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Heating and cooling: Accumulator tanks

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1 to 17 of 17

  1.  
    • CommentAuthorminisaurus
    • CommentTimeAug 25th 2021
     
    I’m really liking some of the new accumulator tanks I get to work with, and am starting to think that all properties that need/have radiators and/or underfloor heating should have at least one. In my work we have CTC, Nibe, Polar-something - all great products - usually about 1.5m high, 0.5m diameter.

    Our local district heating company are now in the game also, with this: https://www.midroc.se/en/rodoverken/reference-projects/energy-storage-in-gothenburg
    56 m high, 23 m wide, 22 000 m3 capacity, multiple heat sources, hopefully means they’ll no longer have to burn oil on the very cold winter days :bigsmile:
  2.  
    Posted By: minisaurusI’m really liking some of the new accumulator tanks I get to work with, and am starting to think that all properties that need/have radiators and/or underfloor heating should have at least one.

    I can't see the point of an accumulator tank for systems that have an on demand heat source e.g. gas boiler, oil boiler or even some pellet boilers.

    Batch burners like wood gasifiers or the Swedish CHP mentioned in the article would need accumulators to solve the problem of heat supply being mismatched to heat demand.

    The losses of an accumulator will negate their use for situations other than where the heat source requires them.
    •  
      CommentAuthordjh
    • CommentTimeAug 26th 2021
     
    I thought in a UFH system the floor slab itself serves the purpose of an accumulator?
    • CommentAuthorowlman
    • CommentTimeAug 26th 2021
     
    Doesn't that require that you have a "controllable" heat source on the input side?
  5.  
    Posted By: djhI thought in a UFH system the floor slab itself serves the purpose of an accumulator?

    No. UFH is thermal mass which will store heat and release it slowly and in an uncontrolled manner to the space above. An accumulator will store heat and then it will be taken out by some other function (DHW or some type of space heating) on demand. The problem is that between putting the heat in to an accumulator and taking it out there are (unwanted) losses that can't be eliminated only reduced by insulation.
    • CommentAuthorminisaurus
    • CommentTimeAug 26th 2021
     
    Posted By: Peter_in_Hungary
    I can't see the point of an accumulator tank for systems that have an on demand heat source e.g. gas boiler, oil boiler or even some pellet boilers.

    Aren't we, on this forum, trying to move away from carbon negative systems such as gas and oil boilers? :bigsmile:

    We find them useful for e.g. solar heating, and for heating with heat pumps when electricity prices are lowest - you can program the pump to read in current tariffs and react accordingly.

    We also hope to start loading tanks with the "waste" heat from fridges and freezers.

    Where we have heat pumps, we also strive to connect our heat pumps only TO accumulator tank(s), and then drive the heating and hot water FROM the tank(s); this results in a much happier heat pump, lower running costs, fewer maintenance events, and more constant radiator and hot water temperatures.

    I'm not sure the losses are so big nowadays, and when the tank is within the building envelope, your radiator thermostat would close earlier, your ventilation system will recycle any heat leaked :bigsmile:
    • CommentAuthorminisaurus
    • CommentTimeAug 26th 2021
     
    Posted By: owlmanDoesn't that require that you have a "controllable" heat source on the input side?

    No, we have e.g. solar. Göteborgs energi (in the article) store waste heat from oil refining and the sewage works. Hope I understood your question.
  8.  
    Minisaurus
    Your opening statement said that all properties with radiators or UFH should should have accumulators without qualification as to the type of heat source. this bit I disagree with. No point in having an accumulator with a gas combi boiler.

    Posted By: minisaurusAren't we, on this forum, trying to move away from carbon negative systems such as gas and oil boilers?http:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt=":bigsmile:" title=":bigsmile:" >

    We find them useful for e.g. solar heating, and for heating with heat pumps when electricity prices are lowest - you can program the pump to read in current tariffs and react accordingly.

    No problem with this. In fact I would agree. however for most people affordability has to come in to the equation and not many can afford to chuck out the gas combi boiler in favour of a heat pump (with or without an accumulator) just to move away from fossil fuels

    Posted By: minisaurusWhere we have heat pumps, we also strive to connect our heat pumps only TO accumulator tank(s), and then drive the heating and hot water FROM the tank(s); this results in a much happier heat pump, lower running costs, fewer maintenance events, and more constant radiator and hot water temperatures.

