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Heating and cooling: Adding extra insulation to a thermal store

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  1.  
    Hi
    I have an Gledhill Torrent thermal store and I find the temperature will drop by 12 ' C overnight with no use (from 60'C). Is this normal? All the pipes (hot water and solar antifreeze) are insulated with solar insulation. What is the best additional insulation to use. Would the recycled glass bottle roll insulation be suitable?

    Many thanks Dave
    • CommentAuthorGaryB
    • CommentTimeMay 27th 2012
     
    It looks as though I will be getting an ASHP and thermal store this summer and I intend to insulate the TS to the hilt.

    The recycled glass bottle insulation roll is cheap and easy to fit, the one recommendation I would suggest is to use a foil faced finish, which reduces the radiant heat emission by 30-35%. Wrap the pipework in foil finish as well for the same reason.

    I used Thermawrap foil bubble wrap to start and then discovered a Homebase foil faced underlay for laminate flooring which is cheaper per m2 than the Thermawrap and would look neater on cylinders and straight pipes.

    My modifed 210 litre solar DHW cylinder drops 7 deg C over a 12 hour period but that is starting at 80 deg C at 6 pm.
  3.  
    Hi GaryB

    Thanks for the tip on the foil. I have some foil backed polystyrene insulation that goes behind radiators. I can use this directly against the current insulation foil side in and then add a layer of the recycled glass bottle insulation. I have a couple of rolls of the wide foil tape left over from installing Super foil insulation. I can wrap the insulated pipes with this.

    Cheers Dave
  4.  
    I thought foil needed a transition to work also know as an air gap? Hence, foil out for low emissivity.
    •  
      CommentAuthordjh
    • CommentTimeMay 29th 2012
     
    Posted By: daveseavistaI have an Gledhill Torrent thermal store and I find the temperature will drop by 12 ' C overnight with no use (from 60'C). Is this normal? All the pipes (hot water and solar antifreeze) are insulated with solar insulation.

    It sounds like you may have some thermally-induced circulation within the pipes. Do any of the pipes feel warm/hot at a significant distance from the store?

    This often happens with pipes that run upwards or slope upwards. It can even happen with horizontal pipes. But it shouldn't happen with pipes that run downwards away from the tank.
  6.  
    Hi Gotanewwife
    Thanks for the tip on foil out. I had not had chance to fit it so goo timing.

    djh I will check the pipes. The solar pipes are very short less than 500 mm to the pump station. It is flexi stainless and is at a similar level to the input / output to the pump station. From the pump station there is about 1 meter of flexi SS to the input of the panels and about 2 meters of flexi SS from the output. The panels are higher than the pump station. The hot water output has a 3 meter run that has a very slight drop on it. Is the circulation within the hot water circuit or the solar circuit?

    Cheers Dave
  7.  
    Hi,

    I dont have the details to hand these days but i recall one solar / store supplier had some inserts that went into the pipes on exit from the store specifically to prevent the extraneous circulations. Just a small / slight cone. Would add a bit of extra flow resistance but wouldnt be overcome by natural convections.

    Cheers, Mike up North
    •  
      CommentAuthordjh
    • CommentTimeJun 6th 2012
     
    Posted By: daveseavistaIs the circulation within the hot water circuit or the solar circuit?

    It can happen in any circuit. But it is easy to test - if a pipe gets hot for a significant distance then you're losing heat through it.
    • CommentAuthorborpin
    • CommentTimeJun 6th 2012
     
    Posted By: djhIt sounds like you may have some thermally-induced circulation within the pipes. Do any of the pipes feel warm/hot at a significant distance from the store?

    This often happens with pipes that run upwards or slope upwards. It can even happen with horizontal pipes. But it shouldn't happen with pipes that run downwards away from the tank.
    Interesting - never heard about that. Had an issue in my last house and that would be the answer why. So all pipes should bend down from the tank before they go off elsewhere?
  10.  
    In theory yes - a downward bend should stop a thermally-induced circulation ( otherwise thermo-siphon or gravity circulation to give other names ) starting, but if a flow is in progress via a pump them switching off the pump is not guaranteed to stop the flow even with a downward bend if the thermally-induced circulation is strong. A spring loaded non-return valve (NRV) in the circuit will stop thermally-induced circulation but will require a small extra amount of pressure from the pump (to overcome the spring). Care has to be taken placing NRVs as they are closed in the reverse direction and this could have consequences in pressure balancing / relief depending on the circuit.
    •  
      CommentAuthordjh
    • CommentTimeJun 7th 2012
     
