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    • CommentAuthorDemonR
    • CommentTimeFeb 2nd 2017
     
    I wonder if any of the very well-informed people on this forum would be able to offer advice on my proposed new heating set-up? I’m shortly going to be building a new house in SW France, near the Pyrenees. It’s pretty hot in the summer, and can be quite cold in the winter – down to -7C at night (although often quite warm again during the day). The house is being built with 200mm SIPs panels so will be airtight and very well insulated.

    There is no gas, so I am currently planning to have underfloor heating with a heat pump as the principal heat source. The house will be built on a 1m high foundation wall, so the land in front of it will have to be built up to the threshold (there’s a hill behind) so additional excavation for a GSHP using slinkies should therefore be minimal – more just a question of backfilling. I’m hoping therefore that the cost difference between a GSHP and ASHP will not be too great, and I can justify the additional cost for a GSHP, with its higher efficiency in the winter.

    We’re keen to have a log fire in the large open plan living room/dining room/kitchen (16m x 6.5m) for aesthetics and as a source of radiant heat on cold nights. This would also mean that I can specify a smaller heat pump capable of dealing with minimum temperatures of, say, 0C, in the knowledge that I can bump up the heat with the wood burner when occasionally necessary. So my thinking is that we get a Waltherm Zebru 15 Kw gasification wood stove installed in the main room, which could feed a thermal store together with the heat pump. We would heavily insulate it to send as much of the heat as possible to the store rather than the room, and also zone the underfloor heating in the living room so it could be turned down or off when the wood burner was running.

    In the summer, indirect coils for domestic hot water at the top of the stratified thermal store would be pre-warmed by the heat pump and topped up as necessary by an electrical emersion heater. In the winter, the wood-burner could often do this job.

    Solar thermal panels seemed initially attractive as a third source of heat for the thermal store, but they seem to have rather fallen out of favour as the cost/benefit is analysed against low maintenance PV Panels, whose costs continue to fall rapidly. So if I have any solar energy input to the system, I’m imagining adding PV further down the line, the output of which could then be sold back to the grid or fed to an additional low-voltage electrical immersion heater in the thermal store.

    Does all this seem sensible and logical - or am I mising anything?
  1.  
    Hi
    I had a look at you proposed stove via google and it seems to be a stand alone stove i.e. not capable of being built in, so I am not sure how you would propose to "heavily insulate it to send as much of the heat as possible to the store rather than the room". I would suggest you run this past the manufacturers to see what they say.

    Have you run a wood stove before? You will need about 4kg of wood per hour to achieve the rated output of the stove. Don't underestimate the amount of dust and dirt that a frequently used wood stove can create in a house.

    The thermal store (TS) recommended on the Walltherm website was 1000lts. which is twice as large as the "nominal size" usually discussed here (a ratio of 1:50) I would suspect that you would need to run the stove for quite a long time to get the TS up to a good temperature, no problem with that - except that at the same time you will be putting up to 4.5kw to the room whilst you are trying to heat the TS and IMO you would have to leave the room after a few hours or open some windows!

    Zoning the UFH to allow the wood stove to better function sounds like a good idea but given the time lag in UFH response I am not sure how practical this will work out. That is how much in advance of the wood stove being lit or being allowed to go out would you need to turn on or off the UFH, I would be surprised if something like a room stat. would be able to control the situation to a constant comfort level and I suspect that some malice aforethought with manual intervention might be needed.

    The general advice here is to separate DHW and space heating where possible as the two have such differing requirements and times, especially with UFH. My system has a separate DHW tank and TS and I am very glad that I separated them (after advice from here some time back).

