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    • CommentAuthorjrw
    • CommentTimeApr 28th 2018
     
    Hello All
    Long-time reader, first time posting here - I have looked at numerous related posts on the forum but I would really appreciate some informed opinion on my specific situation.

    I currently have a 45kW biomass boiler (logs) heating a 2,500L thermal store, which heats my radiators and DHW cylinder. Due to a change in circumstances I am not consistently around to shift logs, feed the fire, clean it out, etc. So I need to install another heating system, either to replace the biomass system entirely or to add heat to the thermal store when I am not around to get/keep the fire going.

    Option 1
    Bin the biomass boiler and thermal store and install oil.
    Option 2
    Keep the biomass and thermal store, but add oil/ashp/gshp heating the thermal store.

    I don't want to go for oil, but I know that the heat pump options will not give me the temperatures for my DHW and radiators. But might they be sufficient for 9 months of the year (I am in South West Wales), with boost from the biomass required when it gets properly cold?

    To complicate the situation, I am also looking at a 6kW solar PV system which could divert excess power to an immersion heater in the DHW and/or thermal store. Does anyone have any experience of the real-world benefits of this set-up?

    Thanks
  1.  
    First a question - How do you source your wood for the boiler, bought in or own forest. If it is bought in how do you buy it, boiler ready or do you have to cut and split. These questions alter the economics of option 1. (which ever one I presume you have enough space to season for 2 years)

    I wouldn't use a heat pump to heat the TS to run the CH. I would run the CH directly from the HP to save the TS losses and increase the size of selected rads to cope with the low temps provided by a HP?

    An oil boiler could directly heat the TS and could be set up to kick in if the TS temp. dropped to a predetermined level.

    The PV system potentially takes the DHW out of the equation but in the winter would the PV system give enough for the DHW and leave anything over for the CH?

    If you can use the PV for the DHW then all you need is to sort the CH. If you can get enough PV then up size the rads and go for a heat pump and bin (sell?) the biomass.

    Any chance of increasing the insulation etc. to reduce the spend on heat pumps/PV
    • CommentAuthorjrw
    • CommentTimeApr 29th 2018
     
    Hello Peter, thanks for the response. I will try to give you a bit more detail...
    On average it is about a 60:40 split between bought-in seasoned split logs and logs that I source, process and season myself. My stocks for next winter are low so I will buy in maybe 90%, but I am taking down several trees this summer which should see me through most of the winter 2019/20. It is all down to opportunity. It is great to save some money, but fuel cost is not the main factor in this case.

    I need to be able to keep the house heated when I am away for a week. During a cold snap, a hot TS lasts for about a day and a half. The easy route is to get rid of the biomass and TS and install an oil boiler. But when I originally installed the TS I had plans to boost it with solar thermal/excess power from a wind turbine (both projects indefinitely shelved at the moment). I am reluctant to go back to 100% fossil fuel if there is a feasible alternative. I am hoping for realworld feedback from people who have combined heat sources with a large TS.

    I will look at the practicality and cost to increase radiator sizes in selected areas to run on lower temp water from a HP.

    How much the PV will contribute to DHW in the winter is an excellent question, I am hopeful that someone on the forum will have some experience of this. Maybe that's a separate thread? At the moment I am using immersion heaters to top up DHW to meet demand when the TS temperature is low!

    Increasing the insulation and draught proofing the house is an ongoing project. Reducing the heat demand in the house is a priority. I have done SAP calculations before, but the house is such a mishmash that you can get any result you want. It certainly feels better with modern double glazing 80% installed. I plan to open another thread shortly on the Renovation section about insulating mixed construction walls and roof. I am busy looking through past threads on details to remove cold bridges and close gaps without compromising vapour permeability.
    • CommentAuthorowlman
    • CommentTimeApr 29th 2018
     
    You appear to have an almost identical system to me and I too am considering my next step as my ageing, but still perfectly functioning, boiler plods on. Like you I've researched lots of alternatives. I also have plenty of solar plus solar thermal for DHW
    Q. Does your accumulator tank have other indirect heating inputs, e.g. coils, immersions etc.?
    Q. How old is your log gas boiler?
    •  
      CommentAuthordjh
    • CommentTimeApr 29th 2018
     
    Posted By: jrwHow much the PV will contribute to DHW in the winter is an excellent question

    PV essentially isn't worth considering during the winter. On good days it might contribute something but there will be periods of several days, if not weeks, when it contributes nothing. Then quite quickly through March or April it switches to providing enough and through the summer there's a large excess.
    • CommentAuthorjrw
    • CommentTimeApr 29th 2018
     
    owlman
    I have a 6 kW immersion installed to heat the top 1/3 of the TS for emergencies. This is where I might divert any excess PV generated power once my DHW is up to temp in the summer.
    I also have an unused DHW coil at the top of the accumulator that could be relocated to a lower port on the TS and repurposed to heat with solar thermal, but I haven't progressed that at all.

