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    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    Currently renovating old stone-built detached house, insulating walls internally with 100mm multipor.
    We have 2 solar thermal panels fitted (Viessman 200), and a Morso DB15 boiler-stove heating a 300l heat store. Heat store also has 3kW immersion heater.

    It's our first full winter in the house after living in a caravan-in-a cabin for most of the previous four and a half years... so even with problems this is luxury..!

    We're using half the house at the moment:
    - the kitchen where the boiler-stove is fitted (5.2m x 4.3m x 4.5m to the apex ceiling); this room also has a 2.6kW radiator
    - the bathroom, which has a 1.1kW radiator, also the heat leak radiator for the stove and low power (290w) electric UFH
    - bedroom, which has piped UFH at 150mm spacing, and is 12m2

    We find that the stored heat from running the stove the previous day (and topping up with the immersion heater for a couple of hours if necessary from 5am to 7am) brings the heat store temp up to about 60c middle to top. The temp in the bottom depends on how hot it is fromn the previous evening.

    This gives us enough HW for 2 good showers and some left, but is insufficient to run the CH.
    We find the temp in the lower half of the heat store - where the CH water is drawn from - drops quickly as soon as we run the CH.
    We can get the whole store up to 60c+ after running during the day, so that we can run the CH for an hour or so in the evening. The bathroom usually stays very warm without the radiator drawing any heat from CH and the kitchen radiator thermostat is turned right down to conserve CH supply heat, so we're only using the CH to warm the bedroom via the UFH. We find we can run the CH for an hour or so and that will bring the temp up about 1degree - usually sufficient to warm the (bare concrete!) floor - but that will drop the temp in the heat store, maybe unless we have the Morso running flat out... It's the Pure Wood model and we're still experimenting with wood supplies and getting to the point where we have sufficint well-seasoned wood stored for a whole winter.

    There is never enough heat stored in the morning to be able to run the heating.
    Part of the issue is that even if we get the thermal store really hot by the evening - and we have had it up to 80c a few times - it will lose that heat by the morning. Some along the copper pipe runs, some from brass fittings around the heat store.

    We are meeting with the heating engineer soon to look at options. One possibility may be retro-fitting a backup LPG boiler and somehow linking it to the system: there are spare tappings in the heat store toward the top and at the bottom, although it would involve dismantling some of the finished work on the house.
    Another option I'm wondering about is somehow installing an additional heat store and linking this in to the existing one, using those same tappings, to give a much larger overall volume of water, and of potential stored heat. This would have to be located in the utility room, which is on the ground floor, so below the existing heat store.

    We will have to look at the feasibility of the LPG boiler with the heating engineer, but I thought I would ask here about the idea of a linked second heat store. Does anyone here know if that's feasible, and how we might control temperature and flow of water between the two?

    A couple more questions related to this situation:
    1. Following guidance (mistakenly) that came with the stove, we fitted a pipe thermostat to the boiler-to-heat-store output pipe rather than a flue thermostat. This is set at about 65c but it means that, if the water at the bottom of the heat store is over 60c, water continues to circulate through the stove for some time after the fire has died down, which can deplete stored heat.
    A flue thermostat would be better but the heat store is on the other side of a 700mm thick wall, a metre or so (with a floor/ceiling between) above the boiler-stove. All the walls are finished (lime plastered) and all the pipe work connected so we can't run a cable from a flue thermostat to the heat store.

    Does anyone know of a wireless solution, by which we could connect a flue thermostat to a wireless transmitter in the fireplace and the signal be received on the other side of the wall where the heat store is located?

    Lastly, any suggestions for the best way to insulate all those brass components on the heat store and pipework?

    Ian
  1.  
    First is there a reason that you are insulating internally rather than externally? For lots of reasons external is better.

    Have you done the heat load calculations for your house, both now and after the insulation?

