of the cylinder's coil, it fills the bath to the top, then takes about 1/2 hour to fully heat up again, however, when you say heat the cylinder directly, would I need to cut two additional flanges into the tank then? That sounds ominous. Also, due to the setup of the house, I really WANT a hot water loop around the kitchen, there are 4 taps in total on an insulated 22mm line (2 in the kitchen, 1 in the laundry room, 1 in the downstairs bath), and the washing machine, so an electric heater isn't an option. I already have a circulator pump that I bought for a different purpose, and it would be extremely easy to run a 15mm return from the last sink to the cylinder, that's my reasoning.
     
  17. SimonH2

    SimonH2

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    I'd say, forget the complicated switchover arrangement - it gives you all the failings of a combi, plus additional complexity. It also means that you lose the separation that is (IMO) the only reason to consider staying indirect - because a large cylinder does need a large quantity of inhibitor (check the manufacturers data sheets to find out how much, you can't go off the "enough for a large system" statement on the packaging).

    Going direct will allow you to use the entire boiler capacity to reheat the store. Not only that, but reheat will be top down if you design the system right - so you don't have to wait for the whole store to reheat, just enough to supply your immediate needs.

    Yes, fitting extra flanges will be required. I'd suggest as a first measure to see if the immersion heater will unscrew. If it will then that gives you a large hole to access the insides of the cylinder by and you can fit proper flanges - should be a doddle.

    If you have a mix of rads and UFH - then consider whether it's possible to separate them. The UFH will quite happily run off the "lukewarm" water from part way up the tank, while running the rads from (say) 2/3) of the way up.

    Yes, storing hot water will lose heat. Good lagging will minimise this, plus ensure that the heat lost is contributing to the heating load rather than being wasted. When I was doing the thermal store in the flat I actually did measurements - ran it from the immersion heater, let it settle for a couple of days, then measured consumption over a couple of days. Standing losses were around 80W.
    By comparison, in the house next door with a BG branded Worcester Bosch combi, with it's "keep warm" function enabled, it used the equivalent of 160W in gas to keep itself warm. That can be turned off, but then you can have to wait up to a minute for hot water when it's cold.
    So it's not as simple as it might sound - especially when you consider the comfort factor, and the amount of water that can be wasted while waiting for the tap to get hot. In fact, the effect is so well known on combis that there's a valve made just to economise on that.

    On that, you may find it beneficial to have a surface stat on the PHE and fire the pump occasionally during "occupied hours". I don't have experience with heat banks, so I don't know how long they take to heat the PHE up from cold.

    Dan, doesn't everyone use the hot tap in bursts ?
    Certainly when washing up, wash some stuff, rinse it, put it on the drainer, wash more stuff, rinse it, put it on the drainer, ... and to coin a phrase rinse and repeat until finished. The alternatives are to leave the tap running (wasteful), or not to rinse anything until you've finished (but then you don't rinse the insides of cups and mugs etc).
     
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