Thermo siphon.

[eabadger]

Hello, I am fitting a wood burning range in our pre eco house accommodation, I was going to use thermo siphon for heat flow from back boiler, but am unsure of the minimum height difference, the tank is a standard household 120ltr copper indirect unit with the immersion replaced with a secondary coil, in summer this had our solar homemade panels connected to it, I now plan to connect this to our current central heating system a 5kw Ebersparcher hydronic water heater (and very good they are to) so the wood burning range will be about 3mtrs away from the indirect tank coil but on the same or similar level, I can raise tank a bit if I empty it, but how much is enough?

I have a second option, the range is built in to a concrete block built “tower” with concrete capped lid, flu exit through the centre of  a concrete pipe, what about I fit another secodnhand tank up there and then somehow pump water back to first tank, bit like a simple (bodged) heat store?

The range has 5kw to water output, we have currently 5 radiators and DHW off the 5kw diesel heater, plan is to leave rads on diesel heater, so if we need instant ish heat we can switch it on.
I may take a rad off the thermo siphon for each bedroom area.

Thanks in advance for any info on thermo siphon, want to get it done next week pre santa’s visit.

Forgot to say, reason for not wanting pump is no mains power, all off grid, so would sooner not use inverter to power pump, unless we can start siphon going on pump then switch it off?

steve

[artful_bodger]

I am also off grid.  I nearly installed a pumped solar HW system before I realised that the antifreeze would degrade in the sunlight.  This is ok until the winter when you need antifreeze and then the water expands and breaks your panel while you sleep.  Sure I could keep topping up the antifreeze, but doesnt fit the eco bit.

I've been looking at in/direct thermosyphon.  My recollection of a gravity HW (gas) system I once had is that length is little problem (warm up time really) but height is important.  The hot leg has to rise to the bottom of the cylinder (I think)  My gut feeling is your indirect coil in place of the immersion will not allow thermosyphon, but you've already had this working this so I'll stop now. 

A small domestic central heating pump (or circulator as they really are) will probably move the water enough, and at 60W, not too much of a challenge for the inverter.  Make sure it's on the cold leg, and kill it when the outlet gets to 80C.

I used a pump to run a counter-chiller for beer cooling that had marginal head.  Once that started, you could turn the pump off, but there was head!

I'm currently looking at a floor mounted solar HW system, with a built in pressurised tank.  It all lives outdoors and has 50mm insulation on the tank.  It's designed for outdoors, but I cant help feeling it'll freeze with only 50mm insulation.

[kevtin101]

As long as you come from the top of the woodburner outlet and to the top of the immersion coil it will circulate, the more vertical the better to aid flow and it may if too horizontal get very hot on the outlet. I run at a very slight angle for the first 5 metres from mine and it works fine.

Regards Kevin.

[Billy]

I run gravity/thermo syphon with a rise of 50mm per 1000mm over 10 metre horizontal.  My pipe size is inch and a quarter.  The boiler output is only 2kW, nevertheless I have had no issues with over heating or bangs as pockets of steam are generated.  Thermo syphon will go anywhere apart from below the height of the return to boiler connection given time assuming the design is right ish.

Before the days of pumps the black art of gravity systems was amazing and they had special pipe fittings to allow miracles to happen like running feed and return in the same pipe.   

I have tried to emulate the old timers by using a flow/return loop and teeing off to the cast iron high water content rads with a valve to balance the heat.  Brilliant and no power needed.

I still reckon to use the same or even less fuel than a friend who has just fitted a condensing boiler, when you factor the cost of leccy.  Ok, my system is not automatic and I have to be there to fettle, clean and light the system but hey, what else am I good for?

An old house of mine had a WBS with gravity to the tank coil and pumped central heating.  During the night with the pump off the system kept the upstairs warm by gravity - toasty.

I love themo syphon me.

billy
 

[WhitePJ]

Before the days of pumps the black art of gravity systems was amazing and they had special pipe fittings to allow miracles to happen like running feed and return in the same pipe. 

Interesting...
Can you enlighten me - Why would you want to do this, and how does it work? (with respect to gravity systems). Surely it would simply act as a heat exchanger?

[Billy]

Greetings WhitePJ,

I was researching old heating systems and came across the tenement systems in New York.  Pipe sizes were measured in several inches and I guess one pipe saved money and space.  I think there is a boundary layer and the hot and cold don't mix too well.  There must be some mixing for sure with moving water but you get stratification on hot water tanks.....  The fittings allowed flow from the top and return to the bottom of the pipes on horizontal runs using Phelp's tees I think.