    Having a combined space heater and heat store heater is not the best option because you will get a conflict where the store needs to be heated and the room with the stove is already too hot. With 9kW to water and 6kW to room I would expect the room to overheat before the store is up to a decent temperature – especially when you have the insulation done

    The nominal calculation for a heat store is 50:1 so with 9kW to water I would expect a 450ltr heat store. A heat store fully charges would be about 90deg. top to bottom but if you can’t get the store much above 60+ then adding an extra store won’t help much. Seasoned wood is key to wood burners and the efficiency drops through the floor if the wood is not seasoned properly (less than 20% moisture)

    It is not a good idea to have the hot water (DHW) and the CH coming from the same store because the temperatures and times of usage are too different. However if you must combine them then the usual way to plumb a heat store is for the stove to heat the store and the DHW is taken from the top 1/3 or 1/4 of the store and the CH is taken from the lower part. In this way the CH never depletes the DHW. The CH should not be run from the stove but only from the heat store. The heat store will act as the heat sink and when the store is up to temperature you either take heat out (via CH (can be automated)) or more usually you stop putting fuel on the fire.

    If your stove is not on a gravity system it should have a load valve (Laddomat or similar) to maintain the temperature of the stove at 70 deg. minimum. Stoves run below this will not be efficient and will clog up the chimney with tar condensate as the fire will be too cold.

    A 300ltr store IMO is not really enough to get DHW and any decent amount of CH out of it as there is just not enough energy in it, especially if you are only heating it to 60+. It is possible to run two stores in either series or parallel (depending upon the physical location of the stores) but it is not the best option. The key to getting the best out of a store is stratification. If the CH pumps the return in too fast then the water in the store will be all mixed up and you finish up with a store full of luke warm water. The CH should have a mixer valve at the store with the flow set at about 50deg – 60 deg so that the CH only takes enough heat out of the store to maintain this temp., whilst recirculating the return and so makes the most of the stored heat. (A problem if you are only heating the store to 60+)

    With regard to the stove cooling down the store when the fuel has run out, with my system (which has a laddomat loading valve) the boiler heats up to 70deg. and then the store starts to heat up. When the whole store is at 70deg. then both the store and the boiler rise in temp. together until the store is up to temp.. The trick is to have the fuel run out at about the same time as the store is up to temp.. That part is experience. I have found that the easiest way to stop the boiler cooling the store is to manually switch off the Laddomat (which has the boiler/store circulating pump contained within).

    Oh and in my experience wireless and 70cm stone walls don’t work together well, or at least with my basalt rock stone walls.

    So where to from here
    Can you post up your heat loads to give us a clue about what is needed.
    Add insulation to the store and insulate the pipes with good quality pipe insulation
    When you speak with the heating engineer make sure he knows about wood burners, heat stores loading valves and other magic associated with solid fuel burners. The average gas trained heating engineer doesn’t know too much about the best way to use/install these systems.
    If you put in an LPG boiler then this should supply the store with the CH coming off the lower part of the store with the DHW at the top as described above. A thermostat on the store would control the firing of the LPG boiler
    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    Hi Peter,
    Thanks very much for taking the time to give this detailed response - much appreciated.

    Re internal insulation, it's aesthetics: I've attached a couple of pictures. We didn't want to spoil this lovely old house and 100mm multipor insulation on all the external walls, 100mm PIR insulation + glasswool on ceilings and 100mm under floors seemed a good compromise solution.

    Controls on the boiler to heat store:

    Pipe thermostat on the boiler output to heat store switches on boiler circulation pump, set to about 65c.

    Esse load valve on the heat store: return to the stove mixes hot water coming from boiler with gradually increasing amounts of cooler water from the heat store. Gate only opens when temperature at the valve is 61c or more, ensuring that water returning to the stove is hot.

    The store is stratified, as you describe, with DHW at the top and CH lower: heating feed is about 3/5 of the height of the store, with DHW feed above that. Heating return is about 1/4 of the way up and the solar coil connections are below that. CH flow (out) doesn't have an additional mixer valve.

    Heat loads:
    (a) The part of the house being used now
    Bathroom, about 9m2: 1.1kW
    Bedroom, 12m2, UFH at 150mm spacing: approx 1kW at 45c (?)
    Kitchen (where boiler-stove is): 2.6kW
    -----------------------------
    (b) Not yet in use:
    Sitting room: 2kW
    Main bedroom: 1.3kW
    Study, 12m2, UFH as above: approx 1kW
    Utility room: 1kW

    Total = 10kW approx.

