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  1.  
    Hello, I'm trying to work out which is the most efficient way to connect a heat pump to my thermal store. It's a large store with two sections and a central baffle, so it ought to be possible to take heat from it from a relatively low temperature (20-30º) at the very bottom, to more like 40º in the middle.

    I'd assume that taking heat at the very coldest would be the most efficient, but then the ASHP wouldn't be able to heat it more than a few degrees, so it would need to heat the entire body of the tank to a decent temperature in order to be able to get anywhere near a temperature of about 50-55º which is what we'd need for a bath or shower. It would also be good to have a portion of cold water at the base of the store to gather any heat on sunny days using the existing solar thermal.

    I also assume that it would need to send the warm water to the portion above the baffle, otherwise there would be no way of maintaining stratification.

    So my current thinking is to take it from the higher part of the lower section (just below the baffle) and return it to the higher section just above the baffle.

    I'd appreciate any thoughts or comments!
    • CommentAuthorcjard
    • CommentTimeFeb 20th 2020 edited
     
    When you say baffle, do you mean heat exchanger?

    Is the tank arranged as two separate tanks one stop the other?

    Perhaps a link to the website shop for your store would help

    I'm almost certain you wouldn't want to shower or bathe at 55 degrees

    ASHP will generally provide a flow of water that is 7degrees warmer than what it's fed with; if yours differs from this the flow rate should be adjusted. Where you choose to place your heat exchanger in the tank will affect how much water you can heat and how much stirring of the tank the heating will cause. In dhw mode mine seems reasonably effective at stirring the tank, to the extent that if my tank has settled to a temperature gradient overnight and there is just enough warm water for a shower at the top, SWMBO feeling the tank (alas on an insulated section until I realised why she always through the we're out of hot water) in the middle and deciding the HW needs to be activated, and turning the ASHP to DHW will spread the vestiges of warm water at the top throughout the upper third of the tank making it no good for a shower

    As most people don't have the luxury of choosing where in the tank the heat exchanger goes and simply have it occupy the bottom third they don't have much choice over how much volume to heat. If your tank has a mechanism that offers choice you'll need to decide how much water you'll need regularly and whether to modify your habits (eg to shower at night after using the warm air in the afternoon to achieve a more rapid tank heating than the cold morning air if you time your ASHP to provide for a morning shower)

    There are possibly gains to be had in choosing a good time to run your ASHP from a humidity point of view too - when the air temp is between 0 and 6 degrees (ish, I can't quite remember) the ASHP is more likely to ice up and have to put energy into driving itself. Warmer than this and the air moisture probably won't freeze on the ASHP, cooler and it's probably already frozen on something else (sub zero air can be quite dry)

    Regards target temps, most ASHP will do 60 as a programmed limit, but you'll notice the rate of warming slow down above 52 ish. You should strive to store water as cool as you can rather than as warm as you can because it's easier for an ASHP to generate and the temperature differential to the ambient is lower resulting in lower losses
  2.  
    Would that start a pumped circuit going from the lower section, through the ASHP, into the upper section, and *downward through the baffle*? IE mixing the top and bottom sections.

    The heat pump is going to struggle to get the water to 55degC without an immersion top up, which may be better done separately from the heat pump, IE immersion in the store.

    Am assuming you have ufh that the ashp can run at lowish temperature, drawn from the middle of the store, with a dhw coil or exchanger at the top?

    So circulate the bottom half of the store through the heat pump, to say 45 deg to feed the ufh without immersioning. The very top of the tank heated by immersion to shower temperature. Or even do the immersion heating directly, as a topup instant heater on the fresh water pipe that goes from the store coil to the shower. As heat is drawn from the top of the store, it's replaced by ASHP-heated water from the bottom, ready to be boosted by immersion. (Or whatever else you have, woodburner, solar thermal, etc.)

    IIRC, our ashp didn't like to be fed water that was already heated hotter than the ashp could run, as that would stop it's condenser working.
    • CommentAuthorcjard
    • CommentTimeFeb 20th 2020
     
    @Will why run the UFH off the TS when the ASHP will provide a controlled target temperature water feed to run the UFH in nonDHW mode?
  3.  
    Posted By: cjardWhen you say baffle, do you mean heat exchanger?

