121 m square, 500w, is it possible

[camillitech]

Having spent most of my school years messing about I'm not very good with sums  and having a pathetic internet connection combined with the attention span of a gnat I thought I'd ask you guys 

We're about to build a 121m square single storey super insulated house with UFH which I initially wanted to heat with wood, dumped power from the wind/hydro and perhaps a gas back up.

However much as I love wood stoves having used them for twenty five years and having limitless birch on my doorstep I'm not getting any younger and this house is to spend my dotage in

So what I'm investigating is using what I already have and what's available nearby. What I've got is a 1500lt fiberglass tank and a heat exchanger that will fit inside it. what I am about to embark on is another hydro project with a 500w 24/7 potential, probably much more over the winter.

Is it possible to heat such a house with 500w 

Cheers, Paul   

[stephendv]

Depends on your definition of "super insulated"   If you build it according to passivhaus standards it will require 15kWh/m2/year to heat = 1815kWh for the year.  Stab in the dark that you need heating 6 months of that year, so that's 180 days = 10kWh/day = 420W constant power.  And that's not counting with the dumping from your wind turbines, and you could always supplement with a small wood burner if you really, really need it.

If you model the house using the PHPP software you'll get a much better month by month view of when and how much heat you need.

[billi]

Paul

a small heatpump perhaps that draws 500 watt and generates 2-3 times heat

[HalcyonRichard]

Hi,
    This cold winter my consumption on the worst days was about 100kWh of gas, this is 4 kW constant input. Assuming the boiler is 75% efficient this is 3kW constant for the worst days. The bungalow is about 120 m2. It is double glazed, loft insulated, cavity insulated, and partially under slab insulated. I think with a very expensive update I could probably halve this. So 1.5 kW constant input. So it is in the ball park to heat most of the winter with the worst days needing a top up. You really need to quantify two variables.

1/ What will be the actual energy input on average and maximum per day.

2/ The actual energy generated - "probably much more over the winter" could mean you easily cope or you need a small top up.

You are very lucky - I'm turning a little green with envy 

[Baz]

Don't forget solar gain, even where you are, but then another power drain is DHW. If you insulate internal walls too you can arrange a core that needs far less for extreme weather.

[camillitech]

Hi Stephen,

I'm going for 500mm equivalent bin the roof and 300mm in the walls, only one door, traditionally proportioned windows and no silly patio doors. I know I may loose out on solar gain but draughts and wind chill are where most of our heat will go. This is why I'm reluctantly ditching the wood burner  no chimney and one less hole and thermal bridge. There will be more than the 12Kwh per day available as I'm planning 2 to 3 Kw of PV and another 1kw turnip  but the surplus from these may come in fits and starts, which is why I was going to make a thermal store with these.



 

[A.L.]

hello,

You will need excellent air tightness, perhaps 2m³/hr/m²@50Pa pressure (about 0.1ach unpressurised), and a  good mvhr. The house will self-heat down to perhaps 7°C and need 80W/°C below this

[camillitech]

Sorry chaps, just been caught out be the friggin stupid no preview and limited time to modify a post bawloxx, I didn't mean to post that 

I meant to post something completely different and I'm now really pi55ed off having spent hours writing and inserting pictures so I'm going off in a huff 

[MarkB]

Is it possible to heat such a house with 500w 

If 500W is the max that is available then the important point here is the peak load (which could be sustained over a few days).

