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.




    • CommentAuthortopher
    • CommentTimeOct 5th 2020
     
    I installed a ground source heat pump some years ago and the adventure, that I enjoyed, is documented on this forum. Now I have the idea that I could install an Air Source Heat Pump in a UK domestic situation.

    I need about 15 kW output and ideally 70 degrees for rads and hot water, from a single phase supply. At this early stage I am thinking that I remove the existing gas boiler and put the ASHP in its place. I have an Evohome controller which I hope I can continue to use.

    These are my questions. . . .
    1. Who are the best manufacturers to look at?
    I have found a Daikin product can provide 70 degrees water output
    2. Where can I find a list of experienced installers?
    • CommentAuthortony
    • CommentTimeOct 5th 2020
     
    Insulate first, it will loose a lot of efficiency and cost too much running at 70C no matter which make
  1.  
    We had a Daikin 'HT' ashp in a previous house. It was effectively two heat pumps in series with different refrigerants. The first is mounted outside as usual and heats to ~30-40deg, the second is mounted inside and boosts the heat level up to ~70deg. We had it as a drop-in replacement for an oil boiler so it fed the existing radiators.

    We were overall pleased with it. The two heat pump circuits are fairly lightly loaded, so overall the CoP was pretty good for the times, though better is available now. There was plenty of heat even in deep Scottish winters. It was cheaper to run than oil, got RHPP and RHI and paid back its cost in savings, and slashed our CO2 footprint.

    We had insulated the house so the existing radiators were bigger than necessary, so the heat pump only ran at 70+ degC when heating the place up from cold, then modulated to a lower temperature.

    The indoor circuit was a bit noisy to have inside the house. It failed after 7years which was not that long, we were moving out so didn't replace it.

    For our next house, we are watching how the market develops. There are some coming out with new refrigerant mixtures that give reasonable CoP even at radiator/DHW temperatures (someone linked to a Valiant one). The market for air-air heat pumps is also interesting as it is marginal whether its worth retrofitting underfloor heating in our place. We won't bother with another two-stage heat pump but would look at another Daikin.

    To claim RHI or GHG if you are eligible, you need to use a MCS installer, they have a list, but others have found it cheaper to DIY. Legally you need a aircon technician with a F Gas certificate to do any work on the refrigerant circuit but a plumber or sparky could do the water or electric connections.
    • CommentAuthortopher
    • CommentTimeOct 6th 2020
     
    Tony, Thanks for the tip. I have insulated with thick stuff in the loft, rock wool cavity wall and new 'A' rated double glazing. I drive an EV too.
    • CommentAuthortopher
    • CommentTimeOct 6th 2020
     
    Hi WillInAberdeen,

    Very useful, thanks.

    Do you need to use an MCS installer to get the £5k grant?
    • CommentAuthorjfb
    • CommentTimeOct 6th 2020
     
    15kw seems very large - how big is the property?

    Rads would need to be oversized if you want to keep the temperature down.
    • CommentAuthorEd Davies
    • CommentTimeOct 6th 2020
     
    I'm with jfb here, unless the property is really large I'm skeptical that 15 kW and 70 °C is really needed.

    In this somewhat leaky pile of rocks I'm renting in Scotland I've been logging various parameters for a while, including the flow temperature to the radiator in the small bedroom I use as my study. The maximum flow temperature is typically just over 50°C and the worst average for a whole day last winter (Nov, Dec, Jan, Feb, Mar) was 37.54 °C on Boxing day (2019-12-26). For all other days it was less than 36 °C.

    Since the heat output from a radiator is more-or-less linear with excess over room temperature this implies that the same heating effect could be got from running the heating continuously at, say, 40°C rather than intermittently at 50°C.

    The nominal output of the boiler [¹] is 18 kW but even on Boxing day it only actually ran for 34.6% of the time so for continuous operation a 6 kW boiler would have done fine. Mostly even that would modulate or switch off at least some of the day; for the climatological winter (Dec, Jan, Feb) the overall average boiler run time was a hair under 25%.

    Caveats: this is a small (two-bed Victorian/Edwardian 1.5-storey semi) house, I keep the “study” cosy but the rest of the house might be a bit cool for others' tastes. OTOH, it has decent DG but the rest of the insulation is a bit perfunctory.

    From what I've seen plumbers tend to over spec boilers; they're much more likely to get a callback if the boiler's not powerful enough than if it's too powerful, the cost increment is not large and they can make a small markup on that anyway.

    With heat pumps a lot more care is needed to pick an appropriate power and flow temperature hence, I think, the horror stories of vastly over-expensive installations and/or installations which don't perform well either because of the design or the way they're operated - e.g., people expecting them to heat the house to very short order.


