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  1.  
    We have a large thermal store with solar thermal. This is set up to heat the top portion first, then when this is up to 60°, it heats the bottom. This works really well. I'd like to do the same with the water from our stove. However, we only have 3 possible ports. At present the top and bottom are connected, which is fine, though it takes a while to heat up from cool. I'd like to use the top and middle until up to say 60°, then switch to bottom. I'd like an automatic valve rather than a motorised valve, but with only 3 ports, I'm not sure how to achieve it. Is it possible? I'd appreciate any ideas.
  2.  
    First thoughts are...

    - assume feed from stove to top connxn, return to stove from bottom connxn. Free connxn at TS mid point

    - use a thermostatic mixing valve, set to say 50oC, with the mid connxn feed to the HOT inlet of the TMV, and the bottom connxn feed to the COLD. The blended outlet returns to the stove (via the existing temp regulator).

    This should act as follows... The stove will draw from the mid of TS, as the TMV initially sees cold water coming to the HOT inlet, and so keeps the COLD inlet closed (no water drawn from base of TS).

    As the top half of the TS warms, the TMV will see increasingly hot water into the HOT inlet, to the point that it starts drawing cold from the base of the TS, and closing back the draw from the mid connxn.

    Think the above through for various scenarios in your setup, and try to find any faults.

    TMV's tend to have mesh strainers, which can clog with bits from stove jackets etc, so I'd fit a strainer on the outlets from the TS, to catch these bits, and let you flush them periodically (depending on how much crude you find). Otherwise you'll end up with no return flow to the stove, and all sorts of issues arising from that.
  3.  
    ComeOnPilgrim
    A couple of questions,

    Is the stove to thermal store a direct or indirect connection?

    Is the stove to thermal store gravity circulation or pumped?

    Do you have a loading valve (laddomat or similar) on the stove?
  4.  
    Hi Peter: direct, gravity, no loading valve. Hence I'd prefer something simple if possible.
  5.  
    @greenpaddy, is there a mixing valve that works like that? I'm familiar with a single input and two outputs.
  6.  
    Posted By: ComeOnPilgrimHi Peter: direct, gravity, no loading valve. Hence I'd prefer something simple if possible.

    So if it is gravity then you should not be putting restrictions (= valves) in the circuit.

    I don't think that putting valves etc. in the circuit will do much to help. Gravity tends to be self regulating (which is why a loading valve is not fitted on gravity circuits). With a gravity circuit the hotter the stove the faster the circulation will run and the cooler the stove the slower - so you will be taking heat out at the best rate for the configuration you have.

    If you pump the circuit then you may get out (a bit) more heat but you will need to put in a loading valve to avoid over cooling the stove and you can't get out more heat than the stove produces anyway and a pump stands more chance of upsetting the stratification in the TS whereas a gravity circulation won't disturb the stratification (ever).

    At the moment with the top feed and bottom return the hot water from the stove goes to the top of the TS (and stays there) and cold from the bottom of the TS returns to the stove (whereas with the solar thermal (which I presume is indirect and pumped) the water circulates through a coil (?) and returns to the panels usually with some residual heat).

    If you modify the gravity circuit to feed to the top and return from the middle at the start of the heating cycle you will probably make things worse because you will reduce the heat difference sooner than a bottom return and it is the heat difference that drives the gravity circulation.

    In order to get the TS hotter at the top quicker you need to put more heat in which IMO means getting more heat out of the stove although some improvement may be possible by improving the pipe routing but if the gravity works this will probably only be marginal.

    Do you have any other heat load on the stove (e.g. CH radiators)? If so these need the correct pipe work to ensure that the TS gets priority.
  7.  
    Köszönöm @peter. I agree, I was worried about putting a valve on the circuit for that reason. However, I think the flow is currently too quick and I think it is mixing up the water in the TS. I think this as the stratification is generally very good, but when the stove is on, the whole tank gets to around the same temperature. Ie, if the top is at 50° and the bottom is 30° prior to the stove operating, the TS then becomes, say 40° top and bottom. In addition, the water coming into e stove is quite cool, whereas going out of is much hotter (I can feel it on the pipes). This may be due to the height of the TS which is about 5m above the stove.
  8.  
    Forget the TMV I suggested above. It works on a pumped system, not on a gravity system (thanks PiH for asking the obvious question :shamed:). To answer your question however, TMV's have 2 inputs, and one output, which is where the "mixing" comes in, just like your shower valve.

    My pumped systems result in the top always getting hotter than the base, which is likely a result of the loading arrangement which prevents any water going to the TS until the stove is supplying consistently 45oC ish water. I also use a temperature differential controller (aka solar controller) to hold off the circ pump till the stove is hotter than the TS...not implying that I think you should change to pumped.