    As Peter says, but also note that you don't need two pipes to get circulation. Warm water can travel along the upper half of a pipe to a point where it can lose heat, and the resulting cooler water circulates back along the lower half of the same pipe.
    •  
      CommentAuthorJSHarris
    • CommentTimeJun 7th 2012
     
    <blockquote><cite>Posted By: djh</cite>As Peter says, but also note that you don't need two pipes to get circulation. Warm water can travel along the upper half of a pipe to a point where it can lose heat, and the resulting cooler water circulates back along the lower half of the same pipe.</blockquote>

    We once lived in an old house that had a single pipe thermo syphon central heating system that worked like this. It used very large bore cast iron pipes that allowed both the flow and return to use the same pipe!

    I have to say it was not effective, and seemed to be almost uncontrollable, in as much as it was either blazing hot or freezing cold. The heat source was a big cast iron coke burning boiler in the basement that was equally temperamental.
    • CommentAuthormikeb
    • CommentTimeJun 7th 2012
     
    On the subject of tank insulation: My 3 tanks -total 2000 litres - were supplied with insulated jackets and caps. All the pipework is lagged with foam however I am still unsure about the overnight heat loss which is only apparent at this time of the year when the boiler is fired every 3 days. On the night of a burn I record a temperature of around 96c which drops by about 24c overnight (between 9pm and 6am) with no heating and little hot water usage. This is with an outside night temp of about 11c.

    Is this normal?

    Is there any merit in boxing in the tanks and filling with insulation? If so - what insulation would you recommend and what depth of fill can I allow?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 7th 2012 edited
     
    It is an exponential loss and by having 3 tanks you have a much larger surface area.
    You may find, and this would take a bit of experimentation that if you raise the temperature to 80°C, it still drops to about the same temperature (72°C)
    Give it a go and see what happens.
    • CommentAuthorowlman
    • CommentTimeJun 7th 2012
     
    Posted By: mikeb................ On the night of a burn I record a temperature of around 96c which drops by about 24c overnight (between 9pm and 6am) with no heating and little hot water usage. This is with an outside night temp of about 11c.

    Is this normal?

    It does seem excessive. When you say 96C, is that top to bottom?, and on each of the three tanks. the temp drop may just be a "settling" of the "whole". - or - What sort of heating controls do you have, with some, if you just turn the thermostat down effectively stopping the CH, the pump will still continue the circulate according to it's timer programme, albeit cold water, but there may still be minor ammounts of the accumulator water entering the circuit and "churning" the lot.
    • CommentAuthormikeb
    • CommentTimeJun 7th 2012
     
    The temp sensor is fitted 12" from the top of the middle tank (the tanks are 60" ).
    The heat loss on the second night averages about 19c.
    When the heating is off the pumps do not circulate.
    Any thoughts on extra insulation?
    To be fair to the tank insulation -it never feels hot to the touch and when the boiler is out the room is only warm.
    • CommentAuthorowlman
    • CommentTimeJun 8th 2012
     
    My "distant" guess is that when you think the whole 2000l is fully charged, it isn't. A small digital temp readout with a stick on temp sensor poked through the insulation on each of the other two tanks would give you a better gauge of when they are fully charged. If my assumptions are correct and the charging process is a bit odd, due to the fact you have three tanks, then during the night they thermally equalise themselves hence the apparent temp drop on the middle tank. Surely if the insul. is doing it's job it shouldn't feel hot. If you eyeball the other issue first then it may be you don't need extra insul.
    Hope this helps, Mike
  18.  
    Hi,

    mikeb - Although not directly related to heat loss, what’s the configuration for loading the 3 tanks. Is it done sequentially one after the other or simply through a common header? Likewise for discharging.

    Cheers, mike up North
    • CommentAuthormikeb
    • CommentTimeJun 8th 2012
     
    Owlman, I have a couple of sensors I can try - where's the best place for the - top -bottom - middle?

    Mike (Up North) I'm not sure - they are linked sequentially but I'm not sure if the heat follows a sequence or feeds all three equally.

    I'm reading the blurb for some tanks which claim 2c-5c loss overnight - surely this is a brave claim or is my system all to cock?

