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?

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.

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

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.

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.

@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.

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.

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?

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).

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

A output of 2kW to the room will overheat most rooms if you have a reasonable level of insulation.

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.

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.

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.

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.)