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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.

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    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2021
     
    Posted By: BowmanFWIW we have a 300L thermal store with ASHP coil (not connected - a bit short of funds) but heated via immersion heaters. The DHW is heated via PHX, we can get a thoroughly decent 10 minute shower when the store is at 50C.

    Ours is 250 L. The bottom immersion is powered from PV. The upper (i.e. middle) immersion is powered from the mains. It heats the top half of the tank if there's no sun. So if there's no sun we have half a tank (125 L) at 60°C and that gives us two showers. I don't know whether it would give us more - on the odd occasions we have visitors in winter I tend to override the time switch so the immersion will reheat the water after a shower. Too embarrassing to run out of hot water to experiment :shamed:
    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2021
     
    WillInAberdeen wrote:

    "See eg https://trystanlea.org.uk/heatpump2020

    "Annual domestic hot water COP: 3.83"

    I'm always impressed by the amount that Trystan manages to get done and by the quality of the reports he presents :cool:

    I don't quite understand his COP figures though. He says:

    Annual space heating COP average: 4.2
    Annual domestic hot water COP: 3.83
    Standby electricity consumption: 95.1 kWh
    Combined space heating and DHW COP (not including standby): 4.11
    Combined space heating, DHW and standby COP: 3.91

    But then the third graph below supposedly claims to show the data behind the last figure 3.91 IIUC. But when I add the heights of all the purple bars and divide by twelve I get 3.57. I don't understand why there's a discrepancy?
    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2021
     
    Replying to myself! I asked Trystan and he told me my obvious error: "you can't add monthly COPs together to get the average annual COP as the summer COPs reflect relatively little heat energy and winter COPs reflect a lot. You have to divide total annual heat output in kwh by total annual electric input in kWh to get the correct value."

    So now I understand :bigsmile:
    • CommentAuthorGareth J
    • CommentTimeOct 24th 2021 edited
     
    Posted By: djh
    Posted By: BowmanFWIW we have a 300L thermal store with ASHP coil (not connected - a bit short of funds) but heated via immersion heaters. The DHW is heated via PHX, we can get a thoroughly decent 10 minute shower when the store is at 50C.

    Ours is 250 L. The bottom immersion is powered from PV. The upper (i.e. middle) immersion is powered from the mains. It heats the top half of the tank if there's no sun. So if there's no sun we have half a tank (125 L) at 60°C and that gives us two showers. I don't know whether it would give us more - on the odd occasions we have visitors in winter I tend to override the time switch so the immersion will reheat the water after a shower. Too embarrassing to run out of hot water to experimenthttp:///newforum/extensions/Vanillacons/smilies/standard/shamed.gif" alt=":shamed:" title=":shamed:" >


    If heating an immersion with PV via a diverter, like an iboost or equivalent, you can wire in, using a separate thermostat and a relay, a setup to heat the top of the tank first, then flip to heat the bottom. I do this with mine which helps get a smaller proportion of water up to shower temperature with the lower PV days.

    Would become a bit trickier to achieve a boost like you do currently though.
    •  
      CommentAuthordjh
    • CommentTimeOct 24th 2021
     
    Posted By: Gareth JIf heating an immersion with PV via a diverter, like an iboost or equivalent, you can wire in, using a separate thermostat and a relay, a setup to heat the top of the tank first, then flip to heat the bottom. I do this with mine which helps get a smaller proportion of water up to shower temperature with the lower PV days.

    Would become a bit trickier to achieve a boost like you do currently though.

    Yes, my guiding light was KISS - simplicity.
    • CommentAuthorGareth J
    • CommentTimeOct 24th 2021
     
    Posted By: djh

    Yes, my guiding light was KISS - simplicity.


    A lot to be said for that!
  1.  
    Posted By:

    Unfortunately, the Gledhill torrent doesn't use a coil but instead is direct. As ASHP use glycol, you can't replace the boiler with a heat pump.

    Can't seem to find any heat pumps that don't use glycol, so looks like the gas boiler might be sticking around a while longer!

    .


    The glycol is not necessary for the operation of an ASHP - it's only there to protect the ASHP if it breaks down in cold weather. If it's operating correctly, it will initiate a cycle to stop itself freezing. We have a Samsung ASHP that runs on simple water for environmental reasons. If it broke down during a cold spell, I could put a blanket over it or something. Otherwise if it breaks, you've invalidated the warranty!
    • CommentAuthorEd Davies
    • CommentTimeOct 24th 2021
     
    Or a long-enough power cut.
    •  
      CommentAuthordjh
    • CommentTimeOct 24th 2021
     
    Do split ASHP (as opposed to monobloc) need frost protection in a similar way? Or are they inherently frost-proof?
  2.  
    No. Yes. :wink:

    Incidentally, many rural homes have externally mounted oil boilers, in a casing outside the home, equivalent to a monobloc heat pump. There is a frost stat in the casing which fires it up if it gets too cold, but is no protection against long power cuts. Seems not to be a problem.
  3.  
    Posted By: djh(125 L) at 60°C
    A shower at 5l/min and 38degC would run off that for (125l / 5lpm *[60-10]/[38-10]) = 45 minutes which is enough for nine people each to have a 5-minute shower. Some people use more than 5l/minute and take longer than 5 minutes but you get the idea.