    Ian
    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    They're my own images
    •  
      CommentAuthordjh
    • CommentTimeFeb 1st 2018
     
    Nice looking building. It's not a water mill is it? The arrangement on the back looks unusual.
    • CommentAuthorIanCD
    • CommentTimeFeb 1st 2018
     
    Yes, this building used to be associated with a mill. This building, 'Mill House', is right by a stream...
    in fact the end wall of the low lean-to at the right of the first picture actually drops down into the stream, which is just out of the picture there.

    A lot of this view at the back is the result of extensive rebuilding: we replaced windows, added the deck and rebuilt the roof of the lean-to; the porch at the front of the house in the other picture is also a recent addition.

    The Mill itself was opposite this house; it was supplied by a leat drawn off a few hundred yards up stream. It might originally have been a fulling mill, converted to / rebuilt as a "good and sufficient water Grist (corn) Mill" as well as building a new house as conditions of a lease granted in 1781. The mill has been derelict for over a century but lime scale from the old mill wheel can still be seen behind the garage in the house opposite.

    Ian
  2.  
    Nice building - I can see why you want form over function regarding external building.

    For the heating
    For the heat loads was it the size of the rads. you have quoted or the heat loss through the fabric and ventilation?

    Posted By: IanCDThe store is stratified, as you describe, with DHW at the top and CH lower: heating feed is about 3/5 of the height of the store, with DHW feed above that. Heating return is about 1/4 of the way up and the solar coil connections are below that. CH flow (out) doesn't have an additional mixer valve.

    By stratification I mean the temperature gradient of the water in the store. Hot at the top, cold at the bottom and an undisturbed temperature gradient between the two.
    If you have 70 deg. top to bottom and you take out CH water from 3/5s up and return the water at say 40 deg into 1/4 way up then you will be mixing 40deg with 70deg making say 55deg. so your water at 1/4 is now 55deg. If the water came in at the bottom of the tank then you would still have 70deg at the 1/4 level and 40deg at the bottom.This 40deg band would then move up the store as more return water came in and you would maintain your 70deg output for longer.

    Based on the figures of where the connections are on the store you have about 120lts for DHW and 100lts for CH. 120lts for DHW should be OK - providing it does not get mixed by the force of the CH return mixing up the water, but 100lts for CH might just about warm the rads.

    So the stove should provide stored DHW but for CH you will need the stove to be running. You should try to get the store temp. a bit higher and have a mixer valve on the DHW t bring it down to safe temps. as this makes the supply of hot go further. The same applies to the CH part with a mixer for the UFH temp.

    For times when the stove is not running e.g. in the morning, when you have flu, or away for the weekend or just can't be bothered to light the stove - then you will need an alternative to the wood stove - enter the LPG boiler. This can be plumbed into the store either direct or through a coil (if you have a spare coil in the store) and probably best if plumbed into the middle of the store roughly where the CH in/out is, although with a 300 ltr store it probably doesn't make much difference.

    The electric immersion heater should be just above the CH output because you don't want to be heating the CH water with the electric immersion heater.

    I am presuming
    1 the DHW is indirect via a coil in the store
    2 the stove is a direct connection to the store
    3 the CH is a direct connection to the store
    4 the solar is indirect via a coil
    5 there are suffucent connection options on the store to enable the connection you want
    • CommentAuthorIanCD
    • CommentTimeFeb 2nd 2018
     
    Thanks again Peter... looks like the LPG boiler is what we need
    The figures I quoted are the size of the rads, and calculated output from the UFH based on an online article. The rad sizes were based on output requirements given by the heating installer and heat loss calculations by the radiator supplier (which were based on type of house, location, insulation, etc.). They were a compromise between their two figures where they differed, which wasn't a lot.

    DHW is indirect as you presumed, but via a plate heat exchanger mounted on the side of the store. There is a mixer valve fitted. Return from the heat exchanger to the store is just below the level of the heating return.

    Your other assumptions are all correct, except that the electric immersion heater is actually about 250mm below the CH feed. We only have this on for a couple of hours each morning, running on 'Economy 7', to 'top up' heat for DHW as needed to provide a couple of showers first thing; if the heat store is already at 60+c I think it cycles on / off a few times as the thermostat cuts in and out. The supply to UFH is via mixers for both rooms.

    There are only 2 spare tappings on the heat store, and the upper one is not in an ideal position: about 3/4 of the way up, 30mm or so below the DHW take off. The other is low down, at about the same level as the return to the wood boiler-stove.