    The log gas boiler is 2009 vintage, still going strong. Pretty bulletproof really.

    djh
    Thanks for the insight - I suspected the PV would not contribute in the winter, but I was unsure about capacity in the summer. Sounds like I can mothball the main heating system(s) for the summer months and let the PV deal with the DHW.
    • CommentAuthorjrw
    • CommentTimeApr 29th 2018
     
    So owlman, have you come to any conclusion as to your next step? Any options I haven't considered?
    • CommentAuthorowlman
    • CommentTimeApr 29th 2018 edited
     
    I'm up in N Yorkshire so PV experience may be different from yours, but I concur with djh that even with my 10kW array, in winter it's not enough for reliable full blown heating of a 2.5 ton accumulator. Obviously there are odd days when you can get a reasonable amount but ultimately you'll end up pulling most in from the grid.
    With your 6kW immersion, (I assume that's three phase), you'll have to separate it into 3 x 2kW single phase feeds,- easily done BTW.
    Log Gas Boilers as you must know are ferocious water heaters, with fire grate temps close to 1000 degrees C they heat water at a rate that standard gas or oil boilers I don't think can match, simply because they, ( gas and oil ) are designed for a different form of heating regime. This forced air "Bonfire in a Box" has few equals in that respect. I looked into hooking up my accumulator infrastructure and CH with a simple pressure jet oil boiler, even phoned the manufactures but in the end ditched the idea because I felt I was too far out on a limb control wise.
    I also have a back boiler open fire in the living room, an old leftover, but which for aesthetic reasons I didn't want to loose, so I hooked it up to a gravity towel rad, (great), and a pumped loop to the accumulator coil, which is next to useless, but was cheap to do and worth a try.
    So CH wise where does all this leave me;
    a. Solar: is out,- except for manual sunny day boosts. supplementary resistive heating e.g. UF cables, storage heaters, towel rads, are an option but can't be relied upon in winter.
    b. Automated pellet boiler; not enough storage area, but also I'm doubtful of their efficiency with such a large accumulator.
    c. ASHP/GSHP Air to water; not a big enough water heating temp capacity, and having just re-furbed the place I'm not prepared to change all the rads.
    d. Oil boiler; ( no gas in my village ) a conventional CH system which would mean ripping out a fully functioning and very responsive current CH system.
    e. Air to Air heat pump; slightly unconventional in the UK but possibly cheap enough to suck it and see, and it could be constructed/sized to keep chill of the place in winter and thereby place less pressure on the log gas system. Summer cooling may also be something to consider as a bonus should you need it. The downside is that I still can't use my solar even in summer as the pump is an inductive load and solar diverters will only divert to resistive loads. Still I guess with the pump running the load becomes part of the house baseload so all may not be lost as far as PV is concerned. This is the route I'm following at the moment.
  2.  
    Posted By: djh
    Posted By: jrwHow much the PV will contribute to DHW in the winter is an excellent question

    PV essentially isn't worth considering during the winter. On good days it might contribute something but there will be periods of several days, if not weeks, when it contributes nothing. Then quite quickly through March or April it switches to providing enough and through the summer there's a large excess.

    Do you mean PV? - I thought PV produced (less) in the winter and it was ST that dropped off dramatically.
    Edit - I just did a calculation (pvgis), a 4kWp array would produce half my DHW in January (about 160kWh), all of it in Feb. and Oct. and from Apr. to Sep. about 150% with a bit more mid summer. I did a quick calc for S wales using south facing 6kWp system and got the same in the winter months but more in the summer.

    jrw - Is the house empty when you are away for a week?

    Posted By: owlmanYou appear to have an almost identical system to me and I too am considering my next step as my ageing, but still perfectly functioning, boiler plods on.