    With a typical 8-12kW heat pump, the heat pump can quickly reheat the water even before one customer has dried themselves and the next one has stripped off, so that's not what determines the size of the store. More about storing the heat that will be used during peak-rate periods, or maybe if you need to fill a bath, or want to combine with legacy solar systems.
    •  
      CommentAuthordjh
    • CommentTimeOct 25th 2021
     
    Posted By: WillInAberdeenWith a typical 8-12kW heat pump

    Yes, even 3 kW is a bit big for our house. But I agree with what you say. Our thermal store is all about storing heat, either from PV during the day or from E7 one night until the next evening, plus all the incidentals of course.
  4.  
    How much hot water do you use during a day, do you think? Was trying to work our usage out.

    One shower per person (25l each @38degC) and a sinkful of washing up (10l @ 50degC). None for appliances as ours are cold feed. Occasional bath for kids, 50l @ 38degC.

    Finding it hard to justify storing more than about 60litres of hot water, with reheating overnight and in the afternoon and top ups if required.
    • CommentAuthorphiledge
    • CommentTimeOct 25th 2021
     
    If youve got solar input to your DHW store, either PV or ST, then Id be looking at a signifcant size store to capture what you can through the spring/summer/early autumn. Other than ST, we dont have any heat input into DHW for 6 months of the year so HP/boiler/woodburner not required.

    If youve no solar input available, Id be adding ST/PV before I added a HP.
    •  
      CommentAuthordjh
    • CommentTimeOct 25th 2021
     
    Posted By: WillInAberdeenHow much hot water do you use during a day, do you think? Was trying to work our usage out.

    Dunno ...

    One shower per person (25l each @38degC) and a sinkful of washing up (10l @ 50degC). None for appliances as ours are cold feed. Occasional bath for kids, 50l @ 38degC.

    One shower each for the two of us. Sundry washing up, hand washing and other cleaning. Baths as rare as hen's teeth so can be disregarded I think.

    Finding it hard to justify storing more than about 60litres of hot water, with reheating overnight and in the afternoon and top ups if required.

    Our 250 L thermal store size was based on finding our 150 L (or thereabouts) cylinder at the previous house sometimes ran out but admittedly we used to have more baths, plus a desire to store solar energy for two or three days, plus winter heating purely on E7 except on very rare occasions. I haven't really tried to work out whether a different size or technology would be better; I'll do that in another five or ten or fifteen years whenever I need to replace the existing system. It seems unlikely there'll be any improvement radical enough to justify throwing away a system that's working well.
    • CommentAuthorBenM
    • CommentTimeNov 13th 2021
     
    Thanks everybody for your input into my initial enquiry. I can’t say I fully understand all of it, but certainly good for thought.

    With the gas boiler on the brink of possibly needed replacing (technical fault), I really don’t want to replace it with another. But, looks like just plugging in an ASHP isn’t really that simple. Most certainly a longer term project!
    • CommentAuthorgravelld
    • CommentTimeNov 16th 2021
     
    Posted By: Gareth J
    If heating an immersion with PV via a diverter, like an iboost or equivalent, you can wire in, using a separate thermostat and a relay, a setup to heat the top of the tank first, then flip to heat the bottom.
    I was wondering whether it was possible to have some sort of component, in place of the relay and separate thermostat, that could recognise when the top immersion thermostat tripped and then divert to the bottom?

    Does such circuitry also maintain the resistive nature of the immersion, allowing variable power?

    I also know some diverters also have multiple outputs for this purpose. But i was trying to keep this as simple as possible.
    •  
      CommentAuthordjh
    • CommentTimeNov 16th 2021
     
    Posted By: gravelldI was wondering whether it was possible to have some sort of component, in place of the relay and separate thermostat, that could recognise when the top immersion thermostat tripped and then divert to the bottom?

    Not quite sure what your objection to a basic relay circuit is, or a solid state direct equivalent?

    PS There's no extra thermostat involved, I don't think, just an extra relay.
    • CommentAuthorgravelld
    • CommentTimeNov 17th 2021
     
    I thought the purpose of the "separate" (@Gareth J's word, which I assumed meant separate to the thermostats in the immersion heaters) thermostat was to switch between the two immersion heaters when a target temperature is reached.

    I just wondered if a setup which just switched when one of the integrated thermostats tripped was possible.