    My Guess (don't do that many gravity systems) would be that your stove is getting into thermosyphon mode quite early, due to the large height diff (very effective), but that means it is circulating before the output water has gotten particularly hot, delivering 40oC say to your TS top, drawing 30oC from the base. So as you realise, and I suspect is the point of the question, you are initially adding energy, but losing "useful" hot water.

    "IF" that is the case, then it's less about which part of the TS gets the heated water first, rather a need to hold off the flow from the stove till it's suitably hot. Not sure you can (or should) do that with a gravity feed system. The only "restriction" I ever fit on gravity systems is a flap check valve on the return, to stop reverse flow, when the stove is cold (quite amazing how effective thermosyphon can be when you don't want it to be.)

    I have always wondered about the cool return water to a stove in a gravity system, and the effects on the stove jacket that we try to avoid in pumped systems with load valves.
  9.  
    Thanks @GreenPaddy, and no worries! I wonder whether it might be worth connecting the middle and top ports on the TS to the outlet of the stove? That way, when the water is cool, it should only circulate through the middle port, and not mix up the hot at the tope of the store. Later, when the water is hotter, it would be able to circulate through the top port.
  10.  
    Not sure I see why that would happen by itself. Why would it chose middle over top based on temperature?. You'd need to bring in some form of control logic I suspect.

    Let's assume it did - if you heat a TS at the base, it destratifies, as circulating currents are set up internally. So not sure how much benefit you might get over existing. Also, potentially when the stove is off, you would get a little bit of thermosyphon from top to middle, as the water in the connecting pipes cooled and drop towards the base.

    One short term "manual" solution (which could poss be developed) might be to connect the top and mid to the stove as you suggest, and put a full bore ball valve in the pipe section going to the top only. You could then light the stove with the top section isolated for say an hour, till the stove is feeding water as hot as or hotter than at the top of the TS, then open the valve.

    That would give the "stall" effect of cool water to the top of the TS, but also not be a safety risk, as you always have an open route to the TS from the stove. About £15 for a full bore 28mm, so not a big cost risk. If it causes a load of issues, put the valve back on the mid port, to keep it perm shut.

    If it did improve things for the cold/cool start, It might be you can find a control vale to do that manual bit for you. There is a honeywell v4043B norm open 2 port valve, but I'm not certain how much of a restriction it might cause to the thermosyphon. Ideally full bore ball valve, or even a butterfly would give very limited resistance. You could even buy a motor head, and fit it to the manual valve body, if all else fails.

    The answer is probably NOT in my musings above, but keep focused on the aspect of doing something with the initial cool water from the stove, till it gets to where you want to be. It would be great to stick the fire on for a 1 hour fast burn and boost only the top bit of the TS for a shower say, rather than having to wait till virtually the whole TS is heated, which I think is your current position.

    If you do think of going to pumped, you're looking at about £500 of parts (pump, solar/differential controller, and a TMV to act as a load valve, plus other bits), and review the open vented nature of the design, to confirm the stove is still free to a vent.
    •  
      CommentAuthordjh
    • CommentTimeJan 6th 2020
     
    Posted By: GreenPaddyWhy would it chose middle over top based on temperature?

    Probably because there'd be no thermosyphon effect to drive it up the extra pipework.

    if you heat a TS at the base, it destratifies, as circulating currents are set up internally

    Well he's proposing heating it in the centre, is he not? Not at the bottom.
  11.  
    Thanks both! I guess it depends a bit on what's happening at the moment. The next step is probably just to measure the temperatures at the pipes entering / exiting the stove and entering / exiting the TS when it's running. That ought to give a good idea of where the heat and flow is going. I have a few cheap digital fridge thermometers that I can use. I'll do this now.
    • CommentAuthorborpin
    • CommentTimeJan 7th 2020
     
    Posted By: ComeOnPilgrimI guess it depends a bit on what's happening at the moment. The next step is probably just to measure the temperatures at the pipes entering / exiting the stove and entering / exiting the TS when it's running.
    That is the best way. I discovered that when my boiler started I was getting a slug of cold water into the top of the tank. By fitting a diverter valve, the water circulates through the boiler until the boiler can heat it to a high enough temperature. The valve only lets through water that is hot enought to the tank.
  12.  
    OK, so here are the measurements. I measured (1) TS top / bottom, (2) TS supply top / bottom, (3) Stove top / bottom.