    If you can I would go for PV sooner rather than later. What sort of metering do you have? Over here we have average metering, which means that the balance between PV and grid is sorted out once a year which is nice because there is no need to play with low voltage immersions etc. and all of the PV output can be put against the grid usage and accounted once a year.
    • CommentAuthorEd Davies
    • CommentTimeFeb 2nd 2017 edited
     
    Posted By: DemonRSo if I have any solar energy input to the system, I’m imagining adding PV further down the line, the output of which could then be sold back to the grid or fed to an additional low-voltage electrical immersion heater in the thermal store.
    Technically, most domestic immersion heaters are low voltage in that nominally 230 volt mains is well within the standard definition of low voltage. I suspect that's not what you're thinking of, though.

    Off-grid PV panels are usually run at relatively low voltages, up to about 150 volts or so. On-grid PV strings are usually run at much higher voltages than that, usually well above mains voltage and often up to 600 volts or so. That could be sent directly to a special-purpose immersion (which would still technically be low voltage) but that would be very unusual and rather fraught.

    Normally when a grid-tied PV system is used for heating applications the way it's done is to convert the power to mains as normal then feed that to the heater. Often it's done with a special box which allows the power into the heater to track the output of the PV less any other use in the house so that the total being used is just less than that being generated locally resulting in a very small export to and no import from the grid.
  2.  
    Hi Denman, Your heating spec is overkill for a low energy house, Heatpump + Stove + Solar Thermal with large water tank + PV? I'd increase the insulation to a 300mm airtight wrap with triple glazing and Heat Recovery Ventilation (but not the ducted type), the insulation now becomes your primary heating system and you'll only need a small back-up system. For the south of France I'd opt for a large 40m2 Solar Roof with a combination of PVT and ST Panels as a heating system and back it up with a little electricity.
    • CommentAuthorgyrogear
    • CommentTimeFeb 3rd 2017 edited
     
    Not knowing your location, I took the town of Auch, which gets better-than-average sun (+7% year-round, and +9% in winter).

    so I trust you are building for solar gain !

    (Also sounds like you might have a crawlspace ?) ( = plant room...)

    in which case I'd be looking to include a modified Trombe wall or an aspirated solar facade in my build...

    translation: look for "passive" principles rather than technology !

    gg
  3.  
    Viking House could be right - have you done the heat load figures based on your proposed build specifications?
    • CommentAuthorDemonR
    • CommentTimeFeb 4th 2017
     
    Thanks everyone for your very helpful thoughts. The Waltherm Zebu also comes in an insulated model (https://thecentreforgreenenergy.co.uk/zebru-insulated) but we have not yet done the thermal calculations. Once the building permit has been approved, we'll get those done, and we'll have a better idea if we are heading for overkill. I agree completely that it's better to find ways to keep the heat in rather than let it out through leakage and keep adding more. We'd certainly plan to have a heat recovery and ventilation system, but what is the alternative to a ducted system?

    We're in a village called Aurignac (France 31420) so the figures for Auch are probably not far out. There seems to be a collective view that that in fact solar thermal remains a good option, although of course it won't be very effective in winter.
    • CommentAuthorringi
    • CommentTimeFeb 4th 2017
     
    Posted By: DemonRThere seems to be a collective view that that in fact solar thermal remains a good option


    Not in the UK, where PV is clearly the best option in 99% of the cases, but you are not in the UK, unlike most people reading greenbuildingforum.
    • CommentAuthorEd Davies
    • CommentTimeFeb 4th 2017
     
    Just had a look with PVGIS [¹] at a field just south of Aurignac (to make sure of getting a clearish horizon to the south). With default parameters but 60° elevation panels it gives 2.48 kWh/day in January (worst month), with 3.83 kWh/day in September (best month [²]). In other words, one of the bigger arguments against solar thermal in the UK doesn't apply anything like as strongly.

    In the UK you need a lot of solar thermal area to get much useful heat in winter so consequently finish up with large amounts in summer which can be awkward and potentially dangerous to get rid of. In the south of France it looks like you could get useful amounts for space heating in the winter but, with steep panels, not have terribly much to get rid of in summer.

