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    • CommentAuthorJeff B
    • CommentTimeAug 21st 2017
     
    I have a Newark thermal store (350 litre capacity) with 3 internal coils: one at the bottom (thermal solar), one in the middle (for ASHP input but not used currently) and one from the middle to the top (for DHW – also not used currently). The latter two have an area of 3.2 sq.m. each. (FYI DHW is generated in a separate solar cylinder upstairs).

    Currently the boiler (wood pellet) heats the store direct and the C/H is also direct from the store hence the combined volume of the system is huge. I almost need a second mortgage to buy the Fernox to treat the system! Currently it seems I only have the minimum amount in there (sample has been tested by Fernox lab.) and I am getting hydrogen gassing in the C/H system.

    I would like to use the one of the unused coils for flow/return from the boiler and the other for flow/return for the C/H, so both systems would be direct rather than indirect. I would put an expansion vessel on the C/H side so I could pressurise that part of the system. The boiler side would stay with the existing F&E tank arrangement. This would cut down hugely on the amount of anti-corrosion inhibitor required and also separate the boiler circuit from the C/H circuit.

    My question is: are 3.2 sq.m. coils sufficient for the job? I have spoken to technical at Newark and they say it is but I would appreciate any thoughts from the other experts on the forum e.g. is the whole idea crazy? Originally I was going to fit an external PHE but when I discovered that I had two unused coils in the store I had a rethink!

    The original designer/installer went out of business a while back so no joy there.

    Thanks for any thoughts/ideas.
    • CommentAuthorCWatters
    • CommentTimeAug 21st 2017
     
    I don't see how it cuts down on the Fernox required. The volume of water is still the same, it's just divided up into smaller bits. You only save when it come to draining and refilling the system and shouldn't do that very frequently.
  1.  
    Pellet boilers are fairly responsive (so I understand - not having been involved with any) so why not remove the thermal store from the equation and run the CH straight off the boiler. This removes 350 lts of water, how much water is in the CH?

    Can someone who knows pellet boilers back this up or shoot it down ! (A quick search on google showed mixed opinions - so I guess it depends on the particular system)

    Jeff - You say the combined volume is huge - what is huge? My system has a boiler with 200ltr water in it, a 2000 ltr store and 17 rads. The Thermal store was added a couple of years back and until then it ran without a TS and the system (except the TS) was installed 22 years ago and has never had inhibitor in it. The general advice here is that you don't need inhibitor unless you have thin gauge heat exchangers! Yes I keep thinking about putting inhibitor in (or anti freeze to do the same job) and one day I will come to a decision, until then...............
    • CommentAuthorJeff B
    • CommentTimeAug 22nd 2017
     
    CWatters - you are right. However by breaking the system down into the separate parts I can drain the C/H loop without affecting the thermal store. I need to flush the C/H system as I have a problem with hydrogen gassing and I'm going to add a new radiator and change another radiator at the same time. Hopefully this will get rid of the corrosion problem; at least it will help distinguish which part of the system is causing the problem?
    • CommentAuthorJeff B
    • CommentTimeAug 22nd 2017
     
    Peter - I understand it is good practice to have a buffer tank when using a wood pellet boiler, because the boiler will cycle too much otherwise. Also I wanted to have the advantage of multiple inputs e.g. solar, ASHP etc.
  2.  
    Jeff - short cycling is a function of a balance between demand and supply. If the supply is over large for the demand then short cycling occurs, if the reverse then the supply (the boiler) never switches off. Somewhere in the middle there is an acceptable situation without buffer tank. Then there are the standing losses of a store that may be a consideration. When I did a quick search there was one chap who said his pellet use went up when he added a buffer store. Its not an exact science and each system is different.If you are not using multiple inputs for the coming winter then if its not too problematic it may be worth taking out the store to see what happens.

    You do not say what size the system is but having a pellet boiler, an ASHP and solar (and you put in an "etc." so PV?) is an awful lot of different prime sources that makes me wonder if you would ever get a return on what has to be a substantial capital investment to provide heat to the CH. I presume you are off the gas grid.

    DHW separated out is the general advice given here - I have done that and I am very pleased I did so, IMO best to keep it that way.
    • CommentAuthorJonG
    • CommentTimeAug 22nd 2017
     
    Google VDI and have a look at the European approach to water quality. Provided the system is sealed, fill water soft, iron low enough and PH kept correct no need for inhibitors (provided no ali). This sounds complex but isn't TBH, and often inhibitors can cause more issues than it resolves.

