Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorGarethC
    • CommentTimeNov 9th 2021
     
    Posted By: djhI agree that it's just as easy to fit an A/W system in place of a boiler, so perhaps Gareth did overstate the case a bit, but A/A systems are still noticeably cheaper than A/W systems for some reason, despite being more capable in some regards, in that they can do cooling.


    All interesting points, but this one, and other comments in the thread, struck me. Aren't A/A systems a bit simpler than A/W, even on a 'like for like' (i.e. space heating only) basis? It's been a while since I compared systems, but, for example, iirc A/W needs a heat exchanger box to transfer heat from refrigerant to water (not to mention, this step will inevitably result in some efficiency loss vs A/A).

    And then, while in principle existing pipework could be used extensively by an A/W system, in practise, most examples I've heard of involved installing extremely disruptive and expensive under floor heating, or at very least needed extra radiators and seemed to involve a lot of new pipework. Not to say installing A/A wall units would be trivial.

    I wasn't aware of the one unit limit too. Another kicking for my plans. Best scenario for me would be an outdoor unit on each gable end. Most pipework would be external, with only short runs into the house at appropriate places. Easier, I think, than long runs from one side of the house to the other.
    • CommentAuthorphiledge
    • CommentTimeNov 10th 2021
     
    Posted By: GarethC

    All interesting points, but this one, and other comments in the thread, struck me. Aren't A/A systems a bit simpler than A/W, even on a 'like for like' (i.e. space heating only) basis? It's been a while since I compared systems, but, for example, iirc A/W needs a heat exchanger box to transfer heat from refrigerant to water (not to mention, this step will inevitably result in some efficiency loss vs A/A).

    And then, while in principle existing pipework could be used extensively by an A/W system, in practise, most examples I've heard of involved installing extremely disruptive and expensive under floor heating, or at very least needed extra radiators and seemed to involve a lot of new pipework. Not to say installing A/A wall units would be trivial.

    I wasn't aware of the one unit limit too. Another kicking for my plans. Best scenario for me would be an outdoor unit on each gable end. Most pipework would be external, with only short runs into the house at appropriate places. Easier, I think, than long runs from one side of the house to the other.


    When comparing complexity of A2A vs A2W its probably prudent to include a DHW system as there cant be many houses that dont need DHW and need space heating only. I think the refrigerant to water exchanger is built into monoblock A2W HPs so no separate exchanger unit. AFAIK, a monoblock unit pumps out hot water direct to UFH/rads/TS/DHW tank.

    I dont think its necessary to couple a A2W HP to UFH and that radiators running lower temps will work. It may be that a shuffle of radiator sizes within the house and the addition of a few larger rads is adequate to get heat output. One thing that rarely gets mentioned is that if you are upgrading insulation/draught proofing then the building heat demand goes down and the demand may well be met by the existing rads running the lower temps A2W HPs offer. That would need calculating on a house by house basis but I dont beleive everyone should assume they will have to change rads when a HP is installed.
    • CommentAuthorLF
    • CommentTimeNov 10th 2021
     
    Agree with Phil that likely rads will bemostly fine at lower temperature. May take a little longer to heat whole house from cold but once maintaining heat levels should be ok. Lower air circulation around radiators by convection was will likely be nicer too.
    Is a moderately sorted draft proof house with double glazing, cavity wall and well insulated loft going to be fine. Say 8kW load when heating on. Hot water for combi boiler is about 24kW by comparison on our boiler.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2021
     
    Posted By: GarethCAnother kicking for my plans.
    Not a major kicking though. Just need to make a planning application.

    Posted By: philedgeWhen comparing complexity of A2A vs A2W its probably prudent to include a DHW system as there cant be many houses that dont need DHW
    Except that the DHW requirement can be met in other ways, such as an immersion. I'll grant that for a specific house it's important to work out exactly what solution will be used. i.e. yes you always need to do a full system plan.

    Posted By: LFIs a moderately sorted draft proof house with double glazing, cavity wall and well insulated loft going to be fine. Say 8kW load when heating on. Hot water for combi boiler is about 24kW by comparison on our boiler.
    Combi implies no cylinder so my immediate answer would be 'needs further thought'. You'd need point of use heaters everywhere or centrally, or install a cylinder.
  1.  
    FWIW, looking at my projection of the total cost of ownership of a retrofit A-W heatpump with radiators, 4bed stone house, based on quotes received,

    The cost over 10 years is roughly

    ~20% supply of the heatpump
    ~20% installation including electrics
    ~ 35% electricity usage*
    ~10% replacement radiators
    ~ 5% replacement DHW cylinder
    ~10% servicing


    Based on this, I'm getting less bothered about the issue of bigger radiators, it's not that big proportion of the total, and spending more on radiators should improve SCoP and reduce electricity costs which are the biggest item.

