Is an 800ltr thermal store worth the hassle?

[beelbeebub]

Apologies for yet another thermal store question.

I am just starting on a self build 120m2 bungalow.

It will be well insulated and air tight etc etc.

My calculations estimate a heating demand of ~4-5kw in a cold (0 to -5C) conditions.  This will be delivered via UFH

My heat sources are planned to be

* Mains gas boiler
* Wood fire with back boiler (Woodfire 12i, 2kw to room, 10kw to water)
* Solar thermal panels (which we'll ignore for now)

To tie all this together I was planning a small heat store, say 250l.

As I have a good source of free wood I was hoping to be able meet all my DHW/Heating requirements from the wood stove in the winter, relegating the gas boiler to backup/ can't be bothered to haul wood status.

This will work fine in the day but at night the stove will not run through and I'll have to rely on stored heat.

I have also done some calcs to estimate that if I had a 250l tank of hot (75C) water I could heat my house at 5kw load for around 2-3 hours before requiring heat input.  So my gas boiler would kick in during the early morning.

If I could get say 800ltrs I should be able to "ride through" the night without the need for the gas boiler.

It so happens I have a 180gal (~800ltrs) stainless steel tank (about 1mdia x 1m high) kicking about.  It's a old brewing vat.

It has 4 adjustable legs, 1 large port (4 bolt flange ~ 6"dia) and 1 small threaded port at the top and 2 at the bottom (both 4 bolt flange).  It also has a large bolted access hatch (big enough to climb in) on the side.

It's pressure rated, though to what I have no idea, but it should be a few bar.

Obviously it has no insulation, but that could be added later, either spray foam, lagging or just putting it in an insulated box.

I would probably have to use plate heat exchangers as there are no coils, though I could run the boiler/wood stove and UFH direct off the water in the tank and just use a heat exchanger for DHW.  If I wanted I could mount a coil and/or an immersion heater in the inspection hatch door, maybe I could use those retrofit immersion coils from a solar system has a coil in tank heat exchanger?

For the solar I could use a Willis solar syphon or something.

I could even, using the access hatch, put some kind of stratification apparatus inside (say a partition disc like the akvaterm).

The larger tank does pose some problems with fitting, but nothing major at this stage (I can tweak the layout to suit)

An 800ltr akvaterm tank is over £2000 so I could save a little money by going the home brew route.

Anyway, my question to the forum is:

Should I go with a 250ltr "off the shelf" system, or is the extra capacity worth the faffing involved in "home brew" (no pun intended)?

Thanks

[Justme]

I am doing very similar.
96m2 259L store wood fired Rayburn.

Will you really need to keep the heat the same 24/7? Dont forget the 2kw to room heat.

I am planning on having a morning boost for about 1 or 2 hours & to run the rads for 4 ish hours in the evening.

Will you zone the heat so the bedrooms only get heat when needed? Plus you can down the room temps overnight/ unoccupied times.

[dhaslam]

The large tank is nice to have.    The  woodstove might have a problem heating the full tank but it could get some assistance from the gas boiler  at times.     I have an  850 litre tank plus separate 250 litre DHW cylinder.  The larger  tank will be used entirely for  the heat pump and disconnected  fiom the stove if external heat store experiments work out OK.   The stove, nominal heat output to water 10K, was never able to heat more than the top few hundred litres.

[beelbeebub]

@ Justme:

Yes, I will be lowering setpoint temp at night, though the house is likely to be occupied most of the day (work from home mainly), so I may not need 800ltrs, but 250ltrs will def not be enough and I happen to have an 800ltr tank.  I suppose I could reduce the capacity of the tank by bunging in some polystyrene blocks.

@dhaslam

Interesting you say the wood stove struggles to heat the tank.  If I am using the woodstove throughout the day there should be a surplus of 5 or so kw to heat the tank, so about 8 hours of full burn (so 12 hours of less than full burn) should top the tank off. 

Of course this is just theoretical.  I'd be interested in why you thought your woodstove was struggling.

[Justme]

As you will be at home so much I would have the wood stove on more.

10kw stove running for 12 plus hours per day = a lot of heat. Some can go "direct" to the UFH & some stored till needed. Work out your demand for the stoves off period & size the store to meet that.

Then you only need the store to smooth out the supply & demand so that they match.

[wookey]

It's still the case that to make a really good thermal store you need to DIY it, as no-one makes good quality stores with stratifiers for sensible money, so if you have the opportunity I'd go for it. I don't understand why anyone needs heating overnight unless your house is apallingly insulated (in which case fix it), but I guess it takes all sorts. Is your duvet not thick enough?

[Baz]

Since you mentioned self build just design in some solid brick walls to provide thermal mass that will provide the overnight heat. 2m3 solid brick equates to 1m3 water which holds 1kWh per degree.
Then use the store just for the morning untill you get the fire lit. You can't have too big a store, as the weather improves you just run the sove on alternate days.

[djh]

I am just starting on a self build 120m2 bungalow.

It will be well insulated and air tight etc etc.