    No, I mean a baffle, ie a division between the sections with a gap to prevent disturbance of the stratification.
    Posted By: cjardIs the tank arranged as two separate tanks one stop the other?

    No, it's one big tank with a baffle dividing it into two.
    Posted By: cjardASHP will generally provide a flow of water that is 7degrees warmer than what it's fed with

    That's really helpful information, thank you!
    Posted By: cjardI'm almost certain you wouldn't want to shower or bathe at 55 degrees

    It seems to be okay from our experience. Don't know whether it's because we have a big tank, or two water coils, or a combination of both.
    Posted By: cjardRegards target temps, most ASHP will do 60 as a programmed limit, but you'll notice the rate of warming slow down above 52 ish. You should strive to store water as cool as you can rather than as warm as you can because it's easier for an ASHP to generate and the temperature differential to the ambient is lower resulting in lower losses

    Also sensible points. Thank you!
  4.  
    Posted By: WillInAberdeenWould that start a pumped circuit going from the lower section, through the ASHP, into the upper section, and *downward through the baffle*? IE mixing the top and bottom sections.

    That's a good point. I don't know. One idea might be to have a couple of valves so we could change the points that the ASHP is drawing from.

    Posted By: WillInAberdeenThe heat pump is going to struggle to get the water to 55degC without an immersion top up, which may be better done separately from the heat pump, IE immersion in the store.

    That's also a good point. At present we have an immersion in the top, but heat most of it using a woodburner (that I'm keen to use less, both for environmental and practical reasons).
    Do you happen to know the rough efficiencies at different temperatures? Maybe this is something I can get from the manufacturer.

    Posted By: WillInAberdeenAm assuming you have ufh that the ashp can run at lowish temperature, drawn from the middle of the store, with a dhw coil or exchanger at the top?


    Yes, even when it's really cold, we can warm the house up by circulating water that's less than 40º.


    Posted By: WillInAberdeenSo circulate the bottom half of the store through the heat pump, to say 45 deg to feed the ufh without immersioning. The very top of the tank heated by immersion to shower temperature.


    Wouldn't an ASHP running at very low efficiency still be more efficient than an immersion? I do think (from experience) that 55º in the tank would be fine.

    Posted By: WillInAberdeenOr even do the immersion heating directly, as a topup instant heater on the fresh water pipe that goes from the store coil to the shower.

    I did think about that, but haven't really found a simple device that will heat the flow. They all seem really complicated (and quite expensive).
  5.  
    Here are a couple of schematics. Note that they both show the ASHP running below the baffle.
      Akvaterm with Air Source Heat Pump.jpg
  6.  
    COP for external air temperature seems to be:
    Tj=-7°C COPd (S) 1.89
    Tj=+2°C COPd (S) 3.01
    Tj=+7°C COPd (S) 4.25
    Tj=+12°C COPd (S) 6.78
  7.  
    Posted By: cjard@Will why run the UFH off the TS when the ASHP will provide a controlled target temperature water feed to run the UFH in nonDHW mode?
    Very good point!

    So then the ufh would be completely separate from the TS. The heat pump would run the UFH directly at a nice low temperature for best efficiency, with weather compensation and the like. The TS would only be used for dhw, with the bottom section pre heated lukewarm by solar and ashp, the top section topped-up to shower temperature by immersion, wbs, ashp@low-efficiency.

    Posted By: CoPWouldn't an ASHP running at very low efficiency still be more efficient than an immersion?
    Probably, depending on losses such as defrost cycles, but what I meant was that it's better to heat the ufh water only to a lowish temperature, not to dhw temperature.
    • CommentAuthorcjard
    • CommentTimeFeb 21st 2020 edited
     
    I generally use my ASHP to heat my tank until the returning water to the ASHP is 50 degrees (as measured by the ASHP - I don't yet have a thermometer in the tank). This generally provides enough water for the family, the store being 60 litres per person. Some shower at night, some in the morning. The morning users are at a slight detriment because the DHW is generated in the day (mid day ish usually- it's still manually switched)