    I looked at using an immersion in the existing DHW cylinder, but it obviously uses less electricity to heat at least some of the water using the heatpump, and I think that will work out cheaper over the lifetime as well as greener.

    (*Projections involving future energy prices are inevitably best guess)
  2.  
    Posted By: LFHot water for combi boiler is about 24kW by comparison on our boiler.


    Posted By: djhCombi implies no cylinder so my immediate answer would be 'needs further thought'. You'd need point of use heaters everywhere or centrally, or install a cylinder.

    Combi boiler size is usually driven be the need to supply a shower and how many bathrooms in the house but there isn't much difference in the price of a system boiler or a combi boiler and given that modern gas boilers modulate power I don§t see the need to install a DHW cylinder for gas boiler usage. For any sort of heat pump however a cylinder makes sense. Point of use water heaters need a lot of power, especially for a shower, and you have to factor in the possibility concurrent usage so I can see some houses having power circuit issues.

    The more you dip into this you realise that there is no one size fits all - far from it!
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2021
     
    Posted By: WillInAberdeenI looked at using an immersion in the existing DHW cylinder, but it obviously uses less electricity to heat at least some of the water using the heatpump, and I think that will work out cheaper over the lifetime as well as greener.

    In past houses, I've always regarded an immersion as a very cheap insurance policy against failure of the boiler, or in this case a heat pump. My current thermal store has two independent immersions, which is useful for other reasons but still comforts me as a backup plan.
    •  
      CommentAuthordjh
    • CommentTimeNov 10th 2021
     
    Posted By: Peter_in_HungaryPoint of use water heaters need a lot of power, especially for a shower

    Fortunately, electric showers are widely available. :bigsmile:
    • CommentAuthorrhamdu
    • CommentTimeNov 22nd 2021 edited
     
    Imagine a split air-to-air heat pump installation. But in each room, instead of transferring the heat to air, the heat is transferred direct to a large radiant surface.
    Radiant heating is considered more comfortable than convection. And there would be no fan noise. Also, I think, no need for an electrical connection.
    Does this exist? If not, why not? Engineering issues, or economics?
  3.  
    Posted By: rhamduImagine a split air-to-air heat pump installation. But in each room, instead of transferring the heat to air, the heat is transferred direct to a large radiant surface.

    the heat is transferred direct to a large radiant surface = radiator ??
    So it sounds like an air to water heat pump would (partially) fill this objective although transferring the heat needs a transfer to water first to conveniently transport the heat which is why it is only a partial fix for directly transferring the heat.

    Using an A2W heat pump usually needs an increase in radiator size when converting from conventional central heating and this is some of the engineering and economic issues, which can be overcome by reducing the demand e.g.insulation and/or draught proofing
  4.  
    I'm looking for it too! A refrigerant-to-air radiator, like the one on the back of a fridge, only much bigger, UFH-size. Possibly embedded into the surface of a floor or wall or ceiling.

    In previous discussions, people were concerned about the potential to leak all the refrigerant from the heatpump into the house, but I don't see that risk being different from running refrigerant pipes to a fan unit, or running mains gas pipes through the house to a cooker. Less, if a non-flamable refrigerant is used, or the system charge is low.

    Problem is probably that heating is a niche market, most products were designed for cooling, which requires condensation to be caught.
    •  
      CommentAuthordjh
    • CommentTimeNov 22nd 2021
     
    Posted By: WillInAberdeenI'm looking for it too! A refrigerant-to-air radiator, like the one on the back of a fridge, only much bigger, UFH-size. Possibly embedded into the surface of a floor or wall or ceiling.

    I think the problem is the volume of refrigerant that would be needed. If the refrigerant needs F-gas certification to install then you want to minimise the quantity of it in the system. Whether it's 'mildly inflammable' propane or one of the 'reduced GWP potential' ones, you don't want the possibility of large leaks. Maybe if you used a CO2 heatpump, you could do it?

    Ooh, a quick google gave the first hit https://www.gdhv.co.uk/why-pick-chilled-water-fan-coil-system-over-refrigerant-based-system which says much the same.
    • CommentAuthorEd Davies
    • CommentTimeNov 22nd 2021
     
    How about a panel with heat pipes inside it, maybe using the same fluid as used in solar thermal tubes, so only the bottom strip needs to be heated by the refrigerant?
    • CommentAuthorrhamdu
    • CommentTimeNov 22nd 2021
     
    Posted By: WillInAberdeenProblem is probably that heating is a niche market, most products were designed for cooling, which requires condensation to be caught.