    I am in the same position - in my case it applies to both the boiler and me - ageing, but still (sort of) perfectly functioning

    owlman - are you saying that an oil or gas boiler could not properly heat a TS in the absence of the wood burner running?
    My initial thought was to plumb a conventional boiler in to the TS and use this when wood was not available. If the conventional boiler won't run a TS then it should still be possible to plumb in such a boiler into the CH to run in place of the TS feed.
    •  
      CommentAuthordjh
    • CommentTimeApr 29th 2018
     
    Posted By: Peter_in_HungaryDo you mean PV? - I thought PV produced (less) in the winter and it was ST that dropped off dramatically.
    Edit - I just did a calculation (pvgis), a 4kWp array would produce half my DHW in January (about 160kWh), all of it in Feb. and Oct. and from Apr. to Sep. about 150% with a bit more mid summer. I did a quick calc for S wales using south facing 6kWp system and got the same in the winter months but more in the summer.

    Yes I do mean PV. Dunno about solar thermal.

    I can't speak for where you are; I'm just reporting what my 4 kWp PV does in east anglia where I am. YMMV.
  3.  
    Posted By: jrwHow much the PV will contribute to DHW in the winter is an excellent question

    As djh says PV output in the winter (and summer) will vary by location (and roof angle / direction and a few other things) You can get an idea of what to expect at
    http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php
    If you full in the boxes you get a calculation for your site.
    • CommentAuthorowlman
    • CommentTimeApr 30th 2018 edited
     
    Posted By: Peter_in_Hungary

    owlman - are you saying that an oil or gas boiler could not properly heat a TS in the absence of the wood burner running?
    My initial thought was to plumb a conventional boiler in to the TS and use this when wood was not available. If the conventional boiler won't run a TS then it should still be possible to plumb in such a boiler into the CH to run in place of the TS feed.


    Hi Peter,
    Personally, I think, as you say, you'd have to run it as a separate entity with manual shut offs to isolate the existing biomass system thereby just leaving the CH flow and return to connect to your oil/gas boiler and run the two in tandem.
    I don't think a bog standard domestic oil/gas boiler has the capacity to cope with a large accumulator. To me there seems to be several issues;
    a. The boiler water temperatures may not be high enough, and to fiddle with the boiler stat to accommodate batch burns may render any warranty void. Most biomass systems start to load around the 70-75 degree mark and in my pressurised system I ran tests pushing the accumulator store up to 92 degrees, I normally load it at about 75-80. My gut feeling is oil/gas boiler couldn't cope with that. the result could be that you'd have to settle for a lower running temperature. With CH and/or PHx demand calling at the same time, a domestic boiler would struggle to keep up.
    b. Linking a gas boiler to a biomass loading valve may prove interesting to say the least. so that may have to go.
    c. The pipework runs between the boiler and accumulator, at least in my case, ( i.e. 35 and 42mm ) may need re-configuring.
    d. I'm sure it could be do-able but maybe, my instinct now, only with a commercial grade oil/gas boiler.
  4.  
    Would you be able to plumb in a conventional boiler into the CH with a NRV on both the flow of the conventional boiler and the TS (to prevent back circuits), with a thermostat attached to the TS so that when the TS dropped to a preset temp. the thermostat switched off the TS CH controls and switched on the boiler. In this way the change over would be automatic and would occur when the TS temp was low. As soon as the wood burner was fired up and the TS reached useable temperature then the thermostat would switch off the conventional boiler and switch on the TS CH controls.

    Owlman - I take your point about a domestic sized conventional boiler not being up to running a big TS, you also save the losses associated with a TS which are less of an issue with cheaper wood fuel. (Not an issue if the TS is within the heated envelope).
    • CommentAuthorowlman
    • CommentTimeApr 30th 2018
     
    Thermo controlled non-return valves I guess could work in the way you describe, but quite a few links in the chain. Alternatively, you could charge the accumulator and keep that storage capacity as an immediate back up, running the "manually" isolated gas fired CH when you need the automation aspect, e.g. in your absence. Then switching to the store if/when you desired.
    I guess it depends to some extent on your lifestyle and how much you want to automate everything.
  5.  
    I was thinking of standard NRVs to stop the flow through which ever circuit was not being pumped. I envisaged that either the conventional boiler or the TS would be running, but never both together and the thermostat change over switch changing from one to the other depending upon the TS temp.