    Thanks for all the comments so far -greatly appreciated!
    • CommentAuthorowlman
    • CommentTimeJun 9th 2012
     
    Posted By: mikeb...........Owlman, I have a couple of sensors I can try - where's the best place for the - top -bottom - middle?

    Hi Mike,
    In the long run it may be a suck it and see approach and you may have to experiment but for starters personally I'd put them as the other one, about 12" from the top. If it does prove that this is the cause of the overnight temp drop and you've figured out the loading sequence you can then fine tune the final temp probe locations. It may be that one probe at say near the bottom of the last loading tank is all you need to observe.

    Mike
    • CommentAuthormikeb
    • CommentTimeJun 9th 2012
     
    Owlman,
    Right -I've done that. I'll be firing up later today so it will be interesting to see the results.
    I wrapped the tanks in foil - bubble insulation and I've also lagged everything that's warm!
    Cheers
    • CommentAuthormikeb
    • CommentTimeJun 10th 2012
     
    I'm wondering if I've done the wrong thing in wrapping the tanks with silver foil insulation blanket on top of the purpose fitted jacket. The foil feels quite warm on the surface, is there a possibility that it's acting as a heat sink and actually drawing heat through the jacket?
    i'm now considering boxing in the tanks and filling with Rockwool or similar....
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 10th 2012
     
    More likely the way that you 'feel' the transfer of heat. Adding anything (unless it massively increases the surface area) will act as insulation.
    • CommentAuthorGaryB
    • CommentTimeJun 10th 2012
     
    Mike:

    ST is right, the metallised surface appears to increase the conductive heat transfer when you touch it.

    When open to air, however, the low emissivity of the foil reduces the radiant losses. Take a look at any designer towel radiator data sheets and compare the output of a chrome finish rad with a standard paint finish model - there should be 30-35% lower output from the chrome one.
    • CommentAuthormikeb
    • CommentTimeJun 10th 2012
     
    Thanks for that! It was a hell of a struggle to do the job and I'm glad I don't have to undo it all.
  26.  
    This is inspiring me to add some bubble wrap + foil type insulation we have spare to our thermal store - will it be important to tape the joints?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 11th 2012
     
    Make sure you check any cables can be covered.

    http://www.cable-calculations.co.uk/test/psc.htm
  28.  
    Sorry, don't understand! Could you explain, please, SteamyTea?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 11th 2012
     
    Electrical cables are rated to to take a certain current, at a certain voltage AND up to a certain temperature.
    An immersion heater is usually 3000 Watts, that is 13 Amp at 230 Volts (12.5 A at 240 V).
    A cable also has an internal resistance, so can be thought of as part of the heater. This is usually stated at ohms/(m^2.m). So the longer the cable (all the way back to the consumer unit that is), the more resistance there is.
    There are what is called in the trade (just to confuse the rest of us) things called Installation Methods, they have numbers line 101,102,104. These are how the cable is physically fitted, it may be just in the air (typical between wall isolator and immersion heater), then clipped to some plasterboard and eventually run through a highly insulated wall.
    Now there are rules that says for any thickness of insulation that the cable goes through the cable needs to be derated. Or in English, if a cable that is in free air can take a load of 13 A, when it is in a wall it may only be allowed to take 10 A. But, and this is missed by many, it also only holds true up to certain temperatures. So say your airing cupboard is usually at 25°C, but you cover the cable in 300mm insulation that is now at 50°C (somewhere near the water temperature), you have to recalculate for 3 changes, the installation method (it is not in free air any more), passing though 300mm of insulation and now working at a higher temperature.
    This applies to the whole cable length (back to the consumer unit), not just the bit you have changed. You take the worse cases and calculate on them, not on an average or on any other bit. So assume that the whole cable is in an insulated wall and then has an extra bit of insulation and at 50°C. You cannot rely on the MCB (fuse) to disconnect the circuit as it can still overheat and be below the rating.

    Usually though, immersion heater cables are larger diameter than ringmain (often 4mm^2 rather than 2.5mm^2), but that needs checking.
  30.  
    Thanks!
    So the risk is that the cable could overheat and burn out or start a fire if the immersion is on and it's too well sealed and insulated?
    I'll leave it untaped round the immersion cable then. Presumably sensor cables are too low current to matter?

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