    I'm not sure the losses are so big nowadays, and when the tank is within the building envelope, your radiator thermostat would close earlier, your ventilation system will recycle any heat leaked

    There aren't many modern houses that have the space (that would/could easily be sacrificed) for an accumulator. My own accumulator (2000 lts) is outside the house and there is no way it and the boiler would fit anywhere in the house so the losses are losses.

    Posted By: minisaurusWe also hope to start loading tanks with the "waste" heat from fridges and freezers.

    Useful only in the non-heating season, otherwise heat from the fridge heat pump goes into the house to reduce the heating demand. Again I would question the financial case for this equipment and I think that most people would want to see a cost benefit for this added level of complication and expense.
    • CommentAuthorowlman
    • CommentTimeAug 28th 2021
     
    Posted By: minisaurus
    Posted By: owlmanDoesn't that require that you have a "controllable" heat source on the input side?

    No, we have e.g. solar. Göteborgs energi (in the article) store waste heat from oil refining and the sewage works. Hope I understood your question.


    Hi minisaurus,
    My reply was a direct response to djh and his thinking that UFH and slab heating may be considered a form of accumulator, ( which in any case is not ).

    I have an 2000l accumulator inside my house envelope as you suggest; but in order to accommodate it and a log gas boiler I had to sacrifice half of my integrated garage.
    My own accumulator has three different heat inputs, and I did consider using an A-W, HP input too, but eventually ditched the idea in favour of partial home A-A. I also run a plate heat exchanger for DHW which is excellent.
    I agree that the concept of a central hot water reservoir with multiple inputs is lucrative, although some of the more nebulous heat sources, I suspect, would require more install costs, than any cost benefits they may give.
    Like many such innovative ideas, it often requires a home that is built around that concept, then it'll work fine.
  10.  
    Posted By: minisaurusabout 1.5m high, 0.5m diamete
    Posted By: minisaurusfor heating with heat pumps when electricity prices are lowest
    This is the bit I am wrestling with. That size of cylinder stores 300l of water and is normally used for domestic/sanitary hot water in the UK.

    If it's heated with an ASHP to say 50degC max, and I draw off heat to users at 40degC, the heat I can store
    = 300l * 4.2kJ/kgK * 10degDeltaT
    = 3.4 kWh

    Now that isn't anywhere near enough to heat my house all day. Looking at a 12kW ASHP, that store is about 15minutes heat, which would be handy to help the ASHP cope with a defrost cycle but not to carry it over peak electricity price hours.

    And typical standing losses will waste maybe 0.5kWh of that each day, and it reduces the CoP to run the ASHP at 50degC when 40degC would do.

    Minisaurus, how many hours of heating does that size store help you with?

    As was suggested, if you look at the property build or purchase cost per m2 of floor space, it gets quite expensive if you give up space to have a big tank indoors.

    Heat from the ASHP this month would cost 6p/kWh peak rate or 3p/kWh off-peak, so that size store would save about 10p/day or £40/year. Those rates will change of course, but even so it's hard to see it paying for itself financially in its lifetime.
    • CommentAuthorminisaurus
    • CommentTimeAug 30th 2021 edited
     
    Posted By: WillInAberdeen
    Minisaurus, how many hours of heating does that size store help you with?

    Answer: 24/7

    The hot water in the tank is used for radiator heat; the heat from that water heats the DHW. CTC explain it much better than I can: https://ctc-heating.com/products/accumulator-tanks/ctc-hot-water-storage-tanks

    “ CTC’s tanks store radiator water instead of hot tap water in order to ensure optimal operation of the heat pump and that you always have fresh hot water. DHW heating is accomplished via finned copper coils, which are effectively heated by the radiator water in the tank.”

    I have a few circa 200 m2 properties with CTC ASHP pumps and tanks - I think 300 L, might be 500 L; I’ll check tomorrow; we get whole months of minus temperatures in February; it all runs like a dream; I’ll try and dig out kWh figures as well.

    Re. size, well they’re not hugely different to a Uk DHW tank (which you’re getting rid of anyway because the ack tank is a bettter solution :)
    • CommentAuthorminisaurus
    • CommentTimeAug 30th 2021
     
    Posted By: owlman
    I have an 2000l accumulator inside my house envelope as you suggest; but in order to accommodate it and a log gas boiler I had to sacrifice half of my integrated garage.

