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Green Building Bible, Fourth Edition
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    •  
      CommentAuthorDamonHD
    • CommentTimeMay 21st 2020
     
    http://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html#returnWillis

    The aim is to be able to selectively draw some space heat from electricity when its carbon intensity is less than that of using gas directly. No storage. No HP.

    It's meant to be cheap and simple. (Combi is a Potterton Performa 24 FWIW.)

    Rgds

    Damon
    • CommentAuthorEd Davies
    • CommentTimeMay 21st 2020 edited
     
    Please consider standard rant ranted: https://edavies.me.uk/2016/07/grid-intensity/
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 22nd 2020
     
    Note that I already reference your item *from my page* and don't particularly disagree with your argument.

    So, for your purposes I may choose to turn on the Willis when the grid is having to constrain renewables and low demand is the reason and transmission capacity isn't the reason.

    Is there a problem from a plumbing pov?

    Rgds

    Damon
    • CommentAuthorGreenPaddy
    • CommentTimeMay 23rd 2020
     
    Can't immediately see an issue with increasing the return temp (within sensible limits), when the gas burner is not in operation. Only issue if it were in operation might be that you would reduce the ability to condense.

    I'm wondering about how you control the circulation pump (which appears to be internal to the boiler), separately from the gas burner. Apart from perhaps a short overrun on the pump, would the boiler not shut down the circ pump when the call for gas heating stops?

    If you can control the circ pump separately, then why not put the elect heater in the flow line? I'm sure you'll have thought about these already, so I'm genuinely interested in your thought process. I often end up tweaking systems, to meet specific interests/setups of clients - always happy to steal others' ideas.
  1.  
    If you are doing electric resistance heating, wouldn't it be better to heat only the room you are using, directly with a panel or fan heater, rather than losing heat to the air that's inside the boiler heat exchanger?

    If you connect to the plumbing, make very sure the heater has a safety cutout as well as a thermostat, and the electric heater should safety-cutout before the boiler does.

    What happens if the electric heating comes on whilst the combi is operating in DHW mode - does the flow through the electric heater stop - so the slug of water sitting in the heater gets way too hot?

    In the unlikely event that the supply ever exceeds demand for renewable energy in London, I guess earth.org would go for the best discretionary use of the available exergy to power a device such as washing machine, dishwasher, freezer etc which you would have run anyway at another time?

    Then you'll heat something to a high temperature (cook something, fill a flask from the kettle). Next best would be to heat something to a moderate temperature (hot water tank, dry some clothes quickly). Last resort would be heating your space to low room temperature.

    Edit: could you get an air-to-air heat pump, and if you have to move house, take it with you?
    IME redevelopment often happens several years later than the original plans..
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 23rd 2020 edited
     
    Hi both,

    Thanks for your thoughts!

    The boiler is non-condensing. Given that it works and the house is due to be pulled down in a few years, and I don't really want to be burning things at all, I don't intend to upgrade to a condensing unit.

    The pump is in the boiler and will only fire up when there is a call for heat. I do that rad-by-rad with Radbots, ie empty rooms don't call for heat as much or at all. So that's where hot water will end up going.

    And so I don't mind if the pump stops when the call for heat stops. There is no more need for heat. The stat in the Willis heater will cap the water temperature in the Willis. There is a safety cutout too.

    I think that the return is (marginally) better than the flow so as to shut down the gas sooner if the Willis heater can do the job entirely by itself, ie heat demand <= 3kW and all other conditions met. Quite likely when only one rad at a time is on.

    Radbot calls for heat room by room already, and making use of the existing rad poses no extra fire risk nor takes any extra space.

    Note that right now the National Grid has had to issue orders to (be ready to) turn off renewables in GB, so the 'using excess RE better is better than gas' case is already with us. I do separately time the use of (eg) our dishwasher to best match my available generation in summer or grid RE status in winter.

    Fitting an ASHP would be tricky here, partly because of planning and house layout. I'm not utterly ruling it out, but I'd like to try this cheaper shading over from gas with the boiler that I have.

    I hope that answers your questions!

    Rgds

    Damon
    • CommentAuthorGreenPaddy
    • CommentTimeMay 23rd 2020
     
    I'm a bit comfused (by this and life in general). Are you planning for the gas boiler to modulate, so that the willis will run at the same time as the gas is burning, with the gas boiler modulating down as a result of the higher return temp?

    I had understood it would be gas OR elect. ie. you would switch over to elect, with gas going off.
  2.  
    Not sure I buy the PR stuff in the press about excess renewables- when I check the stats on earth.org I see that there is still lots of gas and import on the grid - currently 5GW= 22% of grid and I have never seen this come to zero (did you?) or anywhere close.

    So it's not correct to say that 'using excess renewables is better than burning gas' because the marginal generation unit right now is either UK gas or (indirectly) German coal fired.

    (Edit to add: earth.org shows that grid intensity is currently 89% correlated with gas fired generation, indicating that gas is currently the marginal source.)