    Pellet boilers definitely need buffers to allow a battery to supplement heating as it gets up to temp, to absorb excess heat during the shut down and ember burn out phase and to minimise the effects of cycling, this can be influenced greatly by the buffer size, emitter design, blending valves and means by which the buffer is piped.

    In order to assess the coils suitability you would need to know the kw capacity that they are capable of delivering, and head loss to ensure that the delta across the appliance connected to it is correct for its operating parameters.

    If you ask for Mark at Newark he should be able to assist. Direct is best though because there is no loss of efficiency in terms of transfer and no resistance which increases pump consumption, plus flow rates can be lower because turbulent flow is less of requirement.
    • CommentAuthorJeff B
    • CommentTimeAug 22nd 2017
     
    Peter - I'm sure pellet consumption must be higher when you have a TS because you are keeping a large volume of water at 70 or so degrees, 350 litres in my case. My boiler has a max output of 25 kW and can modulate down to 5kW so cycling is not an issue in my case. I've never not had a TS so I don't know what would happen if I tried to run the boiler without one. I never did install any other heat source hence I have two spare coils in my TS! Yes, we have no mains Gas here.
    • CommentAuthorJeff B
    • CommentTimeAug 22nd 2017
     
    JonG - thanks for your comments. I will try to contact Mark at Newark. All I know is that each coil has a surface area of 3.2 sq. metres. I have tried to calculate t he heat transfer using the classic equation but I get some absurdly high figure running to many megawatts! I'm away from home at the moment otherwise I would post my calculation here.

    I have a very complicated system (imo anyway!) but the company that designed it has gone out of business some time back so no help there. The main issue is the hydrogen gassing which is worrying because there must be corrosion taking place. I can bleed the top radiator every day if I choose to and every time gas is emitted. There is an automatic air bleed valve in the flow pipe and that hisses every time the central heating is running. There is no problem with the f&e tank - it has been checked.
  3.  
    Posted By: Jeff BMy boiler has a max output of 25 kW and can modulate down to 5kW so cycling is not an issue in my case. I've never not had a TS so I don't know what would happen if I tried to run the boiler without one.

    You could ask the boiler manufacturers if they consider a TS to be necessary (and if so what size) with the model of boiler you have.

    Do you know the gas is hydrogen rather than air being drawn in form somewhere. A daily bleed of gas seem a lot.
    • CommentAuthorJeff B
    • CommentTimeAug 23rd 2017
     
    Peter - yes, it burns with a blue flame! I did wonder if air was being drawn in oxygenating the water leading to corrosion so we carefully replumbed th f&e tank just to be sure. This is why I am concerned to get to the bottom of this and cure the problem.
    • CommentAuthorJeff B
    • CommentTimeAug 30th 2017
     
    JonG - the coil is rated at 110 kW. Would you say that that is man enough to supply 15 radiators with a rated output of 18.4 kW, assuming the thermal store temperature is maintained at 70C? The flow rate would be of the order of 0.5 litres/sec.
    • CommentAuthorJeff B
    • CommentTimeSep 18th 2017
     
    Further to my original post (21st August) I can report that the mods to the thermal store have been completed and all seems to be working well. Mark at Newark Copper Cylinders has been very helpful.

    The boiler flow goes through a double helix coil (2 x 6m) and the C/H flow goes through a single 12m coil, so both boiler and C/H connections are now indirect and pressurised. The thermal store is now effectively a buffer tank with its own header tank, so the water therein stays therein. The boiler circuit and the C/H circuit can now be worked on independently. The next stage is to add a system cleanser and then give the system a good flush before refilling and adding corrosion inhibitor. Hopefully that will be the end of the gassing problem!
    • CommentAuthorJeff B
    • CommentTimeSep 26th 2017 edited
     
    OK, the “reconfigured” connections to/from thermal store seem to have done the job I intended.