    [¹] Why TF do we call them boilers? If they ever boil anything something has gone dreadfully wrong.
  2.  
    Topher, you are probably better off going for the RHI than the GHG grant but take qualified advice on that. Both require MCS installers.

    IIRC there wasn't much reason not to go for an oversized heat pump, it cost us about 5% extra for a model with 33% more kW than the next model down the range. It can run for fewer hours per day if oversized, and extra capacity was welcome when we came home to a cold house. Just need to be sure the radiators and circ pump can dissipate all of the heat.

    It was worthwhile reducing the operating temperature because that did improve CoP, both in real life and also for RHI purposes. The control software automatically ran the ashp at lower temperatures when less heat was needed.
    • CommentAuthortopher
    • CommentTimeOct 6th 2020
     
    WillinAberdean, Ed, jfb,

    Thanks all. Very useful comments.

    I am learning. I think I overstated the heat required. I was being simplistic and just finding an equivalent to the gas boiler. Wrong.
    • CommentAuthorbhommels
    • CommentTimeOct 6th 2020
     
    Posted By: WillInAberdeenTopher, you are probably better off going for the RHI than the GHG grant but take qualified advice on that. Both require MCS installers.

    From a recent briefing on GHC I understood you can have both, where the upfront GHG will be deducted from your RHI. Sounds like a pretty good arrangement or am I missing something?
    •  
      CommentAuthordjh
    • CommentTimeOct 6th 2020
     
    Posted By: Ed DaviesWhy TF do we call them boilers?

    I would guess that the modern devices can trace their lineage back to the large-bucket-over-an-open-fire devices that really did boil water and indeed were used to produce steam at pressure.

    To add: 'Meaning "vessel for boiling" is from 1725; specific sense "strong metallic structure in which steam is generated for driving engines" is from 1757' according to https://www.etymonline.com/word/boiler
    • CommentAuthorbhommels
    • CommentTimeOct 7th 2020
     
    Posted By: djh
    Posted By: Ed DaviesWhy TF do we call them boilers?

    I would guess that the modern devices can trace their lineage back to the large-bucket-over-an-open-fire devices that really did boil water and indeed were used to produce steam at pressure.

    You are probably right. And boilers in the literal sense are still around: a steam boiler is a good solution when you need to distribute heat around an entire hospital or such like.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 7th 2020
     
    Americans call their domestic boiler 'furnace' - image of half-naked stokers shovelling coal into the behemoth!
  3.  
    Posted By: fostertomAmericans call their domestic boiler 'furnace' - image of half-naked stokers shovelling coal into the behemoth!


    Heating devices that heat the air are called furnaces, but for people with "hydronic heating" (which is what Americans call radiators), they do, indeed, call them boilers. Some people even call heatpumps (almost universally air distribution on the heating side (as most people need cooling too) "furnaces" as well.

    As for the sizing of 15kW - that's far far too high I'd think. My ancient not-well-insulated stone house had a 12kW GSHP that would meet the house's heat load with an external temperature of -14C ... and this was around 200 m^2 ...

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 7th 2020
     
    Thanks Paul for the cultural update!
  4.  
    An ASHP or GSHP is literally a boiler. But so is a fridge.

    A 'Gas Boiler' is a physical contradiction, but an 'Oil Boiler' is correct.
    • CommentAuthorEd Davies
    • CommentTimeOct 7th 2020
     
    Technically correct is the best sort of correct, as they say but i think they're usually being sarcastic :tongue:
    • CommentAuthorgravelld
    • CommentTimeOct 11th 2020
     
    Posted By: WillInAberdeenAn ASHP or GSHP is literally a boiler. But so is a fridge.

    A 'Gas Boiler' is a physical contradiction, but an 'Oil Boiler' is correct.
    can you explain this? Sounds like learning material...
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2020
     
    Posted By: gravelldcan you explain this? Sounds like learning material...

    Well, if you think about it, it is pretty obvious. Q: What does a boiler do? A: Boils things. QED
    • CommentAuthorEd Davies
    • CommentTimeOct 11th 2020 edited
     
    Posted By: gravelldcan you explain this? Sounds like learning material...
    His point is that a heat pump [¹] (whether in an ASHP, GSHP, fridge or elsewhere) causes its refrigerant to boil as it comes out of the jet into the evaporation part of its cycle. Similarly, an oil boiler causes the oil to boil after it comes out of the jet to that it can burn quickly. Likewise, a candle could be considered a boiler, the wax melts then evaporates off before it actually burns. A gas “boiler” doesn't need to do this, because its fuel is already a gas, so it's not a boiler.

    [¹] One that uses compression and expansion of a fluid with a gas/liquid phase change, not e.g., one that moves heat using the Peltier effect.
  5.  
    Specifically: A: a boiler boils a liquid, to turn it into a gas.

