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Heating and cooling: ASHP water heater for passiv h

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  1.  
    • CommentAuthoralant
    • CommentTimeOct 31st 2021
     
    We are building to passiv standard (or thereabouts) and looking at masonry stove with boiler for winter heating (very low radiant heat o/p) and hot water. The issue is what to do about summer time hot water and was considering an ASHP water heater eg. https://www.ariston.com/en-uk/products/air-source-heat-pump-water-heater/air-source-heat-pump-water-heater/nuos-plus-wifi-uk/. This costs about £2k which seems significantly cheaper than a conventional ASHP.
    The above would require a separate store / tank for the stove input. I guess a simpler solution would be be 1 thermal store with immersion heater for summer use and fed by stove boiler in winter time?
    Any other possibilities?
    • CommentAuthorrevor
    • CommentTimeOct 31st 2021
     
    Posted By: alantThe above would require a separate store / tank for the stove input. I guess a simpler solution would be be 1 thermal store with immersion heater for summer use and fed by stove boiler in winter time?
    Any other possibilities?


    I think this would be a good approach but consider having a solar thermal input into the store if you have suitable roof.
    • CommentAuthorEd Davies
    • CommentTimeOct 31st 2021
     
    Posted By: alantASHP water heater eg. https://www.ariston.com/en-uk/products/air-source-heat-pump-water-heater/air-source-heat-pump-water-heater/nuos-plus-wifi-uk/
    Is the air this heat pump sources from indoor or outdoor? It looks a bit like a typical extract-air HP and the text is not clear with no mention or picture of the external unit. The technical data includes a specification for “min volume of the installation room 30 m³” which would be weird of an external unit. Do you actually want to extract 650 m³/h from your house, if that's what it does?
    •  
      CommentAuthordjh
    • CommentTimeOct 31st 2021
     
    Posted By: alantWe are building to passiv standard (or thereabouts) and looking at masonry stove with boiler for winter heating (very low radiant heat o/p) and hot water. The issue is what to do about summer time hot water and was considering an ASHP water heater eg. https://www.ariston.com/en-uk/products/air-source-heat-pump-water-heater/air-source-heat-pump-water-heater/nuos-plus-wifi-uk/. This costs about £2k which seems significantly cheaper than a conventional ASHP.
    The above would require a separate store / tank for the stove input. I guess a simpler solution would be be 1 thermal store with immersion heater for summer use and fed by stove boiler in winter time?
    Any other possibilities?

    If you're building to PH standard, why bother with a masonry stove and its related emissions problems, not to mention the hassle of stoking it?

    Summer DHW provision is not a problem. As revor says, you could use solar thermal or equally you could use PV, which some (me included) think has some advantages.

    In our PH, we use direct electric space heating, which is not as good as a HP, but is a lot simpler and cheaper. This year I've noticed that the mains immersion is coming on already whilst we're nowhere near needing space heating yet. Something to do with the number of sunny days and the outside temperature, I expect. i.e. clouds keep it warm enough we don't need space heating but mean there's not enough sun to heat the water.

    Posted By: Ed DaviesIs the air this heat pump sources from indoor or outdoor? It looks a bit like a typical extract-air HP and the text is not clear with no mention or picture of the external unit. The technical data includes a specification for “min volume of the installation room 30 m³” which would be weird of an external unit. Do you actually want to extract 650 m³/h from your house, if that's what it does?

    Well it specifies an installation minimum temp of 1°C whilst the minimum air temp is -10°C so I suspect an external intake. Also 650 m³/hr is an enormous amount of air to try to take from a whole house, let alone a single room! So I suspect an external air source and a fair amount of noise as also indicated in the spec.
    • CommentAuthoralant
    • CommentTimeNov 4th 2021
     
    Hi folks,
    Many thanks for your thoughts. I had thought it was a completely internal unit which could utilise internal heat which would be ideal for summer use....but i think i would need to get more details as it not entirely clear.

    Also good point about the flow rate and i am not sure how that might interact with MVHR in an airtight building. I guess the sound level would be about bearable as it would be on a different floor to the main living area and i might not have it running during the night.

    Revor - we have considered solar and that would work with the thermal store, but its a flat roof and it would look a bit out of place. Also we are located in the North of Scotland and we have received feedback from a few claiming that solar thermal wasn't really delivering the goods up here.

    DJH - I take your point about the masonry stove and the emissions. We would only use a properly designed "kachelofen" type stove which burns at very high temps and produces significantly less particulates. We are in a rural area and hopefully be much less of an issue than in a built up area. The ones we are familiar with in Germany get loaded with one charge and burnt with no need to open the door. It might only get loaded once every 2 - 3 days. We have a regular supply of timber too which influences things.

