Are you just concerned with having hot water and a heat dump rad for safety or do you want to run other rads for heating as well?
If you are space heating with rads as well as hot water then all the rads can function as a heat dump and if you balance the pipes then everything will get hot. Without electricity the need for heat dump changes since you are not designing for safety in the occasional power cut but rather the norm!
If you only want to heat the water and have a bit of warmth whilst taking a shower then you can control the heat output by forethought when loading the stove, the need for a heat dump in this case changes to be almost unnecessary.
The design of the circuit will depend on the levels, stove on ground floor with tank upstairs or in loft gives one design, all on same level gives another design (and other problems)
By balanced pipes I mean if you come out of the stove with 28mm then tee off with 15mm for a rad 1 then continue with 22mm The next tee would be 15mm for rad 2 and continue with 15mm through the last rad with 15mm out. When picking up the return from rad 2 go to 22mm then pick up the return from rad 1 and go to 28mm back to the stove. This devides the water and therefore the heat equally (assumes equal sized rads). Variations can be made according to the size of the rads but always try to balance the flow and return pipe sizes to share the water (heat) to the size of the rads.
I don't know where you are but depending on location, depth of pocket and skill level then black iron pipe work might workout much cheaper than copper.
A bit more info about what you are trying to fit up would be helpful.
Peter

You don't need any control really. The circuits will work in parallel. Turn the radiator off if you just want hot water.

ListysDad - You can drop below the level of the stove return (i.e. below the floor) but only once. That is if you drop below the floor to get to the rad then the stove return height would have to be above the output of the rad. A gravity circuit can go up down up, but it will fail if it goes up down up down. If you imagine the circuit as a circle - even if mis-shaped - then the stove can be anywhere on the circle except the top, however the closer to the bottom the better it will work. The pipe connected to the top of the stove determines the direction of flow. As Sune said you want your flow and return pipes to rise from the stove, this is the ideal but if this is not possible the return pipe can drop below stove and then come up to it.

In theory you get better performance from a rad if it is top in bottom out but its only marginal unless you are dropping the supply from above. It is also better to have a cross flow rather than in and out on the same side but this will also work.
All pipes on a gravity circuit must slope but bear in mind the comment above regards direction of up and down slopes. this is to help flow but more importantly to avoid any chance of air locks as this will stop gravity circulation dead.

The advantage of down sizing the pipe work as the circuit progresses and then up sizing to balance the return is primarily one of cost. the difference between 28mm , 22mm and 15mm copper is significant! Down sizing works well especially if the vertical rise of the circuit is as much or close to the length of the horizontal but can cause problems if the circuit is marginal.

You can prioritise the DHW by circuit design. The ultimate prioritisation would be to put the DHW and rad in series, the first in line takes all it wants! otherwise if the DHW tank is above the rad then tee off below the tank input to the rad, the lower the more prioritisation will be given to the tank, especially if teed off in a smaller diameter pipe.

With only one or two rads and a DHW tank I would not have any controls at all, control is by the amount of wood you put on the stove, but definitely put a pressure relief valve on the stove output. If the expansion tank is not connected to the highest point on the system (probably the DHW tank input) then a bleed valve will need to be installed at the high point.

As it is a small system, if the stove is not too large an output to water and you have at least as much vertical as horizontal then 22m should work, much cheaper than 28mm. Swept elbows and tees will help if you can get them.
Peter

Do you need heat in the area of the heat dump rad. If not then the heating rad can function as a heat dump. Given the ratio of vertical to horizontal it might be better to mount the heating rad verticaly (i.e. 90 deg from its design) this will raise the start of the heat loss area to above the stove and thus aid circulation. If you do this then a non finned rad would be better . Also given the ratio of V to H I would not put the heat sink rad in series (assuming you need it) but tee off before entering the tank creating a heating loop with both rads as heat sink. The DHW circuit would be stove - tank - return to stove, the heating circuit would be tee off DHW circuit - rads - return to stove. Assuming the DHW circuit is much shorter than the heating circuit this will give priority to the DHW
Your proposal above suggests both rads in series which would give no to minimal heating until DHW is done.