    [¹] http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

    [²] Surprisingly late in the year, is there some seasonal reason for cloud in July/August or is it just high temperatures reducing panel efficiency? Nope - I think it's the high elevation I've put for the panels, reducing it back to 30° puts the peak in July as you'd expect.
    • CommentAuthorgyrogear
    • CommentTimeFeb 5th 2017
     
    FWIW, here is a solar DHW calculator for France.
    http://ines.solaire.free.fr/ecs_1.php
    For Aurignac, use "PAU"...

    "bonne chance"

    gg
    :smile:
  4.  
    @Ed Davies. You don't mention what size of array you have based your calculations on. Remember that the UK limits on domestic PV arrays are unlikely to apply in France so the OP may well have the ability to cover their entire annual electricity usage with PV depending on whether or not there are limits on array size there and the space they have available for a suitably oriented array.
    • CommentAuthorEd Davies
    • CommentTimeFeb 5th 2017
     
    Posted By: Chris P BaconYou don't mention what size of array you have based your calculations on.
    Posted By: Ed DaviesWith default parameters…
    PVGIS defaults to 1 kW so the calculations were kWh generated per kW of nominal PV so effectively equivalent hours of generation per day. The main thing, though, was to get an idea of the summer/winter ratio of insolation for solar thermal [¹].

    (AFAIK there are no UK limits on domestic PV arrays other than 50 kW to be considered “micro-generation” for permitted-development purposes. That's a) pretty academically large for domestic installations and b) only for PD so not likely to be relevant to a new build where the PV would likely be included in the planning permission anyway.

    There is the 16 A/phase limit which it can be difficult to convince the DNO you'll stay within if you're connecting all your panels to the grid and they're much larger than 4 kW and you only have one phase. You need a diverter they approve of to do that. Also, there's the FiT value break at 4 kW but that's also pretty academic now even if it still exists, I'm not sure and don't care enough to look.)

    [¹] approximately due to temperature effects on PV and different absorption of oblique sunlight by PV and solar thermal (particularly evacuated tubes).
    • CommentAuthorEd Davies
    • CommentTimeFeb 5th 2017
     
    Posted By: gyrogear: “FWIW, here is a solar DHW calculator for France.
    http://ines.solaire.free.fr/ecs_1.php

    That's potentially rather confusing unless my French is even more wobbly than I think. It's oriented to producing a fixed amount of domestic hot water (DHW) per day so if you say you want 35 °C water (which might be sensible for space heating) rather than the default 60 °C it says you produce less energy on an average January day whereas common sense says you'd expect to harvest more because the lower temperature means lower losses from the panel.
    • CommentAuthorgyrogear
    • CommentTimeFeb 5th 2017 edited
     
    Hmm, I don't like promoting bad gear, Ed (!) so I checked it out...

    If set up for a south-facing collector at 70° tilt, requiring 200 liters of water at 60°C,
    for January, it says the requirement is 367 kWh, and the gain is 102 kWh, so the solar fraction is 27.9%
    (so dividing 367 by 65, he would need 6m2 of collector...)

    same collector set-up, but water required at 30°C,
    for January, it says the requirement is 151 kWh, and the gain is 102 kWh (still...) , so the solar fraction is 67.7%

    (The "IGP" = "irradiance global dans le plan" = "received irradiance for OUR collector plane" (= south & 70°C), which is 65 kWh/m2 for the month of January)...

    (so I think it works...) :shamed:

    gg
    •  
      CommentAuthordjh
    • CommentTimeFeb 5th 2017
     
    Posted By: gyrogearIf set up for a south-facing collector at 70° tilt, requiring 200 liters of water at 60°C,
    for January, it says the requirement is 367 kWh, and the gain is 102 kWh, so the solar fraction is 27.9%
    (so dividing 367 by 65, he would need 6m2 of collector...)

    Am I right in assuming that the figures are for a 1m² collector? If so, why doesn't he need 367/102 m² = 3.6 m²?