    This has been conflated with negative prices by the press. Negative prices don't mean there's excess renewables - they mean that generators with long-term contracts are having to be paid to break them - a commercial effect not a carbon effect. If not enough contracts are curtailed, then National Grid will intervene, as they have given notice.

    A better signal to switch on an electric heater would be when you detect that both gas generation and electricity imports are zero. Edit: or that grid intensity is currently weakly correlated with gas or imports
    •  
      CommentAuthorDamonHD
    • CommentTimeMay 23rd 2020 edited
     
    @WillInAberdeen This is not just "stuff in the press", see here:

    https://bmreports.com/bmrs/?q=transmission/systemwarning

    Demand has been higher than forecast so maybe none of the ODFM levers have needed pulling yet...

    So no, excess renewables may well be turned off if not used for the next little while. Various inventives are encouring use, I suspect, such as -ve pricing for Octopus Agile customers. (There'll still be gas at the margins for inertia for now, whatever else happens.)

    (-ve prices may also mean that it may be worth paying back some subsidy per MWh in the market to keep getting the rest of it.)

    @GreenPaddy I'm hoping that most of the time when the Willis is willing that the gas will barely run at all. When lots of rads are on for (say) the start of a very cold day, I think that demand may at least briefly exceed 3kW and the gas will need to run a bit. My 24kWh boiler is clearly already massively oversized for a heat demand typically less than 3kW, so yes, it's likely to run heavily modulated down if it runs.

    So no, the aim is NOT to switch entirely from gas, but gradually shade over to it as the grid becomes greener. It's what a fancy research grant application to BEIS might call a "cross-vector hybrid heating system"!

    Rgds

    Damon

    PS. Note the .uk on the end of my site name earth.org.uk!
  3.  
    Damon, as you know (but the press don't!) the ODFM notices don't mean there's 'excess renewables'. They mean that excess generation overall (incl fossil) has been contracted and NG are looking for the cheapest way to persuade some generators to switch off. The renewable generators have low marginal costs so the ODFM is potentially a cheap way for NG to persuade them to turn off, even though they have been contracted to generate. As you mentioned, the marginal generation is still gas fired.

    Sorry for the wrong URL, and thanks again for maintaining the electricity data, definitely my go-to source for these kinds of thoughts!
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Probably best to take the issues in the last few posts to the smart meters thread rather than derail this one as well, unless somebody wants to start a specific topic?

    @Damon, seems like a good plan to me, but I'm not an expert in the relevant technologies so don't take it as a guarantee!
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    @djh and others: another very basic question or two since no one has recoiled in horror so far!

    1) "External immersion heaters are designed to directly heat domestic hot water (DHW) in a vented system."

    My rad circuit is not vented so far as I understand it, but there is an expension vessel there and an emergency pressure relief value, so I am assuming that this would be OK. But please say if not.

    2) "Because the Willis Immersion is such a rapid water heater, it is not recommended for use in hard water areas."

    Hardness is not going to be an issue as new mains water is not getting in as a source of calcium except for top-ups.

    But are there other corrosion or similar issues that I should think about?

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: DamonHDMy rad circuit is not vented so far as I understand it, but there is an expension vessel there and an emergency pressure relief value, so I am assuming that this would be OK. But please say if not.

    I'm guessing your quotes come from some Willis paperwork? Sorry I haven't read it. I think you fail the first part of the quote as well (i.e. it's not DHW you're heating), so I suppose a direct question to Willis would be the best course of action.

    are there other corrosion or similar issues that I should think about?

    I expect so, but they will depend on what materials are present in the circuit and what the circulating fluid does contain exactly, so I doubt us random folks on the intertubes can give an answer. Again, your question to Willis might answer that.
    • CommentAuthorowlman
    • CommentTime7 days ago
     
    Just a thought Damon, but instead of a Willis heater with its foibles, could you not use trace heating on the return, or flow or both and possibly switch them as required. I've no idea on cost comparisons though.
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    I imagine that I can't get nearly enough oomph with trace heating? Especially as there is very little exposed pipework.

    But show me that I'm wrong!

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    I think trace heating is a red herring, but any other inline heater would be possible if there are problems with the Willis (which I'm not aware there are).
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    Thanks!

    (And, to be clear, I mean Willis-like inline heater rather than a specific brand. I'm not sure if Willis even continues to make such things...)

    Rgds

    Damon
    • CommentAuthorEd Davies
    • CommentTime7 days ago
     
    There are people who use Willis-like (maybe actually Willis) heaters as their sole source of heat for UFH so they're not obviously completely incompatible with non-DHW applications. Whether they'd work well with steel radiators and the associated inhibitors, etc, and whether the UFH systems I'm thinking of are vented (unlikely) or unvented (more likely, I think) are separate questions.

    It could well be that those UFH systems are unvented but the total volume is less than the limit for special rules for unvented systems.
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