Plastic pipe can be used, the problem with plastic pipe is that it sags when hot, this means that air locks can form and with no pump they can not be got rid of. The way around this problem is to support the pipe along its whole length by say fixing a tile batten to the wall and laying the pipe on this. Don't forget all pipes must slope to aid circulation and to be self bleeding.
Peter

The control for the DHW temp would be the loading or / and control of the stove. If the stove is shut down a bit or has a partial load in it then it will produce less heat, this in turn will produce less flow on the gravity circuit all of which means less heat to DHW and rads. When the DHW is at the same temp as the stove then all the heat will go to the rads.

Wood stoves should not be run completely shut down as this will cause excess pollution and the tarring up of the chimney. However you can run the stove on partial loads or partially shut down without too many problems as gravity tends to be self regulating and the circulation will shut down as the temp falls so you will not over cool the fire in the same way as with a pumped system.

What do you mean by a std. dia. coil, dia. of the coil or dia of the pipe from which the coil is made. Most coils are made from 1" pipe but even if it was 3/4" it would still work similarly if the coil was a bit shorter it would also still work, just that in either case the DHW would just take a bit longer to get to the same temp as the stove and whilst it is taking its time more heat would go the rads.

Use a blending valve for all the DHW. They are normally set for 60 deg. and you really don't need even this temp. for a shower, otherwise you will come out looking worse than a lobster! In fact even with a blending valve it is worth having a thermostatic shower tap.

The temp you can get the water to will be high, up to boiling depending on how you design the system. The rads should match the output of the stove so this should stop overheating to boiling point or steam. But in any event the running of the system is in your hands. it would be silly to have the whole system humming at about 95 deg and then put a full load on the stove on max throttle. Experience will show how long things take to heat up and what sort of fuel loads go with what building temps.
Peter

Thanks Peter.

<blockquote><cite>Posted By: Rachel Parry</cite>The water is coming from the mountain and held in a tank which is higher than 30 feet above the cabin. Water pressure is low to medium - i was hoping to avoid installing a cold water header tank ...would this work ?
</blockquote>
30ft gives you about 1 bar of pressure. It sounds like you will be using the mountain tank as the header tank for the heating system. I don't think this is a good idea, it would be a risk of blockage if the pipe froze or some other contamination blocked the pipe and you lit the stove - then BANG! Also you would potentially be mixing the heating water with the potable water because of the single feed pipe. If its not for the heating system but just for the potable water then OK - but then some questions about the water source. Washing up water and shower water should be drinking quality. Is the water source a piped spring (best) an open spring (a bit dodgy) or a stream (not a good idea as dead animals can end up in streams)

<blockquote><cite>Posted By: Rachel Parry</cite>problem 1 = space. there is no attic, the ceiling is low and there is very little 'spare' space.
I was hoping that the hot water cylinder might be able to go under the work-surface in the kitchen area(i.e. NOT installed a position higher than the stove). The pipe would be able to go upwards from the back boiler for about 4 ft and then down into the cylinder..would this work ? if so where would the pipe tee off to the heat dump radiator ? and what stops the water getting stuck at the high point of the pipework?</blockquote>

That's a bit marginal. You would have to tee off at the high point to the header tank otherwise you will never get rid of the airlock. The success of the circulation would be in getting enough heat loss in the outflow pipe to get a flow started. The problem with the heat dump rad will be that if it is higher than the tank then the rad will have priority and you might never get hot water in the tank. Classically the rad would tee off just after the stove. In theory you could put the rad after the tank and in series, horizontally mounted, bottom entry and exit as this would give the priority to the DHW but with such a marginal setup I would be nervous about it working properly.

It would be better to have the tank input above the stove. One option could be to dispense with the heat dump rad and replace it with the cylinder (If you can cope with it sticking out more than a rad) and remove the insulation from the bottom part of the cylinder. This will give you hot water at the top of the cylinder warm at the bottom and the bottom of the cylinder also functioning as a heat dump surface.

The plastic central heating pipe we have here is rated to 110 deg. (I think) and is either weld or compression fittings, but here I would reckon that copper would be easiest. The problem with iron pipe half way up a mountain with no electricity is that it is more difficult to work with without power tools. Also the availability of both materials and tradesmen who can and are willing to used iron pipework may be an issue.

BTW where is the stone cabin?

thanks Peter ( Sam I think you are responding to someone elses comment - i am not considering using plastic pipes).