    Or am I missing something fundamental?
    • CommentAuthorEd Davies
    • CommentTimeFeb 5th 2017
     
    Posted By: gyrogearand the gain is 102 kWh (still...)
    But if the water is cooler (30 °C rather than 60 °C) we'd expect to harvest more heat because there are smaller losses through the collector.

    Changing this

    “Coefficients des capteurs thermiques : … K = [3.5 ] W/m².°C”

    to zero or 13.5 increases or decreases, respectively, the Apports (Inputs) as expected but changing the target temperature doesn't.

    Posted By: djhAm I right in assuming that the figures are for a 1m² collector?
    The default is 4 m², I think.

    Surface en capteurs thermiques de l'installation : [4 ] m²
    • CommentAuthorGotanewlife
    • CommentTimeFeb 5th 2017 edited
     
    Posted By: DemonRWe'd certainly plan to have a heat recovery and ventilation system, but what is the alternative to a ducted system?
    Don't worry - that's just Viking House trying to sell his FreshR system or whatever it is called now.

    Putting it bluntly: The no way on earth a house
    Posted By: DemonRbeing built with 200mm SIPs panels so will be airtight and very well insulated.

    will new a 15kW wood burning stove!

    To put it into perspective I have a 450m2 house with most of the wall area uninsulated solid stone and half the windows still single glazed leaky 40 year old wood frames without seals and I heat the whole house to 21 deg, and DHW, with a 40kW wood burner which burns for less than 12 hours a day (obviously via a TS).
    • CommentAuthorgyrogear
    • CommentTimeFeb 6th 2017 edited
     
    Posted By: djhI missing something fundamental?


    No, DJH, apologies, but it's ME that is !

    In using the monthly average of 65 kWh/m2, I neglected that he has 4 m2 of collector, as Ed correctly states, and I *also* neglected the collector efficiency, which is 75%.

    Therefore per my figures,
    the received energy is 4 x 65 = 260 kWh per square metre over the month

    and the useful collector output (energy input = "apport") = (260) x collector efficiency of 75%
    = 195 kWh

    However, the exchanger and storage efficiency is only 65%
    so after the collector he actually only gets a useful power input of:

    195 x 65% = 126.75 kWh

    Monthy DHW req't = (200 l x 60°C) = (12,000 kCal) x 1.1622 = (= 13.946 kWh) per day x 31 days = 432.33 kWh

    So according to me the solar fraction ("couverture") is 126.75 / 432.33 = 29.3%

    This is obviously at odds with the computer ! (which obviously takes account of added factors of air and water temperatures...)
    :cry:
    gg
  5.  
    Putting it bluntly: The no way on earth a [new] house
    will new a 15kW wood burning stove!


    Exactly. My ancient (1898)sparsely insulated house in Montreal has a heatload of 15kW when the temperature is -23C outside (and 21C inside). This is for 190M2 livable area plus 95m2 heated basement.

    Paul in Montreal
    • CommentAuthorDemonR
    • CommentTimeFeb 9th 2017
     
    Unfortunately, the Waltherm Zebru seems to be the only avaialble gasification boiler that can also provides radiant heat into the living room. The insulated version offers a nominal Output to Water of 12kW and a nominal Output to Room of 2.1kW. so I was imagining that during a really cold period (when one would actually want a fire in the living room) all the heating for the house could be done with a firing of just a few hours in the evening, with most of the going into the thermal store, and a bit warming the room in the evening. I was thinking - correct me if I'm wrong - that as long as the thermal store was of adequate size, what mattered was how long the boiler was fired for, not just how much heat it produced in any one hour. So we'd head for bed after a coupla hours at the fireside, with a single load of wood now burnt down and a fully-charged thermal store ready to provide the heat overnight and during the following day.