Some crude assumptions just to get a general feeling:

  The house is square - minimal surface area for given floor area
  The house is on one story - as you state
  The roof is flat - minimal surface area for given floor area
  The external walls are 3m tall (2.4 ceiling, plus floor slab plus roof structure) - will almost certainly be more
  The house has no windows/doors - unrealistic, but unless massive solar gain the best case from heat loss perspective
  There are no thermal bridges
  The house is perfectly airtight and ventilation losses are zero - again unrealistic, but let's run with this for now
  Assume insulation throughout to 0.1W/m2.K
  Let's consider worst case ambient air temperature is -10C, ground 10C internal 20C - keeps maths simple

Wall area: 121m2 works out as 11m on a side. Total wall area becomes 11m * 3m high * 4 sides = 132m2
Roof area: 121m2
Floor slab area: 121m2

Wall and roof are at ambient temperature, so let's consider them together = 253m2
  Losses = 253 * 0.1 * (20 - -10) = 759W

Floor slab
  Losses = 121 * 0.1 * (20 - 10) = 121W

Total losses = 880W.

Already we're more than 500W peak, and this is likely to be a very optimistic scenario unless you can design to achieve a serious amount of passive solar gain.

Even at more optimistic external temperatures of 0C you're still looking at 620W.

Don't forget you need DHW as well?

If you want to stand any chance of making this work then you need the very top spec windows and doors, they need to be positioned to maximise the passive solar gain, you need an excellent ventilation system with heat recovery and you need a design with minimal thermal bridges.

One alternative is a large interseasonal store, but I really can't say I've seen anyone make these work effectively.

PHPP is strongly recommended to get a more accurate feel for the peak energy losses and to help you undertsand the trade-offs needed in the design process.

[dhaslam]

You would probably heat the house with 500 watts but it won't heat DHW as well. When you insulate a house very well  the south facing rooms will be warm  but north facing rooms  will need some heat.  I have two north facing bathrooms and they take most of the output from the heating system.    In the coldest weather  the heat pump was off and the bathrooms were heated by  600 watt towel rails running all night.    The towel rails are  dual mode but not yet connected to the heating system.   

[camillitech]

Thanks guys,

I'll try again now I've taken several deep breaths and vented my rage on a rather large gearbox  

The 500w is worse case scenario, and apart from possible freezing, guaranteed. I'm trying to utilize stuff that I already have (if possible) and this,



will fit in that tank nicely giving me just short of 1500lts of storage, I'm not sure how hot it needs to be and the calculations I did came up with between 96hours and 10 days to heat it   but then I wasn't sure how much it needed heating or how much the heating would extract from it  

Baz, DH,

I appreciate that solar gain is important but it's the west coast, exposed to the north and west with little sun at house level for the coldest months so I'm going for traditionally proportioned windows and no silly patio doors. To the uninitiated in 'wet coast' weather this may seem odd but I spend my whole life looking at stunning views, am rarely in the house during daylight so I'm quite happy with smaller 3g windows and well insulated walls.

Mark B,

The dearly beloved wood stove will be going   in the effort to reduce draughts and thermal bridging so an MVHR is on the cards and only one exterior door is planned. What's PHHP by the way.

Billi,

having mistrusted anything that claims to give out three time what you put into it I've never really done any research on heat pumps, any links  

So freezing aside 500w is worst case scenario from the hydro alone ( a 75m head system 1500m away but that's another story) there should be 850w to a kilowatt in the wetter months depending on what cable I use. This turbine will be connected to the thermal store purely for heating the UFH and will be independent of both DHW and the current power supply.

I'm planning 2 to 3Kw PV plus tubes on the SW facing roof and an extra 1Kw turnip and hoping these will between them provide DHW, the shortfall being made up by immersions from my Lister, though Billi will be pleased to know that my aim is to make him redundant  

I don't know if I'm asking the impossible as I've no hard and fast data for what I produce at the moment from the Proven and Stream engine. To give you some idea when I left the house this morning I'd dumped 138Kwh since Tuesday at 18:30, about 12Kwh per day on my fingers   so do you think another turnip, tubes and some PV would be enough to do DHW  

I know it's a bit vague chaps but if it won't then I have to start thinking where to put the chimney that I told my architect I didn't want  

Cheers, Paul
 

[MarkB]

Mark B,

The dearly beloved wood stove will be going   in the effort to reduce draughts and thermal bridging so an MVHR is on the cards and only one exterior door is planned. What's PHHP by the way.