    There is a temperature gauge fitted just above the heating feed, which doesn't appear to do anything other than indicate CH feed temperature and it looks like it's connected via the same size boss as the pipes. I've asked one of the technicians at the heat store manufacturer whether we could use that boss for the LPG boiler feed, so it would be coming in just above the CH feed.

    Which would just leave us with the issue of how tro control the LPG boiler, which we'll need to discuss with the heating engineer.

    This will mean dismantling some of the finished work on the house, and probably includes taking a lime-plastered stud wall apart which encloses various pipes to and from the boiler-stove and heat store. And I hate to think what the additional cost is going to be - we've already spent nigh on £14k on it, and that's excluding the radiators and flue liners..! But at least we'd have a working CH system.
  3.  
    Plate heat exchangers (PHX) are bad news for heat stores. For a heat store to work efficiently you need an undisturbed heat gradient from to bottom and gentle flow rates. A PHX needs a high flow rate on the primary in order to get the heat output to the secondary. This will create disturbance within the heat store and mix up the hot at the top with the cold at the bottom. In your case you will be taking hot water from the top of the store removing some heat through the PHX and then shoving still hot water into the store just below the CH return to mix with the cold area that has never been heated by the immersion heater (which is above this point?) The result of this will be to mix up your hot water to a not very useful cool/warm temperature.

    It is best to separate the CH /space heating and the DHW. The problem is how to do this economically. From an earlier post you are already thinking of getting a second heat store, perhaps use this as the opportunity for separating the DHW from the CH.

    Your DHW will have 4 sources of heat input, solar, wood stove, LPG and electric immersion. Do you need all of them. DHW tanks can have an internal pipe heat exchanger which gets away from the issues of PHXs. With this sort of tank you would have a coil for the solar input at the bottom, a coil for the DHW out at the top and the stove and LPG direct.
    Do the sums on the cost of heating water via LPG or E7 and you might find that it is not worth the hassle to put LPG to heat the DHW, (especially if you are considering PV in the (near) future) in which case I would suggest a twin coil DHW tank, one for solar and one for the stove, a normal immersion heater and the DHW without any heat exchanger – which gives a better DHW flow. And of course your existing store serves the CH only with inputs from the LPG and wood stove.

    It is useful to have a temperature gauge on the heat store, and I would locate the gauge readout close to the wood stove. With the gauge about 1/3 up on the store you will be able to see the temperature and so know when to stop feeding the stove so as not to overheat the store. (My heat store (2000lts) has 5 gauges top to bottom to guide the loading of the boiler with a separate 300ltr DHW tank driven from the heat store. This serves 2 houses)

    Control of the LPG boiler I would expect to be driven by the temperature of the store with the thermostat somewhere towards the top with the flow and return also in the upper half. The wood stove will serve the whole store. In this way the LPG only comes on when there is insufficient heat in the store and then only heats the top of the store. If you have a separate DHW tank with LPG supply then you will need a diverter valve so that either DHW or CH can call for LPG heat but the DHW will get priority.
    • CommentAuthorIanCD
    • CommentTimeFeb 6th 2018
     
    Hi Peter,
    Thanks again... food for thought, and to discuss with heating engineer next week.

    Reading other posts here re improving the insulation of the heat store to reduce overnight temperature drop, I think I may have found a major culprit: the central pipe from the top of the heat store, which goes to to the feed and exansion tank, is very hot: maybe at same temp as hottest part of the store. Something else to look at.

    Ian
  4.  
    Just a point on Plate Heat Exchangers. We have one attached to our 350l heatbank for the DHW and I haven't found any problems with it ruining stratification. I have actually deliberately tried to get it to do so on hot summer days when our solar PV has heated the top third of the tank to 80C and has shut off dut to the immersion heater thermostat. I ran the hot tap in the hopes of mixing up the water to get more out of the solar PV, but it didn't work. The bottom two thirds of the tank remained at the same temperature. In the end I fitted a second immersion heater in the bottom third of the tank, which is activated once the top immersion hits 80C.

