What size cylinder???

[Stochengberge]

Greetings.
We currently have an open fire which we want to but a boiler stove into - more efficient than an open fire, no downdraught's when it isn't lit and "free" heat to the Domestic Hot Water Cylinder.
The Wood Burning Stove will be for weekends and occasional evening use, not run all the time, with a 6-8kW boiler capacity. The DHWC is a 300 litre triple coil affair, with solar already on the bottom coil and the oil boiler on the top. The WBS will eventually connect to the middle coil which is intertwined with the solar coil, heating from the bottom of the cylinder. The trouble is that whilst the WBS is on the ground floor and the DHWC is on the first floor, there is a 9 meter horizontal difference between the two, so there isn't a cat in h3lls chance of thermo-syphoning... And no, we can't move the DHWC!

So, the idea is to put a buffer vessel in above WBS, and use it as a small thermal store. Thermo-syphon up to the store, then when it gets to a temperature that is something like 8 / 10 degrees above the temperature of the bottom of the DHWC a pump is run to transfer the heat - at this stage, no coils are needed in the store as the same water can circulate around the WBS, the store and the DHWC coil. Once the DHWC gets to temperature a 3 port valve transfers the heat to a heat dump in my playroom (aka, the garage!), and some rads in the house.
As part of the circuit will be in the garage (integral to the house, but still a garage), Sentinel X500 (antifreeze) will be added.
We do get power-cuts - so need to retain the WBS as a source of heat when the lights go out...

[u]Question[/u] - What capacity does the thermal store need to be???
I haven't got a huge amount of space (the cupboard it has got to go in is only 450mm deep) and if it's too big, it will be slow to respond and provide heat to the DHWC. Too small and it may try to boil...
As a backup, I may elect to use an indirect cylinder as the thermal store and a Honeywell TS130 valve. If the Thermal Store gets to 95', the TS130 opens, and cold water can be passed through the cylinder coil, dumping itself out into the garden. A waste of water, but that has got to be better than having a bomb in the house! Or is this overkill on an already harebrained scheme?!?!?!?!


Thanks in anticipation.

[dhaslam]

I have a  buffer tank that  is nominally 1000 litre but actually holds about 860 ltres.   Of course it is very, about two metres tall and  about 800 mm diameter plus 100mm insulation  all round.      Originally all the outputs were through one pipe at the top but I have now split the output  so that the DHW is transferred from the top  and the heating from two thirds way up.    One thing that I would change is to put the return from the DHW  tank  near the top because the constant pumping  destroys  the stratification.   

My old stove, rated as  9 kW to water only produced about 2 kW so it couldn't heat the water  down to the  underfloor heating take off point, the heating  circuit just had low temperature return from  the DHW coil.   The new stove , rated at 12 kW outputs about 7 kW to water  so it  heats the top quite quickly.   It would add 20C per hour to the top third if it wasn't for  the transfer to the other cylinder.   

You need to  fit as large a tank  as  can fit , allowing space for access and insulation but  it  seems like it can only be a few hundred litres unless it is oval or rectangular. 

It would be better to at least have the possibility  of supplying space heating   before the cylinder is hot all the way down.      Today for example my DHW cylinder was almost completely heated by the solar panels so the buffer tank is  mostly just supplying heating but is also building up the store for tomorrow.

[Countrypaul]

You have not mentioned the use of a load valve/pump such as the Laddomat. Is there any reason for not using one?
Not sure on what would happen in teh case of a power cut though.

Paul

[clivejo]

The trouble is that whilst the WBS is on the ground floor and the DHWC is on the first floor, there is a 9 meter horizontal difference between the two, so there isn't a cat in h3lls chance of thermo-syphoning... And no, we can't move the DHWC!

Is the 9 metre stretch totally horizontal, with no dips?  For example running the length of the landing.  I have seen this working as a thermo-siphon.  The WBS is on ground floor. Pipe exits and goes straight up to a 90 degree angle, travels about 10 metres totally horizontally (around a stair well) then into the HW Cylinder.   Return follows same path.  Maybe there is a BS or EU standard in place now, but I've seen this working, your pipes need to be well lagged however!  I did notice that in this installation only the flow was lagged.  The return heat-loss was classed as useful heat!  i.e. underfloor heating 

[Baz]

Yes there is no reason that distance won't syphon provided you go straight up out of the boiler. You need 32mm pipe min for the syphon loop (that's regs). My heating revamp will use that sort of distance for the backup syphon loop though I'm planning to use a charge pump (laddomat syle from bits) since I want to have a ground floor only circuit too.

[Stochengberge]

Chaps,
Thanks for taking the time to profer an opinion...
In summary - this is only going to be a weekend / occasional weekday system and having heard too many horror stories with the likes of Ladomats & DB Neutralizers, I have elected to avoid them, as well as the cost implication. And we would still have the same problem with being Power Fail safe...
I'm not too keen on the idea of cutting a pair of holes 32mm+ in the joists under the landing to hope I can get Thermo-syphoning to work to the DHWC. Also the route involves breaching a structural wall already, so I have no guarantee I can get the continuous rise required especially as there are other postings on here saying any hole drilled into a joist should be on the centre-line...

So, I'm currently back to square one of using a buffer vessel. I would love it to be a nice big one, but as I said earlier, I am tight for room. The question I should have asked is "How small a vessel can I get away with"?
Newark do one that is 350x1600mm, add in the 50mm insulation and I'm at 450mm, which is exactly the gap I have. This gives a capacity of 141litres. Is it enough? If the log burner is lit and the power fails will be enough to not boil? (I know it will boil if left for long enough with enough jheat, but there will be standing losses in the system.)
This vessel is going in my daughters Nursery, so I can't have a heat leak radiator in there, as the temperature in this room obviously needs to be controlled. (Part L lagged cylinder should have sufficiently low standing losses not to be a problem.) The only place I can get a Heat leak radiator in is in the loft, on the Expansion circuit under the tank, before it turns back over into the tank.

I have read somewhere that when sizing a Thermal Store, it should be able to absorb half the fuel value of the boiler:-
1000 gallons of water can store 600,000btu's;
I'm looking at an 8KW boiler which equates to 27000btu's.
Therefore, 27000/600000btu's x 1000gallons = 45 gallons.
Half of this gives 23 gallons, or 100 litres. ie, my 141litres is plenty.

I know I'm not actually sizing a Thermal Store, but in the absence of any other scientific facts or formulas, it is about the best I can come up with, unless anyone has any better ideas...
Or have I answered my own question?!?!?!?!?

Also, as an aside, does anyone have any recommendations on whose cylinder to use, or not? Newark, Telford, Gledhill, Heatre, Biasi, the list goes on. Apparently I can't go Stainless as they can't make them narrow enough...


Many thanks.

[Brandon]

i have ordered yet another newark today, they are great pieces of kit.  I would not have a telford.