    An ASHP should be more efficient than an immersion in all but very cold weather but immersions are very cheap to have and you might consider that fitting two or three of them to the tank at different heights would meet a "i need a quick shower in 15 minutes time; I'll just switch the top immersion on now and generate a small amount of hot water quickly" need better than having it take 2 hours for an ASHP to heat 500 litres of water
    • CommentAuthorcjard
    • CommentTimeFeb 21st 2020
     
    Re your schematics I think that the extra valves (manual, even) and pipe work necessary to have all your heat sources totally configurable as above/below baffle would be a small cost and allow you to achieve best results by experimentation.
  8.  
    Posted By: WillInAberdeenSo then the ufh would be completely separate from the TS. The heat pump would run the UFH directly at a nice low temperature for best efficiency, with weather compensation and the like. The TS would only be used for dhw, with the bottom section pre heated lukewarm by solar and ashp, the top section topped-up to shower temperature by immersion, wbs, ashp@low-efficiency.

    Hi Will - just to clarify - you think it is better to have the ASHP connected to the TS? I'm assuming so as it could provide warmish water - 30º-40º at the lower section that would be perfect for the UFH and also effectively pre-heat for the DHW.

    Then I think it would be good to have valves so that the connection points could be manually or electronically switched from the bottom section to the top section (eg to provide DHW if no other sources were available).
  9.  
    Posted By: cjardRe your schematics I think that the extra valves (manual, even) and pipe work necessary to have all your heat sources totally configurable as above/below baffle would be a small cost and allow you to achieve best results by experimentation.

    Yes, I agree with you!
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 21st 2020
     
    I don't know what TS connections you have available, however I have done the following on previous projects;

    - ASHP usually has two flow settings, DHW (say 55oC) and UFH (say 40oC though variable depending on ext temps)
    - fit 3port valves, (or pairs of flip-flopped 2ports) on the ASHP flow and return pipes so you have one route for DHW temperature, and another route for UFH temp
    - feed top of TS with DHW temp flow and just above baffle for DHW temp return.
    - feed just below baffle with UFH temp flow and bottom of TS for UFH temp return.

    That gives you essentially 2 cylinders from the ASHP point of view, but you get substantial pre-warming of the cold mains flow the coil for DHW output. Running the UFH system from the lower part of the TS rather than direct from the ASHP gives buffering, and stops short cycling of the ASHP, (bad news for compressors and power consumption).

    Control the ASHP with a cyl stat at the top of the TS for DHW set to eg. 45oC make on fall with 10oC hysteresis, and another cyl stat in the lower part of the TS for UFH set to eg. 33oC make on fall with 7oC hysteresis. Do not use the UFH demand signal to control the ASHP. The UFH demand signal should only run the UFH circ pump.

    That will give you DHW from 45-55oC only heating the upper part of the TS, and a good volume buffer for the UFH at 30-40oC, along with the above mentioned preheating of the DHW, which effectively increases the potential volume of DHW available to the house, and also means a goodly portion of your DHW energy comes from the 40oC ASHP production period which has the higher COP.

    I design the TS to have the appropriate connxns, baffle position, and sparge bars, but you might not have that luxury as I think yours is already in situ?

    If you can post a diagram of your TS with all connxn points, there may be ways to get an approximation of the above set-up.
  10.  
    Hi GreenPaddy, that's extremely helpful. In the meantime, I've come across the following schematic from Akvaterm (in Finnish) that seems to approximate your design.
      Akvaterm schematic lowres.jpg
    • CommentAuthorborpin
    • CommentTimeFeb 21st 2020
     
    At a slight tangent, there has been a number of posts on the OEM forum re the control of the HeatPump itself to improve CoP which folk here may find interesting.

    https://community.openenergymonitor.org/c/hardware/heatpump/47
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 22nd 2020 edited
     
    My lawyers are in transit to lodge a suit at the Finnish High Court, for copy right infringement :bigsmile:
    • CommentAuthorgyrogear
    • CommentTimeFeb 22nd 2020
     
    Would not increasing the external air intake temperature improve the COP ?

    Such as by pulling the air through a solar absorber.

    I was on the verge of buying an ASHP a few years ago, until I got the price for penetrating my basement walls (which was not included in the deal).