    What is great about the Plate Heat Exchanger is that it's very efficient and it can be set to a specific temperature, which is very useful when your tank is bubbling away at 80C but the water at the tap is a nice 45C (or whatever temperature you want to set it to).
    • CommentAuthorIanCD
    • CommentTimeFeb 7th 2018
     
    Thanks Pile-of-Stone,
    That's helpful... and good to know.

    Ian
  5.  
    Posted By: Pile-o-StoneJust a point on Plate Heat Exchangers. We have one attached to our 350l heatbank for the DHW and I haven't found any problems with it ruining stratification.

    If your heat bank is say 80deg at the top and 40deg at the bottom and your PHX has a temperature drop of say 15 deg then you will be putting water at 65deg into the 40 deg water at the bottom. Given that PHX usually run at a fairly high flow rate this will cause the 65deg and the 40deg water to mix, raising the temperature at the base of the heat bank. The more the PHX runs the more the mixing will occur and so the heat bank will, over time, have a more even heat top to bottom. this will make it harder to get the required output temperature on the secondary.
    Have you measured the temperature at the bottom of the heat bank with the PHX running to see if there is any temperature rise there
    • CommentAuthorgoodevans
    • CommentTimeFeb 9th 2018
     
    Not necessarily Peter, - the water being heated will have come from the outside mains - perhaps 11 to 20 centigrade - If the heat exchanger is efficient the top of the tank water should be cooled to nearer say 25 centigrade after it has passed through the heat exchanger then placed into the bottom of the tank.
  6.  
    Posted By: goodevansNot necessarily Peter, - the water being heated will have come from the outside mains - perhaps 11 to 20 centigrade - If the heat exchanger is efficient the top of the tank water should be cooled to nearer say 25 centigrade after it has passed through the heat exchanger then placed into the bottom of the tank.

    Except that most PHX are designed to have a 15 -20 deg drop from input to output otherwise you can't get the rated output.
    • CommentAuthorgoodevans
    • CommentTimeFeb 9th 2018
     
    You could be right Peter - I have no experience in this area. I suspect that it depends on the system - it looks like the Gledhill systems adjusts it's heat store flow rate to achieve a target output temperature (it has a dedicated pump variable for this) - the hotter the store the slower the flow.
    •  
      CommentAuthordjh
    • CommentTimeFeb 9th 2018
     
    Certainly the Gledhill adjusts the PHE pump flowrate if the DHW demand is slow (i.e. almost closed tap) and it controls the produced DHW temp to a setpoint, which is 60°C IIRC.
  7.  
    Posted By: Peter_in_Hungary
    Posted By: goodevansNot necessarily Peter, - the water being heated will have come from the outside mains - perhaps 11 to 20 centigrade - If the heat exchanger is efficient the top of the tank water should be cooled to nearer say 25 centigrade after it has passed through the heat exchanger then placed into the bottom of the tank.

    Except that most PHX are designed to have a 15 -20 deg drop from input to output otherwise you can't get the rated output.


    I don't understand how the heat exchanger could raise the temperature of mains water from 12 degrees to 60 degrees and yet not significantly reduce the temperature of the HW feed from the top of the tank. Is it not the case that to raise mains water temperature by 48 degrees, it would reduce the temperature of the feed by at least 48 degrees (with a little extra for inefficiencies)? So to use your initial example, 80C from the top of the tank would be reduced to 32 degrees when it is fed to the bottom of the tank, which would cool the 40C water that was stored there.

    I tried to google the temperature drop across a heat exchanger, but couldn't find anything. Do you have a link with details of the 15C to 20C drop you're quoting? If that temperature drop is accurate, then I guess the water pump attached to the heat exchanger is pumping HW through the HX more than twice as fast as the mains water is coming in. We have pretty fierce mains water pressure, but the pump is on the lowest of the three speed settings.

    I think for balance you also have to look at the disadvantages of having a coil inside a thermal store. They must use an external heat exchanger for a reason, otherwise why do it? It's a lot more expensive to fit a pump and plate heat exchanger onto the side of a tank then just put a coil inside. I know the reasons, and for the size of tank I have (350l) and the size of tank the OP has, it would make much more sense to have a PHX. For much larger stores, such as the one you perhaps have, then I'd imagine a coil would be preferable.
  8.  
    Posted By: Pile-o-StoneI tried to google the temperature drop across a heat exchanger, but couldn't find anything. Do you have a link with details of the 15C to 20C drop you're quoting? If that temperature drop is accurate, then I guess the water pump attached to the heat exchanger is pumping HW through the HX more than twice as fast as the mains water is coming in. We have pretty fierce mains water pressure, but the pump is on the lowest of the three speed settings.