    Wow, that sounds amazing - post some pictures? Sounds like you’re also storing radiator water?
  13.  
    Minisaurus, if you can heat the house and DHW "24/7" with 3.4kWh of heat that you stored
    Posted By: minisauruswhen electricity prices are lowest
    then the house is extremely well insulated and wet central heating is not required. But maybe that wasn't what you meant to say?
    •  
      CommentAuthordjh
    • CommentTimeAug 30th 2021 edited
     
    Posted By: minisaurusRe. size, well they’re not hugely different to a Uk DHW tank (which you’re getting rid of anyway because the ack tank is a bettter solution :)

    I got confused earlier about the difference between accumulators and buffer tanks. When I made my comments I was thinking about buffer tanks and people correctly pulled me up that this discussion was about accumulators.

    But that website seems to be talking about buffer tanks and calling them accumulators? Also, if you are going to extract DHW from the tank, then surely it needs to be hotter than if it was purely used to feed the space heating system? So it will adversely affect the COP of the heat pump heating all the water to that temperature and keeping it hot all the time?
  15.  
    Posted By: djhI got confused earlier about the difference between accumulators and buffer tanks. When I made my comments I was thinking about buffer tanks and people correctly pulled me up that this discussion was about accumulators.

    Accumulator tanks, buffer tanks and thermal stores are terms that seem to be used interchangeably.

    Posted By: minisaurus“ CTC’s tanks store radiator water instead of hot tap water in order to ensure optimal operation of the heat pump and that you always have fresh hot water. DHW heating is accomplished via finned copper coils, which are effectively heated by the radiator water in the tank.”

    DHW should leave the tank (or heat source) at between 50 and 60 deg.. In order to do this with a finned coil heat exchanger in the tank the tank temperature would need to be at least 60 deg and possibly a bit higher. If for example you have UFH which would run at about 40 deg. it makes no sense to me to heat the tank to 60 deg with the attendant impact to the COP of the ASHP only to then mix it down to 40 deg.

    Posted By: minisaurus
    Posted By: owlman
    I have an 2000l accumulator inside my house envelope as you suggest; but in order to accommodate it and a log gas boiler I had to sacrifice half of my integrated garage.

    Wow, that sounds amazing - post some pictures? Sounds like you’re also storing radiator water?

    I too have a 2000ltr accumulator tank. We both have log gas boilers but mine has to be outside the heated envelope because of space constraints. Yes we both store radiator water because the log gas boilers are a batch burners not on demand boilers so in order to get efficiency out of the boiler and to avoid pollution (and chimney problems) the boilers need to be run at a high temp. and the resultant hot water stored for later use mixed down to the correct temp. I fire up my boiler once a day in the winter and bring the accumilator up to about 90 deg and then mix it down to 60 deg. It also provides DHW via a coil in the DHW tank rather than a coil in the accumulator, controlled by a differential thermostat to ensure there is no heat transfer back from the DHW tank to the accumulator.

    I do not see the advantage of using an accumulator with an on demand heat source like an ASHP. especially if you are heating the tank up to 60 deg. which must have a detrimental effect on the COP.
    •  
      CommentAuthordjh
    • CommentTimeAug 31st 2021
     
    Posted By: Peter_in_Hungary
    Posted By: djhI got confused earlier about the difference between accumulators and buffer tanks. When I made my comments I was thinking about buffer tanks and people correctly pulled me up that this discussion was about accumulators.

    Accumulator tanks, buffer tanks and thermal stores are terms that seem to be used interchangeably.

    You are one of those who pulled me up on the difference! The floor slab in a UFH system can serve the purpose of a buffer tank. Namely to provide sensible run times for an ASHP, or a boiler.
  17.  
    It can only do that if the UFH zone is operating, IE any actuators or mixing/recirc pump are online, typically because a room thermostat is calling for heat.

    If that room thermostat is satisfied and shuts down the UFH zone while the ASHP is running, then the buffer is no longer available to the ASHP, for example if it requires heat to do a defrost cycle, or if it is programmed to maximize heat during a low tariff period, or it needs somewhere to shunt the residual heat of the refrigerant that it has already boiled. In that case the ASHP may trip out with a fault code.

    It is possible to work round this by setting up the controls such that sufficient UFH zones are permanently online irrespective of room temperature. However some manufacturers are nervous and require the installer to fit an always-open buffer tank or header of 30-100litres. Otherwise, no warranty.

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