    My intent was to draw the intake air through the cavity of a slate-hung facade that gets "quite hot".

    gg
  11.  
    Hi Will - just to clarify - you think it is better to have the ASHP connected to the TS? I'm assuming so as it could provide warmish water - 30º-40º at the lower section that would be perfect for the UFH and also effectively pre-heat for the DHW.


    Horses/courses I guess! I didn't do that, because storing 250litres of water at 30deg in the bottom half of a TS doesn't actually store much heat (3kWh = £0.15 very roughly, so not enough to heat our Victorian house for very long) so wasn't worthwhile for me to spend the cost or floor area on a TS.

    But different for you, as you already have the TS and will still require it for the solar thermal and wbs. So you might want to negotiate with GP's Finnish lawyers for the design rights!

    An advantage of keeping the ashp central heating circuit separate from the dhw/TS circuit, is that the ashp has clever software for weather compensation, so it would like to be left in charge of deciding what temperature it runs at, it will choose the lowest temperature possible at any time, so maximizing efficiency and reducing numbers of stop-start-defrosts (Ours also had 'load compensation' which was even better IME, but relies on calculation of the difference between CH flow and return, which the TS would spoil).

    An advantage of combining the ashp CH with the dhw, is the TS provides a reserve of heat which the ashp can use for defrosts. Ours worked fine without but some manufacturers may insist.
  12.  
    pulling the air through a solar absorber
    Good idea in principle! But the big fan on an ashp pulls through a huge volume of air, many m3 each minute. The solar absorbers would need to heat that volume of air, but not add to the pressure drop across the ashp, which is designed for free air pressure on its inlet and outlet. So additional (even larger) fans would be needed to move the air through the absorber and duct it to the ashp. The power to run these would be set against the gains from the solar absorber.

    Edit to add: if the air behind the slates gets warmer than room temperature, how about blowing it straight into into the house with a bathroom style fan? If not quite warm enough, use it to take the chill off your basement, it'll reduce heat losses through the floor.
    • CommentAuthorgyrogear
    • CommentTimeFeb 22nd 2020 edited
     
    OK, thanks for the technical explanation !

    Posted By: WillInAberdeenif the air behind the slates gets warmer than room temperature, how about blowing it straight into into the house with a bathroom style fan? If not quite warm enough, use it to take the chill off your basement, it'll reduce heat losses through the floor.


    Nice idea, however the air is mucky (years of accumulated slate dust, bits of residual fiberglass insulation, dead insects, various dust etc.

    In tests with a car radiator fan, backed up by a couple of duct fans, I can get 400 m3/hr or more through the cavity.

    The next stage is to install ducting in the basement, to route the air through the crawlspace, before dumping it back outside.
    Still working on the W.A.F.
    :devil:

    gg
  13.  
    Posted By: WillInAberdeenAn advantage of keeping the ashp central heating circuit separate from the dhw/TS circuit, is that the ashp has clever software for weather compensation, so it would like to be left in charge of deciding what temperature it runs at, it will choose the lowest temperature possible at any time, so maximizing efficiency and reducing numbers of stop-start-defrosts (Ours also had 'load compensation' which was even better IME, but relies on calculation of the difference between CH flow and return, which the TS would spoil).

    Thanks Will! I think ESBE do a weather compensating mixer valve that can be attached to the Heat Store to alter the temperature of the water in the UFH circuit depending on the outside temperature, so this might be a way to have the best of both worlds.
    • CommentAuthorgoodevans
    • CommentTimeFeb 25th 2020
     
    I would suggest that getting the best out of the ASHP is to run the flow temperature at the lowest temperature possible for as long as possible.

    For best performance directly couple the ashp to the UFH and run the unit all day if necessary. It makes no sense to use the ASHP to raise the temperature more that the UFH system requires.
    • CommentAuthorTimSmall
    • CommentTimeFeb 25th 2020
     
    What goodevans said, but the same goes for the thermal store as well. Minimum temperature, and only heat sufficient for your needs. This may require some cunning controls.

    Also if you have an option to limit the input power, then set it to its lowest value possible for your needs. Modern "inverter" based ASHP designs operate at their highest efficiency at their minimum continuous power.

    If you can, get one which uses R32 as the refrigerant (not R410a), because they are both more efficient, and also have significantly lower GWP.
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