    I found a spec sheet for various PHX at
    https://www.aosmithinternational.com/wp-content/uploads/Spec-sheet_PHE_UK.pdf
    This quotes a primary temperature of 80/60 with a secondary temperature of 10/60. The flow rates depended upon the size of the PHX but the domestic size would be 2.15m3/h for the primary and 0.86m3/h for the secondary. Now this would be to get rated output, but a typical 1/2" tap runs at about 14 lts/ min (if I remember correctly) which is about 0.86m3/h So this PHX run as per the spec would support a 1/2" tap at full flow continuously.

    But the above PHX puts water back into the store at 60 deg. whih will b mixing with what ever water temperaure happens to be there. Now if you are not demanding the outflow (e.g. tap not fully open) then your smart gizmo pumping the primary can reduce the flow which could reduce the return temperature.

    Posted By: Pile-o-StoneIs it not the case that to raise mains water temperature by 48 degrees, it would reduce the temperature of the feed by at least 48 degrees (with a little extra for inefficiencies)? So to use your initial example, 80C from the top of the tank would be reduced to 32 degrees when it is fed to the bottom of the tank, which would cool the 40C water that was stored there.


    It can't work like that if you want a high output from a small unit. Temperature transfer is dependent on many things amongst them the temperature difference. The greater the difference the faster the heat transfer. The temperature coming from the PHX is nothing like as cool as the mains water going in, if it was there would be very little heat transfer towards the end of the PHX. which is why the primary flow has a designed temp. of 80/60. Think CH radiators, these are designed with a temp. drop of 20 deg across the rad and the tables for output will quote kW output at (typically) 80/60 or 70/50 and if you step away from this you loose output. (which is why you can't run rads at UFH temps without dramatically increasing the size)

    They fit PHX to heat stores because either you can't retro fit a coil to the store or you need a PHX to get the rated output. (rated output is one thing actuality may be different) DHW coils are not simply copper coils but are usually ribbed tubing to create turbulence to get better heat transfer. (and they are not cheap)
    •  
      CommentAuthordjh
    • CommentTimeFeb 10th 2018
     
    Posted By: Peter_in_HungaryThe temperature coming from the PHX is nothing like as cool as the mains water going in, if it was there would be very little heat transfer towards the end of the PHX. which is why the primary flow has a designed temp. of 80/60.

    The PHE that we're discussing is the Gledhill GT017. Unfortunately, I can't find any specs online but I stongly suspect that its specs are considerably different to the example you found.

    The PHE takes its primary water from the hottest part of the tank at the top and returns it to the coldest part at the bottom. As far as I know, its a counterflow design, so at one end the hot primary water is meeting DHW at near supply temperature (50-60°C or so) and at the other end the cooler primary water is meeting the incoming mains water at maybe 10°C. As far as I know it is usual to design such systems so the temperature difference between the primary water and the secondary water over the length of their counterflow is fairly constant.

    The temperature of the water being returned into the bottom of the tank is just under 30°C according to my thermometer. So yes, it is warming the bottom of the tank a little bit and that means that everywhere in the tank will be at least thirty degrees, and reduces the effective size of the store a bit, but it certainly isn't returning 60°C water to the tank. Indeed, during the winter I only heat the top of the tank to 60-65°C, so it would be physically impossible.
  9.  
    All PHXs run counterflow, they would be too inefficient otherwise. The amount of mixing you would get will depend upon the flow rate of the primary pump (which is driven by the required temp. vs. the achieved temp - correct?) and the return temp vs. the temp at the bottom of the tank. If you only draw a small amount of water at a time then mixing will be minimal, if you draw a large bath mixing could be significant. If in the summer when the top of the store is 80 and the bottom not heated then putting 50 deg water (assumed same 30 deg drop) would be detrimental to the overall usability of the heat contained within the store.