This site gives some ideas of what they got up to way back with the fancy fittings and names.  I can't find the original paper, I'll keep trying, it was strange reading.

http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating-continued.shtml

billy
 

[eabadger]

Hello again, thanks for replies, I appear to have not been too clear, I want the woodburner on the standard cylinder coil, the 1” conections in to 28mm, I was going to run the diesel heating in to the top secondary coil.

Both outlets on wood burner are on the horizontal, so I assume I choose the flow and return?
I may fit pump to start process, inverter is a big 3.5kw constant sine jobbie so not bothered about load, but inverter amp overhead is a big thing at present, when inverter is out of standby for a small load it can draw more current that load, make sense?

Thanks again for help, I have to work tomorrow, then back home, then only 10 days of bodging to get all install pre santa!! Will let you know how it works.

Who knows about wood burning flues………..

[Billy]

I did have a larger "gravity coil" fitted to the tank when I ordered it but if you have somewhere to dump the heat when the tank is good and hot I see no problems.

billy
 

[dhaslam]

There is a lot to be said for the  second cylinder.  Apart from  the comfort of having the extra capacity for the boiler   you should be  save  a bit on fuel for the  Ebersparcher.   The only thing is that you would need to use a differential controller  for the heat transfer down to the  other cylinder.       

[Brandon]

as long as the return (bottom) of the coil in the cylinder is above the bottom of the boiler, it will circulate provided that there are no air locks or obstructions.  I have just finished a gravity heating circuit both upstairs and downstairs (up and over) for a client, works well.  The larger you can keep the pipes the better.

[eabadger]

Thanks to all who replied, finished install last night and now have hot water, the boiler makes some alarming noises until it settles down to a sound not dissimilar to a distant steam train, normal?
Distance from cylinder to boiler is now 2.2mtrs with a rise on both feed and return of 25cm, header tank is at a head of about 2.5mtrs with expansion pipe about 20cm above this.

Now just got to plumb ebersparcher back in to secondary coil, thinking of doing this in series, so ebersparcher picks up some heat from dhw on its way to radiators, can always re plumb if it doesn’t work like a heat store, second coil is pretty high so may do?

Thanks again for replying.

[Billy]

What, like a kettle coming to the boil?  No thumps and bangs I hope.  A gentle hissing or singing while things get moving is ok but we don't want steam really, specially a loaded Britannia Class 4:6:2 up a steep incline no matter how far away it is.

Gets coat and tiptoes away quietly......

billy
 

PS.
Excellent, and before Christmas too, well done.   

[Baz]

You need to start the fire off slowly to give the thermosyphon time to get going and avoid hot spots that cause localised boiling. Thumps and bangs due to boiling /condensing cause pressure pulses that can start leaks. We had this in our house in the fifties with an old iron piped system that was piped out horizontal in both directions. My poor mother was plagued with the resulting leaks and never wanted a wet system again.

[clivejo]

Both outlets on wood burner are on the horizontal, so I assume I choose the flow and return?

If its making a noise, I think you have got them mixed up!  They might be plumbed internally (i.e. the return pipe continues in the boiler and deposits the water deep down at the base of the boiler)

One needs to be the flow and the other the return.  When you light the fire which one gets warm first?  This will be your flow which needs to continue upwards to the top of the cylinder coil.  The bottom of the coil needs to be on a downwards path to the return on the boiler.  Any kind of dips in the pipes can cause a thermo-siphon to not work properly.

[eabadger]

Hello again, I think issue was my reading the badly translated instruction wrongly or rather correctly.
I was feeding the lower firebox and did as said removed top firebox grate and filled up for cooking, but when re-read they then said this was for heating only, so reinstated top grate burn on top grate only and now only noise is when siphon is starting once the return starts getting warm proving water flow all noises goes, but water is mega hot, loads of steam at bath time, need to fit a radiator to disapate some heat.
Bad news with the ebersparcher diesel heater, the flow is wrong, output on 15mm return on 22mm, cant be bothered to sort today, got a cold, so will change on boxing day, rads are warming but no way as much as before.

If any one else ever uses a hydronic ebersparcher heater, I found out how to regulate the fuel used, mine was thirsty and using over 1ltr of red diesel per hour, now set up and less than .6ltr per hour, 5kw output is nice when plumbed properly, so about 30p per hour to run.
Have also run the heater on veg oil, makers say not to, but other than the stupid factory filter clogging it was fine, smell of chips amused the kids.