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Reverse flow up gas boiler from Neutraliser/heat Store

Discussion in 'Plumbing and Central Heating' started by picitup, 21 Jun 2008.

  1. picitup

    picitup

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    Well I guess I need to log the data, get my backside down the plumber's merchants tomorrow and give it a try.

    I don't think it's the pump problem you suggest though, as I'm also running the stove at the same time as the heat store is pumping and the stove can keep up with the light DHW demand, so the heat store pump is always running.

    Even if this doesn't fix the DHW tail-off, it is certain to make it all more efficent!

    Hope a bird doesn't land on our gas boiler flue pipe when I'm running the store! I'll check out front tomorrow for fried seagull... :D

    Ta muchly

    Steve
     
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  3. Onetap

    Onetap

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    If I understand your ASCII graphic correctly, the neutraliser is in the Airing cupboard, the heat store in the loft.

    1) The F&E tank must be raised above the heat store cylinder.
    2) The gas boiler, the indirect DWS cylinder and the radiator circuits must all have their own pumps.
    3) You don't show, or seem to have, any heat dump radiators for the wood burner. The heat dump seems to be dependent on the heat store pump running; if correct, I don't think this is allowed.

    The whole idea of the neutraliser is that it's all at the same pressure, so there's no pressure difference between the connections. The heat store pump can't cause flow through the gas boiler circuit or vice versa.

    If the gas boiler pump is not running, the circulation could only be caused by the gravity circulation.

    Is the gas boiler open vented? Does it have a separate F&E tank, or cold feed & OV connections to the F&E tank above the store?

    If it is an older conventional flue type, as I previously thought, it probably started life with a F&E tank attached. If there are now pumps and the asociated valves between the boiler & the OV & cold feed pipes, I think that it might need to be equipped as for a sealed system; it may not be suitable for this.
     
  4. Norcon

    Norcon

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    Onetap wrote

    But you did say…….

    In response to the OP’s question……..

    Without knowing what type of check valve he was referring too.
    It may have been one of these valves he was referring to…

    http://www.johnsonvalves.co.uk/non return.htm

    Which state the minimum opening pressure.

    Yet you go on to suggest the use of two port valves when a far simpler solution is available.

    It also depends on the minimum opening pressure of the check valve !.


    A selection of valves with minimum opening pressure stated.
    http://www.johnsonvalves.co.uk/non return.htm

    This company advocates the use of check valves to prevent unwanted gravity circulation……….
    http://www.systemlink.ie/about-systemlink.asp

    Apologies if I have offended you. I think you may be referring to the other thread in which I advocated a different solution to yours regarding the African job.
    The OP never did return to verify whether or not the supply to the garden tank should remain potable or not. :(
     
  5. Onetap

    Onetap

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  6. picitup

    picitup

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    Hi Onetap

    You do understand my diagram correctly and everything is where you said it is.

    1) It is
    2) They all have their own pumps, apart from the indirect hot water cylinder. This is higher than the neutraliser and thermosyphons out of the neutraliser and throught the coil.
    3) The system dump is, as you say, dependant on the heat store pump running. Is this too fragile a safety check? I guess if the heat store pump failes, the heat store would vent into the F&E tank.

    The neutraliser does have lots of different pressures flowing. When the gas boiler is running, the boiler pump pumps water through the neutraliser, the CH pump does the same and the heat store pump takes over from the gas boiler pump, so lots of different flows at different times. Is that bad?

    I don't know much about boilers, but it's a Worcester R40 40KW condensing boiler which is about 4 years old. I'm pretty sure it doesn't have its own F&E tank.

    Cheers

    Steve
     
  7. picitup

    picitup

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    Hi All

    I've recorded some data and posted it up here:



    Unfortunately, the wire popped off the gas boiler return sensor so I still don't have the flow direction! Bummer I'll have to do it again.....

    Any road up, the graph from this morning clearly show quite a heat leak, with the flow up the gas boiler being much better than the DHW coil!

    I'll do the test again and post up the results with direction too. I don't want to work on the system and find I have the flow direction wrong!

    Cheers

    Steve
     
  8. Agile

    Agile

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    My understanding of the "heat dump" which is required for solid fuel boilers is that it should be fail safe.

    Thet means that it cannot reply on electricity as there might be a power cut!

    It should not require a pump either as that can stick or fail.

    That only leaves gravity circulation!

    Tony
     
  9. picitup

    picitup

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    Hi Tony

    OK now it's dumb question time.....

    If the stove siphons directly into 200+ litres of water, is it a big problem? I think I'd really have to go at some pace with the fire to get the store to boil.

    I've seen diagrams of the stove going into the DHW coil, and see you need a dump here. The safety factors on my system are as follows:

    1) The stove has a stat in the back boiler which limits the flow temperature. When the water gets up to the setting on the dial, a flap over the air intake starts to close, restricting the burn.

    2) The system is electrically dumped via a pump to all rads at 85 degrees

    3) The cylinder can vent into the F&E

    4) Using my noggin. I have an LCD display by the stove showing heat store temp and various flows and returns and I never heat the store over 80 degrees.

    Are these measures all ok when used together?

    One comment that did cause me some worries though is there is a normally open motorised valve on the stove return. When the flow is less than 40, this closes to stop thermosyphoning overnight and back boiler corrosion I'm told. When the stove flow is over 40, the valve opens and allows flow. Someone told me these can stick and fail closed.:(

    What do you think?

    Cheers

    Steve
     
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  11. Agile

    Agile

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    What is your F&E tank made of?

    If the store can boil then the F&E tank should not be made of plastic.

    If your installation meets the requirements of the manufacturer then its probably OK !

    Tony
     
  12. picitup

    picitup

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    Hi Tony

    I'll check with the manufacture once I've fully documented our current system.

    I took some more readings and proved the flow is in reverse around the boiler - up the into the flow and down out of the return. Here's the graph:


    I've bought a 22mm spring loaded check valve and will fit this in a location which would make replacing it with a motorised valve easy (when it doesn't work!!), but probably tomorrow as I need to do some proper work now :D

    Cheers

    Steve
     
  13. Agile

    Agile

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    the pressure presented by gravity circulation is very low and I cannot immagine any spring loaded non return valve being suitable!

    And the F&E tank?

    Tony
     
  14. picitup

    picitup

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    Hi Tony

    Ops sorry, forgot! The F&E tanks is plastic - can you buy metal or other ones that are boil resistant?

    I don't understand why the check valve wouldn't work - the heat leak is in the opposite direction to the boiler flow, so when the low pressure gravity feed occurs, it would be blocked by the valve.

    Of course, I could be being dumb here. Any road here's a nice diagram:


    [code:1]
    .......Boiler
    ...|--------------|
    .........|Flow
    .........|
    Gravity..|..Boiler
    ...^.....|...|
    ...|.....|...|
    ...|.....|...V
    .........|

    [/code:1]

    Does this make sense to fit the NR valve in the normal boiler flow direction (ie down?)

    Cheers

    Steve
     
  15. Agile

    Agile

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    Its normal to use a galvanised F&E tank with a metal overflow pipe!

    The metal overflow is often overlooked and brings down the ceiling when there is a a problem!

    Tony
     
  16. picitup

    picitup

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    Hi

    I guess a galvanised tank is the next thing on the agenda then. Did my thinking for the NR valve make sense? Do you think it has a chance of working?

    Cheers

    Steve
     
  17. Agile

    Agile

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    I think its not going to work! But dont let me stop you from trying it. I could be wrong!

    Have you tried a calculation to see what the gravity equivalent head might be?

    Tony
     
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