    I would like to ask for advice on an issue that has arisen; namely, there is some thermo-siphoning going on. The C/H flow pipe leading from the thermal store is very hot (when the C/H is off) and thus is transferring heat from the TS up to the attic where the microbore manifolds are located. (We live in a dormer bungalow). I think I need to install a non-return valve. Access to the vertical pipework is extremely difficult owing to the way it was installed in the first place (I have not disturbed this) but access to horizontal runs near the TS is easy. Can I put a non-return valve in a horizontal position? Also can it go in the flow pipe when the pump is on the return as it is currently or do I need to install the pump in the flow too?
  4.  
    Non return valves (NRV) intended for pumped circuits are spring loaded so can be orientated in any direction. (NRVs for gravity circuits must be mounted horizontally). It is usual to have the NRV next to the pump but I think this is just convention.
    • CommentAuthorJeff B
    • CommentTimeSep 26th 2017
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>Non return valves (NRV) intended for pumped circuits are spring loaded so can be orientated in any direction. (NRVs for gravity circuits must be mounted horizontally). It is usual to have the NRV next to the pump but I think this is just convention.</blockquote>

    Peter - the pump is in the return but I need the NRV in the flow. Will this work? I assume the pump suction effect is the same as the forward flow effect i.e. it will still provide enough oomph (technical term) to operate the NRV?
    •  
      CommentAuthordjh
    • CommentTimeSep 27th 2017
     
    Another way to stop thermosyphoning is to install a 'London loop' in the pipe. i.e. choose a horizontal section of the pipe run close to the store, and insert a 6" downward loop of pipe into the length. That breaks the upward flow path for hot water.
    • CommentAuthorJeff B
    • CommentTimeSep 27th 2017
     
    <blockquote><cite>Posted By: djh</cite>Another way to stop thermosyphoning is to install a 'London loop' in the pipe. i.e. choose a horizontal section of the pipe run close to the store, and insert a 6" downward loop of pipe into the length. That breaks the upward flow path for hot water.</blockquote>

    Thanks for that tip - I've never heard of that one! I suppose as a belt and braces I could put an NRV in the line at the same time, having cut the pipe. Do you have any thoughts about my question to Peter in the post above i.e. do I have to move the pump into the flow rather than leave it where it is (i.e. in the return) if I want to put the NRV in the flow? I'm not sure if the pump suction effect will be sufficient to overcome the spring in the NRV.
    •  
      CommentAuthordjh
    • CommentTimeSep 27th 2017
     
    Posted By: Jeff BDo you have any thoughts about my question to Peter in the post above i.e. do I have to move the pump into the flow rather than leave it where it is (i.e. in the return) if I want to put the NRV in the flow?

    Sorry, no idea.
    • CommentAuthorJeff B
    • CommentTimeSep 27th 2017
     
    <blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: Jeff B</cite>Do you have any thoughts about my question to Peter in the post above i.e. do I have to move the pump into the flow rather than leave it where it is (i.e. in the return) if I want to put the NRV in the flow?</blockquote>
    Sorry, no idea.</blockquote>

    OK, fair enough. Thanks.
  5.  
    The pump should be able to work where it is with the NRV on the flow. After all the NRV adds resistance but this is no different to any other resistance in the circuit caused by pipe length, bends and valves half shut etc..

    The London loop works but I doubt that 6" would be enough. If the pipe is hot through use then I would expect the thermosyphion to continue after use especially if there is a 'good head' driving the thermosyphion.
    • CommentAuthorJeff B
    • CommentTimeSep 27th 2017 edited
     
    Peter - thanks for your further response. I have room for about a 12 inch loop and if I put an NRV in that as well, then that should just about do it, do you think?! The NRV could either go in the horizontal bit at the base of the loop or in one of the vertical legs (probably the latter might be better in case any gunk collects at the bottom of the loop). Jeff
  6.  
    The loop won't be needed if you put in a NRV
    • CommentAuthorJeff B
    • CommentTimeSep 27th 2017
     
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>The loop won't be needed if you put in a NRV</blockquote>

    Peter - I am not known as the "belt & braces" Dad for nothing!
    • CommentAuthorJeff B
    • CommentTimeSep 29th 2017
     
    djh and Peter: just to let you know that the combination of the "London Loop" and the NRV have done the trick! I put the NRV in the "up" leg of the loop. Of course I'l never know if both were necessary but the for the cost of four elbows and two ball valves to isolate the loop (in the event of failure of the NRV), well....! Will now be continuing to monitor my pellet consumption to see what difference this has made.

    Thanks both for your help and also JonG for the tip about Mark at Newark (who was a great help too).
  7.  
    OMG that was quick !! SWMBO would love things to happen here with such speed !
    •  
      CommentAuthordjh
    • CommentTimeSep 30th 2017
     
    +1 Well done!
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