    As regards indoor air quality, i would be more worried by the neighbours house (upwind of us) and the MVHR inputting the smoke from their Aga.

    We are also considering PV's to power an immersion for summer use. A heat pump would obviously make more efficient use of the electricity generated but its another "big machine" that costs a fair bit of money and adds to the complexity etc. Thinks i am going round in circles again!
    •  
      CommentAuthordjh
    • CommentTimeNov 4th 2021
     
    Posted By: alantWe would only use a properly designed "kachelofen" type stove which burns at very high temps and produces significantly less particulates.

    Good to hear. Make sure to also check the thermal calculations. A stove can deliver a lot more heat into a single room than is needed in a passivhaus. Maybe it will be installled to distribute the heat more, but with a batch burning stove you'll need to absorb the heat into the fabric of the house and the more evenly you can do that the better things will be.
  7.  
    Posted By: alantI take your point about the masonry stove and the emissions. We would only use a properly designed "kachelofen" type stove which burns at very high temps and produces significantly less particulates. We are in a rural area and hopefully be much less of an issue than in a built up area. The ones we are familiar with in Germany get loaded with one charge and burnt with no need to open the door. It might only get loaded once every 2 - 3 days. We have a regular supply of timber too which influences things.

    I have 3 masonry stoves (until a couple of years ago I had 4) They are traditional in Hungary. From my observations in their use if you have one in a near PH house with the air tightness that that implies you will need an external air supply. I also doubt that you will be able to get 2 - 3 days between firings as the ones I have used are v. hot after the firings but loose their heat over the next 12 hours. If you want to store more heat than the typical masonry stove then it will need to be more massive i.e. bigger with more mass (thickness) in the walls and even then I doubt that 2 - 3 days would be achievable. They also take up a lot of space and tend to dominate any space in which they are built.

    Alant - do you have anyone yet who can build your planned stove? They are a specialist craft to do the job properly and at least over here they are now expensive to get built. I would have my doubts about finding the necessary skills in Scotland, especially as you will need (IMO) an external air supply built in.
    • CommentAuthoralant
    • CommentTimeNov 6th 2021
     
    Hi Peter,
    Thanks for your comments. There is an experienced stove mason in Scotland http://www.stovemason.com/. Perhaps not every 2 or 3 days but i don't think we would fill more than once a day / evening. It would produce about 1.5 kw of radiant heat so quite low level and we have the option of diverting excess heat into producing hot water.
    • CommentAuthorbxman
    • CommentTimeNov 26th 2021 edited
     
    How much hot water do you actually use from your taps during the course of a day ?

    I think you might be surprised if you were to find out .

    and again measure just how much you draw off in the process of getting what you need .
    all of which will have been heated to some degree when it passes through the heating devise

    I find I use less than 5 litres a day which I provide for with a 2 Kw electric Kettle

    Under sink heaters will do the job in most cases without wasting to much energy .

    I have an electric shower but no bath.
    • CommentAuthorowlman
    • CommentTimeNov 26th 2021 edited
     
    Posted By: bxmanHow much hot water do you actually use from your taps during the course of a day ?

    I find I use less than 5 litres a day which I provide for with a 2 Kw electric Kettle

    Under sink heaters will do the job in most cases without wasting to much energy if you find you are using greater quantities .




    I think there is often an assumption that we somehow need to be storing large quantities of domestic hot water, -"just in case". A hangover from earlier times I guess.
    Similar to bxman's experience above, I find that my solar thermal cylinder,- ( PV on a simple diverter would work just as well, better still point of use DHW, ),- just needs a short burst of the top immersion to provide all the DHW the two of us need.
    The often complex shifting of water from a space heating device to a heat exchanger/coil is outdated for what ultimately is a very simple task, i.e. heating clean water. The immersions in my cylinder are just bog standard ones, but immersions come in many variants. Even in a central DHW supply scenario a re-designed cylinder with high efficiency immersions could mean an even faster response than my own situation.

    As in the discussion about A-A and A-W heat pumps I thing we need to separate space heating and DHW heating and treat each on individually. This is one of the reasons I have uncertainty about the, one type fits all, A-W direction of travel regarding heat pumps.
    •  
      CommentAuthordjh
    • CommentTimeNov 26th 2021
     
    Posted By: owlmanAs in the discussion about A-A and A-W heat pumps I thing we need to separate space heating and DHW heating and treat each on individually. This is one of the reasons I have uncertainty about the, one type fits all, A-W direction of travel regarding heat pumps.