    Heat stores supplying CH have mixer valves between the flow and return to reduce the amount of hot returned to minimize the problem of mixing the store to an overall warm
    •  
      CommentAuthordjh
    • CommentTimeFeb 11th 2018
     
    Posted By: Peter_in_HungaryIf in the summer when the top of the store is 80 and the bottom not heated then putting 50 deg water (assumed same 30 deg drop)

    In the summer the whole of the tank is heated in my case (by PV panels). I don't think the drop will be the same because that would imply the temperature differential increased through the length of the counterflow. Seems much more likely to me that the primary flow would reduce to keep the temperature differential constant. But that's just my guess, I don't actually know. Maybe I'll measure it this summer :bigsmile:
    • CommentAuthorgoodevans
    • CommentTimeFeb 11th 2018 edited
     
    Posted By: djhThe temperature of the water being returned into the bottom of the tank is just under 30°C
    Well this is good news - it means that the boiler supply will always allow full condensing.

    With a stores with eternal plate exchangers like the Gledhill I see the following disadvantages.

    1) The bottom of the store may not be the coolest - therefore some extra, useless, heat loss from the tank in the summer months (but cost of that loss vastly reduced if solar pv used).

    2) a complicated system means more parts to fail. But the tank itself is therefore very simple and all the other bits sit outside the tank and can be replaced.

    Strangely the Gledhills come in at prices less than those of a simple finned coil store. Other than disadvantage 1 above, is there really a problem of the bottom of the tank being warm providing it is cool enough to allow the boiler to reach full condensing mode. Not perfect stratification but good enough.
    •  
      CommentAuthordjh
    • CommentTimeFeb 11th 2018
     
    Posted By: goodevansOther than disadvantage 1 above, is there really a problem of the bottom of the tank being warm providing it is cool enough to allow the boiler to reach full condensing mode. Not perfect stratification but good enough.

    I don't know. My system is fully electric and I try to get the whole tank as hot as possible when the energy is free.
  10.  
    Posted By: djhI don't know. My system is fully electric and I try to get the whole tank as hot as possible when the energy is free.


    I'm the same and now I have the second immersion attached to my iBoost, I should be able to get two thirds of the tank toasty hot. I do look at the bottom third of the tank now with regret, though I think 233l of water at 80C should be enough for most of my HW needs this summer :).
  11.  
    Posted By: goodevans
    With a stores with eternal plate exchangers like the Gledhill I see the following disadvantages.

    1) The bottom of the store may not be the coolest - therefore some extra, useless, heat loss from the tank in the summer months (but cost of that loss vastly reduced if solar pv used).

    2) a complicated system means more parts to fail. But the tank itself is therefore very simple and all the other bits sit outside the tank and can be replaced.


    There are also disadvantages with Thermal Stores (internal coil exchangers):

    Circulation of water in a a thermal store is driven by convection currents caused by the cooling effect on the stored water of the cold water flowing through the heat exchanger. These convection currents are relatively gentle, and proportional to the amount of heat being drawn from the store. Thus the cooled water tends to fall gently to the bottom of the store leaving hotter water higher up, nearer the heat exchanger. As the store continues to supply heat to DHW the cooler layers of water extend further up the cylinder until at some point, when much of the heat exchanger is surrounded by cooler water, the temperature of DHW drops below 40°C. At this point there will still be some hot water near the top of the cylinder, so the thermal store is not using the stored hot water with perfect efficiency. In a heat bank water can be drawn from the very top of the cylinder to pass through the heat exchanger, so all the stored hot water should be available for heating DHW.

    Thermal Stores may be more prone to scaling-up in hard water areas. When there is no demand for DHW the water in the heat exchanger coil is raised to the temperature of the Thermal Store, whereas in a Heat Bank the heat exchanger cools towards ambient temperature when there is no DHW demand. Also, even if the heat exchanger of a Heat Bank does become scaled up it can be removed for descaling or replaced if unserviceable, whereas the heat exchanger of a Thermal Store is an integral part of the appliance.

    These were from a wiki I found online. The wiki also mentioned the issue with hotwater being pumped from the top of the store to the bottom, but said this can be mitigated by restricting the flow through the heat exchanger and pump with a thermostatic valve.

    Swings and roundabouts and each type has their merits and disadvantages. The advantage of both is that you get mains pressure hot water with zero risk of legionella and you can couple multiple heat sources together.
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