    Thanks for all the solar capture calculations, BTW - I'll try and get my head round all that next!
    • CommentAuthorringi
    • CommentTimeFeb 9th 2017
     
    A output of 2kW to the room will overheat most rooms if you have a reasonable level of insulation.
    • CommentAuthorDarylP
    • CommentTimeFeb 9th 2017
     
    ...and air-tightness!:cool:
  6.  
    The bottom line is if want a wood burner to heat the TS then you can't have one that heats the room as well - you have to have one that goes in a plant room......but you will be using a GSHP so such a system would be rather silly. If you want an occasional fire for an hour or 3 on cold nights and/or for aesthetic reasons then you want the smallest, simplest wood burner you can get (about 4kW) with none of that expensive, ugliness and and space consuming heating of water built in.

    However if you are remotely serious about building a well insulated house with good air tightness then you will always feel cosy and warm and end up using the wood burner less than 5 times a year in the first year and less thereafter. Also remember that really cold nights mean lots of sun in the days = loads of solar gain.
    • CommentAuthorDemonR
    • CommentTimeFeb 9th 2017
     
    This is helpful. As a long time resident of a leaky draughty six bedroom Victorian villa, I'm probably very bad at judging just how little heat I'm going to need. Clearly I'm going to have to wait until I get the calculation before making decisions. But it sounds like I am going to find myself moving towards a sysytem of solar thermal and GSHP, with a little wood burner for effect (I'm never going to dissuade my wife in this front).
    •  
      CommentAuthordjh
    • CommentTimeFeb 9th 2017 edited
     
    In our well-insulated house, we are forcefully discovering that how much sun there is matters much more than what temperature it is outside. Cold and sunny, we're fine. Overcast as at present and we need some heat.

    edit: to be clear, I have a 400 W heater on at present to keep the temperature in the house up.
    • CommentAuthorgyrogear
    • CommentTimeFeb 9th 2017
     
    Posted By: djhwe are forcefully discovering that how much sun there is matters much more than what temperature it is outside.


    +1 - I have a 7.5 kW wood stove, it's a gem, but it warms like peanuts, compared to my DG Low-E windows ! Once the sun is on the windows, at a guess, it is 4X the power of the woodstove !

    Which is why I just "need" a bit of electric of a cold morning, so I don't have to faff with the stove and the coffee at the same time ! LOL

    gg
  7.  
    Posted By: GotanewlifeDon't worry - that's just Viking House trying to sell his FreshR system or whatever it is called now.
    Its not my FreshR, Vaventis own the rights but I was involved in its development. FreshR is the only Heat Recovery Ventilation system available with a less than 10 year payback on investment. Fitting FreshR instead of ducted HRV reduces the heating demand of a Passive House by 60%, a similar effect to wrapping the walls/floor and roof with 100mm of extra insulation.
  8.  
    Posted By: GotanewlifeThe bottom line is if want a wood burner to heat the TS then you can't have one that heats the room as well - you have to have one that goes in a plant room......but you will be using a GSHP so such a system would be rather silly. If you want an occasional fire for an hour or 3 on cold nights and/or for aesthetic reasons then you want the smallest, simplest wood burner you can get (about 4kW) with none of that expensive, ugliness and and space consuming heating of water built in.
    However if you are remotely serious about building a well insulated house with good air tightness then you will always feel cosy and warm and end up using the wood burner less than 5 times a year in the first year and less thereafter. Also remember that really cold nights mean lots of sun in the days = loads of solar gain.
    +1
    • CommentAuthorringi
    • CommentTimeFeb 10th 2017
     
    Personally I would also give up on the thermal solar and just put on as much PV as possible, then in a few years time add a battery so that for 10 months of the year the heat pump cost nothing to run. (Unless using VH's roof with "free" integrated thermal solar.)

    As to using the FreshR I would consider it if the layout of the building worked well for it, but it start to become very expensive if you need to fit more than two of them due to your layout. (UK building regs are written in a way that gives issues with the FreshR, but you are not in the UK.)
    • CommentAuthorgravelld
    • CommentTimeFeb 10th 2017
     
    What would you do if your building layout didn't work for it ringi?

    I'm asking because I've come to the same conclusion.
   
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