It sounds from what you are saying that I should install a header tank (appropriate height for copper cylinders.)

I think there is a possibility to put the HWC in the shower room, hung on the wall, this would mean it would be higher than the stove. Am i right in thinking that the 'radiator heat dumper' would have its own closed circuit from the HWC and back ? If so am i right in thinking the pipework needs to go up and around like the stove circuit ? ( Not sure if this would be possible with the new height of the HWC if the out pipe has to come from the top of the HWC.

I still don't fully understand the size requirement for the cylinder. I understand one person uses about 50 a day ? How much more does the cylinder have to hold on top of this ? (ie can I get a 60 capacity cylinder -12 in diameter ) or would it work better to get a 15 x 36 in one ?

Crusoe - Rachel said

Posted By: Rachel ParryThe water is coming from the mountain and held in a tank which is higher than 30 feet above the cabin. Water pressure is low to medium - i was hoping to avoid installing a cold water header tank ...would this work ?

Rachel - The amount of water you store and heat and store will depend upon usage. OK one person might use about 50 a day, but this is for 'normal' living. Is your small stone cabin for extended stays or occasional use. As regards DHW if you count 25l. per shower and you want successive showers then you will need a bigger tank than if you are prepared to light the fire to heat the water between showers or to ensure shower + washing up.

A couple of questions - is this in the UK, and as the water is coming from the mountain note that washing up water and shower water should be drinking quality. Is the water source a piped spring (best) an open spring (a bit dodgy) or a stream (not a good idea as dead animals can end up in streams) If none for these what sort of water is it.

Posted By: Peter_in_HungaryThe amount of water you store and heat and store will depend upon usage. OK one person might use about 50 a day, but this is for 'normal' living. Is your small stone cabin for extended stays or occasional use. As regards DHW if you count 25l. per shower

The temperature is important,
25lt at 80°C and 50lt at 44°C, assuming incoming water is at 8°C amounts to the same amount of energy. You have to mix both with cold water to get a lower shower temp (this assumes no tank and pipe losses).

What this amounts to really is how much water can you heat, to what temperature and in what time.

So say you have a woodburner that is 5kW that has a 2kW back boiler that can deliver water at 90°C.
And you are willing to allow for 20 minutes initial warm up period and happy to put some logs in if needed.
In the following hour you can deliver up to kWh of energy at 90°C. Your incoming water is at 8°C.
How much can you heat up to 50°C and 80°C
2 kWh is 7200000 J
Water takes 4180 J for 1 kg to go up in temperature by 1°C

So to raise 1 kg to 50°C
1(kg) x 42 (°C) x 4180 (J/(kg.K) = 175560 J

You have 7200000 J to play with

720000 / 175560 = 41 kg

To raise to 80°

1(kg) x 72 (°C) x 4180 (J/(kg.K) = 300960 J

7200000 / 300960 J = 24 kg

So now you need to find a cylinder that fits where you want it to and do the sums.

And Heat and Temperature are not the same thing (in the real world of SI Units)

Posted By: crusoeLike saying I'd like a KG of Coca-Cola

They buy it in Fluid Ounce in the USA.

Posted By: crusoeAnd 2KW back boilers are unheard of

I was shown one the other day, was a 5kW stove with a back boiler, I asked the output of the back boiler and was told it was 2 kW. Personally I would have no idea from looking at it, but the installer told me what it was and then showed me a plate by the consumer unit that had it written on it.
But assuming that the stove was 5kW, has 6 sides, 4 that radiate/conduct heat, one that mainly convects heat, and one that is at the base and does little other than catch dust, so the one that has the back boiler on can grab up to a quarter of the heat available, or around 1.25 kW ?

It is physically small, about 18" (0.4572 m) all round and in a living room.

It is interesting this litre, cubic metre, volume thing. When it comes to the physics of Specific Heat Capacity, the kilogram is used (sometimes the gram, but that is not an SI unit).
The mass of something is is related to the amount of 'stuff' in it, the amount the 'stuff' moves, and the mean speed of that movement accounts for temperature, pressure and volume, a bit of jiggery pokery with algebra to rearranging the units soon leads back to the basic SI units.

SHC changes depending on the temperature of the water also, so I always presumed it was because the weight could remain a constant, whereas the volume was a variable?