    Start:

    43 / 42 ------ 32 / 32 ------ 35 / 31

    30 minutes:

    43 / 42 ------ 43 / 35 ------ 48 / 35

    1 hour:

    45 / 42 ------ 50 / 39 ------ 57 / 39

    2 hours:

    53 / 42 ------ 52 / 40 ------ 56 / 40

    3 hours:

    55 / 44 ------ 51 / 41 ------ 57 / 41
  13.  
    So my conclusions from the above:
    (1) Stove can heat incoming water around 20º
    (2) Water loses about 5º getting from the stove to the TS (about 6m vertically, and 4m horizontally)
    (3) Water supply stays pretty constant from the bottom of the stove.
    (4) There is decent stratification, at least when the stove is burning at a constant temperature. This is not exactly what I've seen, so I wonder what happens when the stove cools a bit so that the water supply to the TS is slightly cooler than immediately before - perhaps this mixes up the tank?
  14.  
    Might be easier to read
      Screen Shot 2020-01-07 at 21.34.18.png
  15.  
    Posted By: ComeOnPilgrim(2) Water loses about 5º getting from the stove to the TS (about 6m vertically, and 4m horizontally)

    Are the pipes from the stove to the store insulated ?? With about a 10m run this is about the same as a small radiator, if they are not insulated then perhaps insulate them and save most of the loss on flow and return.

    Posted By: ComeOnPilgrim(4) There is decent stratification, at least when the stove is burning at a constant temperature. This is not exactly what I've seen, so I wonder what happens when the stove cools a bit so that the water supply to the TS is slightly cooler than immediately before - perhaps this mixes up the tank?

    When the stove cools to the point that it is cooler than the store the circulation will stop. As the stove progressively cools the circulation will progressively slow.

    What is the size of the store and what is the size of the stove - to water and to the room (air)?
  16.  
    Posted By: Peter_in_HungaryAre the pipes from the stove to the store insulated ?? With about a 10m run this is about the same as a small radiator, if they are not insulated then perhaps insulate them and save most of the loss on flow and return.

    Yes, the pipes are insulated all the way.

    Posted By: Peter_in_HungaryWhen the stove cools to the point that it is cooler than the store the circulation will stop. As the stove progressively cools the circulation will progressively slow.

    Are you sure? My feeling was that the temperature differential in the stove might be sufficient to push the water through the TS, even if the water entering the top of the TS is slightly cooler than the water actually in the top of the TS.

    Posted By: Peter_in_HungaryWhat is the size of the store and what is the size of the stove - to water and to the room (air)?

    Store is 1500 litres. Stove is 5Kw to room and 10 Kw to store (or thereabouts).
  17.  
    I think I'm going to try connecting the top and middle TS inputs to the pipes to and from the stove. I'll leave the bottom connection, but openable in case it is needed to cool the stove.
    • CommentAuthorphiledge
    • CommentTimeJan 10th 2020
     
    Something you could try once youve got the gravity loop from the centre of the store is to have a pumped return from the bottom of the store to the cool return tapping on the stove, pumping via a 2 port zone valve and lockshield valve or similar restrictor. As your only looking for low flows this leg to yhe stove could be in 15mm pipe. Fit a tank stat just above the centre tapping and wire the stat to open the zone valve and wire the zone valve to run the pump when the valve is open. When middle of the tank is up to temp via gravity, the stat will open the valve and start the pump to circulate from the bottom of the store and heat the rest of the tank. Use the restrictor to lkmit the flow so the stove isnt overly cooled

    A further control enhancement would be to put a second tank stat at the bottom of the store to inhibit the pump when the bottom is up to temp

    If water circulates from the pump and short circuits back up the gravity return to the middle tapping there will still be hot water moving by gravity as theres cool water at the stove inlet to replace the water rising up the hot pipe
  18.  
    Posted By: ComeOnPilgrimI think I'm going to try connecting the top and middle TS inputs to the pipes to and from the stove. I'll leave the bottom connection, but openable in case it is needed to cool the stove.

    If you run on the top and middle connections with the bottom closed off then the bottom half of the store will never get heated by the stove.

    Looking at your chart above - after 3 hours the top of the store is at 55 deg and the bottom has just started to rise. Assuming the heat has just reached the bottom after this time and making the assumption that most of the store above this point is close to 55 deg then the store temp. has risen by 12 deg.. Using the formula
    volume in litres x 4 x temperature rise in degrees centigrade / 3412
    to calculate the energy used I get a figure of 21kW received by the store in the 3 hours.
    Now that is probably a bit generous because I don't know the actual temps of the water throughout the store. An even gradient of temperature might give an average rise across the store of 7.5 deg which means that the stove produced 13kW received by the store in the 3 hours.
    The reality is probably somewhere between the 2

    Was the stove running flat out during the measurement times?

    If you continued to pile wood onto the stove I would expect the store temp. to continue to rise and if you stop fuelling the stove I would expect the gravity flow to slow and then stop when the store top temp is above that of the stove top.
    This can be checked with temperature monitoring as you did above.

    BTW whilst you are pouring heat into the store as described above is the room containing the stove getting over warm? And for how long can you heat the store before the room becomes uncomfortable?
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