    Heat pumps and fridges work by boiling cold liquids, to absorb heat energy. Then they compress and condense the resulting gas, to give out the heat energy at a warmer temperature.

    Oil boilers atomise their liquid fuel into tiny droplets, which can boil in the flame, to burn as cleanly as possible.

    Gas boilers are the odd ones, as no liquids are actually boiled. The fuel arrives as a gas already, and the CH water remains a liquid.

    However, they do condense some of their combustion exhaust gases, so you get physical tangles such as: "A condensing gas boiler.. !"

    As Ed gently points out, this discussion does score quite high on the pedantometric scale.... but his question did start it off!
    • CommentAuthorEd Davies
    • CommentTimeOct 11th 2020
     
    Note, though, that just because a liquid turns into a gas doesn't mean it's boiling. The boiling point is the temperature where the equilibrium (“saturated”) vapour pressure of the gas is greater than the pressure in the liquid so evaporation can occur within the body of the liquid rather than just at the surface. It's, at least in principle, possible for an atomized liquid to fully evaporate quite quickly without actually boiling. I've no idea if that's what happens in any of the cases mentioned here.
  6.  
    The temperature in a flame is sufficient to boil kerosene (not just to evaporate it). But the droplets pass through the flame rather quickly, so they have to be small, so that they can get completely heated and boiled before they pass on through as soot. The short time scale and length scales, mean there are all kinds of other surface effects and temperature gradients and pyrolysis going on, to make it more complex than 'simply' boiling as I described it.

    In contrast, the pool of molten wax at the top of a candle is not hot enough to boil, it's actually at its melting/freezing temperature. It can evaporate from its surface, but not enough to sustain a flame. The wick gives the molten wax some extra surface area, and lifts it up closer to the flame, where things get hotter and evaporation goes faster.
    • CommentAuthorgravelld
    • CommentTimeOct 12th 2020
     
    Hah, thanks! I guess I wasn't thinking of the fuel, or medium of heat transfer, I was just thinking of the water in the hot water circuit. Also I didn't properly consider the gas/liquid differences.
    • CommentAuthorJeff B
    • CommentTimeOct 13th 2020
     
    "Pedantometric scale" - like it! Must remember that one!
  7.  
    Simple definition: anything that heats water is a boiler (whether it makes steam or not); anything that heats air is a furnace.

    Boiling is not talking about the fuel or the refrigerant :)

    Paul in Montreal.
    •  
      CommentAuthordjh
    • CommentTimeOct 13th 2020
     
    Posted By: Paul in MontrealBoiling is not talking about the fuel or the refrigerant :)

    On your definition, it's not talking about anything at all :bigsmile:
    boiler and furnace are just two arbitrary strings of letters used to name two classes of device.

    And it doesn't answer Ed's original question:

    Posted By: Ed DaviesWhy TF do we call them boilers? If they ever boil anything something has gone dreadfully wrong.
    • CommentAuthorbarney
    • CommentTimeOct 13th 2020
     
    I guess we call them boilers because historically that's what they did - ie raise steam

    In some cases, many "boilers" would actually boil the heat transfer fluid - we just keep the pressure high enough not to allow that to happen (like your car radiator) - eg I could point to one system that uses mineral oil operating at about 180C being circulated through various coils on air handling plant and a few other heat emitters that would be described as High temperature High pressure - when contrasted with LTHW which tends to describe Low Temperature Hot Water systems used for domestic (or similar) heating systems

    It's just a game of energy transfer - using LTHW on a large site with long primary runs (district hospital with central energy perhaps) would lead to unfeasibly large pipe runs compared to pushing HTHW or steam and taking advantage of the far larger Delta T to the heated space, via the emitters (which are also consequently smaller)

    Regards

    Barney
    • CommentAuthortopher
    • CommentTime6 days ago
     
    Next week I have an appointment to see a Domestic Renewable Heat Incentive installer. I have done the rough estimate on my house and the amount is £960 pa for 7 years. I remember reading about a lump sum, has that now gone?

    I wonder if anyone had done the calculation of installation cost, versus annual savings with Gov heat incentive to see how long it takes to break even.

    Any tips on subjects that I should be sure to mention gratefully received.
  8.  
    The original introduction of the RHI scheme was delayed a few times and there was a lump sum as a placeholder while we were waiting (many years ago now). There's also consultation about RHI reverting to a lump sum in a few years time, less generous than the amount you are anticipating! We switched over from oil which was quite expensive then, so it broke even within the RHI period, but oil is currently very cheap due to Covid so hard to speculate.

    Apart from the obvious stuff like where they will site it, which model, flow temperature, etc - ask the installer about warranty and whether this is linked to a maintenance contract (ours was, and it was expensive). Ask about how they will run cable from the fusebox. Ask how they plan to heat hot water and what the control system will be like.
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press