PHPP is the PassivHaus Planning Package. It's essentially a clever Excel spreadsheet into which you can enter all your design data and it will estimate your losses, energy usage, peak heating load etc.

It's a little daunting to start with, but if you have any sort of engineering/analytical mind and can drive Excel to a basic level it is very powerful. It's much more accurate than SAP and easier to use than most thermal modelling solutions.

More details: http://www.passivhaustagung.de/Passive_House_E/PHPP.html

Demo and ordering details here: http://www.passivhaus.org.uk/index.jsp?id=672

It's not cheap at first glance, but easily worth its money in the long run.

I'd also strongly recommend joining the AECB. Membership gives you access to some additional forums where you can ask questions about how to enter specific details into PHPP and discuss trade-offs.

If you want to avoid any thermal bridges then this book is also useful when you get to the detailed design stage:

http://www.amazon.co.uk/Passivhaus-Bauteilkatalog-Details-Passive-Houses-Konstruktionen/dp/3211994963/ref=sr_1_1?ie=UTF8&s=books&qid=1300022447&sr=8-1

Mark B,
I'm planning 2 to 3Kw PV plus tubes on the SW facing roof and an extra 1Kw turnip and hoping these will between them provide DHW, the shortfall being made up by immersions from my Lister, though Billi will be pleased to know that my aim is to make him redundant  

If you are putting up plenty of tubes then consider getting them close to vertical (ideal is lattitude plus 20 degrees to horizontal) to maximise winter gain. You'll have more than enough in summer, and will want to avoid having to have a heat-dump.

[Baz]

I think the fibreglass tank is an economy too far. It won't last into your dotage plus the temperature is limited. However you have an abundance of rocks so why not use them as a store. Given that your energy source is electric you can heat rocks to twice the temperature of water. By volume rocks have half the storage per degree but the high temp makes up for it.
Solar gain can be achieved as a side effect of your lean-to greenhouse and conservatory. You will want a conservatory so that you can sit in the sun in your rocking chair out of the cold wind.

[skyewright]

I'm planning 2 to 3Kw PV plus tubes on the SW facing roof

On the PV side, is it safe to assume you've tried plugging your figures into PVGIS?

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

If you've not used it before, opt for "Climate SAF" in the "Radiation Database" dropdown, and 10% as "Estimated system losses".

With our WSW facing array (installed last August 30th), so far every month has been above the PVGIS estimate. February was only just above (101%). So far March is running at just 59% (has been as low as 51%), but experience so far is that sun is a bit like wind, i.e. you don't see many "average" days, instead you get quite a lot of below average ones, plus a few sparkling days that really boost the figures. So far this month we've only seen the estimated average on 3 days.

I post our monthly generation figures to BDPV (http://www.bdpv.fr/fiche_utilisateur.php?util=skyewright N.B. The inverter is actually a 3.6 not a 3.0). If you'd find more detail useful just let me know and I'll sort some out for you.

If/when we get around to putting up some tubes, my current thoughts trend to doing as MarkB suggests and put them vertical (or nearly so) on our SSE facing gable end.

[HalcyonRichard]

Hi,
   If you are going this high a standard in insulation then normally insignificant heat inputs become significant. An adult will "output" about 100 watts two will be double this with no physical activity  . Also most if not all electrical consumption ends up as heat. My 4 kWh electrical consumption per day is equivelent to 166 watts continuous. Also as stated earlier solar gain can be considerable. I personally would have a couple of small wood burners as a last chance backup/top up. These could ease the design constraints. Covering 90% of heating needs with the occasional use of wood burner(s) is a lot easier than trying for 100% coverage without them. Also reliability could be a problem. If an inverter or turbine fails it may take days to sort out. The stoves would cover for however long it took to repair any system breakdowns.

Regards Richard