    Good point about the condensation. Also, air movement always increases heat loss from human bodies unless the air is very warm indeed. So a fan is great for cooling.
    I raised the idea of refrigerant-filled radiators because a fan is less good for heating. It may transfer heat efficiently to the room air. But if the room air is moving, people may still be shivering.
    • CommentAuthorGarethC
    • CommentTimeNov 23rd 2021
     
    There's a scheme called Interface Knowledge Connection where you can suggest research topics for universities. I think they're crying out to explore innovation in the heat pump area. If there are topics you guys think are worth exploring, I could maybe pull together a research proposal. I've already drafted one on the topic which is essentially 'why aren't there fan assisted radiators for sale to the specification I want?' But I haven't submitted it yet.
    •  
      CommentAuthordjh
    • CommentTimeNov 23rd 2021
     
    Posted By: Ed DaviesHow about a panel with heat pipes inside it, maybe using the same fluid as used in solar thermal tubes, so only the bottom strip needs to be heated by the refrigerant?

    Now that sounds like a genius idea, Ed. Maybe use low-pressure water as the working fluid? Or propylene? Or something fancy https://www.hindawi.com/journals/ijp/2021/5546508/

    But then again, why not just use a regular water-filled radiator heated by a pipe filled with refrigerant?
    • CommentAuthorowlman
    • CommentTimeNov 23rd 2021
     
    Or a radiator made of cheap heat absorbing transfer medium e.g. ceramic/glass with the refrigerant pipework "mesh" built in at the moulding stage thereby only leaving an interconnection to the HP to be made. That would be easily recyclable and avoids extra fluids altogether.
    Shoot me down if it's a daft idea.
  5.  
    Yep, that's similar to how the refrigerant radiator on the back of the fridge works. The refrigerant pipes only have to be very skinny and spaced far apart, compared to a water-filled radiator, because the transfer of latent heat from the condensing refrigerant is much more intensive than in a water radiator, which deals with sensible heat. The surface where heat gets transferred to air needs to be many times bigger than the refrigerant pipes' area, this surface area is provided by a fine metal mesh on the fridge, but any conductive material would do (cement fibreboard with an aluminium backing perhaps?). Ceramic or aluminium would be good as they'd protect the refrigerant pipes against getting bashed (which is one reason they go on the back of the fridge).

    It's not visible but we have to remember that the radiator on a fridge or an aircon indoor unit, is mostly filled with gas - the condensed liquid just gets in the way so is drained out, asap.

    So an indoor heat pump radiator would contain vastly less volume of refrigerant than a conventional CH radiator, and it would be mostly contain low-density gas. So whereas a CH radiator contains kilogrammes of water, the heatpump radiator would contain grammes of refrigerant. Less mass than the refrigerant contained in the supply/return pipework, and not significantly more than the refrigerant in a conventional fan-forced aircon indoor unit, given the same surface area of refrigerant tubing to give the same heat transfer.
    • CommentAuthorowlman
    • CommentTimeNov 23rd 2021
     
    If it was ceramic for example it could even be made as huge wall panels. The ceramics industry already make decorative wall tiles up to 8' x 4' and larger.
    • CommentAuthorRobL
    • CommentTimeNov 23rd 2021
     
    The hot side of a heatpump is at high pressure - eg if propane or similar would be at ~15bar (twice a really thin racing bike tyre). That probably means a mechanically strong shape (eg copper pipes), or it will be expensive, or it might deform. Whatever the magic gas is, the consequences of leakages are much greater than if water leaks from a radiator - for this reason generally interfaces are usually minimised, and often soldered or brazed (hotter and stronger but otherwise similar to soldering). Propane is flammable, so there is a maximum mass allowable indoors depending on the room size, and most other gasses used have high global warming CO2 equivalencies in the order of 1000x CO2 - so can only be used by somebody certified.
    As it's a phase change thing, I think you want the flow to be gas in at the top, liquid out at the bottom.

    Personally, I think A2W/W2W heatpumps are fine things that work well with well insulated homes and (big enough)radiators/ufh in the uk - they're just higher price due to low supply right now. The phase change material needs a heatexchanger somewhere - BPHE (brazed plate heat exchangers) are only £50 or so, and can couple to a conventional water circuit (likely that's what's in an A2W heatpump instead of the fan & indoor unit of a cheaper A2A unit).
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press