    I'd agree with this assessment. But I think the attraction of the A/W one ring to rule them all approach is in the case of replacing a gas boiler that does both, although that only really applies in the case of an existing boiler plus existing hot water cylinder. In the case of replacing a combi, I think the possibilities open up again.
    • CommentAuthorRobL
    • CommentTimeNov 26th 2021
     
    We use around 3kWh/day of energy via DHW on average, found by solely heating with a monitored immersion for a few weeks. 4 of us, and some unknown proportion of that will be tank losses. I see the future of DHW one of reducing what is ultimately waste; heating water to flush it down a drain. There must be a better way!
    I think it will eventually be normal to filter and re-use hot water, to improve the "efficiency" of taking a shower, the dominant DHW use. Here's a link to one I found - I'm sure I've seen similar before. I've no idea if it's legal to plumb in the UK, somehow I doubt it. They claim 70-90% energy saving, and 90% water saving - you won't find that sort of efficiency benefit from a heatpump!
    https://showerloop.org/
    • CommentAuthorowlman
    • CommentTimeNov 26th 2021
     
    That's and interesting link Rob; As a woodworker I do like the "stitched" geodesic construction on the cubicle. Re using grey water via filtration could be a way forward for sunny climes but with our own maritime climate with usually plenty of water the economics may be less so.

    I agree Dave, re. the boiler replacement scheme shoehorned into an existing plumbing situation;- not very good IMO. However, replacing/linking/designing it with a similar concept of DHW storage is one place the idea falls down. Pipework losses most likely diminish the good HP efficiencies, and as you allude if the A_W is used solely to drive space heating, ( ideally UFH ), then the DHW supply, "possibilities open up", but it becomes a simple problem to solve, with lots of more efficient solutions.
  14.  
    Been thinking about this problem too.... Need to make a decision!

    Looking at the lifecycle costs and CoP, I'm looking at saving ~£2500 of electricity by using the CH heatpump with the cylinder we already have, rather than buying in the extra electricity for point-of-use or immersion water heating.

    That £2.5k saving is easily going to pay for the extra plumbing to connect the heatpump up to the cylinder.

    But if I had to replace the cylinder with a heatpump coil, that would eat into the savings, I might be keener on immersion heating then.

    The heatpump capacity is many kW so recovers the cylinder temperature much faster than an immersion, I might only need a small cylinder as it would reheat between users.

    If I went for point of use showers, I'd have a lot of electrical connection work so am not so keen on that, maybe just for sink taps which never run for long anyway.

    The savings are not going to be enough to pay for new PV or ST, even if I had the right roof aspect.


    This also depends what happens to electricity prices, if they go up then a heatpump will save more money. If they get more variable (pricey at peak times) then it would be good to heat water during off-peak and store it in a cylinder.

    From the green point of view, heatpumps use only a third of the primary energy and carbon that immersions do, but there's a lot of embodied carbon in a stainless cylinder, so not sure which is best as electricity continues to decarbonise.

    In conclusion - one size doesn't fit all.
    •  
      CommentAuthordjh
    • CommentTimeNov 26th 2021
     
    Posted By: WillInAberdeenIn conclusion - one size doesn't fit all.

    Agreed.

    From the green point of view, heatpumps use only a third of the primary energy and carbon that immersions do, but there's a lot of embodied carbon in a stainless cylinder, so not sure which is best as electricity continues to decarbonise.

    A lot will also depend on how recovery of materials by recycling goes. There's a lot of material in a stainless cylinder but I'd hope it was fairly easy to recover and reuse that steel. Do you, or anybody else, happen to know what the current UK process is for that kind of recycling? Does it happen at all? Do they have to remove all the brass and copper parts first? etc. Similar questions for an immersion. There's a lot less material in them, but I can't see any alternative to a manual disassembly to separate the materials before reuse? But then heat pumps are even more complicated so maybe we need to compare the immersion against that?
  16.  
    Surely much depends upon what you have.
    My DHW is by off peak mains tank immersion with the on times decided by the elec. co. (guaranteed min. 8 hours / day). The tanks tend to last for ever, I have one tank that has been in use for 25 years and was bought second hand, they just need periodic replacement of the immersion unit when it gets too much limescale. circa 5 years.
    With such a system in use it may make little sense to toss it out and replace it at some expense both financial and carbon with a system that won't use off peak electricity so arguably more detrimental to the grid than the existing system even if it is powered by a cop 2-3 (for DHW) heat pump,
    I haven't done the ROI of changing over but I suspect the years would be disappearing over the horizon.

    The same goes for storing the DHW rather than point of use instant heaters. The same arguments about grid usage as above and I have found that the storage losses run to about 1 kWh / day per cylinder and the immersion heaters are between 1200W and 1800W so much less power over a longer period which also probably helps grid loadings (apart from being about 30% cheaper / kWh)

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