    If the change over is automated and driven by the TS temp - which is driven by lighting the wood burner then that eliminates having the wrong valve/tap turned for the system in use. - Light the wood burner and it happens safely, it just avoids any 'senior moments'
    • CommentAuthorowlman
    • CommentTimeApr 30th 2018
     
    I don't know how efficient or reliable, presumably motorised, NRVs would be. Having said that I use a motorised, thermo controlled, variable valve to regulate water temperature in my system. However, it's never fully shut off just variable. Shutting off pressurised water may require a bigger beast, but I guess they are out there, try Acaso.
    • CommentAuthorTimSmall
    • CommentTimeApr 30th 2018
     
    It's possible to run a heatpump from PV-produced electricity if you like. What temperatures do you need for DHW and heating, and do you have an idea of the max heating demand in kW?
    • CommentAuthorArtiglio
    • CommentTimeApr 30th 2018
     
    From JRP’s , original post, it seems as though his absence is the need for an alternative. As such whilst not ideal and assuming there is a bit of notice as to when he comes and goes. An lpg or oil boiler in tandem plumbed with valves to isolate the store (as suggested above), would perhaps be the simplist option. Set up for oil/gas when away, back to logs on return, worst case scenario is you lose the energy in a recently charged store and have a need for multiple burns on return if the store has dropped too far ( ie cold).
    Depending on how dhw is provided, pv to a dhw cylinder may cover hot water requirements through the summer and offset some gas/oil use other times.
    Depending on the length of time the heating is from gas, it may be possible to run from manifolded cylinders ( assuming that is still acceptable) solving problem in short term at reasonable cost, allowing additional insulation , pv, st, gshp, ashp to be explored in the future.
    • CommentAuthorjrw
    • CommentTimeApr 30th 2018 edited
     
    Artiglio - yes, it is the fact that I am working away from home more frequently that is the primary reason the past winter was a real struggle trying to keep the house heated with the log gas boiler. I have at least a few day's notice so I can get the TS up to 80oC which gives me a day and a half. After that the supplementary heat source needs to kick in. Whether that is automatic or a flick of a switch is fine. Manually closing some valves and opening others in sequence is not something I want to delegate while I am away!

    TimSmall - DHW is probably 3 showers/day and some washing up. I allowed for 2kW in my initial calculations. The total heat demand by my calculation is 27.8kW.

    PiH and owlman - I have had different opinions from different heating technicians on whether it would be OK to operate an oil boiler and the log gas boiler in tandem or whether it would have to be separate circuits with an interlocked switchover. Both suggested a 35kW system boiler as suitable and that ties in with my estimates.
    If I were to use the TS for both, then if I can only get 70oC from the oil boiler then that still gets me radiators and DHW. A lower rated oil boiler is going to have to run longer that a 45kW log gas boiler to charge the TS anyway.
    • CommentAuthorjrw
    • CommentTimeApr 30th 2018
     
    Thanks for the link on PV output PiH.
    It has calculated 5.45kWh in December (not much excess power probably - the house is occupied through the day so there is always a base load). The 27.4kWh available in June should be enough to have 'free' hot water for the evenings. April through August are above 20kWh, which should be enough to provide a useful boost.
  6.  
    In a similar position thinking of old age and revamping our wood fired central heating. At the moment heating throughout the house provided by Dunsley Yorkshire boiler to piped radiators. This is at present being modified to UFH on ground floor retaining radiators upstairs but still using the same boiler. As part of the UFH screed I am also laying electric mats so retaining the option of solar/hydro heating option.
    • CommentAuthorjrw
    • CommentTimeMay 2nd 2018
     
    Update
    I am getting a quote for a GSHP now. Interesting discussion on the practicality of combining the biomass burner with a new GSHP with the contractor. He also concluded that it is either/or - it is not possible to integrate a HP system with the existing TS because of the discrepancy between the 80oC+ output from the biomass boiler and the 45oC from the HP.
    We debated the benefit of running two entirely separate, parallel systems with the GSHP feeding the heating circuit directly. But every time we thought we had come up with a way to do it, we realised that there was a way for it to go badly wrong with a single component failure or human error.
    So I will brace myself ready for the quote (promised for next week). It will have to include increasing radiator sizes. If I decide to go ahead there will be a second-hand biomass system on offer.

    renewablejohn
    from my research, the wet UFH will make switching to a GSHP or ASHP at a later date a more realistic possibility. Good forward planning!
    • CommentAuthorBeau
    • CommentTimeMay 3rd 2018
     
    45c for the output of the GSHP is not going to be cheap running. Ours almost never has to get over 30c and I doubt the COP is better than 4.
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