My calculations estimate a heating demand of ~4-5kw in a cold (0 to -5C) conditions.

4 kW in 120 m2 is 33 W/m2. Just for comparison, the upper limit for a Passivhaus is 10 W/m2. So you certainly appear to have scope to increase the insulation. And the old mantra says insulation, insulation, insulation ...

In a well-insulated house, as long as it has some thermal mass like Baz says, the thermal time-constant is so long that it won't go cold overnight (you compare how fast you're losing heat - 5 kW - and how many kWh/degree of thermal mass you have to see how fast it cools down). So as wookey says, you shouldn't need any heating overnight.

It so happens I have a 180gal (~800ltrs) stainless steel tank (about 1mdia x 1m high) kicking about.  It's a old brewing vat.

Anyway, my question to the forum is: Should I go with a 250ltr "off the shelf" system, or is the extra capacity worth the faffing involved in "home brew" (no pun intended)?

I'd go along with wookey - just do it. Ultimately, it depends on your attitude to faffing.

[dhaslam]

Interesting you say the wood stove struggles to heat the tank.  If I am using the woodstove throughout the day there should be a surplus of 5 or so kw to heat the tank, so about 8 hours of full burn (so 12 hours of less than full burn) should top the tank off. 

Of course this is just theoretical.  I'd be interested in why you thought your woodstove was struggling.

As far as I can see the reason why the stove wouldn't produce enough heat  was  the  limited surface area of the boiler.   My previous stove had  a much larger water jacket, something like eight times larger  area exposed to  water and the heat  output was roughly eight times greater as well.    The other problem was that running the  stove all day would consume  30-40 kilos of coal or  about 50 kilos of wood just to produce about  40Kwh of heat, some  of which was wasted overheating the room.   Conesequently it normally ran only  for about six hours in the evening.    I am reluctant to replace it with another boiler stove that may not do much better.  In any case solar heating  would cost a lot less  and take a lot less work. 

[djh]

Actually, one qualification to my 'Just Do It'. If you believe that RHI will survive and be implemented  and you think you could qualify, then you may want to get MCS-approved products installed by MCS-approved installers   I doubt your brewing vat would qualify 

[Baz]

For comparison my normal 60's bungalow with cavity wall insulation and only normal other features barely drops 4 degrees overnight worst case but benign Home Counties weather profile.
If the store has to be MCS certified you might need a 100l one and a piece of uncertified 'pipe' of 800l capacity.
With your own design you have the opportunity to be more inventive. For example if you can accomodate an 18 in thick dividing wall this could become a big flat tank. Don't forget to incorporate all wiring in hollow skirting and ducts so that it is easy to add fibre optic broadband and multiple voltage supplies in the future.

[beelbeebub]

Regarding insulation, I agree, insulate first.

I'm not sure I can put much more insulation in, I've got 200mm of rockwool (or similar) in the walls and 200mm of wool plus 40mm of XPS in the roof.  The floor has 150mm of XPS, the windows are all between U(w) of 1.3 and 1.8.  

I don't want to go any higher on the cavity thickness as it starts to give me stupidly thick walls, my little bungalow will look like a fortress!

The target air tightness is less than 0.6ac/h

When run through the SAP model (ok not perfect but at least give an indication) it scored 89, without any PV or wind whirygigs.

As for thermal mass the walls and internal walls are block work (although probably aerated blocks, mainly to reduce the chances of back problems) and the floor will be 100mm of exposed screed.

So the house will have a large thermal mass.

The 4-5kw figure comes from a crude "fag packet" calc.  The goal is to find the limiting case and ignore the slight intangible (i.e. difficult to calculate things) effects that might make things better.

So things like, lower internal temp at night (although the external temp is also lower so the delta T may stay the same or increase), wearing a jumper (which I intend to do), incidental gains from people, lights etc, are all ignored.  If my calcs show that I can "ride through" the night in the worst case, then in reality I should be able to do it easily.

Packaging the thermal store is easier with an off the shelf system, the stainless vat is a real pig to fit in (doesn't fit through doors!) and the tinkering aspect is vaguely appealing, although less so in the dead of winter when the system stops working!
 

[dhaslam]

I think you will get away much lighter than 4-5kw peak heat requirement.  A lot depends on passive heat, mainly from south facing windows in winter.   My bungalow is about 100sq metres bigger but with similar insulation and the peak heat requirement is 1-2 kw. My calculation before building were much higher.   
   

[Justme]

You can't have too big a store,

Dont be mad, of course you can. What use is 1000L of luke warm water? It might hold the same stored energy as 250L of hot water but its not going to be much actual use is it?

Do the sums & work out the size needed to store the energy you need to store. Then make sure that your heat sources can provide that amount of heat.

A big store on a small boiler or solar array would be mad, a small store on a big boiler would be mad. Size it right not just big is better.

[djh]

Regarding insulation, I agree, insulate first.
I'm not sure I can put much more insulation in
The target air tightness is less than 0.6ac/h

Sound like you're serious 

When run through the SAP model

Get yourself a copy of PHPP and run your design through that. It'll give you a better idea of performance.