    I've probably misunderstood.
    • CommentAuthorphiledge
    • CommentTimeNov 17th 2021
     
    Posted By: gravelld
    I just wondered if a setup which just switched when one of the integrated thermostats tripped was possible


    If the top immersion has a separate thermostat in the head, then yes it is. Disconnect the short wire from the stat to the heating element so the stat and element are separate. Wire a supply to one side of the stat and the other side of the stat to the coil on a changeover relay. Supply is likely to be 240vac unless you want to use a different control voltage.
    Wire the PV diverter to the common on the relay contact. Wire the NO of the contact to the top immersion direct to the heating element and the NC of the contact to the stat in the bottom immersion. Add neutrals/earths as needed.
    • CommentAuthorGareth J
    • CommentTimeNov 17th 2021 edited
     
    It is possible to do this using the integrated 'stats, as above, using a NO/NC relay; like this for example;
    https://www.ebay.co.uk/itm/2-Pole-20-Amp-Contactor-Din-Rail-Mount-Relay-Unit-Module-1NO-1NC-UK-Supply-/363467166450?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=710-127635-2958-0

    I made my first one this way. Make sure the relay you use is 20a rated. 16a should do in theory but it's a hard life changing over full chat 3kw of whatever flavour electricity those iboosts put out.

    However, as the standard, built in 'stats will be closed until hot, that means that the coil of the relay will be energised until the top of the tank is hot. Which is fine, but will mean the relay coil is buzzing away all winter and a lot of summer. Not really a problem as the coil draw is tiny, it's just not that elegant.

    When I recently changed the cylinder, I opted to run a separate, external thermostat because you can get No/Nc ones that have terminals that give the option to close when up to temperature. I used Horstmann HTC2. These are rated at 16a and should, in theory, carry the current of the iboost directly without the need for a relay at all.... however, I tried that and the stat gets quite warm. Not a a safety issue warm but, within the tank foam, warm enough to throw the accuracy of the setpoint temperature out, causing the changeover to happen too early....so back in with another relay, in order to take the current of the iboost away from the thermostat and let it measure accurately. At least with that the relay coil is only energised when it's driving the bottom immersion.

    Bit of a journey with it all but it works neatly and, despite my waffling, is easy to wire up.
    • CommentAuthorGareth J
    • CommentTimeNov 17th 2021 edited
     
    Alternatively, the simple option is an "i-boost +" or similar, which has two outlets and does switching for you.
    • CommentAuthorgravelld
    • CommentTimeNov 17th 2021
     
    Thanks, that's really helpful. My diverter can do that, in theory, but the firmware needs updating and as they are out of business now (Energeno Optimmersion) I wanted to avoid risk of bricking the device.

    I will have to talk to an electrician though because this is beyond me.
    • CommentAuthorgravelld
    • CommentTimeNov 17th 2021
     
    Back to the original subject...

    I'm not sure if i understood whether the conclusion is that a separate pre heat store was better or more efficient? Given Trystan Lee's experience, isn't it simpler just to use the HP to heat to a low level? Although it looks to me like he has a UVC (https://trystanlea.org.uk/heatpump-oneyear).

    Is there an aspect of TS operation, e.g. the requirement to heat to quite a high temperature to allow the mains coil to absorb enough energy, which is mandating this pre heat idea?

    There's a chance I'm choosing a UVC soon and, while I realise that's different to a TS, I want to try to understand different systems so I can fully grok (understand) the different dimensions and variables.
    • CommentAuthorphiledge
    • CommentTimeNov 18th 2021
     
    Posted By: Gareth J

    However, as the standard, built in 'stats will be closed until hot, that means that the coil of the relay will be energised until the top of the tank is hot. Which is fine, but will mean the relay coil is buzzing away all winter and a lot of summer. Not really a problem as the coil draw is tiny, it's just not that elegant.

    AFAIK most people would keep at least the top of a DHW cylinder hot so they have hot water available. In that case the top stat would be satisfied and the relay deenergised for much if not most of the time. In winter, if not year round when needed, the primary means of heating the cylinder would ensure hot water with the PV diverter adding what it can
  5.  
    Posted By: gravelldGiven Trystan Lee's experience, isn't it simpler just to use the HP to heat to a low level? Although it looks to me like he has a UVC (https://trystanlea.org.uk/heatpump-oneyear" rel="nofollow" >https://trystanlea.org.uk/heatpump-oneyear).

    I dipped into the above link and saw

    "we have only needed 2-3kW of heat output and flow temperatures of 30-34°C rather than the maximum of 5.5 kW and flow temperature of 40-43°C ............. When it's really cold (-1°C to 2°C) running the heat pump continuously at 30°C flow temperature for 24h+ seems to be sufficient to maintain a comfortable indoor temperature."

    Is it better to run the heat pump for longer periods to get a better COP when you consider there will be standing running inefficiencies / hour and increased maintenance from the longer run times. Perhaps a break even point?
    • CommentAuthorGareth J
    • CommentTimeNov 18th 2021
     
    Posted By: philedge

    AFAIK most people would keep at least the top of a DHW cylinder hot so they have hot water available. In that case the top stat would be satisfied and the relay deenergised for much if not most of the time. In winter, if not year round when needed, the primary means of heating the cylinder would ensure hot water with the PV diverter adding what it can


    That's not my experience and not how I have used diverters. You need the setpoint of the immersion thermostat you're feeding into higher than the "normal" temperature in order for useful energy to have somewhere to go. I.e. if you heat water to 60C with a boiler for DHW, need to set the immersion setpoint at over this temperature. In an attempt to maximize free storage, in reality it's as hot as can be gotten away with. Approaching 80C.
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